Dr Oberst works as Senior Lecturer at the newly founded Centre for Audio, Acoustics and Vibration (CAAV). In 2017 he received the highly prestigious Junior Research Prize awarded by the European Association of Structural Dynamics (EASD) in the category "Development of Methodologies for Structural Dynamics". This prize follows the JSPS Award (2016) nominated by the Australian Academy of Science and an Australia Award/Endeavour Postdoctoral Research Fellowship (Australian Government, 2015) to visit the Imperial College London/Rolls-Royce Vibration University Technology Centre for 6 months. He worked as Chief Investigator on a prestigious DFG Priority Program (SPP1897) in Applied and Theoretical Mechanics at the Technical University Munich in 2016 and as Research Associate and Space Engineer (Mechanical) at the UNSW Canberra (2011-2016). Dr Oberst received his PhD in Mechanical Engineering from the University of New South Wales in Dec 2011 on his research on friction-induced instabilities in automotive brake systems.
- 2017 European Association of Structural Dynamics (EASD) Junior Research Prize in "Development of Methodologies for Structural Dynamics"
- 2016 JSPS Fellowship /Australian Academy of Science (revoked Fellowship to take a permanent position as Senior Lecturer)
- 2016 Grant, DFG Priority Program "Calm, Smooth and Smart" (SPP1897) - sole CI
- 2015 Australia Award/ Endeavour Postdoctoral Research Fellowship (Australian Goverment, 2015)
Can supervise: YES
Current research: Dr Oberst's research focuses on studying ‘Complex Dynamics’ using acoustic and vibration signals. In detail this research includes
- Nonlinear dynamics and Nonlinear time series analysis
- Bioacoustics and insect communication using microvibrations
- Mechanical engineering (friction-induced vibrations: brake squeal, hip squeak,...)
- Uncertainty analysis and statistics applied to dynamical systems
- Nonlinear oscillations in hydro-thermal geological systems modelled as open-flow chemical reactors
Overview of Sebastian's research activities and papers
Teaching experience in Dynamics, Thermofluids, Introduction into vibration, Acoustic Noise
Oberst, S. & Tuttle, S. 2018, 'Nonlinear dynamics of thin-walled elastic structures for applications in space', Mechanical Systems and Signal Processing, vol. 110, pp. 469-484.View/Download from: Publisher's site
Driven by the need for multi-functionality and increasing demands for low mass and compact-stowing, unfolding, self-deploying or –morphing smart mechanical structures have become popular space engineering
designs for flexible appendages. Extensive research has been conducted on the use of tape springs as hinge deployment mechanisms for space booms, solar sails, or optical membranes or directly for used as antennas.
However, the vibrational behaviour of tape springs and its related dynamics have rarely been addressed in detail, even though missions are underway with similarly flexible appendages installed.
By conducting quasi-static bending tests on a tape spring antenna, we evidence hysteresis behaviours in both the opposite- and equal sense bending directions. Apart from the well-known snap-through buckling, the
structure exhibits torsional buckling in the equal sense bending direction before collapsing. Micro-vibrational excitation triggers nonlinear jump phenomena and the period-doubling route to chaos. Using a computational tape spring model and simplified environmental loads similar to those encountered in near-Earth orbits, coupling between the first bending and torsional modes generates a dynamic instability which is predicted by a complex eigenvalue analysis step. The current study highlights that high perturbation sensitivity and system-inherent nonlinearities can lead to stability issues.
In the course of designing a spacecraft with thin-walled appendages, system-level trade-offs are routinely performed. Since it is unclear how severely the vibrations of flexible appendages might affect their proper
functioning or the control of the spacecraft, it is of paramount importance to validate experimentally thin-walled structures thoroughly for their dynamic and stability behaviours.
Oberst, S., Baetz, J., Campbell, G., Lampe, F., Lai, J.C.S., Hoffmann, N. & Morlock, M.M. 2018, 'Vibro-acoustic and nonlinear analysis of cadavric femoral bone impaction in cavity preparations', International Journal of Mechanical Sciences, vol. 144, pp. 739-745.
Oberst, S., Baetz, J., Campbell, G., Lampe, F., Lai, J.C.S., Hoffmann, N.P. & Morlock, M. 2018, 'Vibro-acoustic and nonlinear analysis of cadavric femoral bone impaction in cavity preparations', MATEC Web of Conferences, vol. 148, pp. 14007-14007.View/Download from: UTS OPUS or Publisher's site
Owing to an ageing population, the impact of unhealthy lifestyle, or simply congenital or gender specific issues (dysplasia), degenerative bone and joint disease (osteoarthritis) at the hip pose an increasing problem in many countries. Osteoarthritis is painful and causes mobility restrictions; amelioration is often only achieved by replacing the complete hip joint in a total hip arthroplasty (THA). Despite significant orthopaedic progress related to THA, the success of the surgical process relies heavily on the judgement, experience, skills and techniques used of the surgeon. One common way of implanting the stem into the femur is press fitting uncemented stem designs into a prepared cavity. By using a range of compaction broaches, which are impacted into the femur, the cavity for the implant is formed. However, the surgeon decides whether to change the size of the broach, how hard and fast it is impacted or when to stop the excavation process, merely based on acoustic, haptic or visual cues which are subjective. It is known that non-ideal cavity preparations increase the risk of peri-prosthetic fractures especially in elderly people.
This study reports on a simulated hip replacement surgery on a cadaver and the analysis of impaction forces and the microphone signals during compaction. The recorded transient signals of impaction forces and acoustic pressures ( 80 s - 2 ms) are statistically analysed for their trend, which shows increasing heteroscedasticity in the force-pressure relationship between broach sizes.
TIKHONOV regularisation, as inverse deconvolution technique, is applied to calculate the acoustic transfer functions from the acoustic responses and their mechanical impacts. The extracted spectra highlight that sys- tem characteristics altered during the cavity preparation process: in the high-frequency range the number of resonances increased with impacts and broach size. By applying nonlinear time series analysis the system dy- namics increase in c...
Oberst, S., Niven, R.K., Lester, D.R., Ord, A., Hobbs, B. & Hoffmann, N. 2018, 'Detection of unstable periodic orbits in mineralising geological systems', Chaos, vol. 28, no. 8.View/Download from: UTS OPUS
Worldwide, mineral exploration is suffering from rising capital costs, due to the depletion of readily recoverable reserves and the need to discover and assess more inaccessible or geologically complex deposits. For gold exploration, this problem is particular acute. We propose an innovative approach to mineral exploration and orebody characterisation, based on the analysis of geological core data as a spatial dynamical system, using the mathematical tools of dynamical systems analysis. This approach is highly relevant for orogenic gold deposits, which - in contrast to systems formed at chemical equilibrium - exhibit many features of nonlinear dynamical systems, including episodic fluctuations on various length and time scales. Feedback relationships between thermo-chemical and deformation processes produce recurrent fluid temperatures and pressures, and the deposition of vein-filling minerals such as pyrite and gold. We therefore relax the typical assumption of chemical equilibrium and analyse the underlying processes as aseismic, non-adiabatic and inherent to a hydrothermal, nonlinear dynamical open-flow chemical reactor. These processes are approximated using the Gray-Scott model (GSM) of reaction-diffusion as complex toy system, which captures some of the features of the underlying mineralisation processes, including the spatio-temporal Turing patterns of unsteady chemical reactions. By use of this analysis, we demonstrate the capability of recurrence plots, recurrence power spectra and recurrence time probabilities to detect underlying unstable periodic orbits, as one sign of deterministic dynamics, and their robustness for the analysis of data contaminated by noise. Recurrence plot based quantification is then applied to three mineral concentrations in core data from the Sunrise Dam gold deposit in the Yilgarn region of Western Australia. Using a moving window, we reveal the episodic recurring low-dimensional dynamic structures and the period doubling rout...
Oberst, S., Tuttle, S., Griffith, D., Lambert, A. & Boyce, R. 2018, 'Experimental validation of tape springs to be used as thin-walled space structures', Journal of Sound and Vibration, vol. 419, pp. 558-570.
With the advent of standardised launch geometries and off-the-shelf payloads, space programs utilising nano-satellite platforms are growing worldwide. Thin-walled, flexible and self-deployable structures are commonly used for antennae, instrument booms or solar panels owing to their lightweight, ideal packaging characteristics and near zero energy consumption. However their behaviour in space, in particular in Low Earth Orbits with continually changing environmental conditions, raises many questions. Accurate numerical models, which are often not available due to the difficulty of experimental testing under 1g-conditions, are needed to answer these questions.
In this study, we present on-earth experimental validations, as a starting point to study the response of a tape spring as a representative of thin-walled flexible structures under static and vibrational loading. Material parameters of tape springs in a singly (straight, open cylinder) and a doubly curved design, are compared to each other by combining finite element calculations, with experimental laser vibrometry within a single and multi-stage model updating approach. While the determination of the Young's modulus is unproblematic, the damping is found to be inversely proportional to deployment length. With updated material properties the buckling instability margin is calculated using different slenderness ratios. Results indicate a high sensitivity of thin-walled structures to miniscule perturbations, which makes proper experimental testing a key requirement for stability prediction on thin-elastic space structures. The doubly curved tape spring provides closer agreement with experimental results than a straight tape spring design.
Stender, M., Tiedemann, M., Hoffmann, N. & Oberst, S. 2018, 'Impact of an irregular friction formulation on dynamics of a minimal model for brake squeal', Mechanical Systems and Signal Processing, vol. 107, pp. 439-451.
Friction-induced vibrations are of major concern in the design of reliable, efficient and comfortable technical systems. Well-known examples for systems susceptible to self-excitation can be found in fluid structure interaction, disk brake squeal, rotor dynamics, hip implants noise and many more. While damping elements and amplitude reduction are well-understood in linear systems, nonlinear systems and especially self-excited dynamics still constitute a challenge for damping element design. Additionally, complex dynamical systems exhibit deterministic chaotic cores which add severe sensitivity to initial conditions to the system response. Especially the complex friction interface dynamics remain a challenging task for measurements and modeling. Today, mostly simple and regular friction models are investigated in the field of self-excited brake system vibrations. This work aims at investigating the effect of high-frequency irregular interface dynamics on the nonlinear dynamical response of a self-excited structure. Special focus is put on the characterization of the system response time series.
A low-dimensional minimal model is studied which features self-excitation, gyroscopic effects and friction-induced damping. Additionally, the employed friction formulation exhibits temperature as inner variable and superposed chaotic fluctuations governed by a Lorenz attractor. The time scale of the irregular fluctuations is chosen one order smaller than the overall system dynamics. The influence of those fluctuations on the structural response is studied in various ways, i.e. in time domain and by means of recurrence analysis. The separate time scales are studied in detail and regimes of dynamic interactions are identified. The results of the irregular friction formulation indicate dynamic interactions on multiple time scales, which trigger larger vibration amplitudes as compared to regular friction formulations conventionally studied in the field of friction-induced vibr...
Oberst, S., Bann, G., Lai, J.C.S. & Evans, T.A. 2017, 'Cryptic termites avoid predatory ants by eavesdropping on vibrational cues from their footsteps.', Ecology letters, vol. 20, no. 2, pp. 212-221.View/Download from: UTS OPUS or Publisher's site
Eavesdropping has evolved in many predator-prey relationships. Communication signals of social species may be particularly vulnerable to eavesdropping, such as pheromones produced by ants, which are predators of termites. Termites communicate mostly by way of substrate-borne vibrations, which suggest they may be able to eavesdrop, using two possible mechanisms: ant chemicals or ant vibrations. We observed termites foraging within millimetres of ants in the field, suggesting the evolution of specialised detection behaviours. We found the termite Coptotermes acinaciformis detected their major predator, the ant Iridomyrmex purpureus, through thin wood using only vibrational cues from walking, and not chemical signals. Comparison of 16 termite and ant species found the ants-walking signals were up to 100 times higher than those of termites. Eavesdropping on passive walking signals explains the predator detection and foraging behaviours in this ancient relationship, which may be applicable to many other predator-prey relationships.
Oberst, S., Marburg, S. & Hoffmann, N. 2017, 'Determining periodic orbits via nonlinear filtering and recurrence spectra in the presence of noise', Procedia Engineering, vol. 199C, pp. 772-777.View/Download from: UTS OPUS or Publisher's site
Oberst, S., Lai, J.C.S. & Evans, T.A. 2016, 'Termites utilise clay to build structural supports and so increase foraging resources.', Scientific reports, vol. 6, p. 20990.View/Download from: UTS OPUS or Publisher's site
Many termite species use clay to build foraging galleries and mound-nests. In some cases clay is placed within excavations of their wooden food, such as living trees or timber in buildings; however the purpose for this clay is unclear. We tested the hypotheses that termites can identify load bearing wood, and that they use clay to provide mechanical support of the load and thus allow them to eat the wood. In field and laboratory experiments, we show that the lower termite Coptotermes acinaciformis, the most basal species to build a mound-nest, can distinguish unloaded from loaded wood, and use clay differently when eating each type. The termites target unloaded wood preferentially, and use thin clay sheeting to camouflage themselves while eating the unloaded wood. The termites attack loaded wood secondarily, and build thick, load-bearing clay walls when they do. The termites add clay and build thicker walls as the load-bearing wood is consumed. The use of clay to support wood under load unlocks otherwise unavailable food resources. This behaviour may represent an evolutionary step from foraging behaviour to nest building in lower termites.
Oberst, S., Zhang, Z. & Lai, J.C.S. 2016, 'The Role of Nonlinearity and Uncertainty in Assessing Disc Brake Squeal Propensity', SAE International Journal of Passenger Cars - Mechanical Systems, vol. 9, no. 3.View/Download from: UTS OPUS or Publisher's site
Copyright © 2016 SAE International. Despite significant progress made in the past 20 years in discovering some of the mechanisms of brake squeal, it remains difficult to predict the underlying friction-induced instabilities reliably. Most numerical analyses are based on linear deterministic analyses of structural vibrations such as the complex eigenvalue analysis (CEA). However, nonlinear multi-scale processes govern friction contact with high sensitivities to operating and/or environmental conditions. In addition, uncertainties in the material properties and boundary conditions such as contact and friction laws are rarely considered. Hence, it is quite common to underpredict or overpredict the number of instabilities and extensive brake noise dynamometer tests are still required in industry to ensure acceptable brake noise performance. In this paper, simplified finite element brake models are used to illustrate the role of nonlinearity in brake squeal. By using nonlinear time series analyses, forced response calculations, dissipated friction work and acoustic radiations, unstable pad modes have been found to be responsible for the instantaneous mode squeal which, although observed experimentally, cannot be predicted with the traditional linear CEA. By considering coupled spring-mass-damper oscillators representing a pad on a sliding plate, the role of uncertainties of contact stiffness and friction laws in brake squeal is examined using probabilities of the positive real part of complex eigenvalues and positive friction work. The implications of nonlinearity and uncertainty for brake squeal predictions are discussed. Suggestions on how the new insights gained into nonlinearities and uncertainties can be exploited for practical brake squeal analyses in industry are proposed.
Zhang, Z., Oberst, S. & Lai, J.C.S. 2016, 'On the potential of uncertainty analysis for prediction of brake squeal propensity', Journal of Sound and Vibration, vol. 377, pp. 123-132.View/Download from: UTS OPUS or Publisher's site
© 2016 Elsevier Ltd Brake squeal is a source of significant warranty-related claims for automotive manufacturers because it is annoying and is often perceived by customers as a safety concern. A brake squeal analysis is complex due to changing environmental and operating conditions, high sensitivity to manufacturing and assembly tolerances as well as the not so well understood role of nonlinearities. Although brake squeal is essentially a nonlinear problem, the standard analysis tool in industry is the linear complex eigenvalue analysis (CEA) which may under-predict or over-predict the number of unstable vibration modes. A nonlinear instability analysis is more predictive than CEA but is still computationally too expensive to be used routinely in industry for a full brake finite element model. Also, although the net work analysis of a linearised brake system has shown potential in predicting the origin of brake squeal, it has not been extensively used. In this study, the net work of an analytical viscously damped self-excited 4-dof friction oscillator with cubic contact force nonlinearity is compared with the instability prediction using the CEA and a nonlinear instability analysis. Results show that both the net work analysis and CEA under-predict the instability because of their inability to detect the sub-critical Hopf bifurcation. Then, the uncertainty analysis is applied to examine if it can improve instability prediction of a nonlinear system using linear methods and its limitations. By applying a variance-based global sensitivity analysis to parameters of the oscillator, suitable candidates for an uncertainty analysis are identified. Results of uncertainty analyses by applying polynomial chaos expansions to net work and CEA correlate well with those of the nonlinear analysis, hence demonstrating the potential of an uncertainty analysis in improving the prediction of brake squeal propensity using a linear method.
Oberst, S. & Lai, J.C.S. 2015, 'A statistical approach to estimate the LYAPUNOV spectrum in disc brake squeal', Journal of Sound and Vibration, vol. 334, pp. 120-135.View/Download from: UTS OPUS or Publisher's site
© 2014 Elsevier Ltd. All rights reserved. The estimation of squeal propensity of a brake system from the prediction of unstable vibration modes using the linear complex eigenvalue analysis (CEA) in the frequency domain has its fair share of successes and failures. While the CEA is almost standard practice for the automotive industry, time domain methods and the estimation of LYAPUNOV spectra have not received much attention in brake squeal analyses. One reason is the challenge in estimating the true LYAPUNOV exponents and their discrimination against spurious ones in experimental data. A novel method based on the application of the Eckmann-Ruelle matrices is proposed here to estimate Lyapunov exponents by using noise in a statistical procedure. It is validated with respect to parameter variations and dimension estimates. By counting the number of non-overlapping confidence intervals for Lyapunov exponent distributions obtained by moving a window of increasing size over bootstrapped same-length estimates of an observation function, a dispersion measure's width is calculated and fed into a Bayesian beta-binomial model. Results obtained using this method for benchmark models of white and pink noise as well as the classical Henon map indicate that true Lyapunov exponents can be isolated from spurious ones with high confidence. The method is then applied to accelerometer and microphone data obtained from brake squeal tests. Estimated Lyapunov exponents indicate that the pad's out-of-plane vibration behaves quasi-periodically on the brink to chaos while the microphone's squeal signal remains periodic.
Oberst, S. & Lai, J.C.S. 2015, 'Nonlinear transient and chaotic interactions in disc brake squeal', Journal of Sound and Vibration, vol. 342, pp. 272-289.View/Download from: UTS OPUS or Publisher's site
© 2015 Elsevier Ltd. In automotive disc-brake squeal, most numerical studies have been focussed on the prediction of unstable vibration modes in the frequency domain using the complex eigenvalue analysis. However, the magnitude of the positive real part of a complex eigenvalue is an unreliable indicator of squeal occurrence. Although nonlinearities have been shown to play a significant role in brake squeal, transient nonlinear time domain analyses have rarely been applied owing to high computational costs. Here the complex eigenvalue analysis, the direct steady-state analysis and the transient nonlinear time domain analysis are applied to an isotropic pad-on-disc finite element model representing a simple model of a brake system. While in this investigation, in-plane pad-mode instabilities are not detected by the complex eigenvalue analysis, the dissipated energy obtained by the direct steady-state analysis of the model subjected to harmonic contact pressure excitation is negative at frequencies of pad modes, indicating a potential for instabilities. Transient nonlinear time domain analysis of the pad and disc dynamics reveal that in-plane pad vibrations excite a dominant out-of-plane disc mode. For intermittently chaotic pad motion, the disc dynamics is quasi-periodic; and for chaotic motion of the pad, a toroidal attractor is found for the discs out-of-plane motion. Nonlinear interactions between the pad and the disc highlight that different parts in a brake system display different dynamic behaviour and need to be analysed separately. The type II intermittency route to chaos could be the cause for the experimentally observed instantaneous mode squeal.
Oberst, S. & Lai, J.C.S. 2015, 'Pad-mode-induced instantaneous mode instability for simple models of brake systems', Mechanical Systems and Signal Processing, vol. 62, pp. 490-505.View/Download from: UTS OPUS or Publisher's site
© 2015 Elsevier Ltd. All rights reserved. Automotive disc brake squeal is fugitive, transient and remains difficult to predict. In particular, instantaneous mode squeal observed experimentally does not seem to be associated with mode coupling and its mechanism is not clear. The effects of contact pressures, friction coefficients as well as material properties (pressure and temperature dependency and anisotropy) for brake squeal propensity have not been systematically explored. By analysing a finite element model of an isotropic pad sliding on a plate similar to that of a previously reported experimental study, pad modes have been identified and found to be stable using conventional complex eigenvalue analysis. However, by subjecting the model to contact pressure harmonic excitation for a range of pressures and friction coefficients, a forced response analysis reveals that the dissipated energy for pad modes is negative and becomes more negative with increasing contact pressures and friction coefficients, indicating the potential for instabilities. The frequency of the pad mode in the sliding direction is within the range of squeal frequencies observed experimentally. Nonlinear time series analysis of the vibration velocity also confirms the evolution of instabilities induced by pad modes as the friction coefficient increases. By extending this analysis to a more realistic but simple brake model in the form of a pad-on-disc system, in-plane pad-modes, which a complex eigenvalue analysis predicts to be stable, have also been identified by negative dissipated energy for both isotropic and anisotropic pad material properties. The influence of contact pressures on potential instabilities has been found to be more dominant than changes in material properties owing to changes in pressure or temperature. Results here suggest that instantaneous mode squeal is likely caused by in-plane pad-mode instabilities.
Oberst, S., Nava-Baro, E., Lai, J.C.S. & Evans, T.A. 2015, 'An innovative signal processing method to extract ants' walking signals', Acoustics Australia, vol. 43, no. 1, pp. 87-96.View/Download from: UTS OPUS or Publisher's site
© 2015 Australian Acoustical Society. Eusocial insects such as bees, ants and termites communicate multi-modally using chemical, visual, tactile and vibrational cues. While much work has been done on chemical and visual communications, the tactile and vibrational communication channel is somewhat neglected. Recent research indicates that structural vibrations caused by ants can be used to identify their activity level. However, these structural vibrations are caused by the response of the substrate excited by ants walking. The objective of this study is to determine the footprint of ants walking by separating the response of the substrate from the walking signal. The vibration of the substrate (in this case, a wooden veneer) caused by ants walking is measured by a laser vibrometer in an experimental setup isolated from environmental vibrations. By filtering the recorded vibration signal using a technique based on the dynamics in phase space followed by deconvolution from the response of the veneer using Tikhonov regularisation, the ant's walking signal is extracted and its nature determined.
Zhang, Z., Oberst, S. & Lai, J.C.S. 2015, 'Instability analysis of friction oscillators with uncertainty in the friction law distribution', Proceedings of the Institution of Mechanical Engineers, Part C: Journal of Mechanical Engineering Science, vol. 230, no. 6, pp. 948-958.View/Download from: UTS OPUS or Publisher's site
© Institution of Mechanical Engineers 2015. Despite substantial research efforts in the past two decades, the prediction of brake squeal propensity, as a significant noise, vibration and harshness (NVH) issue to automotive manufactures, is as difficult as ever. This is due to the complexity of the interacting mechanisms (e.g. stick-slip, sprag-slip, mode coupling and hammering effect) and the uncertain operating conditions (temperature, pressure). In particular, two major aspects in brake squeal have attracted significant attention recently: nonlinearity and uncertainty. The fugitiveness of brake squeal could be attributed to a number of factors including the difficulty in accurately modelling friction. In this paper, the influence of the uncertainty arising from the tribological aspect in brake squeal prediction is analysed. Three types of friction models, namely the Amonton-Coulomb model, the velocity-dependent model and the LuGre model, are randomly assigned to a group of interconnected oscillators which model the dynamics of a brake system. The complex eigenvalue analysis, as a standard stability analysis tool, and the friction work calculation are performed to investigate the probability for instability arising from the uncertainty in the friction models. The results are discussed with a view to apply this approach to the analysis of the squeal propensity for a full brake system.
Ant behaviour is of great interest due to their sociality. Ant behaviour is typically observed visually, however there are many circumstances where visual observation is not possible. It may be possible to assess ant behaviour using vibration signals produced by their physical movement. We demonstrate through a series of bioassays with different stimuli that the level of activity of meat ants (Iridomyrmex purpureus) can be quantified using vibrations, corresponding to observations with video. We found that ants exposed to physical shaking produced the highest average vibration amplitudes followed by ants with stones to drag, then ants with neighbours, illuminated ants and ants in darkness. In addition, we devised a novel method based on wavelet decomposition to separate the vibration signal owing to the initial ant behaviour from the substrate response, which will allow signals recorded from different substrates to be compared directly. Our results indicate the potential to use vibration signals to classify some ant behaviours in situations where visual observation could be difficult.
Choice tests are a standard method to determine preferences in bio-assays, e.g. for food types and food additives such as bait attractants and toxicants. Choice between food additives can be determined only when the food substrate is sufficiently homogeneous. This is difficult to achieve for wood eating organisms as wood is a highly variable biological material, even within a tree species due to the age of the tree (e.g. sapwood vs. heartwood), and components therein (sugar, starch, cellulose and lignin). The current practice to minimise variation is to use wood from the same tree, yet the variation can still be large and the quantity of wood from one tree may be insufficient. We used wood samples of identical volume from multiple sources, measured three physical properties (dry weight, moisture absorption and reflected light intensity), then ranked and clustered the samples using fuzzy c-means clustering. A reverse analysis of the clustered samples found a high correlation between their physical properties and their source of origin. This suggested approach allows a quantifiable, consistent, repeatable, simple and quick method to maximize control over similarity of wood used in choice tests.
Oberst, S., Lai, J.C.S. & Marburg, S. 2013, 'Guidelines for numerical vibration and acoustic analysis of disc brake squeal using simple models of brake systems', Journal of Sound and Vibration, vol. 332, no. 9, pp. 2284-2299.View/Download from: UTS OPUS or Publisher's site
Brake squeal has become of increasing concern to the automotive industry but guidelines on how to confidently predict squeal propensity are yet to be established. While it is standard practice to use the complex eigenvalue analysis to predict unstable vibration modes, there have been few attempts to calculate their acoustic radiation. Here guidelines are developed for numerical vibration and acoustic analysis of brake squeal using models of simplified brake systems with friction contact by considering (1) the selection of appropriate elements, contact and mesh; (2) the extraction of surface velocities via forced response; and (3) the calculation of the acoustic response itself. Results indicate that quadratic tetrahedral elements offer the best option for meshing more realistic geometry. A mesh has to be sufficiently fine especially in the contact region to predict mesh-independent unstable vibration modes. Regarding the vibration response, only the direct, steady-state method with a pressurised pad and finite sliding formulation (allowing contact separation) should be used. Comparison of different numerical methods suggest that a obroadband fast multi-pole boundary element method with the Burton-Miller formulation would efficiently solve the acoustic radiation of a full brake system. Results also suggest that a pad lift-off can amplify the acoustic radiation similar to a horn effect. A horn effect is also observed for chamfered pads which are used in practice to reduce the number and strength of unstable vibration modes. These results highlight the importance of optimising the pad shape to reduce acoustic radiation of unstable vibration modes. © 2012 Elsevier Ltd. All rights reserved.
Brake squeal has become an increasing concern to the automotive industry because of warranty costs and the requirement for continued interior vehicle noise reduction. Most research has been directed to either analytical and experimental studies of brake squeal mechanisms or the prediction of brake squeal propensity using finite element methods. By comparison, there is a lack of systematic analysis of brake squeal data obtained from a noise dynamometer. It is well known that brake squeal is a nonlinear transient phenomenon and a number of studies using analytical and experimental models of brake systems (e.g., pin-on-disc) indicate that it could be treated as a chaotic phenomenon. Data obtained from a full brake system on a noise dynamometer were examined with nonlinear analysis techniques. The application of recurrence plots reveals chaotic structures even in noisy data from the squealing events. By separating the time series into different regimes, lower dimensional attractors are isolated and quantified by dynamic invariants such as correlation dimension estimates or Lyapunov exponents. Further analysis of the recurrence plot of squealing events by means of recurrence quantification analysis measures reveals different regimes of laminar and random behaviour, periodicity and chaos-forming recurrent transitions. These results help to classify brake squeal mechanisms and to enhance understanding of friction-related noise phenomena. © 2010 Elsevier Ltd. All rights reserved.
Despite substantial research efforts applied to the prediction of brake squeal noise since the early 20th century, the mechanisms behind its generation are still not fully understood. Squealing brakes are of significant concern to the automobile industry, mainly because of the costs associated with warranty claims. In order to remedy the problems inherent in designing quieter brakes and, therefore, to understand the mechanisms, a design of experiments study, using a noise dynamometer, was performed by a brake system manufacturer to determine the influence of geometrical parameters (namely, the number and location of slots) of brake pads on brake squeal noise. The experimental results were evaluated with a noise index and ranked for warm and cold brake stops. These data are analysed here using statistical descriptors based on population distributions, and a correlation analysis, to gain greater insight into the functional dependency between the time-averaged friction coefficient as the input and the peak sound pressure level data as the output quantity. The correlation analysis between the time-averaged friction coefficient and peak sound pressure data is performed by applying a semblance analysis and a joint recurrence quantification analysis. Linear measures are compared with complexity measures (nonlinear) based on statistics from the underlying joint recurrence plots. Results show that linear measures cannot be used to rank the noise performance of the four test pad configurations. On the other hand, the ranking of the noise performance of the test pad configurations based on the noise index agrees with that based on nonlinear measures: the higher the nonlinearity between the time-averaged friction coefficient and peak sound pressure, the worse the squeal. These results highlight the nonlinear character of brake squeal and indicate the potential of using nonlinear statistical analysis tools to analyse disc brake squeal. © 2010 Elsevier Ltd.
Merz, S., Oberst, S., Dyleiko, P.G., Kessissoglou, N., Tso, Y.K. & Marburg, S. 2007, 'Document Development of coupled FE/BE models to investigate the structural and acoustic responses of a submerged vessel', Journal of Computational Acoustics, vol. 15, no. 1, pp. 23-47.
Gong, S., Wang, X. & Oberst, S. 2018, 'Non-linear analysis of vibrating flip-flow screens', MATEC Web of Conferences, 3rd International Conference on Design and Manufacturing Engineering, EDP Sciences, Monash University, Melbourne, Australia.
Lai, J.C.S., Oberst, S. & Evans, T.A. 2018, 'Termites use vibrations to eavesdrop on predatory ants', Internoise 2018, Chicago, Illinois, USA..
LU, S., Oberst, S., Zhang, G. & Luo, Z. 2018, 'Comparing complex dynamics using machine learning-reconstructed attracting sets', Colloquium on Irregular Engineering Oscillations and Signal Processing, TUHH, Hamburg, Germany.
LU, S., Oberst, S., Zhang, G. & Luo, Z. 2018, 'Order patterns recurrence plots and new quantifications to unveil nonlinear dynamics from stochastic systems', International Conference on Time Series and Forecasting 2018, Granada, Spain.
Oberst, S. 2018, 'Nonlinear Dynamics: Towards a paradigm change via evidence-based complex dynamics modelling', Noise and Vibrations Emerging Methods, Ibiza, Spain.
Oberst, S., Baetz, J., Campbell, G., Lampe, F., Lai, J.C.S., Hoffmann, N. & Morlock, M. 2018, 'Vibro-acoustic and nonlinear analysis of cadavric femoral bone impaction for cavity preparation in hip implants', MATEC Web of Conferences, EDP Sciences.
Oberst, S., Lai, J.C.S. & Evans, T.A. 2018, 'Excitation signal extraction of ant walking pattern under the influence of noise using a biomechanical bipedal mathematical model', 2nd International Symposium on Biotremology, San Michele all'Adige, Italy.
Zhang, Z., Oberst, S. & Lai, J.C.S. 2018, 'Instability analysis of brake squeal with uncertain contact conditions', International Congress on Sound and Vibration, Hiroshima, Japan.
Hobbs, B., Ord, A., Oberst, S. & Niven, R.K. 2017, 'Nonlinear episodic and chaotic behaviour of orogenic gold systems', 18th Annual Conference of the International Association for Mathematical Geosciences, Freemantle, Australia, pp. 1-1.
Oberst, S., Baetz, J., Campbell, G., Lampe, F., Lai, J.C.S., Hoffmann, N. & Morlock, M.M. 2017, 'Vibro-acoustic and nonlinear analysis of cadavric femoral bone impaction', MATEC Web of Conferences, International Congress on Engineering Vibration, EDP Sciences, Sofia, Bulgaria, pp. 1-6.
Oberst, S., Lai, J.C.S. & Evans, T.A. 2017, 'Extracting critical information from ant and termite substrate vibrations', Invertebrate Sound and Vibration 2017, Ebsdorfergrund-Rauischholzhausen, Germany.
Oberst, S., Lester, D., Niven, R.K., Ord, A., Hobbs, B.E. & Hoffmann, N.P. 2017, 'Application of recurrence plot quantification to mineralising systems in geology', 7th International Symposium on Recurrence Plots, São Paulo.View/Download from: UTS OPUS
Oberst, S., Niven, R., Ord, A., Hobbs, B. & Lester, D. 2017, 'Application of recurrence plots to orebody exploration data', Target 2017, At University Club, University of Western Australia.View/Download from: UTS OPUS
A recurring question in mineral exploration is to predict the spatial distribution of ore-bearing
minerals, to determine both (i) the economic value of a prospective deposit, and (ii) the optimal
locations for mineral discovery (Jébrak M, 1997). In hydrothermal gold deposits such as the
Imperial deposit, in the Yilgarn of Western Australia, the gold exhibits a highly variable spatial
distribution, causing great difficulties for ore recovery and mine planning; the hypothesis is that
such variability results from orebody formation by a non-equilibrium open chemical reactor
process with complex chemical-flow-heat-mechanical couplings (Ord et al., 2012, 2016; Lester et
al., 2012). With this insight, an important goal is to develop new computational methods to analyse
drill-core data to extract key features of the underlying dynamical system, so as to shed light on
the orebody formation mechanism(s). In order to discover whether prediction is possible in such
systems we need to establish whether the patterns of alteration and mineralisation are intrinsically
random or have an underlying deterministic origin.
Ord, A., Hobbs, B., Munro, M., Oberst, S. & Niven, R.K. 2017, 'Evidence-based geological models: how to comprehend big data', 18th Annual Conference of the International Association for Mathematical Geosciences, Freemantle, Australia, pp. 1-1.
Ord, A., Oberst, S., Niven, R. & Hobbs, B. 2017, 'What do we do with all these data? Ore Exploration Using Modern Technology', Gold17@Rotorua, Rotorua, New Zealand., pp. 66-69.View/Download from: UTS OPUS
Stender, M., Oberst, S. & Hoffmann, N. 2017, 'Why mechanical machines should be treated as complex systems', Recurrence Plot Symposium, Sāo Paulo, Brazil.
Nonlinear time and recurrence analysis have proven to be highly successful for a very diverse field of nonlinear dynamical processes. Typically, this data driven method is applied to systems that cannot be modelled in a bottom-up manner (i.e. model-based) due to complexities or uncertainties involved but can be quantified via measurements. Well-known examples range from natural sciences, medicine and biology (physiology) to finance and earth science to only mention a few. Focus is on analyzing the output, i.e. observations instead of trying to mathematically detail the systems.
Friction brakes, turbo machinery and many other highly sophisticated mechanical machines reveal a plethora of unwanted and possibly safety critical noise and vibration phenomena, which today's engineers can only partly relate to operational conditions or system configurations. Analysis approaches are mostly limited to the concept of assembling models of little parts: geometrical, material and dynamical properties of each part are determined, from which a mathematical model of the complete machine is created following the pure form of linearity (superposition principle). This bottom-up approach has been applied successfully in many disciplines with the aid of advanced material models, discretization strategies, model order reduction methods and high performance computing. However, system inherent nonlinearities and their interaction, multi-physics, different time and length scales and unknown operational conditions often cause large discrepancies between experimental results and the mathematical models for numerous engineering structures, making high-fidelity modelling of a synthesized machine an ambitious task.
Considering those modeling challenges in machine dynamics and the limited modeling improvements made in recent years, we pose the provocative question: why do we not as engineers look at a machine and its dynamics as if it was a brain, a stock market or a planet's climate system, i....
Oberst, S., Zhang, Z., Campbell, G., Morlock, M., Lai, J.C.S. & Hoffmann, N. 2016, 'Towards the understanding of hip squeak in total hip arthroplasty using analytical contact models with uncertainty', Proceedings of the 45th International Congress on Noise Control Engineering, Internoise Congress, http://pub.dega-akustik.de/IN2016/data/index.html, Hamburg, Germany, pp. 5539-5549.View/Download from: UTS OPUS
© 2016, German Acoustical Society (DEGA). All rights reserved.Osteoarthritis in hip joints affects patients' quality of life such that often only costly orthopaedic surgeries i.e. total hip arthroplasty (THA) provide relief. Common implant materials are metal alloys, steel or titanium-based, plastics such as ultra-high molecular weight polyethylene, or biocompatible alumina and composite ceramics. Hard-on-hard (HoH) bearing articulations, i.e. ceramic-on-ceramic, or hard-on-soft combinations are used. HoH implants have been known to suffer from squeaking, a phenomenon commonly encountered in friction-induced self-excited vibrations. However, the frictional contact mechanics, its dynamics related to impingement, the effect of socket position, stem configuration, bearing size and patient characteristics are poorly understood. This study gives an overview of the state of the art biomechanical research related to squeaking in THA, with a focus on the effects of friction, stability, related wear and lubrication. An analytical model is proposed to study the onset of friction-induced vibrations in a simplified hemispherical hip stem rubbing in its bearing by varying the contact area. Preliminary results of the complex eigenvalue analysis and stick-slip motion analysis indicate that an increased contact fosters the development of instabilities, even at very small values of the friction coefficient owing to large local contact pressures.
Zhang, Z., Oberst, S. & Lai, J.C.S. 2016, 'Influence of contact condition and sliding speed on friction-induced instability', Proceedings of the 23rd International Congress on Sound and Vibration, International Congress on Sound and Vibration: From Ancient to Modern Acoustics (ICSV), International Institute of Acoustics and Vibration, Athens, Greece.View/Download from: UTS OPUS
Brake squeal, defined as audible noise above 1 kHz, is triggered by energy provided in the contact area between the pad and the disc and friction-induced instabilities. Owing to customers' demand of reducing vehicle noise and the increasing use of light composite materials in cars, squealing brakes remain a major concern to the automotive industry because of warranty-related claims. The prediction of disc brake squeal propensity is as challenging as ever. Although friction-induced instabilities are inherently nonlinear and during squeal the brake system's operating and environmental conditions keep changing, mostly linear and steady state methods are used for the analysis of brake squeal propensity. While many different instability mechanisms have been identified, their interactions and the resulting dynamics are not yet fully understood. Linear instability predictions suffer from over- and under-predictions and have to be complemented by extensive noise dynamometer or in vehicle tests. Recent studies indicate that frictional contact is multi-scaled in nature, highly sensitive and inhomogeneous. Very high local pressures and partial contact separations in the contact interface further complicate its numerical modelling. By studying an analytical model of 3 3 friction oscillators using three different friction laws (Amonton-Coulomb, the velocity-dependent and the LuGre friction model) in point contact with a sliding rigid plate and incorporating uncertainties in the contact condition, robustly unstable vibration modes have been identified in our previous research. Here, the number and the combination of friction oscillators engaged in contact are randomised to model imperfect contact. In addition, the effect of the variation in the plate's sliding velocity on the in-stability analysis is investigated with randomised friction coefficient of the Amonton-Coulomb friction model. Results of instability prediction and net work calculations are used to illustrate the s...
Oberst, S., Griffin, D., Tuttle, S., Lambert, A. & Boyce, R.R. 2015, 'Analysis of thin curved flexible structures for space applications', Acoustics 2015 Hunter Valley, Conference of the Australian Acoustical Society, Hunter Valley, NSW, Australia.View/Download from: UTS OPUS
With the advent of affordable nano-satellite designs (off-the-shelf payloads, standardised launch geometries), increasingly enterprises, governmental agencies and universities have started developing their own space programs to explore the environment of Low Earth Orbits. Thin, flexible and unfolding/deployable structures are common space engineering antenna and solar panel designs owing to their lightweight and ideal packaging characteristics, which are, however, difficult to experimentally validate in a 1-g environment. Further, curvatures or discontinuities to increase functionality without violating prioritised design criteria may lead to system-level trade-offs: stability issues arising from buckling in combination with micro-vibrations which feed back to the satellite's attitude behaviour. It appears that the literature lacks a systematic investigation of these aspects. On-Earth experimental validations (static experiments, model updating) are the starting point for studying the response to static/dynamic loading of thin curved flexible structures such as deployable high frequency antennas. Linear and nonlinear buckling modes owing to varying loadings (aerodynamic drag, solar radiation pressure, residual gravity and magnetic body forces) are found together with a high sensitivity to torsional modes' frequency changes under micro-vibrational forcing.
Williams, J.J.R., Zhang, Z., Oberst, S. & Lai, J.C.S. 2015, 'Model updating of brake components' influence on instability predictions', 22nd International Congress on Sound and Vibration, ICSV 2015, International Congress on Sound and Vibration, Florence, Italy.View/Download from: UTS OPUS or Publisher's site
Customers perceive brake squeal as a major annoyance in their automobiles' acoustic performance. Squeal is self-excited, friction induced audible noise above 1 kHz and one of the strongest cost drivers in noise vibration and harshness departments of automotive manufacturers. In order to reduce expensive and time-consuming dynamometer and road vehicle tests, numerical complex eigenvalue analysis has become popular in predicting brake squeal. However, one difficulty in assessing the prediction quality apart from the linearisation of the system is the complexity of the brake system to be modelled. Using structural finite elements the computer model is often insufficiently detailed, insufficiently damped or insufficiently experimentally validated so that instabilities causing brake squeal are over-predicted. Here we present the process of updating components of a brake system's squeal prediction and the improvement in modelling using updated material parameters and a Rayleigh damping model by applying a rigorous mesh refinement study and different friction laws.
Zhang, Z., Oberst, S. & Lai, J.C.S. 2015, 'Instability analysis of coupled friction oscillators with uncertainties in contact conditions', 22nd International Congress on Sound and Vibration, ICSV 2015, International Congress on Sound and Vibration, Florence, Italy.View/Download from: UTS OPUS or Publisher's site
Although brake squeal is a significant noise, vibration and harshness (NVH) issue which incurs significant cost in the automotive industry, its prediction is still difficult. This is because brake squeal is essentially a nonlinear phenomenon and traditional complex eigenvalue analysis (CEA) is a linear method. In addition, there are many uncertainties in a brake system such as material properties, operating and contact conditions which cannot be determined accurately with confidence. Here, the influence of uncertainties in contact conditions on the instability of an analytical model consisting of 3x3 coupled oscillators in point contact with a sliding rigid plate is analysed. The uncertainties in contact conditions considered are: percentage of contact, stiffness and friction laws for the contact (Amonton-Coulomb, relative velocity dependent and LuGre law). The instability is analysed in the frequency domain by randomising these three uncertainty parameters. The results will be discussed with a view to applying this approach to the analysis of the squeal propensity for a full brake system.
Zhang, Z., Oberst, S., Williams, J.J.R. & Lai, J.C.S. 2015, 'Improving Brake squeal propensitiy prediction by model updating', Acoustics 2015 Hunter Valley, Conference of the Australian Acoustical Society, Hunter Valley, NSW, Australia.View/Download from: UTS OPUS
Brake squeal as a significant warranty-claim related costs problem to the automotive industry is difficult to model numerically and analyse because of inherent nonlinearities, uncertainties in material properties, contact and boundary conditions, and system complexity. Often, model components are linearised and not experimentally validated. Sophisticated contact or friction models as well as stiffness in joints are often not considered owing to difficulties in experimental validation. In this study, a full brake system is modally updated at the component level and then at the subassembly level (pad assembly alone, pad in bracket). Squeal prediction using the complex eigenvalue analysis on a finite element model of the system is compared to squeal results from a noise dynamometer test. The results are discussed with respect to further refinement of the modelling approach and improvements to brake squeal prediction.
Oberst, S., Nava-Baro, E., Lai, J.C.S. & Evans, T.A. 2014, 'An innovative signal processing technique for the extraction of ants' walking signals', INTERNOISE 2014 - 43rd International Congress on Noise Control Engineering: Improving the World Through Noise Control, INTERNOISE 2014 - 43rd International Congress on Noise Control Engineering: Improving the World Through Noise Control.
Eusocial insects such as bees, ants and termites communicate multi-modally using chemical, visual, tactile and vibrational cues. While much work has been done on chemical and visual communications, the tactile and vibrational communication channel is somewhat neglected. Recent research indicates that structural vibrations caused by ants can be used to identify their activity level. However, these structural vibrations are caused by the response of the substrate excited by ants walking. The objective of this study is to determine the footprint of ants walking by separating the response of the substrate from the walking signal. The vibration of the substrate (in this case, a wooden veneer) caused by ants walking is measured by a laser vibrometer in an experimental setup isolated from environmental vibrations. By filtering the recorded vibration signal using a technique based on the dynamics in phase space followed by deconvolution from the response of the veneer using TIKHONOV regularisation, the ant's walking signal is extracted and its nature determined.
Zhang, Z., Oberst, S. & Lai, J.C.S. 2014, 'A stochastic approach to predicting brake squeal propensity', 21st International Congress on Sound and Vibration 2014, ICSV 2014, 21st International Congress on Sound and Vibration 2014, ICSV 2014, pp. 629-636.View/Download from: UTS OPUS
Brake squeal as a significant noise, vibration and harshness (NVH) issue to the automotive industry is triggered by friction-induced self-excited vibration. Validating theoretical predictions using analytical or numerical models against experiments is difficult because the test results are often not repeatable even under apparently similar operating conditions. The poor repeatability of brake squeal could be attributed to the nonlinearity of the dynamics involved and the uncertainties associated with material properties, boundary conditions (such as contact pressure, temperature, stiffness, exact area of contact) and operating conditions. In this paper, a stochastic approach to predicting brake squeal propensity is examined using an analytical model of a popular 4-DOF friction oscillator with constant friction coefficient. Instability of this model is first estimated using the conventional linear complex eigenvalue analysis (CEA) and compared with calculations of positive friction work. The sensitivity of this deterministic model to variations of parameters such as spring stiffness and damping coefficient is studied. To account for uncertainties in the exact values of parameters, the analytical model is studied using polynomial chaos expansions with beta distribution on a set of Jacobi polynomials. Probabilities for instabilities based on positive friction work are determined and the implications for estimating squeal propensity in a full brake system are discussed.
Zhang, Z., Oberst, S. & Lai, J.C.S. 2014, 'Instability prediction of brake squeal by nonlinear stability analysis', INTERNOISE 2014 - 43rd International Congress on Noise Control Engineering: Improving the World Through Noise Control, INTERNOISE 2014 - 43rd International Congress on Noise Control Engineering: Improving the World Through Noise Control.View/Download from: UTS OPUS
Prediction of brake squeal as unwanted high frequency noise above 1 kHz remains a challenging problem despite substantial research efforts in the past two decades. Brake squeal, triggered by friction-induced self-excited vibration, can be caused by many different and interacting mechanisms with nonlinear origins in material properties and boundary conditions. Although brake squeal is essentially a nonlinear phenomenon, the standard industrial practice for prediction of brake squeal relies on the linear complex eigenvalue analysis which may under-predict or over-predict the number of unstable vibration modes. Brake squeal can be considered in nonlinear dynamics terms to be caused by a friction-induced self-excitation driven into instability and oscillating in a limit cycle through super-critical Andronov-Hopf bifurcations. In this paper, a nonlinear stability analysis that may be applied to a full brake system is examined using an unforced 4-DOF friction oscillator with cubic nonlinearity. The local bifurcation behaviour of this model is studied using the normal form theory and the nonlinear stability boundary is evaluated. Differences between results of linear and nonlinear analyses are discussed and the limitations of the linear analysis are highlighted. The energy provided by friction and consumed by damping is calculated by multiple scales method to provide a physical explanation for instability generation.
Baro, E.N., Oberst, S., Lai, J.C.S. & Evans, T.A. 2013, 'A signal processing method for extracting vibration signals due to ants' activities', 42nd International Congress and Exposition on Noise Control Engineering 2013, INTER-NOISE 2013: Noise Control for Quality of Life, International Congress and Exposition on Noise Control Engineering, Innsbruck, Austria, pp. 3631-3640.View/Download from: UTS OPUS
Many software algorithms have been developed to track ants by analysing recorded videos. On the other hand, the feasibility of using vibrations measured at the substrate to classify ants' behaviour has not been examined before. A method is developed to separate vibrations owing to ants' activities from the substrate's response through a filtering/de-convolution procedure. This involves estimating the frequency response of the substrate and applying wavelet analysis to the measured vibrations. A number of responses due to ants' behaviours have been observed: Ants shaking, falling, carrying stones, walking, scratching/biting, tapping hind legs, grooming, and antennation/feeding. Vibrations produced by ants falling, carrying stones, walking and scratching/biting are measurable (i.e, above background noise levels). The proposed method is shown to be successful in classifying activities due to ants falling, ants carrying stones and to a lesser extent ants' scratching/biting. With further refinement, it seems feasible to use vibrations and the proposed algorithm to measure ants' behaviours in bioassays. Copyright© (2013) by Austrian Noise Abatement Association (OAL).
Oberst, S. & Lai, J.C.S. 2013, 'The role of pad-mode instabilities in disc brake squeal', 20th International Congress on Sound and Vibration 2013, ICSV 2013, International Congress on Sound and Vibration, Bangkok, Thailand, pp. 2861-2868.View/Download from: UTS OPUS
Automotive disc brake squeal remains an economically significant and technically challenging problem to solve, owing to customer complaints' associated warranty costs and the many interacting parameters. While industrial practice aims at identifying unstable vibration modes using complex eigenvalue analysis, in this paper, we show how to identify pad-mode instabilities using vibration forced response analysis complemented by acoustic radiation calculations for simplified brake systems in the form of a pad-on-plate model. Our recent results indicate that pad-mode instabilities might trigger so-called instantaneous mode squeal without the necessity of mode coupling. Pad-mode instabilities, which complex eigenvalue analysis fails to detect, are revealed by the dissipated energy spectrum at frequencies where the dissipated energy is negative (i.e. providing energy instead of dissipating energy). Pad-modes seem to radiate locally higher sound pressure depending on the phase shift between the structural vibration and the sound pressure while exciting the underlying plate's or disc's modes. Pad-mode instabilities are shown to be one mechanism of brake squeal. In order to identify pad-mode instabilities, it is beneficial to perform a full range of vibration analysis which includes complex eigenvalue value analysis, forced response and dissipated energy spectra as well as acoustic radiation calculations for a range of different parameters such as friction coefficient, operating pressure, temperature and contact conditions.
Oberst, S.M. & Lai, J. 2013, 'The role of pad-modes and nonlinearity in instantaneous mode squeal', Proceedings of Meetings on Acoustics, Meetings on Acoustics.View/Download from: UTS OPUS or Publisher's site
Disc brake squeal is a major source of customer dissatisfaction and related warranty costs for automobile manufacturers. Although mode coupling is recognised as a mechanism often found in squealing brakes, recent research results show that friction induced pad-mode instabilities could be the cause of instantaneous mode squeal reported in the literature. In this paper, the nonlinear characteristics of instantaneous mode squeal initiated by pad-mode instabilities are studied by analysing phase space plots of vibrations and sound pressure for a numerical model of a pad-on-plate system as the friction coefficient increases. Results show tat as the friction coefficient increases from 0.05 to 0.65, attractors of vibration in the phase space transits from limit cycle to quasi-periodic, showing signs of approaching chaotic behaviour. It is shown here that the correlation of the sound pressure behaviour in the phase-space with structural vibration is crucial to understanding the role of pad modes and nonlinearity in instantaneous mode squeal. © 2013 Acoustical Society of America.
Zhang, Z., Oberst, S. & Lai, J.C.S. 2013, 'Application of polynomial chaos expansions to analytical models of friction oscillators', Annual Conference of the Australian Acoustical Society 2013, Acoustics 2013: Science, Technology and Amenity, Annual Conference of the Australian Acoustical Society, Victor Harbor, Australia, pp. 408-414.View/Download from: UTS OPUS
Despite past substantial research efforts, the prediction of brake squeal propensity remains a largely unresolved problem. The standard practice to predict the brake squeal propensity is to analyse dynamic instabilities using the complex eigenvalue analysis. However, it is well known that not every predicted unstable vibration mode will lead to squeal and vice-versa. Owing to nonlinearity and problem complexity (e.g. operating conditions), treating brake squeal with uncertainty seems appealing. Another indicator of brake squeal propensity, not often used, is based on negative dissipated energy. In this study, uncertainty analysis induced by polynomial chaos expansions is examined for 1-dof and 4-dof friction models. Results are compared with dissipated energy calculations and standard complex eigenvalue analysis. The potential of this approach for the prediction of brake squeal propensity is discussed. © (2013) by the Australian Acoustical Society.
Oberst, S. & Lai, J.C.S. 2012, 'Analysis of disc brake squeal: Progress and challenges', 19th International Congress on Sound and Vibration 2012, ICSV 2012, pp. 2874-2881.
Brake squeal noise has been the subject of intense research efforts owing to concerns of car manufacturers caused by complaints lodged and warranty claim related costs arising from dissatisfied customers. Brake squeal is known to be fugitive, and often not repeatable, even under apparently similar operating conditions. The production of brake squeal is dependent on a large number of interacting parameters, such as the mechanical properties of the brake lining materials, contact conditions, wear, operating pressure and temperature which contribute to its often observed nonrepeatability. In this paper, an overview of the state-of-the-art understanding of brake squeal mechanisms and numerical analysis methods (primarily based on finite element analysis) for the prediction of brake squeal propensity is presented. The question of nonlinearity of brake squeal is raised in terms of analysing the mechanisms and how present solution methods reflect this degree of nonlinearity. This is complemented by a description of current industrial practice in the treatment of brake squeal which is, generally, managed on a case-by-case, trial-and-error basis using expensive equipment and time-consuming noise dynamometer and/or on-vehicle tests. The gaps between theory and industrial practice and, hence, challenges for brake squeal research are identified. Recommendations for bridging these gaps and improving the usefulness of current numerical methods for practical industrial use are proposed.
The prediction of disc brake squeal propensity remains difficult despite significant progress made in the last two decades towards understanding its nature. Most of the numerical analysis of brake squeal is based on linear methods that have found some success in guiding the development of brakes in industry. One popular approach is the complex eigenvalue analysis using finite element models to predict unstable vibration modes. However, the complex eigenvalue analysis may over-predict or under-predict the number of unstable vibration modes and not all predicted unstable vibration modes will result in squeal. Therefore, extensive brake testing in noise dynamometers is required in order to ensure that the noise performance of brakes is acceptable. Although the analysis of brake squeal propensity is primarily based on linear approaches, it has been recognised that the operation of a brake contains a number of nonlinearities such as the excitation through the friction contact between the disc and pad, material properties, and operating conditions. The purpose of this paper is to provide an overview on nonlinearity as one mechanism of the cause of brake squeal and to discuss how such knowledge could be used to develop alternative strategies in numerical prediction of brake squeal propensity. © European Acoustics Association.
Oberst, S. & Lai, J.C.S. 2011, 'Nonlinear friction coupling in disc brake squeal', 18th International Congress on Sound and Vibration 2011, ICSV 2011, International Congress on Sound and Vibration, International Institute of Acoustics & Vibration, Rio de Janeiro, Brazil, pp. 1748-1755.
Friction-induced noise, such as disc brake squeal as a research area of practical importance to the automotive industry, has been investigated for many years. In recent years research focus was on dynamic instabilities such as mode coupling as the most prominent squeal mechanism, rather than on physical or geometric instabilities as trigger mechanisms. However, many trigger mechanisms such as contact and friction nonlinearity are still poorly understood. In brake squeal analysis, pad-dynamics is poorly understood and in-plane radial and out-of-plane pad vibrations are often neglected as most research has been directed towards understanding the dynamics of the rotor as the main structure radiating sound. However, recent research has shown that transient radial in-plane vibrations of the pad might be a novel squeal mechanism. In this study, the transient nature of these radial in-plane vibrations and their influence on the overall vibration and dynamic behaviour is investigated. For this purpose a sinusoidally driven in-plane sliding of a friction-coupled 2-dof oscillator (in the form of a slider over a moving belt) is formulated based on the most stable configuration of a 1-dof dry friction oscillator with continuous/ locking contact using a friction law with constant/static-kinetic/velocitydependent friction coefficient. It is found that due to nonlinear friction-coupling, steady-state in-plane radial vibrations induce a broadband spectrum in the overall dynamics of the friction oscillator with slightly fractal POINCARE section for a small belt's angle. The results indicate the importance of in-plane pad-vibrations as a possible trigger squeal mechanism. Copyright © (2011) by the International Institute of Acoustics & Vibration.
Oberst, S. & Lai, J.C.S. 2010, 'Acoustic radiation of friction-induced pad-mode instability in disc brake squeal', 20th International Congress on Acoustics 2010, ICA 2010 - Incorporating Proceedings of the 2010 Annual Conference of the Australian Acoustical Society, pp. 2158-2168.
Since the early 1920s, disc brake squeal has been an issue for the automobile industry due to dissatisfied customer's complaints and the accompanying warranty costs. Despite a good deal of progress having been made in predicting brake squeal propensity, not all mechanisms are known and brake squeal remains unpredictable and highly fugitive. In recent years, research has been focused on brake squeal due to the mode-coupling type of instability, leaving out the primary friction-induced mechanisms such as stick-slip. In this paper, the acoustic radiation of simplified brake systems, in the form of a pad rubbing on both a plate and disc, is investigated. The radiation efficiency and acoustic power are calculated using the acoustic boundary element method, specifically ESI's Fast Multipole Solver (DFMM) implemented in VAOne. Results show that there exist some frequencies at which squeal occurs but which are predicted by the complex eigenvalue method. These frequencies do not correspond to the frequencies of the rotor modes and are here referred to as 'instantaneous' pad-modes causing a friction-induced instability. The frequencies of these instantaneous modes are dependent on the material properties of the pad and the contact conditions. Radiation efficiency due to pressure variations changes less, than due to friction coefficient variations. Further, it is shown, that pad-modes are acoustically relevant and especially active at lower pressures.
Oberst, S. & Lai, J.C.S. 2010, 'Numerical methods for simulating brake squeal noise', 20th International Congress on Acoustics 2010, ICA 2010 - Incorporating Proceedings of the 2010 Annual Conference of the Australian Acoustical Society, pp. 1505-1516.
Due to significantly reduced interior noise as a result of reduction of noise from internal combustion engine and tyre-road contact noise and the use of lightweight composite materials for the car body, disc brake squeal has become increasingly a concern to automotive industry because of the high costs in warranty related claims. While it is now almost standard practice to use the complex eigenvalue method in commercial finite element codes to predict unstable vibration modes, not all predicted unstable vibration modes will squeal and vice versa. There are very few attempts to calculate the acoustic radiation from predicted unstable vibration modes. Guidelines on how to predict brake squeal propensity with confidence are yet to be established. In this study, three numerical aspects important for the prediction of brake squeal propensity are examined: how to select an appropriate mesh; comparisons of methods available in ABAQUS 6.8.-4 for harmonic forced response analysis; and comparisons of boundary element methods (BEM) for acoustic radiation calculations in LMS VL Acoustics and ESI VA. In the mesh study, results indicate that the mesh has to be sufficiently fine to predict mesh independent unstable modes. While linear and quadratic tetrahedral elements offer the best option in meshing more realistic structures, only quadratic tetrahedral elements should be used for solutions to be mesh independent. Otherwise, linear hexahedral elements represent an alternative but are not as easy to apply to complex structures. In the forced response study, the modal, subspace and direct steady-state response analysis in ABAQUS are compared to each other with the FRF synthesis case in LMS/VL Acoustics. Results show that only the direct method can take into account friction effects fully. In the numerical analysis with acoustic boundary elements, the following methods are compared in terms of performance and accuracy for a model of a sphere, a cat's eye radiator, a pad-on-plate ...
Oberst, S. & Lai, J.C.S. 2010, 'Numerical study of friction-induced pad-mode instability in disc brake squeal', 20th International Congress on Acoustics 2010, ICA 2010 - Incorporating Proceedings of the 2010 Annual Conference of the Australian Acoustical Society, pp. 2146-2157.
Disc brake squeal as a major source of customer dissatisfaction is known to be friction-induced due to the highly non-linear contact of the surfaces between the disc and the pads. Brake squeal remains fugitive and difficult to predict also to some of its squeal frequencies have varying character and cannot always be associated with component modes. By means of structural finite element analysis, a simplified brake system in the form of a pin-on-disc is firstly approximated by a block sliding on a plate. By varying pressure and the friction coefficient, no mode coupling instability is observed and the mechanism extracted is purely of friction-induced nature. Especially in-plane pad motion in direction of and perpendicular to the sliding direction seem to feed-in most of the energy. These modes and their variability due to pressure variation, changes of lining material's elastic components and increased friction coefficient are studied in the following by means of the plate model. Then, it is shown, that these pad modes also exist for a pad-on-disc model with isotropic lining material. A second pad-on-plate model with more realistic lining material is developed which considers changes of elastic constants due to pressure variations. It is found, that changes in elastic properties of the lining material influence significantly the vibrations of the pad modes. The kinetic energy spectrum lifts up with changing pressure and stiffness and that combined effects of pressure synchronised with changing material properties are more severe than could be assumed by the complex eigenvalue method alone. By means of inverse Fourier transform of the response spectrum and non-linear time series analysis it is possible to detect the instability of the pad-on-plate model. The results show that friction-induced instabilities result from non-binding forces between pad and disc, with energy transfer from pad to disc causing dynamic instability, might trigger mode coupling or amplify un...
Oberst, S. & Lai, J.C.S. 2010, 'Uncertainty modelling for detecting friction-induced pad-mode instabilities in disc brake squeal', 20th International Congress on Acoustics 2010, ICA 2010 - Incorporating Proceedings of the 2010 Annual Conference of the Australian Acoustical Society, pp. 1517-1528.
Since the early 1930s, brake squeal has been a problem for NVH departments and the high-pitched noise causes customers to complain and lodge costly warranty claims. Due to its friction-induced nature, material properties and operating conditions, the problem of brake squeal is non-linear and highly complex. In the past, research has been focussed on mode-coupling instability predicted by the complex eigenvalue analysis (CEA). However, for unstable modes not detected by CEA, friction-induced energy fed back by the pad modes, due to the friction coefficient, pressure variations and non-linear material properties, has been shown, by means of non-linear time series analyses and the acoustic boundary element method, to cause friction-induced pad squeal or to amplify the mode coupling of brake components for a pad-on-plate system. It is suggested that pad mode instabilities be treated as a stochastic process defined by a random 3-parameter-space: the mean changes in kinetic energy, frequency and acoustic power caused by changes in pressure or the friction coefficient. It is shown that, for a pad-on-plate system and a pad-on-disc simplified brake system, this stochastic approach enables the probability to be calculated for a specified increase in kinetic energy or a specified change in frequencies, thus allowing the assessment of brake squeal propensity and the development of strategies for controlling brake squeal. Copyright © (2010) by the International Congress on Acoustics.
Oberst, S. & Lai, J.C.S. 2009, 'Non-linear analysis of brake squeal', 16th International Congress on Sound and Vibration 2009, ICSV 2009, pp. 1116-1123.
Transportation noise has received increasingly more attention in the last decade from the environmental point of view but also from customers. In the case of cars, manufacturers have focussed on reducing the overall car's noise level. As a result, engine, gearboxes and the overall drive train have seen a reduction sound emission. Because of this overall reduction, brake squeal becomes even more audible. Brake squeal is a constant annoyance for customers and a significant cost factor for the car industry due to warranty claims. Brake squeal is self-sustained friction-induced noise and various mechanisms have been shown to be capable of causing unstable vibration behaviour, for example, sprag-slip, stick-slip or mode coupling. Usually disc brake squeal is analysed by means of linear methods such as the complex eigenvalue method employed in finite element analysis as virtually an industry standard. However, brake squeal is intrinsically a transient and non-linear process, the geometry is complex, the material properties and contact conditions between brake pads and the brake disc are difficult to be determined exactly. As a result, prediction of brake squeal propensity is difficult to be realised in the near future. In this study, a new approach to brake squeal is undertaken by performing non-linear time series analysis of data obtained from an accelerometer and a microphone of a brake system in a dynamometer. New insight into brake squeal will be discussed.
Oberst, S. & Lai, J.C.S. 2009, 'Numerical prediction of brake squeal propensity using acoustic power calculation', Annual Conference of the Australian Acoustical Society 2009 - Acoustics 2009: Research to Consulting, pp. 111-118.
Both low- and high-frequency disc brake squeal, first studied some 80 years ago, remain of concern to automotive NVH departments due to customer warranty claims. Despite both intensive and extensive research, disc brake squeal is still not well understood. It is a very complex problem which involves many different disciplines, such as tribology, structural vibration, acoustic radiation and dynamic instabilities. While there has been considerable research in the first two areas (tribology and vibration analysis), the prediction of brake squeal through acoustic radiation calculations using numerical methods has remained largely unexplored. In this paper, the influence of the geometrical designs of brake pad on brake squeal is studied using a simplified brake setup consisting of an annular disc in contact with one brake pad. The various configurations of a brake pad studied here has been influenced by those used in the industrial testing of a full brake system. In this study, unstable vibration modes were first identified by the conventional complex eigenvalue analysis of a finite element model of the simplified brake system. Then, the acoustic power was calculated for a range of frequencies and friction coefficients using the acoustic boundary element method. It is shown that the performance of the various pads, in terms of brake squeal propensity caused by their geometric differences, could be ranked based on contour plots of acoustic power with friction coefficient and frequency as the independent variables. These results indicate that the inclusion of acoustic power calculations, following a complex eigenvalue analysis of unstable vibration modes, provides improved prediction of brake squeal propensity.
Oberst, S. & Lai, J.C.S. 2008, 'New approaches for understanding the mechanisms of brake squeal', Annual Conference of the Australian Acoustical Society, AAS'08, pp. 265-272.
Brake squeal has become an increasing concern for the automotive industry because of associated warranty costs and the requirement for the continued reduction of interior vehicle noise. Low and high frequency noises in car brakes, often referred to as brake squeal, are known to be a result of nonlinearity, unstable behaviour and bifurcations leading to limit cycle behaviour. By using the data from a separate experimental study designed to determine the influence of the geometric parameters of brake pads (such as the number and location of slots) on brake squeal noise, we examine two new approaches for providing improved understanding of the brake squeal phenomenon: statistical and nonlinear dynamics analyses. Results of the statistical analysis indicate that the performances of certain pad designs correlate with their levels of nonlinearity. The nonlinear time series analysis reveals that, in the experimental data, not only are limit cycle behaviours present but also a route to chaotic solutions can be observed.
Merz, S., Oberst, S., Dylejko, P.G., Kessissoglou, N.J., Tso, Y.K. & Marburg, S. 2007, 'Development of coupled FE/BE models to investigate the structural and acoustic responses of a submerged vessel', Journal of Computational Acoustics, pp. 23-47.View/Download from: Publisher's site
An analytical model and a fully coupled finite element/boundary element model are developed for a simplified physical model of a submarine. The submerged body is modeled as a ring-stiffened cylindrical shell with finite rigid end closures, separated by bulkheads into a number of compartments and under axial excitation from the propeller-shafting system. Lumped masses are located at each end to maintain a condition of neutral buoyancy. Excitation of the hull axial modes from the propeller-shafting system causes both axial motion of the end closures and radial motion of the shell, resulting in a high level of radiated noise. In the low frequency range, only the axial modes in breathing motion are examined, which gives rise to an axisymmetric case, since these modes are efficient radiators. An expression for the structurally radiated sound pressure contributed by axial movement of the end plates and radial motion of the shell was obtained using the Helmholtz integral equation. In the computational model, the effects of the various influencing factors (ring stiffeners, bulkheads, realistic end closures, and fluid loading) on the free vibrational characteristics of the thin walled cylinder are examined. For both the analytical and computational models, the frequency responses, axial and radial responses of the cylinder, and the radiated sound pressure are compared. © IMACS.
LU, S., Zhang, G., Luo, Z. & Oberst, S. 2018, 'Order pattern recurrence plots unveiling determinism buried in noise'.
Oberst, S., Stender, M., Baetz, J., Campbell, G., Lampe, F., Lai, J.C.S., Morlock, M. & Hoffmann, N. 2018, 'Extracting differential equations from measured vibro-acoustic impulse responses in cavity preparation of total hip arthroplasty'.
Hobbs, B., Ord, A., Oberst, S. & Niven, R.K. 2017, 'Nonlinear episodic and chaotic behaviour of orogenic gold systems'.
- UNSW (Australia): Prof Joseph C.S. Lai; A/Prof Robert Niven; A/Prof Nicole Kessissoglou; Dr Sean Tuttle
- UWA (Australia) A/Prof Theodore A. Evans
- TU Munich (Germany): Prof Steffen Marburg
- Imperial College London & TU Hamburg-Harbug: Prof Norbert Hoffmann
- TU Clausthal (Germany): Prof Stefanie Retka
- TU Dresden & University Clinic Dresden (Germany): Dr Mario Fleischer
- Harvard University: Dr Justin Werfel, Dr Nicole Carey
- Cambridge University (Zoology, Neuroscience): Dr Caroline C.G. Fabre