© 2017 World Scientific Publishing Company. A time-delayed quadratic nonlinear mechanical system can exhibit two coexisting stable bifurcating solutions (SBSs) after two-to-one resonant Hopf bifurcations occur in the corresponding autonomous time-delayed system. One SBS is of small-amplitude and has the Hopf bfurcation frequencies (HBFs), while the other is of large-amplitude and contains the shifted Hopf bifurcation frequencies (the shifted HBFs). When the forcing frequency is tuned to be the sum of two HBFs or the sum of two shifted HBFs, two families of additive resonances can be induced in the forced response. The forced response under the additive resonance related to the HBFs can demonstrate periodic, quasi-periodic and chaotic motion. On the contrary, the forced response under the additive resonance associated with the shifted HBFs may exhibit period-three periodic motion and quasi-periodic motion. Bifurcation diagrams, time trajectories, frequency spectra, phase portraits and Poincaré sections are presented to show periodic, quasi-periodic, and chaotic motion of the time-delayed nonlinear system under the two families of additive resonances.

This paper proposes a hierarchical multi-scale topology optimization method for the design of integrated materials and structures by taking advantage of both cellular composites and functionally graded materials. The topology optimization involves two scales: firstly, macrostructural design using SIMP to generate an overall multilayered layout with free material distribution involving intermediate densities; and secondly, microstructural design to produce periodic cellular composite for each layer, by integrating the numerical homogenization into a level set approach. Thus, the cellular composites will be characterized by variation in microstructures and the corresponding changes of properties over layers. The proposed method can generate new artificial composites similar to functionally graded materials but layer-based, to achieve multifunctional properties for energy absorption, anti-impact, thermal isolation, etc. Several numerical examples are used to demonstrate the effectiveness of this method.

Copyright © 2018 ASME. To study the impact of compliant terrains on the biomechanics of rapid legged movements, a well-known spring loaded inverted pendulum (SLIP) model is deployed. The model is a three-degrees-of-freedom system (3 DOF), inspired by galloping greyhounds competing in a racing condition. A single support phase of hind-leg stance in a galloping gait is taken into consideration due to its primary function in powering the greyhounds locomotion and higher rate of musculoskeletal injuries. To obtain and solve the nonlinear second-order differential equation of motions, the Lagrangian method and MATLABb R2017b (ode45 solver), which is based on the Runge-Kutta method, has been used, respectively. To get the viscoelastic behavior of compliant terrains, a Clegg hammer test was developed and performed five times on each sample. The effective spring and damping coefficients of each sample were then determined from the hysteresis curves. The results showed that galloping on the synthetic rubber requires more muscle force compared with wet sand. However, according to the Clegg hammer test, wet sand had a higher impact force than synthetic rubber which can be a risk factor for bone fracture, particularly hock fracture, in greyhounds. The results reported in this paper are not only useful for identifying optimum terrain properties and injury thresholds of an athletic track, but also can be used to design control methods and shock impedances for legged robots performing on compliant terrains

Understanding the biomechanics of rapid running locomotion plays an important role in comparative biomechanics and bio-inspired engineering and is an integral part of animal welfare.

However, this is not easily achieved using conventional methods of gait analysis: measuring ground reaction forces using a force plate, mainly on irregular granular terrain i.e. greyhounds in racing conditions or in animal's natural habitats i.e. cheetahs in natural terrain. An alternative to measuring forces externally via force platforms embedded in track ways, we can attach inertial measurement units to agile quadrupeds to measure the effects of rapid running and turning.

Here we deployed an IMU equipped with a tri-axial accelerometer on sprinting greyhounds to analyze rapid locomotion behaviors like dynamic banking and turning in conditions equivalent to racing. High speed videography and paw print analysis of the entire race were used for calibration. The results are beneficial in locomotion analysis and welfare of greyhounds

© 2017 National Committee on Applied Mechanics. All Rights Reserved. Greyhounds can travel twice as fast as human athletes, attaining constant average running speeds of ~65 km/h vs ~29 km/h. Their locomotion is also different from human sprinters, and more similar to cyclists. Unlike human sprinters where the muscles powering the locomotion are also supporting the weight, locomotion of greyhound are powered by torque about the hip. Agile, high-speed quadrupeds, such as the greyhound, experience extreme ground-limb contact forces while negotiating turns; leading to an increased susceptibility to injuries. Added to this, rapid, high velocity changes in direction and extreme turning angles magnify the lateral acceleration forces experienced on the limbs and torso. In this paper, the rate of severe musculoskeletal injuries of racing greyhounds at 34 tracks in New South Wales, Australia, were obtained for the year of 2016. The correlation of parameters, namely bend radius, bend camber, bend length and back straight length and the catastrophic injury rate are statistically analyzed . Track injury locations were obtained from race video footage No correlation was seen between catastrophic injury rate and bend radius, bend camber, bend length and back straight length. Analyses revealed the highest injury rate based on location to be at the first turn. Footage lends support to this being caused by the immediate clustering of the greyhounds towards the inner 'lure' rail.' The results of this study support previous findings that greyhounds racing in an anti-clockwise direction most commonly suffer musculoskeletal injuries to their right hind limbs which is consistent with knowledge of the forces that occur on the leading limbs of these dogs as they maintain their speed around bends.

This paper describes the investigation, analysis and proposed solutions to the problem of a regularly failing traction winch that resulted in significant and unacceptable downtime of crucial production machines. The winch's wire ropes fail on average about once a month. The reliability of the machine investigated is critical to the success of the business. A Root Cause Analysis (RCA) of the failing machine was conducted which identified the physical, human and latent root causes of the traction winch failures. The average number of cycles to failure for the wire ropes was only 1088 cycles (equating to approximately 24 days of service).

Physical root causes were identified through theoretical fatigue analysis and visual inspection and scanning electron microscopy (SEM) to examine the fracture surface of the failed wire ropes. The analysis indicated that there were extensive bending fatigue cracks leading to total rope failure. It was concluded that the leading physical root cause of the premature failure of the traction winch was the bending fatigue failure in the wire rope caused by the specification of an undersized sheave and drum in the traction winch design.

Hip resurfacing is proposed as an alternative to total hip replacement (THR) for treatment of osteoarthritis (OA), especially for younger, heavier and more active sufferers. There is however, concern with regards to the incidence of post operative femoral neck fractures. We have investigated, with finite element models, the changes in stress and strain in the femoral neck following hip resurfacing. We have included several different bone material property values representing normal, elderly, osteoarthritic and osteoporotic bone. We have also modelled two different hip implant orientations. We have shown that hip resurfacing may increase the magnitude of stress and strain in the femoral neck, especially in osteoporotic bone. We have also shown that the superolateral offset associated with the valgus orientation, not the valgus orientation itself, may be what reduces the stress and strain in the neck and leads to lower incidence of fracture.