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This paper describes an effective integrated method for estimation of subject-specific mass, inertia tensor, and center of mass of individual body segments of a digital avatar for use with physics-based digital human modeling simulation environment. One of the main goals of digital human modeling and simulation environments is that a user should be able to change the avatar (from male to female to a child) at any given time. The user should also be able to change the various link dimensions, like lengths of upper and lower arms, lengths of upper and lower legs, etc. These customizations in digital avatar's geometry change the kinematic and dynamic properties of various segments of its body. Hence, the mass and center of mass/inertia data of the segments must be updated before simulating physics-based realistic motions. Most of the current methods use mass and inertia properties calculated from a set of regression equations based on average of some population.

The objective of this paper is to present our method of predicting and simulating visually realistic and dynamically consistent human stair-climbing motion. The digital human is modeled as a 55-degrees of freedom branched mechanical system with associated human anthropometry-based link lengths, mass moments of inertia, and centers of gravity. The joint angle profiles are determined using a B-spline-based parametric optimization technique subject to different physics-based, task-based, and environment-based constraints. The formulation offers the ability to study effects of the magnitude and location of external forces on the resulting joint angle profiles and joint torque profiles. Several virtual experiments are conducted using this optimization-based approach and results are presented.

SI property class 12.9 high strength steel bolts were used to investigate the fatigue behavior of bolt threads rolled before/after heat treatment using two different thread profiles and five different preload values. Bolts were 3/8 UNRC-16 (coarse) and 3/8 UNRF-24 (fine) and preloads were taken as 1, 50, 75, 90, and 100% of roll before heat treatment proof stress. Maximum near surface residual compressive stresses, obtained via x-ray diffraction, ranged from -500 to -1000 MPa. Axial loads were applied through the nut and all fatigue failures occurred at the first thread of the nut/bolt interface. SEM evaluation indicated all fatigue crack growth regions contained multiple fatigue facets, while final fracture regions were ductile dimples.

A continuation of the SAEFDE round-robin fatigue test program was conducted to determine the influence of a finer microstructure on monotonic tension, strain-controlled low cycle fatigue, fatigue crack growth, and fracture toughness of A356-T6 cast aluminum alloy. The finer microstructure castings, referred to as material W, were obtained using a water-chilled sand casting procedure. Material W exhibited more desirable ductile behavior than the previous SAEFDE materials X, Y, and Z. Material W exhibited superior smooth specimen low cycle fatigue resistance at both short and long lives, when compared to materials X, Y, and Z. This was due in part to the higher ductility and lower porosity of material W over materials X, Y, and Z. Material W exhibited similar fatigue crack growth behavior, and slightly higher values of fracture toughness at the same thickness when compared to materials X, Y, and Z.

Fracture toughness tests were conducted on the SAEFDE Committee's round-robin A356-T6 cast aluminum alloy materials designated X, Y and Z. Compact type specimens with a thickness of 9.1 and 20.3 mm were tested. Valid Klc values couid not be obtained for 9.1 mm thick specimens but were obtained for 20.3 mm thickness specimens. Due to larger castings, and hence slower cooling rates, a coarse secondary dendrite arm spacing, DAS, of 80 to 90 μm existed in the three materials. Similar Klc values were 18, 16.7 and 17.3 for the A356-T6 materials X, Y and Z respectively. Final fracture surfaces were also similar with predominant cleavage fracture with some localized ductile dimples and secondary cracking.