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The objective of this research was to determine fatigue behavior of SAE 1045 steel subjected to very high tensile mean stress for unnotched, mildly notched, and sharply notched test specimens, and to determine if common S-Nf and ε-Nf mean stress fatigue life models are applicable. High tensile mean stress fatigue tests for R ratios of 0.8 and 0.9 were conducted using unnotched and notched, Kt=1.65 and Kt=3.65, axial loaded SAE 1045 steel specimens with hardness levels of Rc=10, 37, and 50. The monotonic notch strength ratio, NSR, for 5 of 6 test conditions was greater than 1, which allowed many notched cyclic test values of Smax or Sm to exceed the unnotched ultimate tensile strength. Much notched specimen fatigue resistance at these high R ratios was superior to that of unnotched specimens. However, cyclic creep/ratcheting, particularly for Rc=10 and 37, was a predominant cause of failure.

Axial strain-controlled low cycle fatigue behavior was obtained from smooth specimens machined from spokes of A356-T6 cast aluminum alloy wheels. Two different foundries cast the wheels. Three wheels were used from one production run at one foundry and two wheels were used from two different production runs at the other foundry. Specimens from the three wheels of the same production run had essentially the same monotonic tensile properties and low cycle fatigue resistance. Specimens from the two wheels of the different production runs had different monotonic tensile properties and different low cycle fatigue resistance. All these A356-T6 wheel specimens cyclic strain harden with hysteresis loops typically offset to the compression side by five percent or less. The usual log-log linear model for low cycle fatigue adequately described the low cycle fatigue behavior.

The objective of this research was to determine the feasibility of applying strain based fatigue life calculation models, which are commonly used for metals, to smooth SRIM polymer matrix composite axial specimens subjected to variable amplitude loading. A thorough investigation of the monotonic and strain controlled constant amplitude low cycle fatigue behavior of this material was conducted, including the effects of mean strains/stresses on the fatigue life of smooth specimens. Using these results, mean stress life calculations were made on the constant amplitude tests, as well as on smooth specimens subjected to strain controlled variable amplitude loading, using the Morrow and SWT mean stress models. These results were compared to experimental data, and it was found that the correlation between experimental and calculated lives was very poor, for both the constant amplitude and variable amplitude tests.

The results of the SAEFDE Committee's round robin low cycle fatigue test program with A356-T6 cast aluminum alloy indicated that the conventional low cycle fatigue model was not a satisfactory representation of the data. This occurred because the elastic strain amplitude-life curve was not log-log linear and this yielded a non-conservative fatigue life representation at both extremes of long and short lives. This paper involves a reanalysis of the A356-T6 composite all-laboratory data using two additional empirical models. These models are: 1. linear log-log total strain amplitude-life 2. bilinear log-log elastic strain amplitude-life Both proposed empirical models improve the representation of the data compared to the conventional low cycle fatigue model. The bi-linear log-log elastic equation, however, when added to the plastic equation, yields a discontinuous curve with non-conservatism in the region of the discontinuity.

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.

Fatigue crack growth behavior was obtained for the SAEFDE Committee's round-robin A356-T6 cast aluminum alloy program with crack growth rates between 10−11 and 10−6 m/cycle for R-ratios equal to 0.1 and 0.5. Three different mold temperatures resulted in secondary dendrite arm spacings (DAS) that varied from approximately 80 to 90 µm, resulting in only coarse microstructure. Threshold levels, ΔKth, and the Paris exponent, m, were approximately twice the values usually found for wrought aluminum alloys. The influence of R-ratio was quite pronounced and crack closure, as measured with a crack mouth COD gage, did not eliminate all threshold and near-threshold R-ratio differences. Roughness-induced crack closure appeared to be more important than plasticity-induced closure.

The effect of restrictive clothing on functional reach and on balance and gait during obstacle crossing of five normal subjects is presented in this work using motion capture and stability analyses. The study has shown that restrictive clothing has considerably reduced participants' functional reach. It also forced the participants to change their motion strategy when they cross-higher obstacles. When crossing higher obstacles, the participants averted their stance foot, abducted their arms, flexed their torso, used longer stance time, and increased their hip angle in the medial-lateral (Rolling) and vertical (Yawing) directions. The stability analysis of a virtual human skeletal model with 18 links and 25 degrees of freedom has shown that participants' stability has become critical when they wear restrictive clothing and when they cross higher obstacles.

The objective of this research was to obtain and compare fracture toughness, stress corrosion cracking and constant and variable amplitude fatigue behavior of AZ91E-T6 cast magnesium alloy in both an air and 3.5% NaC1 corrosive environment. An additional objective was to determine if commonly used models that describe fatigue behavior and fatigue life are applicable to this material and test environments. Fatigue tests included constant amplitude strain-controlled low cycle fatigue with strain ratio, R, equal to 0,−1 and −2, Region II constant amplitude fatigue crack growth with load ratio, R, equal to 0.05 and 0.5 and variable amplitude fatigue tests using keyhole notched specimens. In all fatigue tests, the corrosion environment was significantly detrimental relative to the air environment. The material was also susceptible to stress corrosion cracking. Low cycle fatigue models and the Paris equation properly represented the fatigue data in both environments.

In the Army mechanical fatigue subject to external and inertia transient loads in the service life of mechanical systems often leads to a structural failure due to accumulated damage. Structural durability analysis that predicts the fatigue life of mechanical components subject to dynamic stresses and strains is a compute intensive multidisciplinary simulation process, since it requires the integration of several computer-aided engineering tools and considerable data communication and computation. Uncertainties in geometric dimensions due to manufacturing tolerances cause the indeterministic nature of the fatigue life of a mechanical component.