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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.

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.

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.

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.

Reliability-based design optimization (RBDO), which includes design optimization in design space and inverse reliability analysis in standard normal space, has been recently developed under the assumption that all input variables are independent because it is difficult to construct a joint probability distribution function (PDF) of input variables with limited data such as the marginal PDF and covariance matrix. However, since in real applications, it is common that some of the input variables are correlated, the RBDO results might contain a significant error if the correlation between input variables for RBDO is not considered. In this paper, Rosenblatt and Nataf transformations, which are the most representative transformation methods and have been widely used in the reliability analysis, have been studied and compared in terms of applicability to RBDO with correlated input variables.