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The effect of stress ratio on the statistical aspects of small fatigue crack growth behavior was studied in a duplex microstructure of Ti-6Al-2Sn-4Zr-6Mo (Ti-6-2-4-6) at 260 degrees C with particular emphasis on incorporating small-crack data into probabilistic life prediction and the influence of stress ratio on probabilistic lifetime limits. A Focused Ion Beam (FIB) was used to machine micro-notches in test specimens, which served as crack-initiation sites and enabled the acquisition of multiple small-crack growth data sets from single experiment. Stress ratios of -0.5, 0.05, and 0.5 were employed, and small-crack growth was monitored by the acetate replication method. Qualitatively, change in the stress ratio produced almost negligible influence on the small-crack growth behavior when plotted as crack growth rate (da/dN) versus applied stress intensity factor range (delta K). A probabilistic method was employed to represent the variability in the small-crack growth behavior and the statistical differences with respect to stress ratio., Preprint to be submitted to International Journal of Fatigue. Prepared in cooperation with Universal Technology Corporation, Dayton, OH. Report contains color.

IN100 is one of the workhorse nickel-base superalloys of the current fleet of turbine engines owned and operated by the US Air Force. Many of the life-limited locations on fracture critical IN100 components are associated with locations of stress concentrations, e.g. bolt holes, fillets, balance flange scallops, etc. The prior work by Jha, Caton, and others examining the fatigue variability of nickel-base superalloys found a link between the minimum fatigue life and fracture mechanics. This prior work only examined the fatigue behavior of smooth fatigue bars. The current effort determined that the link between minimum fatigue life and fracture mechanics is applicable even when only a small volume of the material at the root of a notch is subject to the high stress conditions., Preprint to be submitted to Superalloys 2012.

An analysis of fatigue crack-initiation facets from the perspective of variability in lifetime of a duplex microstructure of Ti-6Al-4V is presented. Fatigue variability behavior of this alloy was marked by an increase in the lifetime variability to almost three orders in magnitude as the stress level was decreased. Crack initiation was found to occur primarily from the specimen surface with only a few exceptions where subsurface initiation was recorded. In most cases, and irrespective of lifetime, crack initiation was accompanied by crystallographic facet formation across primary- particles. Crystallographic characterization of faceted grains and their neighborhood was conducted by sectioning across the facets using either Focused Ion Beam or mechanical polishing, and subsequent Electron Back Scattered Diffraction analysis of the sections. The emphasis in this study was on discerning the factors that distinguish the crack-initiating microstructural arrangements and plausible mechanisms producing a life-limiting failure versus a long lifetime failure under nominally similar microstructure and applied stress level., Prepared in collaboration with Universal Technological Corp., Dayton, OH and Univ of Dayton Research Institute, Dayton, OH.

Shot peening is commonly used to retard initiation and growth of fatigue cracks in fracture critical components. During service, the shot-peened residual stresses may change due to thermal-mechanical loading. This paper describes an approach for characterizing and modeling residual stress relaxation in supersolvus IN100 at 650 degrees C. The model incorporates the dominant creep deformation mechanism, coupling between the creep and plasticity models, and effects of prior plastic strain. Significant shot peen residual stresses are retained even after 300 hours of thermal exposure to 650 degrees C. Fatigue loading resulted in relaxation only in the initial load-unload cycle for the entire stress distribution. Under uniform applied stresses, residual stress reversal occurred for stresses greater than 1000 MPa. In geometries with steep gradients, such as notches, significant compressive residual stresses are retained near the surface even when the local stresses exceed the yield stress., Submitted for publication to the Journal Procedia Engineering.

The research develop a comprehensive understanding of the causes of fatigue variability, especially the minimum life, in a +b titanium alloys and nickel base superalloys. A physics-based life-prediction model has been developed that provides accurate probabilistic fatigue lifetime limits in aerospace materials with respect to microstructural and extrinsic variables, and promises significant reduction in the uncertainty associated with life prediction. The modeling is based on some key findings, common to a range of aerospace material systems, in terms of the fatigue variability behavior. First, it is found that effect of microstructural and loading variables on fatigue variability is manifested as separate degree of influences on the mean fatigue behavior and the life-limiting, lower-tail behavior. Therefore, the lifetime distribution can be represented as a divergence or convergence of these two behaviors as a function of the imposed variables. Second, the life-limiting behavior is shown to be controlled by the crack growth regime, i.e., the crack initiation lifetime is negligible. This separation of mean and the crack-growth-controlled behavior is found to be promoted by the development of an array of heterogeneity scales in the material under any given nominal microstructure and loading condition. These heterogeneity scales, which are related to the underlying microstructural configurations, are suggested to produce sequential failure mechanisms in the order of decreasing heterogeneity level. These vital concepts have been developed with the help detailed investigation of the relationship of the mean and the life-limiting mechanisms to the underlying microstructural drivers. Quantitative analysis of crack initiation sites in terms of size, and spatial and crystallographic orientations of the crack initiating feature and its neighborhood were employed to support this purpose., The original document contains color images. All DTIC reproductions will be in black and white.

We have studied the effects of microstructure and temperature on the fatigue variability behavior of the alpha+beta titanium alloy, Ti-6Al2Zr-4Sn-6Mo (Ti-6-2-4-6). These variables had separate influence on the minimum, and the mean behavior. This was related to perhaps a fundamental aspect of fatigue variability which dictates that at any stress level, the mean is dominated by a mechanism different from the one controlling the lower-tail behavior. As a result in this material, while the mean response was increasingly dominated by the crack initiation regime with decreasing stress level (which is the conventionally expected behavior), at the same time the life-limiting behavior was controlled by the crack growth regime. This produced a very systematic effect of microstructure and temperature on total uncertainty in lifetime depending on the sensitivity of crack initiation and growth regimes to these variables. We suggest a new paradigm to treat the fatigue variability behavior and show that, this can have significant implications for life management especially, with respect to reducing the uncertainty with life prediction and improving the reliability of design life., The original document contains color images. All DTIC reproductions will be in black and white.