Threshold and near-threshold fatigue crack growth (FCG) is critical for the total life prediction as majority of time is spent in this regime. The proposed study includes the fatigue crack growth near-threshold in the time-based subcycle model for fatigue life prediction under arbitrary loading conditions. A novel fatigue-life prediction methodology combining a subcycle fatigue crack growth analysis and equivalent initial flaw size (EIFS) concept is proposed. A previously developed time-based subcycle fatigue crack growth model is extended to near threshold regime and under multiaxial loadings. A new temporal kernel function to include intensity factor corresponding to near threshold region is proposed. The multiaxial load scenario is considered for mixed-mode FCG using a critical plane approach. Model predictions under arbitrary are compared with experimental data from open literatures and internal testing. Most of the predicted fatigue life results lie with error factor range of 2, which shows a good prediction for fatigue life.
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This paper discusses a method that focuses not only on peak stresses but also on stress gradients to rationalize fatigue design using a low-alloy steels. First, fatigue strength reduction ratios are associated with stress gradients rather than stress concentration factors. Next, to verify the stress gradient method, fatigue tests were conducted on hole-notched specimens. Finally, the fatigue life was predicted, considering the stress gradient at the notch root. The predicted atigue lives agreed well with the experimental results. It was confirmed that the fatigue life can be predicted more accurately than the conventional peak stress method.
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Steel structures may experience localized plastic strains, arising from wide range of service anomalies. Regions of accumulated plastic strain are more prone to accelerated stress corrosion cracking and reduced fatigue life. In this work, we systematically analyzed the intergranular corrosion (IGC) under combined oscillatory mechanical loading and active electrochemical environment in a specially designed experimental apparatus. Loading cycles were design to mimic both the low amplitude high frequency vibration loads and the low frequency-high amplitude structural duty cycles. Electrochemical potentials were maintained for active dissolution in moderately alkaline carbonate-bicarbonate solutions and under pre-accumulated plastic strain of 0-4%. We observed grain boundary softening, directly arising from vacncies formed by silicon oxidation. Triangular wedges were formed and correlated with the level of the accumulated plastic strains and the load profile. A three-dimensional elasto-plastic continuum damage mechanics model is developed to account for both, the pre-accumulated plastic strain, and the induced elasto-plastic fatigue strains to accelerate the evolution of damage accumulation. Upto 90% of life reduction is observed with 4% of pre-accumulated plastic strain. These findings can be used to advance the understanding of the combined effect of damage and corrosion on the remaining fatigue life of energy materials.
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The retrogression and re-aging (RRA) and interrupted aging heat treatments (T6I4 conditions) lead to a bimodal microstructure feature within the 7050-aluminum alloy. Microstructural effects have a strong influence on fatigue crack nucleation and short crack growth via the activation of competing mechanisms operating during fatigue crack nucleation. Electron Back Scatter Diffraction (EBSD) technique permits the study of the effects of fatigue-induced crystal defects on the crack path and the pre-existing defect structure. Furthermore, few studies have explored the potential of multi-stage aging treatments on the fatigue crack nucleation paths using the EBSD technique. The purpose of this study is to elucidate the role of the microstructure on small fatigue crack crystallography in samples of RRA, T6I4-65 and T7451 conditions made from AA 7050. Fatigue crack surface crystallography will be determined near the vicinity of crack initiation sites and within the early crack growth regime using focused ion beam (FIB) surface preparation combined with electron back-scattered diffraction (EBSD). All the small fatigue cracks were initiated at the bore-hole surfaces on the gage length of specimen geometry, i.e., double-edge notches with two 4.8 mm diameters. The bore-hole surfaces of samples were electropolished prior to fatigue loading to remove bore-hole surface machining damage.
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