Structures are often subjected to multiaxial cyclic load superimposed with a mean-stress. Thus, both the mean stress effect and the biaxiality effect are needed to be addressed for a proper design. Steels (Cr-Mo) used for such structures often contain manufacturing defects. Mean-stresses were found to control the crack initiation sites: at low mean-stresses, fatigue cracks initiated from the surface, while for high mean-stresses cracks initiated from internal or surface-cutting defects. This transition can be explained by the stress and strain fields around the defects. Moderate stress biaxialities had a beneficial effect on fatigue lives, attributed mainly to a retardation of crack initiation, while equibiaxial tension had a slightly detrimental effect, attributed to a “pseudo size effect”: higher probability for an incipient crack to grow along two possible planes, compared to a single one. An empirical fatigue criteria based on Gerber’s parabola was able to captured the evolution of the endurance limit under the combined effects of a positive mean stress and positive biaxiality.
Structures in offshore applications are often subjected to corrosive environment in addition to complex cyclic loads. Such conditions impose additional constraints while designing them. Some industries use coatings to protect them while some prefer electrochemical process like cathodic protection. In this study, a cathodic potential of -950 mV was used to study the multiaxial corrosion fatigue behavior of these steels. In free corrosion, fatigue lives and the endurance limit were suppressed due to corrosion due to the formation of corrosion pits that favor early, multiple crack initiations. The detrimental effect of corrosive environment was not enhanced by equibiaxial tension. The crack growth mechanism was different: transgranular brittle decohesion in uniaxial loading and mostly intergranular in biaxial tension. Cathodic protection enhances the H-induced embrittlement of the grain boundaries.
A crystal plasticity finite element (CPFE) simulation framework was used to predict the combined detrimental effect of mean stress and defects on the fatigue behavior of aluminum alloy. Experimental data for mean-stress effect on fatigue life and crack growth behavior was obtained on metal inter gas (MIG) welded joints of Al-5083/Al-5.8%Mg alloy plates. A 2D representative model for material’s microstructure was used for the simulations, generated using an anisotropic tessellation algorithm using the EBSD measurements data. A total of 10 different microstructure models were generated for each loading condition. The simulated loadings at different stress ranges and stress ratios (R-ratio) were similar to the experimental conditions for the better comparison of the results. Significant heterogeneity in the distribution of R-ratios and the far-field applied R-ratio was observed. The proposed CPFE simulation framework not only predicted well the effect of defects and mean stress on the fatigue lives, but also the scatter induced in them due to the defects.
In this study, effect of post weld heat-treatment (PWHT) at two different aged temperatures was investigated on the microstructure and the material properties of the aluminum alloy 2024. The plates of AA2024 were welded using friction stir welding process, followed by PWHT at different aged conditions: 190 for 10 hrs and 200 for 10 hrs. Both tensile and cyclic properties were investigated. PWHT at 200$\celsius$-10 hours resulted in significant changes in the microstructure and improvement in the mechanical properties of the welded joint. PWHT resulted in re-precipitation of the precipitates, specifically in the thermo-mechanically affected zone (TMAZ) & nugget zone (NZ) but no significant abnormal grain growth was observed in the nugget zone. A significant improvement in the ductility and the hardness of welded joint was observed for PWHT at 200 – 10 hours. Long crack growth tests were conducted using sinusoidal loading of 10 Hz frequency and at a stress ratio of 0.1. The PWHT joint at 200 – 10 hours resulted in higher threshold stress intensity factor range () as compared to as-welded joint and the PWHT joint at 190 – 10 hours. The observations are explained based on microstructural changes in the FSW joint.