With the extension of design life of engineering critical components, it is thus critical to investigate high cycle fatigue and very high cycle fatigue behaviors of weldments, upon which optimization of welding parameters should be based, and from which the approach to design against long life fatigue is expected. The fatigue behavior of welded joint, both similar and dissimilar, were investigated up to the very high cycle fatigue regime, by justification of specimen sampling method as the basis, fatigue testing with potential influencing factors, i.e., temperature, size, frequency, environment, and material strength level. Results indicated a strong dependence of S-N curve shape, fatigue strength and surface-to-interior crack initiation transition behavior on various influencing factors, while micro-defect-induced cracking was predominant for long life fatigue. Using a multi-scale and full-field approach, based on post-mortem fractography analysis by SEM, FIB/TEM, and EBSD techniques, the inclusion-induced interior cracking mechanisms were found to be associated with inclusion-microstructure interaction resulted plasticity.
We found the fine granular area characteristic of polycrystalline features close to micro-defects was indicative of loading-rate dependence of material hardening and higher fatigue strength. The fine granular area was characteristic of several nano-scale fine grains formed in terms of dislocation cell structures by martensitic laths breakdown, which underpins the mechanistic model of “fragmentation of martensitic laths and formation of dislocation cells”. A physical criterion for formation of fine granular area was proposed as the ratio of accumulation to release rates of cyclic plastic energy was higher than one. A very high cycle fatigue lifing model, the Z parameter model, was finally developed by combining size, location and shape of inclusions. We found a continuously decreasing fatigue strength reduction factor of welds with fatigue lifetime due to the transition of failure mechanisms from geometrical soft zone to micro-defects induced cracking. A sound fatigue design requires careful selection of filler metal and consideration of soft zone and micro-defects. All these inform the significance of combining metallurgical and processing factors and integrating design/manufacturing in designing against fatigue of engineering materials.