Fatigue failure mechanisms and influential factors for aluminum alloy and its welded joint in a high-speed train

Aluminum alloy and its welded joint are widely used in high-speed trains, which are subjected to complex fatigue loadings in service. The fatigue failure mechanisms and influential factors for Base Metals (BMs) and Welding Metals (WMs) subjected to low cycle (dwell) fatigue (R = 0) and (very) high cycle fatigue (R = -1) loads were investigated. The development of cumulative strain in tension-tension low cycle (dwell) fatigue was attributed to "cyclic ratcheting effect", which developed only when the applied maximum stress level is higher than the yield strength. In that condition, the cumulative strains continually developed and resulted in ductile fracture for BMs, but gradually converged to a finite value and resulted in fatigue fracture for WMs. Further, the dwell loading contributed to slowing down the development speed of cumulative strain and extending the fatigue life for BMs. Moreover, the welding processing reduced the (very) high cycle fatigue strengths and shortened the fatigue lives due to the introduction of welding defects, and a model replacing the nominal maximum stress by an equivalent one was proposed for modeling the impact of those defects on fatigue properties, which agrees with the S-N data.