Failure mechanisms and fatigue strength assessment of a low strength Cr−Ni−Mo−V steel welded joint: Coupled frequency and size effects

• The inner nature of VHCF behavior of a low strength welded joint was reported.
• Similar in high strength steels, the localized plasticity resulted in FGA.
• A physical criterion based on energy dissipation was proposed for formation of FGA.
• Coupling of size and frequency effects was required for fatigue strength modeling

Axially loaded push-pull tests of a low strength Cr−Ni−Mo−V steel welds using cross-weld specimens with different sizes were conducted up to the very high cycle fatigue regime under ultrasonic frequency. Results showed that smaller samples had higher fatigue strength with interior cracking predominant at the weld metal. Fine granular area characteristic of polycrystalline features close to micro-defects was indicative of higher fatigue strength with implications of cyclic hardening, and was related to localization of plastic deformation. 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. It was found that the size effect on fatigue strength of the low strength welds was coupled with frequency effect, an issue that both current deterministic and probabilistic approaches were unable to rationalize, was actually dependent on tensile strength as well as extrinsic factors such as cyclic hardening and micro-defect distribution.