Material factors influencing notch tip strain rate

When precracked or notched specimens are used in slow strain rate stress corrosion testing, it is the local strain rate at the crack or notch tip and not the global strain rate or externally applied testing displacement rate that governs the process. This paper presents computational results from an elastic-plastic finite element analysis of notched specimens, demonstrating that the relationship between local and global strain rates depends not only on the specimen geometry but also on the stress-strain curve of the material and on the load applied on the specimen (the latter two factors controlling the plastic strain distribution in the overall geometry, particularly in the vicinity of the notch tip), which implies that local strain rate changes with time, even for a constant global strain rate externally applied, as in the case of slow strain rate tests. It is shown that the evolution (or variation with time) of the notch tip strain rate depends on the spreading of the plastic zone and thus on the constitutive equation of the material.