Piezomagnetism and fatigue: II

The cumulation of damage in test specimens of AISI 1018 steel, subjected to repeated cycles of tension and compression leading to fatigue failure at N f cycles, is correlated with the evolution of stress–strain (σ − ε) hysteresis and piezomagnetic ( B –ε) hysteresis. Specifically, the σ − ε hysteresis loop areas, when plotted as a function of the number of loading cycles N , show systematic variations that can be identified with the three principal stages of fatigue: initial accommodation (i.e. strain softening or hardening), N < N 2 ; accretion of damage, N 2 < N < N 3 , and terminal failure (crack coalescence and growth); N 3 < N < N f . Data from 49 fatigue trials, spanning the range 1219 ⩽ N f ⩽ 250 200, show that the transitional cycles N 2 and N 3 have an approximately invariant relation to final fatigue failure at N f : i.e. N 2 / N f ≈ 12% and N 3 / N f ≈ 90%. Piezomagnetic hysteresis develops in parallel with stress–strain hysteresis and also exhibits transitions at N 2PM and N 3PM corresponding to N 2 and N 3 . Detailed analyses of eight fatigue trials yield the approximately invariant ratios N 2PM / N f ≈ 12% and N 3 PM / N f ≈ 93% where 3561 ⩽ N f ⩽ 189 629.