Piezomagnetism and fatigue: II

Abstract

The cumulation of damage in test specimens of AISI 1018 steel, subjected to repeated cycles of tension and compression leading to fatigue failure at Nf 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 < N2; accretion of damage, N2 < N < N3, and terminal failure (crack coalescence and growth); N3 < N < Nf. Data from 49 fatigue trials, spanning the range 1219 ⩽ Nf ⩽ 250 200, show that the transitional cycles N2 and N3 have an approximately invariant relation to final fatigue failure at Nf: i.e. N2/Nf ≈ 12% and N3/Nf ≈ 90%. Piezomagnetic hysteresis develops in parallel with stress–strain hysteresis and also exhibits transitions at N2PM and N3PM corresponding to N2 and N3. Detailed analyses of eight fatigue trials yield the approximately invariant ratios N2PM/Nf ≈ 12% and N3PM/Nf ≈ 93% where 3561 ⩽ Nf ⩽ 189 629.