Cracking behavior in nodular graphite cast iron under non-proportional cyclic loading

Abstract

The cracking behavior of ferritic nodular cast iron subjected to multiaxial non-proportional cyclic loading was studied. Crack length increased logarithmically during cycling, with a slop (in µm/cycle) as high as the phase-shift angle (δ) was closing to 90°. A non-symmetrical behavior in terms of fatigue life and crack growth relative to δ = 90 was observed. The application of analytical method revealed that at all times of the fatigue cycle, the amplitude of principal stresses is much higher for δ < 90° than for δ > 90° contributing thereby to crack growth accelerating, and shortening finally the fatigue life. There were a vast range of critical crystallographic orientations undergoing maximum normal principal stress for δ < 90° than for δ > 90°. Microscopic studies revealed that fatigue cracks did not have strong preferential directions for high-phase angle loads due to the continues rotation of principal directions, so their propagation did not stop.