Non-proportional axial-torsional loading fatigue tests on nodular cast iron revealed a non-symmetrical behavior of the fatigue life relatively to the phase shift angle δ = 90° between axial and torsion loads. This behavior was in agreement with fatigue crack orientation investigated using optical and electron microscopy. Elasto-plastic finite element model (FEM) were used to assess the crack orientation behavior and it could explain this non-symmetrical behavior by the non-symmetrical values of both σI and σI / σII ratio relatively to δ = 90°. FEM revealed that for the δ < 90° loads, principal stress was applied essentially at critical planes of low angle ϕn, thus occurrence of tensile cracks mode was higher for δ < 90° than for δ > 90° load, which reduces the fatigue life. σI / σII ratio strongly influenced the dominant crack mode. When the applied loads were torsion σI / σII = 1, cracks were observed to occur, equally, in modes I and II. Presence of the nonpropagating failure (mode II) significantly increased the fatigue life. Conversely, for high phase shift, where σI / σII >> 1, crack mode I dominated and crack driving force remained high during the whole fatigue cycle, inducing a lower fatigue limit.
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°c ontributing 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.
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