PSI - Issue 25

A. Gryguć et al. / Procedia Structural Integrity 25 (2020) 486– 495 Andrew Gryuc/ Structural Integrity Procedia 00 (2019) 000–000

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Figure 7 illustrates some quantitative assessment of the morphology of the fracture surface of a specimen which following multiaxial fatigue loading with a high level of non-proportionality (90 ° out of phase). The shear contribution of 0.37 [MJ/m 3 ] to total SED is low, and is in the regime of beneath the transition threshold, and thus as expected, the multiaxial cracking behaviour that is observed is transverse (i.e. 0 ° orientation) as the axial mode supresses the helical cracking mode and dominates the macroscopic cracking behaviour. The fatigue crack initiation location (FCI) is denoted in Figure 7 (b) and (c) and occurs at the outer surface of the sample as that is where the combined stress state is largest. The profile of fracture surface across the entire circumference of the fractured sample is illustrated in Figure 7 (d) and it is very flat in nature for a large proportion in the initiation and propagation phases of the cracking. Especially in the short crack growth the profile of the surface is within 0.1 – 0.2 mm total height variation which confirms the mode 1 failure type which virtually is a purely transverse crack at 0 ° orientation. Even with the inclusion of the final fracture zone the total height variation does not exceed 1.1 mm which qualitatively indicates that the cracking mode remains broadly similar throughout the entirety of the propagation phase.

FCI

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Figure 7 – Quantitative fractography and morphology of fracture surface of a bi-axial specimen Δε/2 = 0.4%, Δγ/2 = 0.5%, 1492 cycles. (a) macroscopic crack plane (outer view), (b) fracture surface (top view) (c) fatigue crack initiation (FCI) detailed view and (d), surface elevation profile across midplane of sample depth along the entire circumference (y-axis is elevation in μm and x-axis is circumferential distance in μm). 4. Conclusions The effect of the multiaxiality and proportionality of loading on the cyclic behaviour and early cracking behaviour was studied for forged AZ80 Mg alloy. Several different loading paths were presented, uniaxial and biaxial with varying levels of non-proportionality. The synergistic effects of each individual uniaxial cracking modes contribution towards the combined macroscopic cracking behaviour in multiaxial loading was investigated. Based upon the results, the following conclusions can be drawn: 1. In multiaxial loading, the axial response is governed by both the dislocation slip and twinning-detwinning mechanisms (below and above the “kink” threshold respectively) which are insensitive to the level of non proportionality in multiaxial loading. In contrast to this, the shear deformation mode is sensitive to the presence of the axial loading when twinning-detwinning is the salient deformation mechanism above the “kink”, but is not sensitive below it. 2. In general, non-proportional loading of AZ80 that is 90 ° out of phase results in the lowest fatigue life for a given multiaxial strain amplitude, and approximately a 50% reduction in life compared with proportional loading. At 45 ° phase angle, at axial strain amplitudes which are below the kink, the life is insensitive to the presence of non-proportional loading. However, at axial strain amplitudes above the kink, an 18% increase in life was observed.