Multiaxial cyclic behaviour and fatigue modelling of AM30 Mg alloy extrusion

Abstract

Multiaxial fatigue characteristics of AM30 Mg alloy extrusion are studied through fully-reversed strain-controlled cyclic experiments including pure torsional and combined axial-torsional at 0, 45 and 90° phase angle shifts. Under pure torsional cyclic loading, AM30 extrusion is realized to exhibit better fatigue properties than AZ31B and AZ61A extrusions, especially in low-cycle fatigue regime. Under proportional axial-torsional cyclic loading, twinning/de-twinning in axial mode results in asymmetric shear hysteresis loop. The effect of non-proportionality of biaxial loading on various aspects of material response is also examined and observed to be depending on the magnitude of axial strain amplitude. Finally, the life prediction capabilities of two critical plane models, i.e., modified Smith-Watson-Topper (SWT) and Fatemi-Socie (FS), as well as Jahed-Varvani (JV) energy-based approach are assessed, employing fatigue life data of AM30 extrusion. Correlation data between experimental and estimated lives are found to lie within narrow scatter band.

On the phase angle role in the shear response of ZK60 Mg alloys under multiaxial fatigue

“…It is noted that the second and half life hysteresis are almost similar, and the hysteresis loop is symmetric with zero mean stress. As also elsewhere reported [9], the dominant deformation mechanism at such shear strain amplitude is believed to be the slip modes of deformation that lead to such symmetric response. The observed shear response revealed considerable amount of plastic deformation with the plastic strain energy density of 0.086 MJ/m 3 , so 0.5% was selected as the shear strain amplitude for the multiaxial tests.…”

“…According to this figure, the highest twin amount (the compressive axial peak) coincides with the peak of shear strain under the in-phase mode; however, shifting the phase angles toward 90° leads to the lower volume of twins at the shear strain peaks. As was elsewhere reported [9], the basal slip is more favourable in the re-orientated grains of the microstructure than the matrix; thereby, higher twin volume fraction yields in a lower torque amplitude needed to accommodate the shear strain under the torsional mode. On the other hand, at the lower strain amplitude, i.e.…”

“…On the contrary, the torque amplitudes at the phase angle shift of 90° remain almost similar regardless of the axial strain amplitude. It is believed that the simultaneous axial deformation affects the shear response [8], [9]. While for the cases of in-phase and 45° out-of-phase, some axial strain is being accommodated at the peak of shear strain, in the case of 90° out-of-phase,…”

“…The multiaxial fatigue characteristics of different wrought Mg alloys have recently been investigated. The effect of phase angle on the mechanical behaviour of these alloys are studied, and it was reported that the non-proportionality has no effect on their fatigue life [6]- [9] . However, to the best knowledge of the authors, the literature on the biaxial fatigue characteristics of wrought Mg alloys is restricted to the AZ and AM families.…”

Fatigue modeling for wrought magnesium structures with various fatigue parameters and the concept of highly strained volume

“…Each work uses different approaches to determine the positive elastic strain energy density and achieves a good agreement between the predicted and experimental life for uniaxial unnotched specimens. Only a few investigations were carried out concerning the fatigue modeling of notched specimens [8,30,32], bending specimens [7,15,28] and the fatigue behavior under dominant multiaxial loading [1,24]. The mentioned researches investigate the influence of stress concentrations, stress gradients and different types of loading on fatigue life separately from each other and achieve good results in fatigue modeling for a specific specimen type (uniaxial unnotched specimen, notched specimen, bending specimen, axial and torsional loaded tubular specimen).…”