Cyclic Characterisation and Plasticity Modelling of AM30 Magnesium Alloy Extrusion
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Date
2017-08-14
Authors
Roostaei, Ali Asghar
Journal Title
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Volume Title
Publisher
University of Waterloo
Abstract
Reducing fuel consumption and, thereby, greenhouse gas pollution has been the main thrust of lightweighting endeavours by transportation industries, particularly, automotive sector. Wrought magnesium alloys, being the lightest engineering alloys, are potential candidates for manufacturing automobile components. That said, more critical testing is still required to assess their mechanical and structural attributes toward new product development for a broad range of applications.
Anisotropic fatigue and cyclic behaviour of AM30 Mg alloy extrusion is investigated by performing fully-reversed strain-controlled tension-compression cyclic tests at strain amplitudes between 0.3% and 2.3%, along extrusion (ED) and transverse (TD) directions. The shapes of half-life hysteresis loops suggest the predominance of slip and twinning/de-twinning mechanisms below and above the strain amplitude of 0.5%, respectively. The twinning/de-twinning occurrence is found to be more extensive during straining along ED, which results in higher asymmetry of hysteresis loops, and thereby, higher induced mean stress. This adversely affects the fatigue resistance and yields lower number of cycles before failure in ED. Optical microscopy and texture analysis are employed to validate the findings. In addition, fracture surfaces are studied by scanning electron microscopy to identify the sources of fatigue crack initiation. Persistent slip bands (PSBs) and twin lamellae interfaces are evidenced as crack initiation sites at low and high strain amplitudes, respectively. Cracks emanated from debonded inclusion interface are also observed. Lastly, estimated fatigue life by the Smith-Watson-Topper (SWT) and Jahed-Varvani (JV) fatigue models are compared with experimental life obtained through this study as well as the ones reported in the literature. The JV energy model is proven to yield better life predictions.
Moreover, 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 SWT and Fatemi-Socie (FS), as well as 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.
Basal-textured wrought magnesium alloys are inherently prone to mechanical twinning/de-twinning during cyclic deformation at room temperature. They, subsequently, exhibit distinctive flow curve attributes, which are impossible to describe using conventional plasticity models. A purely phenomenological plasticity model is herein proposed, in such way that accounts for various asymmetric/anisotropic aspects of cyclic flow response of wrought magnesium alloys. The proposed model entails an isotropic von Mises yield function which evolves in stress space according to a new generalized anisotropic kinematic hardening rule, based on Ziegler’s rule. The phenomenological concept of plastic moduli matrix introduced in the kinematic rule is viewed as the key factor in representing material yield/hardening behaviour in different directions. The components of this matrix can independently be calibrated by conducting uniaxial experiments along each direction. An efficient and stable numerical algorithm is developed and then coded into both MATLAB®, and user material subroutine (UMAT) to use within Abaqus®/Standard finite element software. Thereafter, model validation has been successfully done using two different types of experiments: proportional and non-proportional biaxial axial-torsional cyclic tests on AZ31B, AZ61A, and AM30 Mg alloy extrusions, and notched plate tensile loading-unloading test on AM30 extrusion.
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Keywords
Wrought Magnesium Alloys, Anisotropy/Asymmetry, Cyclic Response, Multiaxial Fatigue, Cyclic Plasticity Modelling, Finite Elements