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Magnesium alloys are of growing research, development and commercial interest for their lightweight characteristics, notably in the automotive sector. Recent results based on experiments and simulations of beam components have shown that finite element (FE) predictions using commercial FE software may significantly overestimate the peak load and load beyond the peak load. This indicates that better deformation and failure criteria are needed for crashworthiness simulation and design of Mg alloys for the development of computer-assisted engineering (CAE) capacity for Mg alloys. In this study, yield and hardening laws for deformation simulation of Mg alloys are reviewed. An isotropic Lode angle dependent von Mises yield and flow model originally used for soil was modified by replacing shear strength with tensile or compressive flow strength for deformation simulation of Mg alloys.

Use of ultra high strength steel (UHSS) sheet in automotive components has potential to simultaneously reduce weight and increase crashworthiness. For crashworthiness design and simulation, constitutive equations are required; however, these are scarce for UHSS. Also, UHSS sheets may suffer unexpected fracture such as shear fracture, and toughness data for UHSS sheets is very limited. In this work, effects of strain rate and temperature on flow stress of two UHSS sheet steels (a dual-phase ferritic/martensitic DP980 and a martensitic boron (B) steel) are experimentally investigated and compared to a simple constitutive equation for structural steels based on thermal-activation theory of dislocation motion. The flow stress of the two UHSS steels obeys a constitutive equation similar to that of structural steels of other microstructures (ferrite, ferrite/pearlite, pearlite, ferrite/bainite, and bainite).

Effects of strain rate and temperature on tensile, compressive and toughness properties were investigated for an extruded Mg alloy AM30 in both extrusion (ED) and transverse (TD) directions. The effects of strain rate and temperature on uniaxial flow stress were reported elsewhere but will be briefly described to facilitate the understanding of Charpy results (i.e., load, deflection and absorbed energy). The effects of loading rate from quasi-static to 5.1 m/s on un-notched and V-notched Charpy load-deflection curves up to maximum load were negligible, but load up to maximum load increased with decreasing temperature. The deflection at rapid load-drop in un-notched Charpy tests increased with decreasing loading rate and increasing temperature between -50°C and 100°C for un-notched ED and TD specimens (i.e., L-T and T-L specimens according to ASTM E 1823 terminology).

Engineering stress analysis of bolt joints requires knowledge of the stiffness of the joints. One convenient way to estimate stiffness of joints is to use effective stress area equations because only part of the joint surface carries load. In this work, the existing effective stress area equations (developed about 50 years ago) were assessed using a washer/joint contact finite-element (FE) model. It was found that these equations may underestimate the effective area and the joint stiffness by as much as 50%. Typical bolt-load retention (BLR) test fixtures were modeled using a commercial FE code. In the simulation, the creep stress-strain behaviour of a magnesium alloy was described by a three-parameter power-law relation that was fitted from the compressive creep test results of a high-pressure die-casting (HPDC) Mg alloy (AM50). The bolt-load vs. time relationships from FE simulation were compared with those obtained experimentally, and they showed good agreement.

For automotive applications at elevated temperatures, the need for sufficient creep resistance of Mg alloys is often associated with retaining appropriate percentages of initial clamp loads in bolt joints. This engineering property is often referred to as bolt-load retention (BLR); BLR testing is a practical method to quantify the bolt load with time for engineering purposes. Therefore, standard BLR test procedures for automotive applications are desired. This report summarizes the effort in the Structural Cast Magnesium Development (SCMD) project under the United States Automotive Materials Partnership (USAMP), to provide a technical basis for recommending a general-purpose and a design-purpose BLR test procedures for BLR testing of Mg alloys for automotive applications. The summary includes results of factors influencing BLR and related test techniques from open literature, automotive industry and research carried out in this laboratory project.