Numerical simulation of AM50A magnesium alloy under large deformation

doi:10.1016/S0734-743X(03)00060-5

International Journal of Impact Engineering

Volume 30, Issue 2, February 2004, Pages 117-142

https://doi.org/10.1016/S0734-743X(03)00060-5 Get rights and content

Abstract

The present research concentrates on the development of a material model for the AM50A magnesium alloy, frequently used in the automotive industry. This computer model was developed and validated through experimental and numerical simulation of standardized charpy and tensile tests, and intensive microstructural analysis. The material model was further used to predict the behavior of an AM50A magnesium alloy steering wheel armature. Experimental and numerical impact tests were performed, correlated and validated by detailed structural analysis of the armature. Three material models were evaluated in the present research including a bilinear approximation, piecewise linear, and Johnson–Cook models. The piecewise linear model presented accurate predictive capabilities with variations explained by microstructural defects existing within the specimen being tested. In addition, a failure criterion was proposed and assessed during numerical simulations of the charpy impact tests.

Introduction

Strength, durability, and fatigue resistant properties, which increase fuel efficiency in vehicles, are the reasons automobile manufacturers have turned their attention to aluminum and magnesium alloys for various load bearing and non-load bearing applications. Safety during crash or impact situations is essential in the development of vehicles and, due to elevated strain rates and large deformation behavior, the above alloys are able to provide these securities.

A steering wheel armature is the skeleton of the steering. This research focuses on the use of the AM50A magnesium alloy as a potential material for the steering wheel armature primarily because of its lighter weight. Armatures are designed so that they are capable of resisting two main loading conditions; cyclic loading and impact [1]. Analysis of loads and their deformational effect on the armature design when it is impacted with another unit is initially done by finite element (FE) modeling. The FE simulations often provide potential design modification, however, prediction of the actual response of the armature is favored. Relevant behavioral prediction encompasses a good correlation between experimental and numerical tests. Research of this type can be extremely advantageous to the automotive industry and yet very little information on the present subject is currently available in open literature.

Aune et al. [2] have studied three unique die-cast magnesium alloys (AZ91D, AM60B, and AM50A) which have been subjected to a range of strain rates from 15 to 130/s. Findings showed that the rate of deformation does not affect elongation. There is, however, rate dependencies for material stress behavior and flow stress, which were found by Aune et al. [2] and Carlson [3], respectively. Carlson's [3] research concentrated on strain rates between the range of 10⁻³ and 10³/s for the magnesium alloy AM60B and since sensitivity to strain depends largely upon the alloy tested, it does not provide conclusive evidence that such effect should be taken into account for the AM50A alloy used in this current research.

While Johnson and Cook [4] have provided a relationship between flow stress in terms of yield stress (σ₀), strain (ε), strain rate $(ε ̇)$ , and temperature (T), which can be seen in Eq. (1), Aune et al. [2] have determined and experimentally tested the parameters for the AM50A alloy without making an allowance for thermal effects in the material: $σ=(σ_{0} +Bε^{n}) 1+C ln ε ̇ ε ̇_{R} 1− T−T_{room} T_{melt} −T_{room}^{m} .$

From the research conducted by Aune et al. [2], the values of σ₀, B, n, and C are 88, 599 MPa, 0.5966 and 0.019, respectively, while $ε ̇_{R}$ takes on a reference value of 1/s when the unit of time is seconds. A small sensitivity to rate effects in AM50A can be observed from the low value of the constant C.

In order to obtain a proper AM50A material model to use in the nonlinear FE software LS-DYNA, experimental tensile testing was completed on standard tensile specimens. The intent of the material model was to precisely predict the deformation of the AM50A magnesium alloy under impact. The material model data was acquired and inputted into a FE model to confirm the relationship between experimental and numerical testing. With the aim of accurately verifying the material model under impact conditions, standard charpy impact tests and steering wheel armature impact tests were conducted and correlated with FE results.

Section snippets

Experimental uniaxial tensile testing and numerical material modeling

Eight tensile test specimens were die-cast in compliance with American Standard of Testing Materials (ASTM) designation B 557M-94 [5], the geometry of the specimens is shown in Fig. 1. The tensile test procedures were obtained from the above ASTM standard (B 557M-94) and were performed on a united test machine. To provide strain data an extensometer was positioned in the gauge region of the specimen. The tests were conducted at a crosshead speed of 0.42 mm/s until 5% strain was reached, at which

Experimental charpy impact testing

Twenty-six un-notched charpy impact test specimens were die-cast by Hydro Norsk [9] according to American Standard of Testing Methods designation E23-98 [10]. Fig. 6 shows the geometry of the test specimens used in the charpy impact tests. Due to the questionable calibration of available test equipment, Hydro Norsk provided data from a charpy impact test performed in accordance to procedures of ASTM E23. The data was presented in the form of a load versus time graph with maximum energy

Experimental steering wheel impact testing

A droptower testing device used in this research project for impact loading on steering wheel armatures is illustrated in Fig. 13. A rigid plate with mass of 57.1 kg was dropped onto the steering wheel armature with gravity acting as the accelerating agent. A 44.5 kN triaxial load cell, mounted directly below the armature, measured impact load between the plate and the armature in the direction of the impact velocity (z-axis direction). The displacement of the rigid plate in the z-axis direction

Conclusions and recommendations

This research focused on the development of a material model for the magnesium alloy AM50A. The material model was developed and verified by performing standardized tensile and charpy tests, and implemented in simulating large deformation in steering wheel impact tests. The following conclusions were drawn from the present research:

(i)
The piecewise linear plasticity material model was found to most accurately predict material behavior in the tensile tests and charpy impact.
(ii)
The tied

References (11)

Ames W, Altenhof W. Observations on the relative performance of magnesium and aluminum steering wheel skeletons with...
Aune TK, Albright D, Westengen H, Johnsen TE, Andersson B. Behavior of die cast magnesium alloys subject to rapid...
Carlson BE. The effect of strain rate, temperature on the deformation of die cast AM60B, SAE paper no. 950425....
Johnson GR, Cook WJ. A constitutive model and data for metals subject to large strains, high strain rates and high...
American Standard of Testing Materials. Standard test methods of tension testing wrought and cast aluminum and...

There are more references available in the full text version of this article.

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