Christopher Salisbury

Experimental and Numerical Study of Electromagnetic Forming of AZ31B Magnesium Alloy Sheet

Wrought magnesium alloys are interesting materials for automotive and aeronautical industries due to their low density in comparison to steel and aluminium alloys, making them ideal candidates when designing a lower weight vehicle. However, due to their hexagonal close-packed (hcp) crystal structure, magnesium alloys exhibit low formability at room temperature. For that reason, in this study a high velocity forming process, electromagnetic forming (EMF), was used to study the formability of AZ31B magnesium alloy sheet at high strain rates. In the first stage of this work, specimens of AZ31B magnesium alloy sheet have been characterised by uniaxial tensile tests at quasi-static and dynamic strain rates at room temperature. The influence of the strain rate is outlined and the parameters of Johnson-Cook constitutive material model were fit to experimental results. In the second stage, sheets of AZ31B magnesium alloy have been biaxially deformed by electromagnetic forming process using different coil and die configurations. Deformation values measured from electromagnetically formed parts are compared to the ones achieved by conventional forming technologies. Finally, numerical study using an alternative method for computing the electromagnetic fields in the EMF process simulation, a combination of Finite Element Method (FEM) for conductor parts and Boundary Element Method (BEM) for insulators, is shown.

Finite element modeling for the prediction of blast trauma

A simplified finite element model of a human torso has been developed to investigate and predict primary blast injury to the lung. The motivation for this approach was to understand the basic origins of blast trauma to the lungs and to create a predictive model for the evaluation of injury and future development of blast protection. The model consists of a two-dimensional slice of the torso at the mid-sternum level with blast loading applied via a coupled Arbitrary Lagrangian-Eulerian approach, allowing for a variety of loads to be considered. In parallel, a simplified model of a sheep torso has been developed for direct comparison to published experimental data on blast injury. Blast loads were applied to the models based on threshold lung damage, and various lethal dose quantities for comparison to expected injury levels based on the Bowen curves. The predicted injury levels based on relative lung pressure correlated well to existing experimental data. Further, the predicted peak chest wall velocities in the model compared well to an existing trauma model and with the expected severity of trauma. Future research will focus on the prediction of lung injury in a complex blast environment, and the development of blast protection.

High strain rate characterization of shock absorbing materials for landmine protection concepts

Numerical modelling of footwear to protect against anti-personnel landmines requires dynamic material properties in the appropriate strain rate regime to accurately simulate material response. Several materials (foamed metals, honeycombs and polymers) are used in existing protective boots, however published data at high strain rates is limited.

Deformation Mechanics of a Non-Linear Hyper-Viscoelastic Porous Material, Part II: Porous Material Micro-Scale Model

Abstract Foam materials are widely used for energy absorbing applications, and are often addressed in a modeling environment at a macroscopic or continuum level by measuring the mechanical properties, which may be size dependent, and implementing the properties in a continuum-level constitutive model. However, foams are known to exhibit a characteristically low wave speed and an understanding of the deformation mechanics of foams at the micro-scale and dependence on morphology are essential to understand the performance of foam material in impact scenarios. In this study, experimental testing and finite element modeling were used to investigate a viscoelastic polychloroprene closed-cell foam at the cell level, subject to large deformation and high deformation rates. A numerical model was created with solid hexahedral elements and a repeated tetrakaidecahedron cell structure using measured foam cell size and wall thickness, and mechanical properties measured from non-porous polychloroprene. The finite element model predictions were evaluated using experimental compression tests on the foam material at high deformation rates. The enclosed nitrogen in the closed cell foam was modeled using an Arbitrary Lagrange–Eulerian method so that this contribution could be included or removed, and demonstrated the significant effect of the enclosed gas on the mechanical response of the foam. The foam cell dimensions were varied to investigate morphological factors including cell size, cell aspect ratio and cell wall thickness. Increasing wall thickness, decreasing cell size and decreasing the cell aspect ratio resulted in increased material stiffness, with wall thickness having the most significant effect. Investigation of the wave transmission speed demonstrated a low value compared to the constituent materials, which was explained by the path of the stress wave through the foam structure and wave reflections within the cells, attenuating the stress wave. The consequence of this low wave speed was non-uniform deformation of the foam sample demonstrating that the measured mechanical properties of porous foams depend on the sample thickness, an important consideration for foam material testing and characterization.

High Strain Rate Behaviour of Aluminium Alloy Sheet

This paper presents results from quasi-static and high rate tensile testing of three aluminum sheet alloys, AA5754, AA5182 and AA6111, all of which are candidates for replacing mild steel in automotive bodies. Tests were performed at quasi-static rates using an Instron apparatus and at strain rates of 600 to 1500 s-1 using a tensile split Hopkinson bar. Additionally, an in-depth investigation was performed to determine the levels of damage within the materials and its sensitivity to strain rate. The constitutive response of all of the aluminum alloys tested showed only mild strain rate sensitivity. Dramatic increases in the elongation to failure were observed with increases in strain rate as well as greater reduction in area. Additionally, the level of damage was seen to increase with strain rate.

Electromagnetic Forming of AZ31B Magnesium Alloy Sheet

Historically, electromagnetic forming technology has mainly been used to form parts from aluminium and copper alloys due to their excellent electrical conductivity and limited formability by conventional methods. However, little research has been carried out in high strain rate forming of magnesium alloy sheets. Therefore, in the current contribution electromagnetic forming experiments are performed for rolled AZ31B magnesium alloy sheet at different temperatures up to 250oC. Two forming operations are studied in this paper, i.e. drawing and bending operations. The final deformations achieved for the different conditions were measured and the effect of both temperature and discharged energy on deformation is shown. Bending experiments at room temperature were recorded by means of a high speed camera and the springback behaviour at high strain rates is evaluated. In one hand, increasing the forming temperature the yield strength of the material decreases while on the other hand, the electrical conductivity and thus the induced forces are also decreased. It is observed that increasing the forming temperature, for a given discharged energy, the maximum height of the deformed part is decreased. However, increasing the discharged energy at warm temperatures, higher deformation values are achieved without failure. Additionally, bending experiments show that springback effect is also decreased at warm conditions. It is concluded that warm electromagnetic forming is a suitable procedure to manufacture magnesium parts.