evices and components are commonly subjected to shocks and impacts, either accidentally or during normal use. The inadvertent dropping of a mobile phone or a bullet penetrating a target are common examples. Hence, knowing how objects and the materials they are made from
respond to sudden, high-intensity loading is essential for both their design and protection. Investigations into the behaviour of materials, products and components subjected to different speeds of loading are being carried out in the Impact Mechanics Laboratory in the Department of
Mechanical Engineering. This enables us to understand how the rate of loading affects the deformation, damage and failure of objects that experience impact and shock. Described below are two such projects related to the human body.
The cervical spine or neck (Fig. 1) is vulnerable to injury arising from events ranging from automobile collisions to pilot ejection. To understand what governs its response, specimens of cancellous bone and ligament were extracted from the neck (Fig. 2) and subjected to dynamic loading at different speeds. Figure 3(a) shows that the stiffness and strength of cancellous bone increases with both bone density and the rate of compression. Test results also indicate that when ligaments are stretched more rapidly, they exhibit a higher stiffness and sustain larger forces for the same degree of extension, as shown in Figure 3(b). Equations relating the load, deformation and loading rate have been developed for use in computer software to model the human body. This will facilitate an understanding of how the human neck responds to different situations involving sudden impact loads.
The other project involves high strength woven fabrics that are often used in body armour and other applications where impact resistance is required. An area of study encompasses determining the dynamic mechanical properties of such material and developing a computer model of fabric woven from yarns. The model is used to investigate parameters such as the ballistic penetration limit of the fabric, energy absorbed during penetration and how the deformation and failure characteristics are affected by projectile speed and impact angle. A gas gun (Fig. 4) and high-speed photography (Fig. 5) are employed sgto obtain experimental data for comparison with computer predictions. Results obtained indicate that the stress-strain and failure properties of the material are affected by loading speed, contributing to different regimes of fabric perforation response, depending on projectile impact velocity. This information is relevant to the design and analysis of fabric-based body armour.
The Impact Mechanics Laboratory also facilitates work in areas such as the use of cellular materials for shock absorption, characterization of dynamic material properties and impact testing of electronic products and components.
