The Behaviour of Impedance-Graded Multi-Metallic Systems under Dynamic Loads

Abstract

The development of protective structures to resist highly energetic dynamic loads such as impact and blasts is a widely researched domain with many civil and military-related applications. With increased emphasis on material, structural and geometrical optimization, a special class of multi-material systems called functionally graded materials have gained significant interest in this field of research. This thesis investigates the performance of a novel multi-metallic system, where the metals are functionally graded in reducing order of their impedance. The scope of the thesis is limited to dynamic loads that generate high strain rates in metals. Under such conditions, metals behave predominantly in a uniaxial state of strain, where the analysis must consider the propagation of stress waves. These stress waves can be in the form of elastic, plastic or shock waves. Hence, Impedance - which is the product of volumetric mass density and wave velocity - was chosen as the function, as it plays a governing role in the propagation of elastic, plastic and shock waves. The main objective of the thesis is to investigate the possibility of attenuating stress waves through the proposed system and a comprehensive experimental, analytical and numerical investigation is carried out to achieve this. The experiments consist of low velocity and high-velocity impact tests and are carried out using a single-stage gas gun test setup, as well as near-field blast trials. The performance of the impedance-graded combinations of steel-titanium, steel-aluminium, steel-titanium-aluminium and steel-brass-aluminium are compared against a monolithic steel configuration. Numerical simulation of these events is conducted using the non-linear finite element software LS-DYNA. One-directional and two-directional wave propagation models with a Lagrangian mesh discretisation are used for the impact events to accurately capture the propagation of stress waves through the solids. In addition to the attenuation of stress waves, the experiments identified the potential of this system in minimizing common failure types under dynamic loads, such as debonding, spalling and scabbing. The reasons behind these findings were explained by referring to the generation of compressive and tensile stresses due to the propagation, transmission and reflection of stress waves. In conclusion, this thesis provides a comprehensive framework to design and apply impedance-graded multi-metallic systems in the field of protective structures.