Metals Response to Multiple Shock Loading Impacts

Impacts with sufficient velocities can generate shock waves which propagate through materials. These shock waves can reflect off free surfaces and interact to cause local regions of high tensile stresses. This can lead to the spall failure of the material, whereby a fragment of material is detached from the bulk. The mechanism of spall failure varies depending upon the material but, for a ductile material, is typically:
- the generation of spall voids within the bulk of the material, termed incipient spall
- the coalescence of the voids
- spall failure
Plate impact experiments are commonly used to study spall caused by impacts. Typically conducted on gas guns, a flyer plate (the impactor) is fired at a stationary target. The velocity, and both the type and thickness of the impactor and target materials can be controlled to generate the desired spall characteristics.
The aim of this project is to study the effect of not one but two plate impacts on a target material. The first impact will be tailored to generate incipient spall. The samples will then be recovered, remounted and impacted by a second shot with the aim to recompact the material, to close the spall voids. To ensure that the target material undergoes the simplest form of shock loading, whereby the shock waves are 1D, shock recovery techniques will be used and the samples soft captured.
The materials that will be investigated are copper and aluminium, both with face centred cubic crystal structures, and titanium which has a hexagonal close packed structure. The objectives of the project are to assess the resultant recompaction, the microstructure and mechanical properties of the target material and link the recompaction characteristics to the properties of the three materials.
The insights gained from this research will fill an important gap in the literature and provide direct insight into the fundamentals of the behaviour of materials which experience multiple impact events. These can be defence/security related concerns, such as the behaviour of blast mitigating structures subject to multiple loadings when a device is detonated. The work will also underpin industrially relevant areas such as shock-welding, one of the potential technologies put forward to meet the demand for higher requirements for welding technology. Shock welding is capable of directly joining a wide variety of similar and dissimilar combinations of metals that cannot be bonded by traditional methods, and so is of particular interest to industries that require optimal lightweight structures such as aerospace and automotive.