High-strain-rate fracture and fragmentation
Supervisor: Dr Daniel Eakins, Co-Supervisor: Dr Bill Proud
Dynamic fragmentation is a process observed in many materials at high strain-rates, characterised by the large-scale breakup of a contiguous body into many smaller pieces. It is widely agreed that material properties, such as microstructural flaw distribution, grain size, and crystal structure, directly contribute to the conditions for fragmentation. Despite rigorous studies over many decades however, the physics linking material properties to fracture and fragmentation of sheet metals or thin walled vessels strained at high strain-rates is still empirical at best, and not sufficiently developed.
Mechanisms of dynamic fragmentation may be investigated in two controlled loading configurations. The first, the radial expansion of a metal cylinder, is an attractive technique for studying high strain-rate ductility since interpretable fracture information in a uniform radial strain-field is obtained around the circumference of the expanded specimen. The second, the radial collapse of a thick-walled cylinder, is also a useful technique since a sustained compressive pulse, of many microseconds in duration, may be applied to the specimen. In some materials such as stainless steels and titanium alloys, this generates the conditions required for adiabatic shear bands to originate at the inner circumference of the collapsing specimen. Use of both of these configurations provides a means to differentiate how loading (tensile vs. compressive) interacts with existing flaws and defects.
The ISP is offering this PhD studentship to study fragmentation of dynamically expanding and collapsing metal cylinders. It is proposed that experiments are designed and performed with the aim of understanding the mechanics that govern the fragmentation process, particularly in materials known to deform at high strain rates by shear localisation. This work will make use of the large-bore gas-gun, which is shortly to be installed at the ISP, through which the symmetric impact of elastomeric material will be used to apply uniform radial deformation to a cylindrical sample. In addition to conventional analyses such as high-speed photography, the student will be encouraged to develop supplementary techniques to obtain detailed information on incipient crack formation. These techniques may include time-resolved speckle photography to visualise large-field discontinuities in material flow at the surface , spatially-resolved velocity interferometry to characterise the initial loading and any early-time non-uniform behaviour, and post-shock x-ray tomography to note the location of sub-surface cracks within the material microstructure. By surveying the response of several materials and loading conditions, the student will develop detailed novel assessments of the dynamic fracture mechanisms in metallic materials to support sophisticated physics-based hydrocode development.