Multiple liquid impact of single-crystal lithium fluoride

Abstract

Lithium¯uoride in its single-crystal form is an interesting material for investigating the development of fracture by multiple liquid impact owing to its well characterized crystal structure. The development of fracture during liquid impact is attributed to the extension of short circumferential cracks produced around the loaded area by the passing Rayleigh stress wave after the impact event. The damage threshold of single-crystal lithium¯uoride is developed using the Cavendish Laboratory's multiple impact jet apparatus as a result of identifying the characteristic fracture annulus associated with liquid impact during a controlled experimental procedure. The observation of damage produced in solids by liquid impact has practical signi® cance in the problems with supersonic aircraft¯ying through rain and in the erosion of turbine blades moving at high speeds through wet steam. } 1. The nature of impact damage Liquid impact on single-crystal materials was ® rst reported by Jolli e (1966). Jolli e impacted single-crystal magnesium oxide (MgO) with methanol, water, oil and mercury droplets then etched to reveal dislocation activity. However, Jolli e did not impact single-crystal MgO beyond its damage threshold velocity but did induce the movement of dislocations. Jolli e noted that dislocation movement due to repeated impact by liquids is caused by the extension of dislocation loops to form slip bands on {110} 458 planes. Research workers at the Cavendish Laboratory (Jackson and Field 1998, 1999a) noticed that the nature of impact damage in single-crystal magnesium oxide due to liquid impact was in the form of slip bands in h100i directions until the material reached its damage threshold limit. When the limit was reached, {110}-type cracks appeared in h100i directions which were presumably formed by the interaction of {110} 458 slip planes. Further studies have revealed a number of important fracture observations in multiple-liquid-impact experiments.In addition to the characteristic slip-band pattern for single and multiple liquid impacts on (001) magnesium oxide, there is a fracture annulus surrounding the central impact zone. When etched and viewed in re¯ected illumination, the cracks within the fracture annulus have the following characteristics.(I) Owing to fourfold symmetry around the [001]impact axis, the damage is the same in eight octants.