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Ballistic impact of rods

PhD Thesis, University of Cambridge, 2000
L C Forde

Abstract

Situations involving impact occur in a range of diverse applications, and safe and cost-effective design for these demands understanding of the behaviour of materials and structures when intense impulsive loading is applied to them. Examples of these applications include building demolition, vehicle crashes, structural failure of industrial machinery (e.g. engine turbine blades, nuclear reactor containment vessels) and protection for police officers, public figures and military personnel as well as orbiting spacecraft, military vehicles, vessels and aircraft. Of the great range of impact conditions, the geometries studied in this work are the end-on impact of a rod, with length much greater than diameter, onto: a) a flat plate, and b) another end-on rod (Taylor impact).

An integrated approach was conducted using the versatile equipment of the plate impact facility at Cambridge, capable of launching a 500 g projectile to 1000 m s-1. Many techniques were available for observation of the plate perforation and Taylor impact experiments, such as high-speed photography, with up to 20 million frames per second framing rate, and flash X-ray, as well as embedded foil stress gauges and velocity interferometry (‘VISAR’) to monitor elastic stress waves in the rods.

Plate perforations were generally performed using the reverse ballistics configuration, where the rod was stationary and instrumented. Manganin stress gauges in the rods have proved to be sensitive to a combination of rod yield strength (strain rates of 102 - 103, but small strains as in plate impact), perforation events (e.g. cutting the stress pulse short) and dispersion effects intrinsic to elastic wave propagation in rods. The experimenter may choose which of the three aspects to study by manipulating the experimental parameters accordingly. High-speed photography was utilised to correlate penetration events, concentrating most on deformation, deceleration and failure of the plate rear surface. This was viewed both in profile and directly (in separate experiments) using mirrors to observe a grid drawn on the rear. Relations between such observations as the degree of plate stretching, the time of plate failure and the thickness of plate material in front of the rod nose, were inferred. Computer simulations of perforation showed reasonable agreement but with a slight overestimation of target strength leading to slower apparent penetration.

The Taylor impact experiments were carried out in rod-on-rod or ‘symmetrical’ configuration, approximately analogous in the centre-of-mass frame to the traditional Taylor test of a rod fired at a rigid target. The advantages of the former set-up are minimisation of friction effects between the rods (as they deform together) and of yield in the ‘rigid’ target. However, the need for co-axial alignment of the two rods is an added challenge. Rod profiles taken from high-speed photography were measured to monitor the propagation of plastic waves along the rod, giving characteristic logarithmic decelerating curves. The profiles themselves were characteristic of the material properties such as strain hardening and strain-to-failure (break-up), as were VISAR traces. Asymmetry between the responses of the two rods involved (moving and stationary) was not observed within the resolution of this technique. A modified Armstrong-Zerilli material model predicted intermediate profiles quite well, for example the position of ‘shoulders’ (bulges), but again with a slight overestimation of the material strength (giving 2 - 6 % difference in maximum diameter) which may stem from a lack of failure modes.

The study of penetration of transparent armour materials, i.e. borosilicate glass (pyrex), investigated more brittle failure mechanisms, including Hertzian cone cracking and comminution due to stress wave damage. Flash X-ray was able to discern mushrooming of the rod and subsequent self-sharpening as the lobes shear away during perforation. The target material retained strength for some time after comminution, giving a significant ‘dwell time’ in penetration, and subsequently failed in blocks in a reproducible pattern.


Thesis available upon request.

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