Laser Shock Effects on Stressed Structural Materials- Experimental Results

Originally from the proceedings of the Sixth DOD Conference on DEW Vulnerability, Survivability and Effects, May 12-15, 1987.
Authored by C. T. Walters, A. H. Clauer, and B. E. Campbell

ABSTRACT
The effects of intense single pulses of 1.06 mm radiation on structural composite materials have been investigated. Fluences in the 1000 – 3000 J/cm2 range were delivered in single pulses with 20 ns pulse widths (FWHM) to thermal coupon and tensile bar type samples in vacuum. Materials studied included Kevlar/epoxy, fiberglass/epoxy, and graphite epoxy uniaxial composites in coated and uncoated conditions. Diagnostics were employed to assess energy partitioning in the interaction and stress wave histories in the material. Post-test sample examination and strength tests were conducted on the tensile bar samples. The diagnostics indicated that most of the beam energy goes into a very hot plasma (300,000 K) which drives a shock wave into the material. The shock wave has a peak amplitude of about 30-40 kbars and attenuates as it propagates through the sample. A synergistic damage effect was discovered wherein the sample fails in tension due to addition of the sample preload stress and the axial component of stress due to the shock wave reflected from the rear surface of the sample. Details of the beam energy partitioning and strength degradation in the samples will be presented.

INTRODUCTION
During the past several years, experimental laser effects research has been in progress to understand the effects of single short-wavelength laser pulses on composite materials in vacuum. In most cases, previous studies have been oriented toward understanding fundamental interactions in unstressed samples at low incident laser fluences (< 200 J/cm2 ) and emphasized measurement of integrated response characteristics such as thermal coupling, impulse coupling, and effective heat of ablation, Q*. Recently, research was undertaken to extend some of these basic results to the more realistic case of laser interaction with stressed samples and, in particular, to look carefully at laser-induced shock enhancement of material damage. This paper summarizes results of an 18-month effort to develop an understanding of the damage and degradation of complex structural materials subjected to intense single laser pulses (20 ns, 1.06 mm) in vacuum while they are under preload stress conditions which might be typical of actual applications. (ref. 1) In the high fluence regime investigated, high plasma pressures generate stress waves which have damaging effects in addition to the simple removal of material by vaporization. Under certain conditions, laser induced stress wave components were found to add to the preload stresses to produce enhanced damage effects.
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