Originally published in Shock Compression of Condensed Matter 1991, S.C. Schmidt, R.D. Dick, J.W. Forbes, D.G. Tasker (editors) copyright 1992 Elsevier Science Publishers B.V.

Authored by Craig T. Walters

Neodymium-glass laser pulses (1.06-mm wavelength, 25-ns pulse width) have been used to generate shock waves with peak pressures in the 5- to 120-kbar range at the front surface of solids. Relatively uniform irradiance levels were employed with circular beam areas in the 0.4- to 1.5-cm2 range and single pulse energy up to 800 J (fluences ranged from 200-2000 J/cm2). At 1000 J/cm2, the resulting peak shock pressure is about 35 kbar. By confining the plasma with a transparent glass overlay, this peak pressure was raised to 120 kbar. The nature of the plasma initiation process has been revealed through careful simultaneous temporal resolution of the beam-power, temperature, and stress-wave details.

INTRODUCTION
High-power laser pulses have been used to produce stress waves in materials for more than two decades. Most of the measurements of stress wave amplitude have been performed with pulse fluences less than 100 J/cm2. Laser exposures with fluences many orders of magnitude greater than this have been conducted, but pressures have not been directly measured in these cases. We report here measurements of stress-wave amplitudes in laser interactions with fluences in the range 200-2000 J/cm2 with and without plasma confinement (transparent overlays). A map of pressures that may be achieved with single laser pulse interactions is presented in Figure 1 in terms of peak power density. The dashed line at high intensity follows the set of ablation pressures estimated by Cottet et al.1,2 in the correlation of thin aluminum foil spallation data. These estimates follow a 0.7 power dependence on intensity. Similar estimates made by Eliezer et al.3 and Gilath et al.4 are shown in Figure 1 by the dotted line for aluminum and the chain dashed line for carbon/epoxy at lower intensity. The open circles are peak pressures measured recently with 20- to 30-ns pulses at Battelle5,6 at the front surface of stress gage packages coated with either graphite or carbon/polymer (black paint). These data agree with the estimate of Reference 3 at low intensity, but rise nearly linearly up to 5 x 1010 W/cm2 in contrast to the model. The solid circles show the effect of confining the plasma at these intermediate intensities with a transparent overlay. These pressures were in the 90 to 120 kbar range and are believed to be the highest directly recorded pressures generated in laser interactions with solids. The solid squares and triangles present the measurements of confined interactions by Fairand and Clauer7 and by Ballard et al 8 for 30-ns pulses. These data are highly consistent up to about 3 x l09 W/cm2 (90 J/cm2). For fluences in the 100- to 1000-J/cm2 range, the detailed nature of the transparent overlay probably takes on increased importance as breakdown processes interfere with energy delivery to the absorbing interface. Our data indicate that some benefit in increased pressure from a correctly designed overlay may be possible at even higher fluences than those investigated, contrary to the plateau seen in the Ballard data. Careful examination of the pressure histories with different coatings and with and without overlays has also revealed some detail of the plasma initiation process as discussed below.

To download the entire article as a pdf: Laser Generation of 100-kbar Shock Waves in Solids

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.

To download the entire article- as a pdf: Laser Shock Effects on Stressed Structural Materials- Experimental Results

Originally published by Shock Waves and High-Strain-Rate Phenomena in Metals, Mark A. Meyers and Lawrence E. Murr (eds.), Plenum Press, New York (1981), pp. 675-702.

Authored by Allan H. Clauer, John H. Holbrook.

A high-energy, pulsed laser beam combined with suitable transparent overlays can generate pressure pulses of up to 6 to 10 GPa on the surface of a metal. The propagation of these pressure pulses into the metal in the form of a shock wave produces changes in the materials microstructure and properties similar to those produced by shock waves caused in other ways. This paper reviews the mechanism of shock wave formation, calculations for predicting the pressure pulse shape and amplitude, in-depth microstructural changes and the property changes observed in metals. These property changes include increases in hardness, tensile strength and fatigue life. The increases in fatigue life appear to result from significant residual surface stresses introduced by the shock process.

INTRODUCTION
The ability of a high-energy, pulsed laser beam to produce recoil pressures from vaporization of metal surfaces was suggested in 1963 by Askaryon and Morez (1). Others verified this effect on unconfined surfaces (2-4). Then Anderholm (5) showed that pressures of gigapascals (GPa) could be obtained at confined surfaces, i.e., surfaces covered by an overlay transparent to the laser beam. This configuration confined the vaporized materials in the vicinity of the metal surface and significantly increased the peak pressures developed. Later, O’Keefe and Skeen (6) investigated the effects of several different transparent overlays and Yang (7) measured the peak pressures developed by a large number of metal absorbers using a glass transparent overlay. Fairand et al (8) and Fairand and Clauer (9) showed comparisons between pressure measurements and calculated pressures including the effects of different transparent overlays and target absorber materials.

Based on the early demonstrations of the potentially significant stress waves developed in metals by high energy pulsed laser beams, Fairand et al (11) showed that beneficial property and microstructural changes could be produced in an aluminum alloy. The success of these first experiments led to the further exploration and development of laser shock processing of metals. This paper reviews all the aspects of laser shock processing: the formation mechanism of the stress waves, calculations for predicting the pressure environment, in-depth microstructural effects and the material property changes which have been observed up to this point in the process development.

To download the entire article- as a pdf: Effects of Laser Induced Shock Waves on Metals

Originally published in Journal of Applied Physics, 50 (3), 1497-1502, (1979).

Authored by B. P. Fairand and A. H. Clauer.

Stress-wave environments generated at a confined surface by a pulsed laser were investigated. Experimental measurements and theoretical calculations demonstrated that confinement of the surface with a transparent overlay provided an effective method of generating high-amplitude laser-induced stress waves in the target material. Peak pressures approaching 10 GPa were possible at laser power densities of several times 109 W/cm2 for laser pulse durations ranging from several nanoseconds to several tens of nanoseconds. These pressures were generated in an air environment at standard conditions, thus enhancing their practical utilization for processing of materials and measurements of material properties. At laser power densities greater than 109 W/cm2, the laser-induced stress-wave environment was controlled by properties of the ionized plasma created near the metal surface. Some enhancement in the amplitude and duration of laserinduced stress-wave environments was observed at laser power densities less than 109 W/cm2 if low thermal conductivity and low heat of vaporization materials were used. Calculations show that peak pressures greater than 10 GPa were possible by superimposing stress waves either through reflection off a high acoustic impedance barrier or through the interaction of stress waves which were generated at different surfaces of a material.
PACS numbers: 79.20.Ds, 62.50.+p, 42.60.-v

I. INTRODUCTION
The generation of high-amplitude stress waves with short bursts of laser radiation was first investigated a few years after the first laser became operational.1-4 These early studies predicted that high amplitude stress waves could be generated in materials by impinging the laser beam on an unconfined surface of the body and vaporizing a small amount of surface material. Later work which involved direct measurements of pressure showed this was not the case and peak pressures typically were less than 1 GPa.5 Subsequently, methods to enhance the pressure environments over the free-surface conditions by modifications in the target surface conditions proved to be successful.6-11 Our interest in this area was stimulated by the need to generate pressures greater than 1 GPa in order to produce significant changes in the in-depth microstructures and mechanical properties of metal alloys.12-14 A comprehensive understanding of the stress-wave environments needed to produce these changes is required for their effective application in altering the properties of materials. This paper presents our studies of laserinduced stress-wave environments with particular attention given to methods of enhancing the magnitude of the stress waves over free-surface conditions. The effects on stress-wave environments from placing transparent confining media on the target surface and addition of absorbent films to the surface are treated. Our results are based on experimental measurements of pressure and theoretical calculations using a one-dimensional radiation hydrodynamic computer code.

To download the entire article- as a pdf: Laser generation of high-amplitude stress waves in materials

Originally published by Applications of Laser Material Processing, 1979.

Authored by A. H. Clauer B. P. Fairand.

ABSTRACT
The effect of high intensity laser induced stress waves on the hardness and tensile strength of 2024 and 7075 aluminum and on the fatigue properties of 7075 aluminum were investigated. Laser shocking of these alloys increases the hardness of the underaged 2024-T351 but has little or no effect on the peak aged 2024-T851 and 7075-T651 or the over-aged 7075-T73. The largest increases in tensile strength were observed in 7075-T73, lesser increases in 2024-T351 and none in 2024-T851 or 7075-T651. The fretting fatigue life of fastener joints of 7075-T6 was increased by orders of magnitude by laser shocking the region around the fastener hole before drilling and assembling. Also the fatigue crack propagation rates were significantly decreased by laser shocking.

INTRODUCTION
The effects of high amplitude stress waves on the micro-structure and properties of metals and alloys have been the subject of numerous investigations. These stress waves are often generated by explosive charges or impact between a projectile and the target specimen. One of the interesting effects is that the shock waves can develop significant plastic strains in the metal with a smaller equivalent change in the specimen dimensions than required by more conventional metal working processes. This led to investigations of the effects of shock treatments on strength and hardness, fracture toughness, stress corrosion cracking, thermomechanical processing, and other properties (1-4). While in some of these areas the benefit derived from shock deformation compared to conventional working is not always clear, it is established that shock waves can produce high dislocation densities with attendant effects on material properties. However, systems design considerations associated with handling explosives or using driver plates for materials processing make application of shock processing difficult.

Another source of high intensity stress waves is the high energy pulsed laser. This potential was first recognized and explored in the early nineteen sixties (5,6). Subsequent work established the conditions for major enhancement of the amplitude of the stress waves, making it possible to plastically deform metals when irradiating in air at standard conditions (7-11). This stimulated interest in using these laser induced pressure waves to alter the properties of materials in a manner similar to high explosive and flyer plate shock deformation of metals and alloys.

This paper presents the results of investigations on the effects of laser induced stress waves on the hardness, tensile strength, and fatigue life of several aluminum alloys. Although some of the results are from a limited number of experiments, they are of sufficient interest in showing the potential of laser shock processing that they are presented here.

To download the entire article- as a pdf: Interaction of Laser-Induced Stress Waves With Metals

Reproduced with permission from B. P. Fairand and A. H. Clauer, “Laser Generated Stress Waves: Their Characteristics and Their Effects to Materials,” Conference Proceedings #50: Laser-Solid Interactions and Laser Processing – 1978, S. D. Ferris, N. J. Leamy and J. M. Poate (eds.) 27-42. Copyright 1979, American Institute of Physics.

Authored by B. P. Fairand and A. H. Clauer.

INTRODUCTION
The potential of using pulsed lasers to generate high intensity stress waves in materials was first recognized and explored by the early nineteen sixties.1,2 Later work established that a major enhancement in the amplitude of the laser generated stress waves occurred if the absorbent surface was covered with a material transparent to the incident laser light.3-8 Stress waves generated under these conditions were found to be sufficiently intense to plastically deform metals and alloys even when the experiments were conducted in a gas environment such as air at standard conditions.9-11 The ability to generate high intensity pressure environments in materials without imposing the constraint of conducting the experiments in vacuum stimulated interest in using these laser induced pressure waves to alter the properties of materials in a manner similar to high explosive and flyer plate shock deformation of metals and alloys. The laser also offered attractive characteristics as a source of high intensity pressure environments which provided added incentive for investigating the properties and applications of laser generated stress waves.

This paper examines the types of high amplitude pressure environments one can generate with a pulsed laser and defines important parameters governing the interaction mechanisms and their impact on the resultant pressures. This analysis is confined to a pulsed neodymium-glass laser because it was the experimental facility used in essentially all of our investigations; however, other lasers with wave-lengths ranging from the infrared to the visible and near ultra-violet have the potential of producing similar environments.

The effects of these stress waves to the surface and in-depth-properties of materials also is investigated in this paper. These effects range from increasing the surface hardness to improvements in yield strength and increases in the fatigue life of other metals.

To download the entire article- as a pdf: Laser Generated Stress Waves: Their Characteristics and their Effects to Materials

Originally presented at Lasers In Modern Industry Seminar, May 23-24, 1978.

Authored by B. P. Fairand and A. H. Clauer.

INTRODUCTION
When light from a pulsed laser is incident on the surface of an absorbent material, part of the light is absorbed and vaporizes a small amount of surface material. The rapid vaporization and blowoff of this material generates a stress wave at the surface. As this pressure pulse propagates into the material, it changes the metal’s microstructure, which is the source of the observed improvements in material properties. This laser shock process has been successfully used to increase the strength and hardness of stainless steel. The strength properties of heat-affected zones in welded aluminum structures have been increased to values up to the strength of the parent material. Recent studies have demonstrated that laser shock processing can also be used to improve the fatigue life and stress corrosion of some aluminum alloys. In general, all alloys which are strain hardenable have a good chance of responding favorably to the laser shock process.

Active investigation of the effects of laser induced stress waves to materials began in our laboratories in 1971. Results of several years of research have led to a good understanding of the pressure environments generated by pulsed lasers and their effects to materials. Certain facets of this area of technology still require additional studies of a basic character. However sufficient knowledge has been gained from past research to realistically look at the use of laser induced stress waves for the solution of specific material related problems. This paper addresses materials problems presently under investigation and looks at other areas where laser stress waves have potential application. As a prelude to the discussion of laser applications, a review is given of the types of pressure environments generated by pulsed lasers. The application section is followed by a discussion of the types of present and future laser systems possessing appropriate parameters for shock processing applications.

To download the entire article- as a pdf: Applications of Laser-Induced Stress Waves

Originally published by Metallurgical Transactions A, 8A, 119-125 (1977).
Authored by A. H. Clauer, B. P. Fairand, and B. A. Wilcox

The plastic deformation produced by laser induced stress waves was investigated on an Fe-3 wt pct Si alloy. The intensity and duration of the stress waves were varied by changing the intensity and pulse length of the high energy pulsed laser beam, and also by using different overlays on the surfaces of the specimens. The resulting differences in the distribution and intensity of the deformation caused by the stress waves within the samples were determined by sectioning the specimens and etching (etch pitting) the transverse sections. The magnitude of the laser shock induced deformation depended on the laser beam power density and the type of surface overlay. A combination transparent plus opaque overlay of fused quartz and lead generated the most plastic deformation. For both the quartz and the quartz plus lead overlays, intermediate laser power densities of about 5 x 108 w/cm2 caused the most deformation. The shock induced deformation became more uniform as the thickness of the material decreased, and uniform shock hardening, corresponding to about 1 pct tensile strain, was observed in the thinnest specimens (0.02 cm thick). 200 ns laser pulses caused some surface melting, which was not observed for 30 ns pulses, the pulse length used in most of the experiments. Deformation of the Fe-3 wt pct Si alloy occurred by both slip and twinning.

HIGH power pulsed lasers generate stress or shock waves in a target sample by subjecting it to very short pulses of intense laser light. The resultant disturbance in the sample is primarily mechanical since the thermal effects are small and confined to a region within a few micrometers from the laser irradiated surface.
The shock wave is created by the momentum impulse imparted to the specimen’s surface when a thin surface layer is rapidly vaporized by the intense laser light. Since this process depends on effectively coupling the laser energy into the specimen’s surface, the magnitude of the pressure is affected by such factors as the surface condition, its reflectivity, and the target material’s sublimation and ionization energy. Also, phenomena occurring within the blowoff material which absorb, reflect, or reradiate energy, such as the formation of a plasma, limit the amount of energy reaching the surface of the specimen. For these reasons, it was anticipated that the modification of surface conditions through the use of various overlay materials, and blowoff confinement by transparent overlays would affect the peak pressures. In addition, modifications of the pulse length and shape could also influence the magnitude and length of the pressure pulse.

Various recent studies have examined methods of modifying the material surface conditions to enhance the magnitude of laser induced shock waves. 1-4 It has been shown that a 7075 aluminum alloy was shock hardened after irradiation with a high power, Q switched laser through a transparent quart overlay covering the target specimen.5 Another study showed that very short (~1 x 10-9 s), high fluence pulses could cause back surface spalling in thin aluminum discs irradiated in vacuum.6

This paper reports on a series of experiments which relate the magnitude and extent of shock induced plastic deformation produced in an Fe-3 wt pct Si alloy to the laser pulse length and power density for several different types of surface overlays. The overlays studied include a transparent overlay to confine the blowoff material to the sample surface and an overlay of lead, which has a low sublimation energy. The etch pit patterns on the Fe-3 pct Si specimens after shocking and sectioning are qualitatively correlated with the measured and predicted shock pressures. A previous paper7 presents the computer code calculations, laser physics, and pressure environments for the experiments discussed here.

To download the entire article- as a pdf: Pulsed Laser Induced Deformation in an Fe-3 Wt Pct Si Alloy

Originally published in Industrial Applications of High Power Laser Technology, Vol 86, 1977. This electronic reprint is made available with permission from SPIE.

Authored by Barry P. Fairand and Allan H. Clauer.

INTRODUCTION
Laboratory studies have established that the mechanical properties of different aluminum and iron base alloys can be improved by laser shock treatment. When the energy from a powerful pulsed laser is trained on the surface of a metal, a high amplitude stress wave is generated. This wave propagates into the material and alters its microstructure, which is the source of the observed improvement in the metal’s mechanical properties. The ability to generate stress waves in materials with short duration bursts of laser energy has been known for some time, (1-6) but it has only been in recent years that these stress waves have been shown to provide an effective method of altering the in-depth mechanical properties of metals. (7) Various methods have been used to increase the amplitude and duration of these stress waves in order to increase the depth
and degree of change introduced into the metal. (8-15) These techniques have generally taken the form of adding to the surface of the material various coatings and layers of material which may be opaque or transparent to the incident laser energy. The most effective method found up till now for increasing the efficiency of converting laser energy into mechanical stress wave energy has involved the use of transparent overlays. This technique has produced pressure with peak values several times greater than the Hugoniot elastic limit of most metals and alloys. When a pressure of this amplitude propagates through a material, the metal is plastically deformed in a manner similar to that observed in explosively shocked materials.

This paper discusses methods of generating high amplitude stress waves in materials with pulsed lasers and demonstrates by selected examples how these stress waves can be used to improve the properties of metals and alloys.

To download the entire article- as a pdf: Use of Laser Generated Shocks to Improve the Properties of Metals and Alloys

Reproduced with permission from B.P. Fairand, A.H. Clauer, R.G. Jung, and B.A. Wilcox, “Quantitative assessment of laser-induced stress waves generated at confined surfaces,” Applied Physics Letters, 25 (8), 431-433, (1974). Copyright 1974 by American Institute of Physics.

B. P. Fairand, A. H. Clauer, R. G. Jung, and B. A. Wilcox Battelle Columbus Laboratories, Columbus, Ohio 43201. Received 22 April 1974; in final form 15 July 1974)

Laser-induced stress waves in iron samples were analyzed by measuring the pressure environment at the back surface of various sample thicknesses. These results were compared with numerical calculations obtained from a one-dimensiona1 radiation hydrodynamics computer code.  The experiments were conducted in an air environment under ambient conditions and the metal surfaces were confined by transparent overlays. Peak pressures exceeding 50 kbar were measured with quartz pressure transducers at a laser power density of about 109 W/cm2. Computer predictions agreed favorably with the experimental results and indicated that peak pressures exceeding 100 kbar could be generated by appropriate modifications in the laser environment and target overlay configuration.

Generation of stress waves in solids using high-power pulsed lasers has been pursued for some time. 1-6 Over the past four years various methods to enhance the magnitude of these stress waves by modifications in the target surface conditions have received considerable attention. 7-13 In an earlier study it was established that the in-depth microstructural and mechanical properties of aluminum alloys covered with transparent overlays were significantly altered when they were irradiated in air by a highpower Q-switched laser. 14 The laser-induced pressure environment was not monitored in those experiments. However, the high dislocation densities observed from transmission electron micrographs strongly suggested that high-amplitude stress waves, i.e., well above the dynamic yield strength of the material, were being propagated in depth. This paper reports on experiments with iron-base alloys where quartz piezoelectric transducers were used to dynamically measure the pressure environment as a function of sample thickness. The measured shape of the pressure pulse and its magnitude were compared to theoretical calculations performed by a one-dimensional radiation hydrodynamics computer code. Additional computer studies were made to assess the effect of different sample surface configurations on stress wave production. These studies confirmed that peak pressures exceeding 100 kbar can be generated with a high-power Q-switched laser.

To download the entire article- as a pdf: Quantitative assessment of laser-induced stress waves