Abstract:
An apparatus for measuring the strength of a pressure pulse created from a laser peening device. The apparatus is reusable, and includes a pressure-sensitive medium, a back-up disk, and a cap, all disposed within a housing having a removable lid. All components of the apparatus are replaceable, thereby allowing an operator to utilize the apparatus more than once despite the harsh environment of laser peening.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the invention.  
           [0002]    The present invention relates to the use of coherent energy pulses, as from high powered pulse lasers, in the shock processing of solid material, and, more particularly, to a reusable gauge for measuring the strength of an energy pulse in a laser peening apparatus.  
           [0003]    2. Description of the related art.  
           [0004]    Laser shock peening is a process for improving the fatigue, hardness, and corrosion resistance properties of materials by focusing radiation on preselected surface areas of a workpiece. Shock peening the workpiece can avoid gross deformation, cracking, and spallation of the workpiece, and nonplanar workpieces can be shock processed without the need of elaborate and costly shock focusing schemes.  
           [0005]    Laser peening, or also referred to as laser shock processing, utilizes two overlays: a transparent overlay (usually water) and an opaque overlay, typically an oil based, acrylic based, or water based, black paint. During processing, a laser beam is directed to pass through the water overlay and is absorbed by the black paint, causing a rapid plasma formation and vaporization of the paint surface and the generation of a high amplitude shock wave. The shock wave cold works the surface of the workpiece and creates compressive residual stresses, which provide an increase in fatigue properties of the part. A workpiece may be processed by producing a matrix of overlapping spots that cover the fatigue critical zone of the part.  
           [0006]    It would be advantageous for maintaining control and consistency in the laser peening process to utilize a pressure gauge that would sense the pressure being applied by the laser to the workpiece. Because of the high pressure associated with the laser peening process, however, gauges currently utilized in laser peening devices are typically single use gauges, and therefore, render a relatively expensive means of determining the strength of the created shock wave.  
         SUMMARY OF THE INVENTION  
         [0007]    The present invention provides a method of gauging the strength of the laser-formed shock wave with a reusable, and therefore, cost efficient device. The invention comprises replaceable components, including a pressure sensitive medium, a housing for that medium, including a screw-on ring to hold internal components in place, a cap for the pressure-sensitive medium, a backup disk, a momentum trap, and a spring.  
           [0008]    In the invention, a housing is removably mated with a screw-on ring, and the pressure-sensing medium is disposed between the a back-up disk and a cap. The cap includes an external surface and an internal surface, the internal surface having a convex shape for ensuring intimate contact with the backup disc and the pressure-sensing medium.  
           [0009]    It is an advantage of the present invention that components can easily be replaced in the gauge by opening the housing, replacing the necessary components, and assembling it back together. By this process, gauge components that wear at a faster rate than their cooperating components can be exchanged without disposing of the entire gauge.  
           [0010]    It is a further advantage of the present invention that the gauge provides a low cost alternative to single use gauges. By providing replaceable gauge components, the invention allows a user to replace only damaged or worn out parts of the gauge, rather than the entire gauge, during the laser peening process.  
           [0011]    The invention comprises, in one form thereof, a thick high-strength steel or ceramic back-up disk for backing up a pressure gauge medium, such as manganin. In the ceramic embodiment of the back-up disk, a circumscribing guard ring is provided for protecting the ceramic core.  
           [0012]    A separate, thin metal disc or cap engages the back-up disk, thereby holding the pressure gauge medium in intimate contact between the back-up disk and the metal cap.  
           [0013]    In the preferred embodiment of the invention, the pressure-gauge-engaging surface of the cap is shaped in a convex manner, so that it holds the pressure gauge medium in close registry with the back-up disk. The preferred embodiment of the invention is further defined to include alignment notches on both the cap and the housing to prevent relative rotation between the cap and the housing to prevent relative rotation between the cap and pressure gauge medium, thereby avoiding damage to the pressure medium.  
           [0014]    Adequate contact is necessary for accurately reading the strength of the shock wave. Reading of the shock wave magnitude is accomplished by the cap receiving the shock wave and conducting the shock wave to the pressure gauge medium.  
           [0015]    The preferred embodiment of the invention further comprises a momentum trap for attenuating and trapping the shock wave, thereby preventing undue damage to the shock pressure gauge system. Without the momentum trap a shock wave could continue to reverberate within the gauge, causing fracture and yielding a shorter life span for gauge components.  
           [0016]    Additionally, a spring is provided between the momentum trap and the housing, for holding the gauge components firmly against the back-up disk. Also, after each firing of the laser, the spring returns the components to their original position. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention will be better understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein:  
         [0018]    [0018]FIG. 1 is a diagrammatic view of one embodiment of a laser shock peening system;  
         [0019]    [0019]FIG. 2 is a diagrammatic view of the laser shock peening device of FIG. 1, incorporated with a shock pressure gauge of the present invention;  
         [0020]    [0020]FIG. 3 is an exploded diagrammatic view of the preferred embodiment of the invention. 
     
    
       [0021]    Corresponding reference characters indicate corresponding parts throughout the several views. The exemplification set out herein illustrates one preferred embodiment of the invention, in one form, and such exemplification is not to be construed as limiting the scope of the invention in any manner.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0022]    Laser shock processing is an effective method of increasing fatigue life in metals by treating fatigue critical regions. The effects of laser shock processing on fatigue of welded specimens has been studied in great detail in “Shock Waves and High Strained Rate Phenomena in Metals” by A. H. Clauer, J. H. Holbrook and B. P. Fairand, Ed. by M. S. Meyers and L. E. Murr, Plenum Press, New York (1981), PP. 675-702.  
         [0023]    For more thorough background in the prior history of laser shock processing and that of high power processing of engineered materials, reference can be made to U.S. Pat. Nos. 5,131,957 and 5,741,559, these patents explicitly hereby incorporated by reference.  
         [0024]    Referring now to FIG. 1, a typical laser shock processing apparatus  10  comprises a target chamber  12  with an opening  14  for a laser beam  16  created by laser  18 , a source of coherent energy. Laser  18 , by way of example, may be a commercially available high power pulse laser system capable of delivering more than approximately 40 joules in 5 to 100 nanoseconds. The laser pulse length and focus of the laser beam may be adjusted as known in the art. As shown in FIG. 1, a workpiece  20  is held in position within target chamber  12  by means of a positioning mechanism  22 .  
         [0025]    Apparatus  10  includes a material applicator  24  for applying an energy absorbing material onto workpiece  20  to create a “coated” portion. Material applicator  24  may be that of a solenoid-operated painting station or other construction such as a jet spray or aerosol unit to provide a small coated area on workpiece  20 . Apparatus  10  further includes a transparent overlay application  26  that applies a fluid or liquid transparent overlay to workpiece  10  over the portion coated by material applicator  24 . Transparent overlay material should be substantially transparent to the radiation, water being the preferred overlay material.  
         [0026]    Referring now to FIGS. 2 and 3, the present invention comprises a shock pressure gauge  40  positioned within target chamber  12  of the apparatus  10 . As shown in FIG. 3, the preferred embodiment of a shock pressure gauge  40  comprises a housing  41  with a screw-on ring  43 , a pressure gauge medium  44  disposed between a metal disc or  46  and a back-up disk  42 , and a momentum trap  48 .  
         [0027]    In one embodiment of the invention, pressure gauge  40  further comprises a momentum trap  48  for dispersing the shock wave after it is passed through cap  46 , pressure gauge medium  44 , and back-up disk  42 . Momentum trap  48  serves to reduce the momentum of the shock, thereby minimizing reverberation within the gauge and consequently preventing undue damage to back-up disk  42 .  
         [0028]    In another embodiment of the invention, a guard ring  55  is provided when back-up disk  42  is comprised of a ceramic material. Guard ring  55  circumscribes back-up disk  42  such that back-up disk  42  is in a spaced relationship from housing  41 .  
         [0029]    Pressure gauge  40  additionally comprises two thin layers of highly electrically resistant material, for providing electrical insulation around the pressure gauge medium  44 . As illustrated in FIG. 3, insulation layer  50  insulates between pressure gauge medium  44  and back-up disk  42 , and insulation layer  52  prevents electrical conduction between pressure gauge medium  44  and cap  46 .  
         [0030]    In accordance with the preferred embodiment of the present invention, pressure gauge  40  further comprises a spring  60  disposed between housing  41  and momentum trap  48 , for holding momentum trap  48  in intimate contact with back-up disk  42 .  
         [0031]    Additionally, cap  46  is preferably shaped to include an external surface  61  and an internal surface  62 , the internal surface  62  being convexly shaped for ensuring intimate contact between pressure gauge medium  44  and the surrounding elements.  
         [0032]    Housing  41  is threadably mated with screw-on ring  43 , such that all of the enclosed components of the pressure gauge  40  can be accessed. Additionally, housing  41  includes an alignment protrusion  54  for engaging with alignment notch  53  of cap  46 . By providing alignment protrusion  54  and alignment notch  53 , the invention prevents the rotation of cap  46 , thereby preventing undue damage to pressure gauge medium  44 . In some configurations, a guard ring  55  circumscribes back-up disk  42  to prevent tensile release waves from entering back-up disk  42  and possibly fracturing back-up disk  42  as it is supporting pressure gauge medium  44 .  
         [0033]    In the preferred embodiment of the invention, the pressure gauge medium is composed of a form of manganin, but it may also be composed of any piezoresistive material, including 86Cu—12Mn—2Ni, 83Cu—13Mn—4Ni, or ytterbium or piezoelectric materials such as PolyVinyDene Fluoride (PVDF). According to the preferred embodiment of the invention, the piezoresistive or piezoelectric material is approximately {fraction (1/10)} of a micron to two millimeters thick.  
         [0034]    Electrical attachments  56  provide current to and from pressure gauge medium  44 , and more substantial leads  58  carry the current beyond the external portion of pressure gauge  40 . Electrical attachments  56  are preferably composed of gold or copper, but other electrically conductive elements are suitable.  
         [0035]    Insulation layers  50 ,  52 , enveloping pressure gauge medium  44 , are preferably each a thin layer of highly electrically resistant material, such as or Al 2 O 3  or SiO 2 . For piezoelectric material this would be Teflon or a similar film or coating.  
         [0036]    Back-up disk  42  is preferably a cylindrically shaped disk that is manufactured of a solid material selected from the group consisting of B 4 C, Al 2 O 3 , Si 3 N 4 , SiC, TiB 2 , Borosilicate, Borofloatä by Schott Glass, and Pyrexä by Corning and hardened steal such as AISI 4340 steel or Vasco 300. Disk  42  is of sufficient thickness to prevent spalling as the pressure pulse travels through the material, and of sufficient strength or hardness to minimize deformation of the surface.  
         [0037]    Cap  46  is preferably manufactured of a spring steel, such as AISI 1070, or a high strength steel such as AerMet 100, Vasco 300, or AISI 4340, and is shaped such that it is in intimate contact with pressure gauge medium  44  and back-up disc  42 . For piezoelectric material, the cap could consist of a polymeric material such as neoflon PTFE. In the preferred embodiment, the internal surface  62  of cap  46  is slightly convexly shaped, such that the concentric portion  68  of internal surface  62  protrudes slightly, forming a tighter engagement between cap and back-up disk  42  on which pressure gauge medium  44  is located.  
         [0038]    The present invention operates as follows. When laser shock processing apparatus  10  emits laser beam  16 , shock pressure gauge  40  receives a pulse from laser beam  16 . Importantly, gauge  40  is positioned within target chamber  12  in such a way as to place all of pressure gauge medium  44  within the cross sectional area of the path of laser beam  16 .  
         [0039]    When the pulse of coherent energy impacts cap  46 , a shock wave is created in the cap  46 . It is this shock wave that is useful in improving the fatigue life of workpiece  20 , and therefore it is desirable to determine the strength of the shock wave. After being impacted, cap  46  conducts the shock wave to pressure gauge medium  44 , wherein a piezoresistive material or piezoelectric, to be discussed further infra, determines the strength of the shock wave.  
         [0040]    During operation, electricity is carried by leads  58  and electrical attachments  56  through pressure gauge medium  44 . As a shock wave travels through pressure gauge  40 , pressure gauge medium  44  responds to the shock wave by altering or impeding the flow of electricity such that the electrical current undergoes a measurable change. The difference in the flow of electricity is measured by an external logic controller circuit or microchip, which converts the measurements into pressure readings. Under pressure piezoresistive material change resistance, and therefore modify the voltage at constant current, and piezoelectric material emit current flow.  
         [0041]    As noted above, the advantage of the present invention is that any component of the pressure gauge  40  may be replaced individually, without disposal of the entire pressure gauge  40 . This feature is particularly important in the laser shock processing environment because of the high amplitude of the shock waves.  
         [0042]    The invention is utilized without a workpiece, or as a sample workpiece, as detailed below. Utilizing the invention without a workpiece, an operator places pressure gauge  40  in the path of laser beam  16 , such that the surface area of pressure gauge medium  44  is within the perimeter of the laser beam  16 . Laser beam  16  is activated, and a resulting pressure reading is sent to a logic controller or chip.  
         [0043]    Alternately, an operator can utilize the invention as a sample workpiece, subjecting cap  46  to an energy-absorbing coating dispensed by material applicator  24  (FIG. 1), and a transparent overlay dispensed by transparent overlay application  26 . Finally, laser beam  16  is activated, and cap  46  is subjected to a high amplitude shock wave much like a workpiece would have been subjected, transferring the shock wave to the pressure gauge medium  46  for pressure determinations.  
         [0044]    While this invention has been described as having a preferred design, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.