Electro-elastic self-scanning crack detector
FIELD OF THE INVENTION 
     This invention relates to apparatus and methods for detection of surface defects in conductive materials and more specifically to contactless self-scanning electro-elastic nondestructive testing techniques capable of rapid inspection of large surface areas to determine in real time the presence, location, number and size of defects present on the surface of a conductive material under test. 
     BACKGROUND OF THE INVENTION 
     It is often necessary to verify in real time the mechanical integrity of materials having large areas to assure satisfactory quality of an article or workpiece. It is especially important to assure integrity and quality in aircraft componentry where the failure of components may have life threatening consequences. In other applications, it is important to inspect or monitor the workpiece or member in real time even while the workpiece or member is subjected to stress. 
     Many non-destructive inspection techniques have been developed in the past. Eddy current inspection techniques, in particular, have been employed for a number of years for the inspection of materials having conductive surfaces, however, known eddy current probes have proved ineffective in differentiating the presence of multiple defects and sensing small surface defects below 0.010. One form of eddy current probe known in the art employs a coil which is scanned over the surface to be inspected. A defect in the surface, of sufficient size, causes a change in the electrical impedance of the coil. The probe produces an output signal indicative of defects proximate to the eddy current probe. An extremely small area coil is needed to permit detection of small defects. The prior art eddy current probes, even of small area, have not been effective in reliably detecting defects of less than 0.010 inches in length. Additionally, the use of eddy current coil probes requires the time consuming and inexacting process of X-Y scanning to complete inspection of a large area surface. 
     One eddy current defect sensor is disclosed a co-pending U.S. patent application of the same inventor and assignee as the present disclosure which is capable of detecting defects smaller than 0.010 inches. In this disclosure eddy currents are induced in a conductive workpiece by one or more wires disposed along a path parallel to a first axis, and defects are sensed with one or more wires disposed along a path perpendicular to and intersecting the first axis. 
     Another apparatus and method for eddy current detection of subsurface discontinuities in a conductive body is disclosed in U.S. Pat. No. 4,271,393 to Hansen, et al. 
     SUMMARY OF THE INVENTION 
     A method and apparatus are disclosed for rapid and efficient non-destructive inspection for cracks, fissures, or other defects of a large area workpiece having a surface of conductive material. A first wire is disposed parallel to and in fixed spatial relation with the surface of the workpiece. In one embodiment, a pulsed electrical current is passed through the wire and current flow through the wire induces eddy currents in the conductive surface parallel to the first wire. A magnetostrictive delay line, such as a nickel wire, is disposed closely parallel to the first wire and substantially equidistant from the surface of the workpiece. The presence of a defect or crack in the workpiece causes transverse eddy currents within the workpiece which magnetostrictively induce elastic waves in the delay line. The elastic waves travel in the magnetostrictive wire and are detected by an ultrasonic sensor. The location of the defect is determined by the time of arrival of the elastic wave at the ultrasonic sensor relative to the time of current pulse generation. 
     The defect detector operative in accordance with the present invention, may be manually scanned over the surface of a workpiece, automatically scanned over the surface of a workpiece, or the workpiece may be positioned with respect to the detector so as to effect full inspection of the full area of the workpiece surfaces. Additionally, the wires may be shaped to conform to a non-planar workpiece surface to permit inspection of such surfaces for defects. 
     In an alternative embodiment, pulsed currents are applied to the workpiece so as to flow along parallel paths directly through the workpiece. The presence of a defect produces transverse current components which electromagnetically couple elastic waves to a two-dimensional magnetostrictive delay line which is parallel to the workpiece surface being inspected. One or more ultrasonic sensors are disposed at selected locations of the delay line. Based on the time of arrival of elastic waves at each of the sensors with respect to current pulse energization of the workpiece, the precise location of surface defects is determinable. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     The invention will be understood by reference to the following description in conjunction with the accompanying drawings in which: 
     FIG. 1 is a pictorial representation of self-scanning line electro-elastic sensor in accordance with the present invention; 
     FIG. 2 is a pictorial representation of the sensor of FIG. 1 in which a coil is employed for sensing of elastic waves; 
     FIG. 3 is a pictorial representation of a sensor in accordance with the present invention in which currents are directly applied through a conductive workpiece; and 
     FIG. 4 is a pictorial representation of a two-dimensional magnetostrictive sheet employed as an acoustic delay line in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In accordance with the present invention, apparatus and method are disclosed that provide for detection and location of cracks, defects, or fissures in the surface of a conductive material, article, or workpiece. The apparatus includes means for applying electric current in the workpiece along a first axis or means for inducing an eddy current in the surface of the workpiece along a first axis and additionally includes a magnetostrictive delay line and an associated sensor coupled to transverse current electromagnetic fields resulting from at least one defect creating elastic waves in the magnetostrictive delay line. 
     Referring to FIG. 1 there is shown a workpiece 10 having a surface 12 to be inspected. Parallel to the surface 12 is a first wire 14 which is disposed parallel to and in fixed spatial relation with the surface 12 of the workpiece 10. A pulsed current 16 produced by a source of current 18 flows through the wire 14. The flow of current 16 through the wire 14 results in an eddy current 20 in the surface 12 of the workpiece 10 parallel to the current 16 but of opposite direction as shown in FIG. 1. A magnetostrictive delay line 22, such as a nickel wire, is disposed parallel to the wire 14 and parallel to and substantially equidistant from the surface 12 of the workpiece 10. An ultrasonic sensor 24 is coupled to the magnetostrictive delay line 22 at one end. The sensor 24 is operative to produce an output signal representative of received elastic waves. The delay line 22 need only be magnetostrictive in the portion corresponding to surface area inspection. Details on a magnetostrictive delay line are shown in U.S. Pat. No. 4,035,762 of the same inventor. 
     A magnetic bias may be applied to the magnetostrictive delay line to enhance its sensitivity. 
     The presence of a defect 26 in the surface 12 to be inspected causes the pulsed eddy currents 20 to flow around the edges of the defect 26, thereby producing transverse eddy currents 28. The transverse pulsed currents 28 magnetostrictively induce an acoustic pulse in delay line 22 which travels as an elastic wave in the delay line 22. The elastic wave travels through the delay line 22 to the sensor 24 and the time of arrival of the elastic wave at the sensor 24 with respect to the time of application of current 16 by the source 18 is indicative of the location of the defect 26. In the embodiment shown in FIG. 1, the longer the time interval between application of current by the source 18 and receipt of the elastic wave by the sensor 24, the farther the defect 26 is located from the sensor 44. It is apparent that the sensor 24 may be coupled to either end of the magnetostrictive delay line 22. 
     In the embodiment shown in FIG. 2, a coil 30 is employed as a sensor, and is disposed about the magnetostrictive delay line 32. The coil 30 is operative to produce an output signal by magnetostrictive interaction with an elastic wave travelling down the delay line 32 induced by a defect in the manner previously discussed. 
     Any form of ultrasonic sensor may be employed such as capacitive sensor, piezoelectric sensor, an optical sensor, or a fiber optic sensor. Additionally, the magnetostrictive delay line may be limited in length to the width of the surface to be inspected and the magnetostrictive delay line may be coupled to an elastic wave transmitting member which is in turn coupled to an ultrasonic sensor. 
     In another embodiment illustrated in FIG. 3, pulsed currents 34 are applied directly to the workpiece 36 by a source of current 38. The current source is connected to a first electrode 40 which is disposed along a selected edge of the workpiece 36 and in conducting relationship therewith. A second electrode 42 is disposed along an opposing workpiece edge and provides a path for return current. The presence of a defect 44 causes transverse pulsed currents 46 resulting from the flow of current around the edges of the defect 44. The transverse pulsed currents produce in a magnetostrictive delay line 48 an elastic wave which travels along the the delay line 48. The elastic wave travels through the delay line 48 to an ultrasonic sensor 50 which is operative to provide an output signal representative of the received elastic waves. The presence and location of a defect in the surface of the workpiece may be determined based upon the time of arrival of an acoustic wave at the ultrasonic sensor 50. 
     The magnetostrictive delay line may be scanned with respect to the workpiece surface or the workpiece surface may be positioned with respect to the delay line to affect inspection of the entire workpiece surface. In the embodiment in which eddy currents in the workpiece are inductively coupled to a wire 14, the wire 14 is scanned along the workpiece with the delay line 22. 
     The surface to be inspected may be other than a flat surface in which case the magnetostrictive delay line and current carrying wire are shaped to the contour of the workpiece surface. 
     A two-dimensional delay line may also be employed to map an area for defects as illustrated in FIG. 4. In this embodiment, a two dimensional sheet 52 is disposed proximate to and equidistant from the surface 54 of a conductive workpiece 56. The sheet 52 is presently shown to be a magnetostrictive sheet, such as nickel. A pulsed current is applied to the conductive workpiece 56 so as to produce discontinuities in the magnetic field resulting from current flow in the vicinity of defect 58 edges in workpiece 56 surface 54 in the manner previously described. Elastic waves are magnetostrictively generated in the sheet 52 at the defect locations, and the elastic waves travel through the sheet 52 to a first edge 50 and a second edge 62 of the sheet 52. Ultrasonic sensors 64 are disposed in elastic wave transmitting contact with the respective first and second edges of the sheet and are operative to produce output signals indicative of received elastic waves. The presence and location of defects in the workpiece 56 surface 54 may be determined based on the time of first arrival of the elastic waves at the sensors 64 disposed along first and second edges 60 and 62 of the sheet 52 with knowledge of the speed of sound in the sheet 52. The ultrasonic sensors 64 may be electromagnetically, mechanically, electrostatically, optically and magnetically coupled to the two-dimensional delay line producing an output signal indicative of travelling elastic waves. 
     Additionally, workpieces which are nonconductive but conductively plated may be inspected in accordance with the present invention by coupling to a single axis or two-dimensional acoustic delay line via electrostatic, optical or thermoelastic means. Examples of such workpieces include gold plated ceramic substrates. 
     The above described invention is illustrative of a novel method and apparatus permitting non-destructive inspection of a workpiece. Other modifications, embodiments, and departures from the present disclosure are possible without departing from the inventive concept contained herein. Consequently, the invention is to be viewed as embracing each and every novel feature and novel combination of features present in or possessed by the technique and apparatus herein disclosed and are limited solely by the scope and spirit of the appended claims.