Signal analysis in leaky lamb wave nde technique

Abstract

A system and method for detecting defects in composite laminate material are described which comprise a first trtansducer for directing an ultrasonic beam of preselected frequency and tone burst excitation along a transmission axis through a sonic coupling bath onto a surface of the material at a preselected angle to generate Lamb waves in the material and leaky Lamb waves reflected therefrom, a frequency modulator for selectively modulating the frequency of the incident beam, and a second transducer for receiving the reflected sonic field including leaky Lamb waves reflected from the material and for providing a recovered and envelope-detected output signal corresponding to the amplitude of the reflected waves.

I claim: 
     
       1. A system for detecting defects in composite laminate material, comprising: (a) first transducer means for directing an ultrasonic beam of preselected frequency along a transmission axis onto a surface of a laminate material at preselected angle to generate Lamb waves in said material and leaky Lamb waves reflected from said surface of said material;   (b) means for exciting said first transducer means with a radio frequency tone burst having preselected burst length and preselected on-off duty factor;   (c) second transducer means for receiving said leaky Lamb waves reflected from said material along a reflection axis and for providing an output signal corresponding to the amplitude of the reflected waves received by said second transducer means;   (d) means for recovering radio frequency signal amplitude information from said output resulting from the tone burst excitation of said first transducer means;   (e) said first transducer means and said second transducer means each disposed in preselected spaced relationship to said surface of said material and to each other such that said transmission axis and said reflection axis intersect within said material a preselected distance below said surface; and   (f) fluid means for sonically coupling said material with said first transducer means and said second transducer means.   
     
     
       2. The system of claim 1 further comprising means for selectively positioning said first transducer means and said second transducer means along said surface of said material in said spaced relationship. 
     
     
       3. The system of claim 1 wherein said fluid means for sonic coupling of said material and said first transducer means and said second transducer means comprises water. 
     
     
       4. The system of claim 1 wherein said preselected frequency is in the range of from about 0.5 to about 15 MHz. 
     
     
       5. The system of claim 4 wherein said preselected angle is in the range of from about 8° to about 75° relative to a normal to said surface of material. 
     
     
       6. The system of claim 1 wherein said preselected burst length is from about 10 to about 200 microseconds. 
     
     
       7. The system of claim 1 wherein said preselected on-off duty factor is from about 0.001 to about 0.20. 
     
     
       8. The system of claim 1 wherein said means for recovering radio frequency amplitude information comprises a gated integrating amplifier. 
     
     
       9. A method for directing defects in composite laminate material, comprising the steps of: (a) directing an ultrasonic beam of preselected frequency along a selected transmission axis through a fluid sonic coupling bath onto a surface of a laminate material at preselected angle to generate Lamb waves in said material and leaky Lamb waves reflected from said surface of said leaky material;   (b) gating said preselected frequency to produce a radio frequency tone burst excitation directed along said selected transmission axis;   (c) modulating said frequency of said beam over a preselected percentage range of said frequency at preselected modulating rate;   (d) receiving said leaky Lamb waves reflected from said surface of said material along a reflection axis intersecting said transmission axis within said material at a preselected distance below said surface, and providing an output signal corresponding to the amplitude of the reflected waves; and   (e) envelope detecting said output signal to remove radio frequency content thereof.   
     
     
       10. The method of claim 9 further comprising the step of (f) audio-frequency filtering said output signal to effect a preselected level of suppression of unwanted signal information.   
     
     
       11. The method of claim 10 further including the step of selectively positioning said first transducer means and said second transducer means along said surface of said material for performing steps of (a) through (f) at a plurality of locations along said surface. 
     
     
       12. The method of claim 9 wherein said preselected frequency is in the range of from about 0.5 to about 15 MHz. 
     
     
       13. The method of claim 12 wherein said preselected angle is in the range of from about 8° to about 75° relative to a normal to said surface of said material. 
     
     
       14. The method of claim 9 wherein said fluid sonic coupling bath comprises water.