A CRITICAL-ANGLE ULTRASONIC TECHNIQUE FOR THE INSPECTION OF WOOD PARALLEL-TO-GRAIN

The objective of this paper is to present a critically refracted longi tudinal wave (L(CR)) technique that allows localized ultrasonic inspec tion of wood parallel-to-grain by accessing only a single side of the material. The L(CR) technique has been widely applied to other materia ls, but not to wood. The chief advantage of the L(CR) technique is tha t ultrasonic waves can be transmitted through wood at frequencies much higher than previously possible (up to 1.5 MHz), leading to potential gains in sensitivity over lower frequency methods. The L(CR) techniqu e was verified using southern pine lumber. Transducer beam characteris tics were examined and the influence of growth ring angle was observed . Ultrasonic wave energy was found to travel near to the inspection su rface. Further, localized growth ring angle was significantly correlat ed to signal amplitude and frequency.