Adaptive Nonlinear Signal Processing for Characterization of Ultrasonic NDE Waveforms. Task 2: Measurement of Subsurface Fatigue Crack Size.

Abstract : A new NDE nonlinear signal processing system has been developed to detect and measure small, subsurface fastener hole fatigue cracks. The system synthesized from nondestructive evaluation (NDE) waveform parameter inputs is capable of detecting and measuring quantitatively subsurface fatigue cracks in the size range of 0 to 279 mils to within 70 percent of their nominally characterized lengths. Previous investigations had achieved a 50 percent detection rate for cracks larger than 30 mils, and no detection capability for cracks smaller than 30 mils. However, the fatigue crack measurement system reported herein is the first known fatigue crack NDE system capable of detection and measurement for this wide range. The NDE waveforms were recorded from sixteen aluminum sample specimen cracks under two different experimental conditions. Series 1 was recorded with the ultrasonic transducer wedge at 20 degrees to the plane of the material surface, and Series 2 with the wedge at 30 degrees. A 10 MHz transducer was used for both series.