ULTRASONIC POLAR BACKSCATTER IMAGING OF TRANSVERSE MATRIX CRACKS

Transverse matrix cracks in [0,90]s, [0,90(2)]s and [+/- 45]s graphite/epoxy laminates were imaged with a 15 MHz spherically focused transducer which was oriented at 27-degrees from the normal to the plane of the laminate and perpendicular to the fiber direction of the cracked ply. The backscattered signal from the micron wide cracks was two to five times stronger than the backscattered signal from the fibers. Polar backscatter C-scans of the coupon shaped specimens showed the cracks clearly enough to be counted up to a crack density of 8 per centimeter (20 cracks/inch).