A new technique for the ultrasonic detection of internal transverse cracks in carbon-fibre/bismaleimide composite laminates

Through-transmission ultrasonic C-scan imaging with inclined focusing transducers in confocal configuration has been used for the detection of partial and internal cracks in a cross-ply, [0/90/0] carbon/epoxy laminate. A number of partial cracks (intersecting only one edge of a coupon) as well as completely internal cracks were found ultrasonically after the sample was subjected to static loading. The validation of the result by cutting the sample along two lines in the 0° direction and checking the presence of the internal cracks with an optical microscope shows a good correlation between the ultrasonic image and the optical data. Experimental results show that in the IM7/5250-4 composite, which contains a toughened BMI resin, initiation of transverse cracks under static loading occurs with the formation of partial cracks not necessarily originating at the edge of the sample.