an ultrasonic probe suited for testing the integrity of sheet metal surfaces around fastener openings is provided with means to center the probe over such openings . both a mechanical centering rod and an electronic display assist the user in centering such probe . once positioned within tolerance limits , phased array ultrasonic beams search for defects within the metal surfaces , allowing for residual offsets in the centering of the probe . also described are test fixtures for calibrating the probe .