Ultrasonic Methods for Crack Detection

Concerning the cracks in the sheets we have to distinguish between surface breaking cracks at the probe coupling surface and those cracks ending at the opposite surface. Mode conversion phenomena in connection with a phased arraybased LLT - technique and the method of secondary echo were investigated toestimate crack depth quantitatively

With the method of secondary echo, longituainal waves arisen by mode conversion after each refledion of the shear wave at the sheet surfaces in half skip distances, propagate nearly parallel to the sheet surfaces and deliver echo indications when impinging at the crack surface. Therefore two characteristic indications can beobserved when the crack is greater than about 30% and only one indication when the crack is smaller than about 30% of the wall thickness.

When the crack starts at the opposite surface, crack depth is determined by varying adequately the angle of incidence for the transmitting and receiving wave. For each depth position within the lower part of the -sheet the combination of transmitting and receiving beam angle is calculated and the expected and measured sound path lengths are compared. The depth position where we have conformitiy between calculated and measured sound path length delivers Greek depth when determining the distance between depth position and surface.

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