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Test for External Defects / Test for Internal Defects

Coupling is realized from the outside for pipe testing with automated ultrasonic testing lines.Defect testing for internal defects is realized according to the principle shown in image 2 by using the corner effect.Testing for external defects is achieved by using a reflection at the internal surface (backwall) to redirect the defect signals back into the probe (see image 3).

Ultrasonic probe
Imperfection
Reflector

 

Grundlagen klBild3

Longitudinal defect detection

Testing for longitudinal defects is generally done by using line focused probes. The probes are focused that way that the probe is tilted on the circumference by the refraction angle α towards the normal vector of the surface. The refraction angle is chosen by means of equation 2 that a transverse wave is spreading within the test material at an refraction angle of 45° to the normal of the surface.  The line-focused sound field is focused parallel to the longitudinal axis of the pipe. Longitudinal defects can be detected in an optimal way by using the corner effect that way. The test is carried out with at least two probes and sound incidence angles to cover both circumferential directions, thus longitudinal defects which are slanted towards therunning diagonally to the surface can be detected as well.

Transverse defect detection

Testing for transverse defects is generally done with line focused probes as well. The orientation of the sound fields is similar than for longitudinal defect detection: the line focused sound field is adjusted parallel to the longitudinal axis of the tube and is slanted in a way that a shear wave in the material under test will be propagating under 45° to the surface normal. Two transducers are used here as well to cover both incidence directions.

Oblique defect detection

Especially the production of seamless steel pipes is prone to the occurence of defects which are oriented under an obliquity angle towards the tube's longitudinal axis. These oblique defects can be detected as well with line focused probes, which are oriented in a way thus their sound fields are oriented parallely to the defects direction.