Preparatory Notes for ASNT NDT Level III Examination - Ultrasonic Testing, UT

The plot to the right shows an example curvature correction curve and DAC
curve. This curvature correction curve was generated for the application of
detecting a #4 flat bottom hole under a curved surface as shown in the
sketch and photograph. An immersion techniques was used generate a
shear wave since the reflective surface of the target flaw was not parallel with
the surface. The DAC curve drops monotonically since the water path
ensures that the near field of the sound beam is always outside the part. The
correction factor starts out negative because of the focusing effect of the
curved surface. At greater depths, the correction factor is positive due to the
increased beam spread beyond the focal zone caused by the surface
curvature.