Figure 5-50. Correct and Incorrect Search Unit Orientation for Finding Cracks with Surface Waves.
Lamb Wave Method.
If the thickness of a test part is less than one wavelength of the sound introduced at the appropriate incident angle,
lamb waves (paragraph 18.104.22.168) travel between the two parallel surfaces of the part. This is a special method that is not
Distance Amplitude Correction (DAC).
When inspecting thick material, a DAC curve should be used to compensate for change in sensitivity with changing
metal travel distance. Many instruments have DAC features built in. If this is the case, follow the instructions in the
operator's manual for establishing a DAC curve. Otherwise, procedures for establishing DAC curves are given below.
Straight Beam DAC.
a. Connect the search unit and instrument to be used in the inspection. Turn the reject control to its