Figure 5-58. Transfer Limits.
Evaluation of Discontinuity Indications.
When a discontinuity indication is found, it is desirable to learn as much as possible about the discontinuity (or
discontinuities). Information on the location, size, orientation and spacing helps in determining the seriousness of a
The location is determined by noticing the position of the indication on the waveform display and comparing this
position to the positions of indications from known reflectors, such as the front and back surface. This is simple for
straight beam inspections and is explained in paragraph 188.8.131.52. For angle beam methods the position is determined by
first determining the angle of the refracted beam and then performing distance calibration. With this information, the
beam path and distance to the discontinuity in the test part can be determined. It is often helpful to use a cross-
sectional sketch of the test part and draw the beam path on the sketch. For surface wave inspections, the location of a
discontinuity is easily determined by wetting a finger with oil and then moving the finger along the test part surface
away from the search unit. Surface waves will be damped by the finger, and the discontinuity signal will be reduced in
amplitude until the finger moves just past the discontinuity. By noting when the discontinuity signal first starts to
increase in amplitude, the location of the discontinuity is determined. A distance calibration can also be easily set up
for surface waves. The search unit is placed on the test part at a known distance away from a reflector, such as an edge
of the test part; or the search unit can be placed at a known distance from a reflector on the IIW block.
The size of a small discontinuity (less than the diameter of the sound beam) is estimated by measuring the maximum
signal amplitude produced by the discontinuity. Information on sound beam diameter is contained in paragraph
184.108.40.206.5. In general, the amplitude from a small discontinuity is proportional to the cross sectional area of the
discontinuity if the discontinuity is oriented normal to the sound beam. Since natural discontinuities usually have