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

4.3: Curvature Correction
Curvature in the surface of a component will
have an effect on the shape of the ultrasonic
beam. The image to the right shows the beam
from a focused immersion probe being
projected on to the surface of a
component. Lighter colors represent areas of
greater beam intensity. It can be seen that
concave surfaces work to focus the beam and
convex surfaces work to defocus the
beam. Similar effects are also seen with
contact transducers. When using the
amplitude of the ultrasonic signal to size flaws
or for another purpose, it is necessary to
correct for surface curvature when it is
encountered. The "correction" value is the
change in amplitude needed to bring signals
from a curved surface measurement to the flat
surface or DAC value.