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

Selecting the right angle beam assembly
The parameters that affect angle beam performance include not only the
(1) beam angle generated by the wedge, but also (2) transducer frequency
and (3) element size. The optimum beam angle will generally be governed
by the geometry of the test piece and the orientation of the discontinuities
that the test is intended to find. Transducer frequency affects penetration
and flaw resolution:
1. As frequency increases, the distance the sound wave will travel in a given
material decreases, but resolution of small discontinuities improves.
2. As frequency decreases, the distance the sound wave will travel increases
but the minimum detectable flaw size will become larger.
3. Similarly, larger element sizes may decrease inspection time by increasing
coverage area, but the reflected echo amplitude from small discontinuities
will decrease. Smaller element sizes will increase reflection amplitude from
small discontinuities, but the inspection may take longer because the
smaller beam covers less area.
These conflicting factors must be balanced in any given application, based on
specific test requirements.