ASME BPVC V 2023

NONMANDATORY APPENDIX N
TIME-OF-FLIGHT DIFFRACTION TOFD INTERPRETATION
N-410 SCOPE
This Appendix is to be used as an aid for the interpretation
of time-of-flight diffraction TOFD ultrasonic images.
Diffraction is a common ultrasonic phenomenon and occurs
under much broader conditions than just
longitudinal-longitudinal diffraction as used in typical
TOFD examinations. This interpretation guide is primarily
aimed at longitudinal-longitudinal diffraction TOFD setups
using separated transducers on either side of the
weld on a plate, pipe, or curved vessel. Other possibilities
include:
a shear-shear diffraction
b longitudinal-shear diffraction
c single transducer diffraction called “back diffraction”
or the “tip-echo method”
d twin transducer TOFD with both transducers on the
same side of the flaw/weld
e complex inspections, e.g., nozzles
N-420 GENERAL
N-421 TOFD IMAGES DATA VISUALIZATION
a TOFD data is routinely displayed as a grayscale image
of the digitized A-scan. Figure N-421a shows the
grayscale derivation of an A-scan or waveform signal.
b TOFD images are generated by the stacking of these
grayscale transformed A-scans as shown in Figure
N-421b. The lateral wave and back-wall signals are visible
as continuous multicycle lines. The midwall flaw
shown consists of a visible upper and lower tip signal.
These show as intermediate multicycle signals between
the lateral wave and the back-wall signal.
c TOFD grayscale images display phase changes,
some signals are white-black-white; others are blackwhite-
black. This permits identification of the wave
source flaw top or bottom, etc., as well as being used
for flaw sizing. Depending on the phase of the incident
pulse usually a negative voltage, the lateral wave would
be positive, then the first diffracted upper tip signal negative,
the second diffracted lower tip signal positive,
and the back-wall signal negative. This is shown schematically
in Figure N-421c. This phase information is very
useful for signal interpretation; consequently, RF signals
and unrectified signals are used for TOFD. The phase information
is used for correctly identifying signals usually
the top and bottom of flaws, if they can be differentiated,
and for determining the correct location for depth
measurements.
d An actual TOFD image is shown in Figure N-421d,
with flaws. The time-base is horizontal and the axis of motion