CEMP-E
TI 809-26
1 March 2000
4. ULTRASONIC TESTING (UT).
a. Method Description. Ultrasonic testing requires specialized equipment to produce and receive
precise ultrasonic waves induced into the steel using piezoelectric materials. The unit sends electric
pulses into the piezoelectric crystal, which converts electrical energy into vibration energy. The vibration
is transmitted into the steel from the transducer using a liquid couplant. The vibration is introduced into
the steel at a known angle, depending upon the design of the transducer, with a known frequency and
waveform. The speed of travel of the vibration in steel is also known. The vibration pulse travels through
the steel until it strikes a discontinuity, or the opposite face of the steel, either of which reflects energy
back to the transducer unit or another receiving transducer. Using a system of calibration and
measurements, the location, relative size and nature of the discontinuity, if any, can be determined by
close evaluation of the reflected signals. Small reflections are generally ignored, unless located in
specific regions such as along edges. Locations of discontinuities can be determined using the display
screen scale and simple geometry.
(1) AWS D1.1 Section 6, Part F provides the UT inspection procedures, including calibration,
scanning methods, scanning faces, and transducer angles, and weld acceptance criteria, including
reflected signal strength, discontinuity lengths and locations for weld discontinuities. Report forms,
generally hand written, are prepared by the UT technician, recording weld discontinuities and other
material discontinuities that exceed the acceptance criteria specified.
(2) More expensive and sophisticated UT equipment can be operated in digital mode, recording
and printing display screen images with input data. Very sophisticated automated UT equipment can
record the transducer location and the corresponding reflections, then use computer software systems to
produce representative two-dimensional images, from various directions, of the inspected area and
discontinuities. Such equipment is rarely used in normal construction inspection applications, but is
available and sometimes used for very complex and critical inspections.
(3) Even with conventional equipment, more complex inspection methods can be used to locate,
evaluate and size weld discontinuities. These techniques include tip diffraction and time-of-flight
techniques, and can be incorporated into project inspection through the use of AWS D1.1 Annex K
provisions. Annex K requires the use of written UT procedures specific to the application, with
experienced and qualified UT technicians tested in the use of the procedures, and also provides for
alternate acceptance criteria in lieu of the tables found in Section 6, Part F of AWS D1.1. Such
provisions are necessary when using miniature transducers, alternate frequencies, or scanning angles
other than those prescribed.
b. Advantages and Disadvantages.
(1) Ultrasonic testing is a highly sensitive method of NDT, and is capable of detecting discontinuity
in welds and base metal in a wide variety of joint applications and thicknesses.
(2) AWS D1.1 provisions are applicable for thickness ranges from 8 mm (5/16 in.) to 200 mm (8
in.) Both thinner and thicker materials may be examined and evaluated using UT, but Annex K must be
used for technique and acceptance.
(3) Although capable of locating discontinuities and measuring discontinuity length, it is less
capable of directly sizing discontinuities or determining discontinuity height without the use of advanced
techniques.
D-7
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