The most commonly occurring defects in welded joints are porosity,
slag inclusions, lack of side-wall fusion, lack of inter-run fusion,
lack of root penetration, undercutting, and longitudinal or transverse
With the exception of single gas pores all the defects listed
are usually well detectable by ultrasonics. Most applications
are on low-alloy construction quality steels, however, welds in
aluminum can also be tested. Ultrasonic flaw detection has long
been the preferred method for nondestructive testing in welding
applications. This safe, accurate, and simple technique has pushed
ultrasonics to the forefront of inspection technology.
Ultrasonic weld inspections are typically performed using a straight
beam transducer in conjunction with an angle beam transducer and
wedge. A straight beam transducer, producing a longitudinal wave
at normal incidence into the test piece, is first used to locate
any laminations in or near the heat-affected zone. This is important
because an angle beam transducer may not be able to provide a
return signal from a laminar flaw.
The second step in the inspection involves using an angle beam
transducer to inspect the actual weld. Angle beam transducers
use the principles of refraction and mode conversion to produce
refracted shear or longitudinal waves in the test material. [Note:
Many AWS inspections are performed using refracted shear waves.
However, material having a large grain structure, such as stainless
steel may require refracted longitudinal waves for successful
inspections.] This inspection may include the root, sidewall,
crown, and heat-affected zones of a weld. The process involves
scanning the surface of the material around the weldment with
the transducer. This refracted sound wave will bounce off a reflector
(discontinuity) in the path of the sound beam. With proper angle
beam techniques, echoes returned from the weld zone may allow
the operator to determine the location and type of discontinuity.
To determine the proper scanning area for the weld, the inspector
must first calculate the location of the sound beam in the test
material. Using the refracted angle, beam index point and material
thickness, the V-path and skip distance of the sound beam is found.
Once they have been calculated, the inspector can identify the
transducer locations on the surface of the material corresponding
to the crown, sidewall, and root of the weld.