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Ultrasonic Testing (UT):

 

UT is a non-destructive inspection method that uses high frequency sound waves (ultrasound) that are above the range of human hearing, to measure geometric and physical properties in materials. Ultrasound travels in different materials at different speeds (velocity). However, the speed of sound propagation in a given material is a constant. There are several ways that sound travels through a material. One type of sound wave, called longitudinal or compression travels about 330 metres per second in air and about 6400 metres per second in aluminium or in steel at approximately 5960 metres per second.

 

To perform UT, electrical energy is converted to mechanical energy, in the form of sound waves, by a transducer. The transducer accomplishes this energy conversion due to a phenomenon referred to as the piezoelectric effect. This occurs in several materials, both naturally-occurring and man-made. Quartz is a naturally occurring piezoelectric material. A piezoelectric material will produce a mechanical change in dimension when excited with an electronic pulse. Similarly, this same material will also produce an electric pulse when acted upon mechanically. An example of the common use of piezoelectric materials is found in the electronic lighters available for starting gas stoves, gas grills, cigarette lighters, etc. In these examples, the piezoelectric crystal is squeezed and released suddenly to result in the generation of an electric spark that jumps across a gap to ignite the gas.

 

To perform UT, the transducer is attached to an electronic ultrasonic set. Following a prescribed calibration procedure, the ultrasonic set will essentially be converted into a measuring device. The ultrasonic set will generate precise electronic pulses that are transmitted through a coaxial cable to the transducer that has been placed using a couplant in contact with the test object. These pulses are of very short duration and high frequency (typically 1 to 10 million cycles per second). This high frequency sound has the ability to be directed in which its shape is like the beam from a torch.

 

When excited by the electronic pulses, the transducer responds with mechanical vibration, which creates a sound wave that is transmitted through the material at whatever velocity is typical for that material. A similar phenomenon can be heard when a metal is struck with a hammer to provide a “ringing”. This ringing is simply a sonic (lower frequency) sound wave that travels through the material.

 

The sound wave will continue to travel through the material at a given velocity and does not return to the transducer unless it hits a reflector (a boundary between two different materials, or a flaw). If that reflector is favourably oriented, it will reflect the sound back to the transducer at the same velocity. When struck by this sound wave, the piezoelectric crystal will convert that energy into an electronic pulse which is amplified and displayed on a cathode ray tube (CRT) of the ultrasonic set as a visual indication to be interpreted by the operator.

 

By using “calibration” blocks of the correct material having specific dimensions and shapes as well as the various controls on the ultrasonic set, the time it takes for the sound wave to travel through the material can be related to the distance the sound has travelled. Consequently, the ultrasonic set allows the operator to control how long it takes for the sound to travel through the material to a reflector and back to the transducer to facilitate the accurate distance of how far the sound has travelled.

 

One of the primary benefits of UT is that it is considered to be a truly volumetric test. That is, it is capable of determining not only the length and location of a flaw, but it will also provide the operator with information as to the type of flaw found. Another major advantage of UT is that it only requires access to one side of the material being tested. This is a big advantage in the inspection of pressure equipment, tanks and piping systems.

 

Another important advantage is that UT will best detect those more critical planar discontinuities such as cracking and incomplete fusion. UT is most sensitive to discontinuities that lie perpendicular to the sound beam. Because a variety of beam angles can be used UT can detect laminations, incomplete fusion and cracks that are oriented such that detection with radiographic testing would not be possible. UT has deep penetration ability. Modern UT equipment is lightweight and often battery-powered making that method portable.

 

The major limitations of this test method are that it requires a highly skilled operator because interpretation can be difficult. Also, the test object surface must be fairly smooth and couplant is required for contact testing. Further limitations are that reference standards are required and that no permanent record of the CRT display is generally available. This test method is generally limited to the inspection of butt welds in materials that are thicker than 6 mm.

 

Limitations

 

• Test surface must be smooth
• Couplant required
• Expensive equipment
• Reference standards required
• Results require interpretation by experienced personnel
• Inspection of welds over 6 mm thick

 

Advantages

 

• Volumetric inspection
• Access to only one side required
• Inspects a variety of thicknesses and weld types
• Portable equipment
• Can detect surface and subsurface flaws
• Can readily size flaws detected
• Subject to orientation, can detect planar flaws reliability
• Non-hazardous to personnel
• Suitable for automation

 

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