Non-destructive evaluation methods, such as ultrasound, x-ray imaging, dye penetration, eddy current, and strain gauge testing, allow for the inspection of structures and equipment without causing damage. These tests can reveal both visible and hidden defects, and are commonly used in industries such as construction and aviation.
Non-destructive evaluation is any diagnostic process for inspecting parts or equipment that does not require the part to be damaged or destroyed by testing. Buildings, bridges or pipelines require routine inspections and cannot be disassembled or damaged while their condition is being determined. Using a non-destructive test allows both visible and hidden defects to be investigated, and the results can be used to determine if further repairs are needed.
High-frequency sounds, or ultrasounds, have been used since the mid-20th century to find hidden defects in metal or masonry structures. Ultrasonic sound waves enter the inspected parts at specific depths depending on the material, and a monitor can display the reflected sound image. Cracks in steel beams, aircraft structural parts, or even hidden defects in human organs can be seen with reflected sound waves.
Another non-destructive evaluation is x-ray imaging, where a part is exposed to a beam of radiation that leaves an image on film similar to a photograph. X-rays can penetrate solid materials or the human body and show hidden damage without the need to open the structure or surgically expose the body part. These tests are often used on bridges where structural damage is suspected but not visible, and the bridge cannot be taken out of service while the test is being performed. Repeated X-ray exposures can cause cellular damage in humans and animals, but routine exposures are considered non-destructive.
Dye penetration tests can be used to find very fine cracks in metals. This non-destructive evaluation is often used in aircraft metal testing and does not cause damage or chemical changes to aluminum parts. The dye will get into the cracks, making them visible to the eye. In some cases, dyes that glow when exposed to ultraviolet light may be used.
Eddy current testing is another non-destructive evaluation method that uses small electric currents produced by a magnetic field to look for hidden structural defects. These tests will only work on metals that can conduct an electric current and are often used on steel structures and piping. An electric coil is placed near the object to be tested and the alternating current passing through the coil creates a magnetic field which produces small electric eddy currents in the test part. Another coil acts as a receiver and will allow the variations of the eddy currents to be measured and displayed on a monitor.
Strain gauge testing is a non-destructive evaluation using small electronic gauges placed on the test part. These gauges measure small changes in the size or shape of a part caused by loads and stresses. Strain gauges do not cause damage to parts, but do require the parts to be clean to allow them to be glued into place. Testing with strain gauges must be performed over a longer period, as stresses occur slowly and changes may not manifest immediately. Wireless systems can be used that will send signals to offsite computers for data storage, allowing the indicators to stay in place without the need for human monitoring.
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