Nondestructive evaluation is any diagnostic process for inspecting parts or equipment that does not require the part to be damaged or destroyed by the test. Buildings, bridges or pipelines require routine inspections, and cannot be disassembled or damaged while determining their condition. Using a nondestructive 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 ultrasonics, have been used since the mid-20th century to find hidden defects in metal or masonry structures. Ultrasonic sound waves enter the inspected parts to specific depths depending on the material, and a monitor can view 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 nondestructive evaluation is X-ray imaging, where a part is exposed to a radiation beam that leaves an image on a film similar to a photograph. X-rays can penetrate solid materials or the human body and show hidden damage without the need for opening the structure or surgically exposing 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 testing is performed. Repeated X-ray exposures can cause cellular damage in humans and animals, but routine exposures are considered nondestructive.
Dyes penetration testing can be used to find very fine cracks in metals. This nondestructive evaluation is often used in aircraft metals testing, and causes no damage or chemical changes in the aluminum parts. The dye will enter the cracks, making them visible to the eye. In some cases dyes can be used that will glow when exposed to ultraviolet light.
Eddy current testing is another nondestructive evaluation method that uses small electrical 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 pipelines. An electric coil is placed near the object to be tested, and alternating current passing through the coil creates a magnetic field that produces small electrical eddy currents in the test part. Another coil acts as a receiver and will allow the changes in eddy currents to be measured and seen on a monitor.
Strain gauge testing is a nondestructive evaluation that uses small electronic gauges placed on the test part. These gauges measure small changes in the dimension or shape of a part caused by loads and stresses. Strain gauges cause no damage to parts, but require that parts are clean to permit them to be glued in place. Testing with strain gauges needs to occur over a longer period, because stresses occur slowly and changes may not show up immediately. Wireless systems can be used that will send signals to offsite computers for data storage, allowing gauges to remain in place without the need for human monitoring.