Non-destructive testing is any material test which does not cause damage to the subject material.
The simplest form of NDT is visual observation. The quality assurance representative will simply look at the material for signs of stress or cracks. Checks may also be performed for foreign material or improper alignment of a joining material.
These checks are normally performed by individuals with specialized training. Some inspection requirements call for specific quantities and colors of lighting. These tests may include validating wiring diagrams to physical layouts or the use of soap bubbles to check for leaks. This is when a non-corrosive soap is placed in deionized water and then applied to a joint or weld bead. The system to be tested is pressurized. Any bubbles which form in the soap film are indicative of a leak. This is easy to perform and can be accomplished with minimum training. However, it is only useful for detecting leaks in pressurized systems.
Another form of NDT is liquid dye testing. This is where a penetrant is applied to a cleaned service. Any extra penetrant is then removed. This may be as simple as wiping it off or as complicated as using an emulsion and spray. After this, a developing compound is applied and a trained technician will inspect for defects. This test is both fast and cheap; however, it is not useful for anything beyond surface flaws.
Another form of NDT is magnetic particle inspection. This is where a ferrous material to be tested is placed in a magnetic field. Any defect will cause magnetic flux leakage. Iron powder is sprinkled along the tested material to provide a visual indication of flux leakage. This is also relatively inexpensive but can only be performed on ferrous materials.
Radiography is another common form of NDT. In this, a strong gamma/x-ray source is placed near the item to be tested. The radiation field observed on the other side of the material will be shielded or allowed to more easily pass based on material defects. This is a moderately invasive procedure that will require special radiation precautions since electromagnetic radiation is harmful to the technician.
Ultrasonic testing is a form of testing where an ultrasonic transducer is connected to the item to be tested in a sink. The transducer vibrates causing sound waves to be transmitted to the material. A diagnostic computer will display the results based on reflection of sound waves on flaws in the material. This is a very high accuracy test that has high penetrating power and can detect extremely small flaws. However, this type of testing can only be performed on certain materials and is very complicated. Where it is possible, it is one of the most preferred types of NDT.
Similar to this, acoustic emission monitoring may also be used to predict material failure. In this type of testing, a transducer is attached to a material to be monitored. These transducers are monitored by a computer which will alert users when a crack begins to propagate when the material is under load. In this way, steps can be taken to reduce the chance of failure or altogether repair the material. In this picture, a vibration sensor has been placed on a pump to indicate abnormal operation. An example of acoustic monitoring can be seen at the top of this article.