Refers to Non-Destructive Testing - liquid penetrant, magnetic particle, ultrasonic, radiographic and other tests and inspections that do not alter the service life.
Non Destructive testing. Any material testing that doesn't destroy material. During the testing process. Note: Usually we think of Ultrasonic Thickness Testing. Also included in NDT is usually dye-penetrant testing, x-ray testing, and electrical resistance testing. Vibration, thermography, look, listen, feel, smell, winding test etc are not included since they are testing components, and equipment, not material.