Nondestructive Evaluation Techniques

Magnetic Particle

Magnetic particle inspection is an NDE technique that evaluates ferromagnetic material. Ferromagnetic material is material that can be magnetized. A ferromagnetic-colored powder is sprinkled onto the material as the material is being electronically charged by a coil. The coil, or yoke, is a handset that sends an electric charge into the material to magnetize it. Any cracks or subsurface defects attract the colored powder and become magnetized. Liquid ferromagnetic material is also a type of magnetic particle inspection. Instead of using powder, a colored liquid is poured over the test material as a charge is placed on the area. The liquid acts the same way as the powder and is attracted to any cracks or subsurface defects.

Dye penetration is an NDE used on any type of material, including metallic or nonmetallic nonporous material. The testing material must be thoroughly cleaned for this type of NDE to work properly. A liquid dye is poured over the testing material and wiped away leaving defective areas that have been penetrated by the liquid dye. Cracks measuring as little as 150 nanometers can be seen by the naked eye during the test. Some dye-penetration liquid is used under white light or ultra-violet light when maximum sensitivity is required.

In ultrasonic NDE, a quality-control inspector uses sound beams to penetrate an object or material. The sound wave has a high-frequency, short wavelength and is detected by an oscilloscope. The oscilloscope shows a pulse on the monitor. The time it takes a wavelength or pulse to travel across the material and to a reflector determines whether there is an internal flaw. The height of the wavelength shown on the monitor determines the size of the defect. The taller the wavelength on the monitor, the bigger the defect.

Ground-penetrating radar is an NDE technique used primarily by construction companies. A radio wave is sent into the ground and this electromagnetic wave is monitored by an antenna traveling along the surface of the ground. Once a defect is detected, the radio wave or pulse strikes the defect and sends an echo back to the antenna and through a cable into a computer system. The different radio echos show up on the computer monitor and can be recorded for further analysis.