Introduction to Non Destructive Testing

Nondestructive testing (NDT) is an interdisciplinary field that allows technicians and engineers to create and implement procedures for the testing of materials to locate and qualify flaws. Inspections are performed utilizing one or more methods that will not hinder the inspected materials, thus allowing return to its service life.

Procedures, specifications and codes set the guidelines for the inspectors to permit a material suitable to return to service. These guidelines will state the allowances in shape, dimension and type of flaws found during an inspection as well as the acceptable procedures for repair processes.

NDT methods are continually being researched and developed, however there are a group that are most often used.  These methods are visual, liquid penetrate, magnetic particle, eddy current, ultrasonic and radiography testing.

Visual Testing (VT)

The most basic and most used method involves using your eyes to look for defects. There are many tools that are utilized to aid in the visual inspection of materials, including mirrors, borescopes, magnifying glasses, cameras and video recorders.

Liquid Penetrate Testing  (PT)

PT involves a specific procedure of coating a material with a visible or fluorescent dye solution then waiting for a specific dwell time. Excess dye is then removed from the surface and application of a specialized developer is applied in visible dye inspection. This developer acts as a blotting agent and pulls out any remaining dye trapped in open-to-the-surface flaws. In fluorescent dye inspection an ultraviolet or black light   is used in place of the developer to make the defect areas easily visible and ready for interpretation.

Magnetic Particle Testing (MT)

MT involves inducing a magnetic field into a ferromagnetic material then applying the magnetized area with dry or liquid suspended iron particles. Both open-to-surface and near-to-surface flaws will disrupt the magnetic field and the iron particles will “bunch up” at the flaw showing its location, as well as general orientation and dimensions.

Eddy Current Testing (ET)

ET involves inducing an alternating current into a conductive material. The currents flow in circles on the surface and slightly below. Flaws, dimension change or change in the conductive and permeability properties of tested material will interrupt the eddy current flow and is interpretable with proper inspection equipment.

Radiography Testing (RT)

RT involves the use of penetrating gamma or X-Radiation to expose images of an object on a film. The resulting radiograph film can be inspected and will show flaws and imperfections in the form of shape and density changes much as a medical x-ray.

Ultrasonic Testing (UT)

UT involves of inducing a material with high frequency sound waves to detect flaws or changes in material densities or properties.  Pulse echo is one technique where the induced sounds reflections or echoes from the surface of a part are read on a receiver to detect flaws.