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Common NDT Methods

 

While there are many different methods of NDT only the more common NDT methods used for the evaluation of materials and welds will be briefly outlined here. These methods are the following:

 

(1) Visual inspection

(2) Liquid penetrant inspection

(3) Magnetic particle testing

(4) Radiographic inspection

(5) Ultrasonic testing

(6) Eddy current testing

 

Visual Testing (VT):

 

VT simply refers to inspection primarily using unaided human vision, but may be enhanced by the use of magnification, borescopes, cameras, or other optical arrangements for direct or remote viewing. It is extremely useful due to its simplicity and low cost. VT is the most commonly applied method, used as a standalone technique or to enhance others such as magnetic particle or liquid penetrant testing.

 

Penetrant Testing (PT):

 

PT originated in the locomotive industry in the late 1800s and has since become invaluable to industries such as oil, gas and aerospace. It is used to detect surface-breaking discontinuities using a low viscosity, highly penetrating oil. The technique is based on the principle of “capillary action”, a process where a liquid is drawn into narrow spaces without the assistance of an external force. It is one of the simplest methods to perform, provides a high degree of reliability and is relatively low cost compared to most other methods of NDT.

 

Magnetic Particle Testing (MT):

 

MT is based on the principles of magnetic induction. A magnetic field is induced into the object under test and particles (powders or liquid suspensions) are applied to the surface. Areas where defects (either surface breaking or near-surface) are present create leakage fields which attract the particles, providing an indication of their existence. Although MT has many advantages, a major limitation is that only ferromagnetic materials such as iron or nickel can be inspected.

 

Ultrasonic Testing (UT):

 

UT is based on the principles of echo location. Short wavelength ultrasonic pulse-waves are transmitted into materials. Waves are reflected by changes in accoustic impedance within the material which are received and displayed as pulses on a cathode ray tube or LCD. The information gathered can indicate cracks, weld grooves and fractures, as well as indicate material thickness or other key characteristics. UT is without a doubt one of the most important methods to date and is indispensable to many industries including aerospace and nuclear power.

 

Eddy Current Testing (ET):

 

ET makes use of electromagnetic induction to inspect conductive materials and is probably the most sensitive of the surface inspection methods available today. ET has a range of uses including flaw detection, conductivity measurements, coating or thickness measurement and efficiency of heat treatment processes. An advantage of ET with regard to flaw detection is its ability to work through non-conductive coatings, making it well suited for maritime and off-shore applications.

 

Radiographic Testing (RT):

 

RT is one of the earliest NDT methods and surprisingly, industrial applications precede medical use. A test object is placed between a radiation source (commonly a radioactive nuclide such as Iridium 192 or Selenium 75) and a detector (radiographic film or imaging plate). Radiation passes through the object and is recorded by the detector, revealing the internal nature of the object. The range of applications and degree of reliability make RT one of the main volumetric methods in use.

 

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