Non destructive testing (NDT), also known as non destructive examination (NDE), is used to examine materials, components and welds in such a way that their properties and usefulness remains unchanged. The most common NDT requirement for equipment providers is to verify welding quality when joining a material. There are five major types of NDT used in weld verification. These categories are: visual testing, magnetic particle testing, radiography, liquid penetrant testing. Let’s cover each in detail below.
Visual Testing (VT)
Visual inspection is the most cost-effective method because it requires no equipment. A visual expectation of a welded joint for porosity and proper weld passes can be quickly performed. All welded joints must be inspected as it is quickest way to spot deficient weld quality.
Liquid Penetrant Testing (LT)
Liquid penetrant testing (LT) coats the subject material with a liquid or fluorescent dye solution. The dye is removed and a developer is applied. The developer draws out any defects that may exist in the material, allowing them to be seen easily. LT identifies surface level defects only and can be used for all types of materials. Fluorescent dyes are developed via ultraviolet radiation and can detect smaller discontinuities than liquid dyes. Liquid dyes do not require darkness, making them a more versatile option.
Magnetic Particle Testing (MT)
Magnetic particle testing (MT) can identify defects (such as cracks) at or near the surface of metals. This method uses ferrous magnetic shavings coupled with the application of a magnetic field. When magnetic flux is introduced to the shavings, defects can be revealed. MT can only be used for ferrous materials, but can detect defects near the surface in addition to surface level defects.
Radiographic Testing (RT)
Radiography (RT) uses radioactive isotopes to determine the internal soundness of welds. Radiography can provide a permanent film record of weld quality It is widely used to locate leaks in welds and can be applied to nonferrous materials where magnetic particle inspection would be useless. This testing method usually requires access to both sides of the welded joint. Although this is a slow and expensive method of nondestructive testing, RT is one of the most highly valued, versatile and widely accepted of all methods.
Ultrasonic Testing (UT)
Ultrasonic testing (UT) uses high-frequency sound waves to detect imperfections in metal materials, as well as changes in properties of the materials that could cause problems or failure of the component undergoing testing. Both surface and subsurface defects in metals can be detected, located and measured by ultrasonic inspection, including flaws too small to be seen by other methods. UT determines the exact position of a discontinuity in a weld and can be used on ferrous and nonferrous materials. It is often suited for testing thicker sections accessible from one side only, and can often detect finer lines or plainer defects which may not be as readily detected by radiographic testing.
Visual inspection, liquid penetrant, magnetic particle, radiography, and ultrasonic testing each have an appropriate time for use. Knowing the practical applications for each of these tests will help you understand how work is deemed safe and fit for use.