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NDT

NON-DESTRUCTIVE TESTING (NDT) is a process of inspecting, testing the materials, components or assemblies for finding the defects in them.

Today modern nondestructive tests are used in manufacturing, fabrication and in-service inspections to ensure product integrity and reliability, to control manufacturing processes, lower production costs and to maintain a uniform quality level. During construction, NDT is used to ensure the quality of materials and joining processes during the fabrication and erection phases, and in-service NDT inspections are used to ensure that the products in use continue to have the integrity necessary to ensure their usefulness and the safety of the public.

It should be noted that while the medical field uses many of the same processes, the term "nondestructive testing" is generally not used to describe medical applications.

NDT METHODS

In NDT, we use the following methods for finding the defects

  • PENETRANT TESTING
  • MAGNETIC PARTICLE TESTING
  • RADIOGRAPHIC TESTING
  • ULTRASONIC TESTING
  • VISUAL TESTING
  • RADIOGRAPHIC TESTING FILM INSPECTION

1. PENETRANT TESTING :

Penetrant Testing is also called as Liquid Penetrant Testing (LPT) or Dye Penetrant Testing. In this testing we use a chemical called Penetrant which can penetrate the material being tested and find defects. Penetrant is used to find only surface defects on the material.

Advantages & Disadvantages :

PT is one of the low-cost method used in NDT.

This method does not require current supply

This method can find only surface defects, it cannot find defects inside the materials which are not open to the surface.

Where is PT used?

PT can be used on different materials like:

  • Metals (aluminum, copper, steel, titanium, etc.)
  • Glass
  • Many ceramic materials
  • Rubber
  • Plastics

2.  MAGNETIC PARTICLE TESTING

Magnetic particle Inspection (MPI) is a non-destructive testing (NDT) process for detecting surface and slightly subsurface discontinuities in ferromagnetic materials such as iron, nickel, cobalt, and some of their alloys. The process puts a magnetic field into the part. The piece can be magnetized by direct or indirect magnetization.

The presence of a surface or subsurface discontinuity in the material allows the magnetic flux to leak, since air cannot support as much magnetic field per unit volume as metals. To identify a leak, ferrous particles, either dry or in a wet suspension, are applied to a part. These are attracted to an area of flux leakage and form what is known as an indication, which is evaluated to determine its nature, cause, and course of action if any.

Advantages and Disadvantages:

  • Magnetic Particle Testing is also one of the low-cost methods used in NDT.
  • But almost every equipment requires current supply.
  • Cannot be used on Nonferrous metals as they cannot be magnetized.

Where is MPT used ?

MPT is used only on the following metals

  • Iron
  • Some Alloys of steel
  • Nickel
  • Cobalt

3. RADIOGRAPHIC TESTING

Radiographic Testing is performed by placing an object between a radioactive source and a film. Once the radiation passes through the object, it created an image on the film based on density differences. Radiation can be produced by using an X-ray machine or a Gamma Ray machine. Industrial X-ray machines use a high amount of current whereas Gamma-ray machines use radioactive isotopes which produce radiation without using any current. This is the reason why RT is one of the most preferred methods in NDT.

Advantages & Disadvantages:

This method can find defects in the surface, near the surface and subsurface area of materials of any thickness

We can directly see the image of the defects.

Technicians should be skilled to operate the machine

Safety is mandatory as it involves radiation.

Where is RT used?

RT can be used on any materials of any thickness. Examples include:

  • Metals
  • Plastics
  • Electronic components
  • Nuclear reactors
  • High-Quality Welding
  • Aerospace Industries

4. ULTRASONIC TESTING :

Ultrasonic testing is one of the most important methods of NDT. In this, we use high-frequency Ultrasonic sound waves to find the defects. The ultrasonic sound has a frequency higher than 20,000 Hertz, so they can easily penetrate into metals. In UT we use Probe which produces the sound of frequency between 0.5 MegaHertz to 50 MegaHertz. It is connected to an Ultrasonic Flaw Detector which displays the location of the defects. These equipment are highly expensive.

Advantages & Disadvantages:

  • In UT we can find the depth of the defects which we cannot find in other methods
  • Can find the surface and near-surface defects
  • The technician should be highly skilled
  • Cost of the equipment is very high compared to other methods

Where UT is used?

Ultrasonic testing is often performed on steel and other metals and alloys, though it can also be used on concrete, wood, and composites, albeit with less resolution. It is used in many industries including steel and aluminum construction, metallurgy, manufacturing, aerospace, automotive and other transportation sectors.

5. VISUAL TESTING

Visual inspection, with or without optical aids, is the original method of NDT. Many defects are surface-breaking and can be detected by careful direct visual inspection.

Optical aids include low-power magnifiers, microscopes, telescopes and also specialized devices such as boroscopes, endoscopes and other fiber-optic devices for the inspection of restricted access areas. These devices can also be used with television camera systems. Much of the success of visual inspection depends on the surface condition and the lighting arrangements. Surface preparation such as cleaning and etching is often used and also surface replicas can be taken for both macro- and microscopic examination. 

Advantages & Disadvantages :

  • Visual testing can be done with the help of simple gauges
  • Technician skill requirement is low
  • But we can find only surface defects and areas where there is visual access.

Where VT is used?

VT is used initially before starting an NDT process. With the help of gauges, we can do testing on almost any material.

6. RADIOGRAPHIC TESTING FILM INSPECTION ( RTFI)

Film inspection deals with the inspection of Films produced after RT. In film inspection, we can learn detail analysis of films based on their density and identify the defects produced in the materials.