TML Consultancy - NDT Level 3 - What is NDT?

What is NDT?

NDT is Non-destructive Testing, also known as NDE (Non-destructive Examination). There are many NDT methods and techniques, and here are brief descriptions of the more commonly used ones. The object of NDT is to find flaws in the material being tested, and the successful application of the test is largely dependent upon the skill of the operator and the equipment used.

Magnetic Testing (MT)
Also known as MPI (Magnetic Particle Inspection). It is a method that can be used to find surface and near surface flaws in ferromagnetic materials such as steel and iron.

The technique uses the principle that magnetic lines of force {flux) will be distorted by the presence of a flaw in a manner that will reveal its presence. The flaw (for example, a crack) is located from the "flux leakage", following the application of fine iron particles to the area under examination. There are variations in the way the magnetic field is applied, but they are all dependant on the above principle.

The iron particles can be applied dry or wet; suspended in a liquid, coloured or fluorescent. While magnetic particle inspection is primarily used to find surface breaking flaws, it can also be used to locate sub-surface flaws. But its effectiveness quickly diminishes depending on the flaw depth and type.

Surface irregularities and scratches can give misleading indications. Therefore it is necessary to ensure careful preparation of the surface before magnetic particle testing is undertaken.

Penetrant Testing (PT)
Also known as Dye Penetrant Inspection (DPI), Liquid Penetrant Inspection (LPI) or Fluorescent Penetrant Inspection (FPI). It is a method that is used to reveal surface breaking flaws by bleedout of a coloured or fluorescent dye from the flaw.

The technique is based on the ability of a liquid to be drawn into a "clean" surface breaking flaw by capillary action. After a period of penetration time, excess surface penetrant is removed and a developer applied. This 'draws' the penetrant from the flaw to reveal its presence. Colour contrast (Or "visible") penetrants require good white light while fluorescent penetrants need to be used in darkened conditions with an ultraviolet light (Also known as black light).

Penetrant inspection can be used on almost any non-porous material. It is essential that the material is carefully cleaned first, otherwise the penetrant will not be able to enter the defect. If surface penetrant is not fully removed, misleading indications will result.

Ultrasonic Testing (UT)
Ultrasonic inspection uses sound waves of short wavelength and high frequency to detect flaws or measure material thickness. It is used to test welds, castings and wrought products, e.g. rolled plate or forgings. Manual and immersion techniques are employed.

Manual testing involves sound waves (pulsed beams of high frequency ultrasound) transmitted via a hand-held transducer, which is placed on the specimen. A 'couplant' is used to enable the sound to pass from the transducer to the component under test. Any sound from the pulse that returns to the transducer (echo) is shown on a screen, which gives the amplitude of the pulse and the time taken to return to the transducer. Flaws anywhere through the specimen thickness reflect the sound back to the transducer. Flaw size, distance and reflectivity can be interpreted.

The immersion technique involves placing the component in a tank of water and passing the sound waves through the water into the component.

More recently developed Ultrasonic Testing techniques are Phased Array Ultrasonic Testing (PAUT) and Time of Flight Diffraction (TOFD). PAUT utilises multi-element probes, which are individually excited under computer control. By exciting each element in a controlled manner, a focused beam of ultrasound can be generated. Software enables the beam to be steered. Two and three dimensional views can be generated showing the sizes and locations of any flaws detected. TOFD is also under computer control and is based on diffraction of ultrasonic waves on the tips of discontinuities, as opposed to the sound being reflected back from the discontinuities. When ultrasound is incident at linear discontinuity such as a crack, diffraction takes place at its extremities in addition to the normal reflected wave.

Because of its complexity considerable technician training and skill is required.

Radiographic Testing (RT)
Also known as x-radiography and gamma radiography. X-rays are produced by high voltage x-ray machines, whereas gamma rays are produced from radioactive isotopes such as Iridium 192 and Cobalt 60. The x-ray or gamma rays are placed close to the material to be inspected and they pass through the material and are then captured on film. This film is then processed and the image is obtained as a series of grey shades between black and white. Gamma sources have the advantage of portability which makes them ideal for use in site working.

X-rays and gamma rays are very hazardous. Special precautions must be taken when performing radiography. Therefore the operator will use these inside a protective enclosure or with appropriate barriers and warning signals to ensure there are no hazards to personnel.

Digital Radiography is a recent form of radiographic imaging. Since no film is required, digital radiographic images are captured using either special phosphor screens or flat panels containing micro-electronic sensors. The images can be digitally enhanced for increased detail and can be easily archived.

There are a number of forms of digital radiographic imaging including:

  • Computed Radiography (CR): digital imaging process that uses a special imaging plate which employs storage phosphors.
  • Real-Time Radiography (RTR): a form of radiography that allows electronic images to be captured and viewed in real time.
  • Direct Radiography (DR): a form of real-time radiography that uses a special flat panel detector.
  • Computed Tomography (CT): uses a real-time inspection system employing a sample positioning system and special software.

Eddy Current Testing (ET)
ET is an electromagnetic technique and can only be used on conductive materials. Its applications range from crack detection, to the rapid sorting of small components for either flaws, size variations, or material variation. Commonly it is used in the aerospace, automotive, marine and manufacturing industries.

When an energised coil is brought near to the surface of a metal component, eddy currents are induced into the specimen. These currents set-up magnetic field that tend to oppose the original magnetic field. The impedance of coil in close proximity to the specimen is effected by the presence of the induced eddy currents in the specimen.

When the eddy currents in the specimen are distorted by the presence of the flaws or material variations, the impedance in the coil is altered. This change is measured and displayed in a manner that indicates the type of flaw or material condition.

Visual Testing (VT)
Also known as Visual Inspection, VT is the one NDT method used extensively to evaluate the condition or the quality of a weld or component. It is easily carried out, inexpensive and usually doesn't require special equipment.

It is used for the inspection of welds and other components where quick detection and the correction of flaws or process related problems can result in significant cost savings. It is the primary evaluation method of many quality control programmes.

VT requires good vision, good lighting and the knowledge of what to look for. Visual inspection can be enhanced by various methods ranging from low power magnifying glasses through to boroscopes. These devices can also be used with television camera systems. Surface preparation can range from wiping with a cloth to blast cleaning and treating with chemicals to show the surface details.

It can identify where a failure is most likely to occur and identify when failure has commenced. VT is often enhanced by other methods of inspection which can identify defects that are not easily seen by the eye.


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