NDT Testing Methods | Non-Destructive Testing Methods

NDT Testing Methods | Non-Destructive Testing Methods

NDT Testing Methods | Non-Destructive Testing Methods

NDT Testing Methods | Non-Destructive Testing Methods

Introduction

'Non-destructive testing (NDT)' is testing materials without destroying them. The materials after inspection do not change in their size, shape, physical or chemical properties. There are some major NDT methods. visual and optical aids, radiography testing, ultrasonic testing, eddy current testing, magnetic particle testing and dye penetrant testing. These are regularly used by industry, Each of these methods, has its own flaw detection capability and therefore no method can replace other methods. These methods are discussed in brief.

Types Of NDT Methods

Commonly employed NDT methods can be broadly divided into two groups:

1: Methods for detection of internal defects

2: Methods for inspection of surface/sub-surface defects

1: Methods For Internal Flaws: 

Given information about defect Location size shape and type of defect in between material inside.

 1 a: Radiographic Method

Basic principle of radiographic testing is shown in figure 1 a. The radiation transmitted through a material, is recorded on an X-ray film. In this method, different types of radiations and films are used, depending upon the type of information required.

NDT Testing Methods | Non-Destructive Testing Methods

Fig. 1 a Principle of Radiography Testing.  

 X and Gamma Radiography:

Radiography testing is most widely used in industry for inspection of welds, castings and assemblies. The image of a material produced on an X-ray film shows optical density (degree of blackening) variation depending on the internal structure of the object.

Industrial X-ray machines in the range of 50-400 k V are used for inspection of metal thicknesses upto 7.5 cm steel equivalents. Betatrons and linear accelerators upto 30 MeV are used for higher thicknesses.

Artificially produced radioisotopes, emitting gamma radiations, such as iridium-192 and cobalt-60 have many advantages over X-ray machines. Equipment used for gamma radiography is compact, rugged and ideal for field work. Iridium-192 and cobalt-60 sources combined together can cover inspection range of 10-200 mm of steel equivalents.

 1 b: Ultrasonic Method

Ultrasound waves are generated by piezo electric transducers which convert, electrical energy to mechanical vibrations and vice-versa. These waves are made to fall on the material to be tested. As the wave travels through the material, it may get reflected, refracted, scattered or transmitted depending upon the structure of the material.

NDT Testing Methods | Non-Destructive Testing Methods
Fig. 1 b : Ultrasonic Methods

Most commonly used frequency range for industrial inspection is 0.5 - 25 mega hertz. Three important methods of ultrasonic testing are pulse-echo, transmission and resonance techniques.

1 c: Pulse-Echo Method

In this method, evenly timed pulse waves are transmitted into the material to be tested. Homogeneous material, the wave travels through the material and gets reflected from the back surface. In case of a defect, the original pulse reflects back from the defect location and returns to the transducer before the return of back surface echo pulse as shown in figure 1 b a part. A single transducer can serve both as transmitter and receiver.

 1 d: Transmission Method

In this method, two separate transducers are used on either side of the material, one as transmitter and the other as receiver (Fig. 1 b. b part) Variation in intensity across the transmitted beam indicates the soundness of the material. Attenuation of the sound beam is indicative of coarse grain structure of the material. When the wave length is comparable to the grain size of the medium, scattering process predominates. Transmission method is less sensitive.

1 f: Resonance Method

This method is mainly useful for measurement of thickness of plates or sheets and also in case of bonded materials. In this method, ultrasonic wave of continuously varying frequency is fed into the material. The frequency is varied till a standing wave is set up within the material, causing it to resonate at the fundamental frequency or multiples of it at a greater amplitude as shown in figure 1.f. The resonance is sensed by an instrument. Change in resonant frequency is an indication of discontinuity.

NDT Testing Methods | Non-Destructive Testing Methods

Fig. 1.f : Principle of Resonance method.

 2: Methods For Surface/Sub-Surface Inspections:

Given information about defect Location size shape and type of defect in material Surface and Sub-Surface.

2 a: Visual and Dye Penetrant Testing Methods

Examination of cracks and other irregularities on the surface under visible light is the cheapest, simplest and quickest NDT method. However, all defects cannot be seen, by unaided eyes. Therefore, techniques to increase the contrast of the discontinuity to make it visible are used. Coloured and fluorescent dyes are made to seep into the surface cracks to provide contrast against the background.

2 b: Rayleigh Wave Testing

Rayleigh waves are ultrasonic shear waves propagated near the surface of a material. Waves of frequencies in the range of 1-10 MHz arc used for detection of surface cracks and other defects. The technique is used, where access is limited.

2 c: Magnetic Particle Testing

This method is applicable only to materials which can be magnetised. The object is magnetised by applying high alternate or direct currents (A.C or D.C) and flow of magnetic powders is observed either in dry or wet process. Surface discontinuity, such as grinding cracks, forging laps and seams, etc. can be easily detected. In some cases, sub-surface defects about one centimeter deep, can also be revealed.

 

Fig. 2 c: Principle of Eddy Current Testing

 2 d: Eddy Current Testing

The method employs alternating currents in the range (50-5000 kHz), and is useful for detection of surface and near surface defects in electrically conducting materials. When a coil carrying alternating current is placed in the proximity of metal specimen, as shown in figure 1.4, eddy currents are induced on the surface layer. Strength of these eddy currents depends on a largenumber of surface variaales.

(Note:- Depth of inspection depends upon type of material and frequency of the alternating current and it is about 1-10 mm.)

2 E: Leak Testing

Leak testing method is used to check fabricated components and systems, for nuclear reactors, pressure vessels, electronic valves, vacuum equipment, gas containers, etc. A leak is passage of a gas from one side of the wall of the container to the other side, under pressure or concentration difference. It is measured as cc/sec.

Depending upon the range of leak detection capability, a number of test methods are available. Some examples are pressure drop/rise, ultrasonic leak detectors, bubble tests and ammonia sensitized paper, with detection capabilities upto 104 cc/sec. Halogen diode sniffer, Helium mass spectrometer and Argon mass spectrometer have detection sensitivities in the range 107 - 10 11 cc/sec.

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