NDT Testing Methods | Non-Destructive Testing Methods
Table of Contents- Table of Contents
- Introduction
- Types
Of NDT Methods
- 1:
Methods For Internal Flaws:
- 1
a: Radiographic Method
- X and Gamma Radiography:
- 1
b: Ultrasonic Method
- 1
c: Pulse-Echo Method
- 1
d: Transmission Method
- 2:
Methods For Surface/Sub-Surface Inspections:
- 2 a: Visual and Dye
Penetrant Testing Methods
- 2 b: Rayleigh Wave
Testing
- 2
c: Magnetic Particle Testing
- 2
d: Eddy Current Testing
- 2 E: Leak Testing
- Table of Contents
- Introduction
- Types Of NDT Methods
- 1: Methods For Internal Flaws:
- 1 a: Radiographic Method
- X and Gamma Radiography:
- 1 b: Ultrasonic Method
- 1 c: Pulse-Echo Method
- 1 d: Transmission Method
- 2: Methods For Surface/Sub-Surface Inspections:
- 2 a: Visual and Dye Penetrant Testing Methods
- 2 b: Rayleigh Wave Testing
- 2 c: Magnetic Particle Testing
- 2 d: Eddy Current Testing
- 2 E: Leak Testing
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.
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.
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.
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.