The non destructive technique is quality control method that does not damage the material or product being tested.
The basic purpose of non destructive testing is to know the quality of material such that it may be considered for a particular job or may be rejected if it is not upto the specifications.
Term ‘Non-destructive testing’ is used in general form as this method uses testing or inspection without changing the structural properties of the material so that it may still be used in future. These have the following
salient features:1. It has no effect on future serviceability.
2. Profits or productivity may be increased after the tests.
3. Safety norms may be applied as per the requirements after the tests.
Non–destructive tests involve the following essential parts:
(i) Application of the testing or inspection material.
(ii) Modification of the materials after identifying the various defects.
(iii) Using suitable device to identify the defects and their positions.
Non destructive tests are generally used to find and identify the defects in the materials while destructive tests pertain to find the mechanical properties like hardness, toughness, compressive strength, yield strength, tensile strength etc.
Some of the important methods used in non-destructive tests are given as follows:
(a) Magnetic Methods
In this method the specimen is magnetized and the distortion of magnetic field due to defects is studied. Using this technique, defects of thickness 0.7 cm to 1.25 cm can be identified.
(b) Electrical Methods
These methods are based on the principle of different materials having different resistivity. In electrostatic method, variation of electric fields can be detected by motion of charged particles.
When a coil carrying high frequency alternating current is moved near a conductor, eddy currents are induced in the conductor. Also the magnetic field is associated with induced currents. Some defects, physical and chemical constituents and other structural variations cause change of resistivity thereby the eddy currents also very resulting in the variations of induced magnetic fields. These variations in magnetic fields can either be detected by search coil or by their effect on coils inducing the currents.
Various techniques used for non-destructive electrical tests are: change of electrical resistance, thermoelectric effect, triboelectric effect, and variation of static field.
(c) Radiographic Methods
Radiographic method uses the technique of X-rays or gamma-radiations. In these days even electron and neutron beams are being used.
On passing through matter, these high radiation beams are modified by various defects in the material. Phenomenon of differential absorption forms the basis of use of radiography. The contrast that is the density difference on the developed film between the image of an area containing a defect and image of defect free part of specimen allows knowing the flaw.
Decrease in intensity ∆I is proportional to the original intensity of the beam and thickness ∆x of the absorber.
Therefore, ∆I = -μ I ∆x
Where μ is constant of proportionality and negative sign shows that with increase of thickness, intensity decreases. Normal X-ray sources can find flaws upto a thickness of 20 cm while for higher thickness, more input voltage may be used to study inner structures. Some of the techniques being used in non-destructive testing using Radiography as under-
1. Weld radiography can find flaws, slag inclusion, incomplete fusion, cracks, pin holes in welds etc. The term ‘Slag’ or non metallic inclusion is used to describe the oxides and other solids which are found along with the weld.
2. Multiple film techniques are used to scan total specimen in one exposure. This avoids retakes of camera.
3. In pipe radiography, three techniques viz. single wall, double wall and variation of double are used.
4. High speed radiography and xero-radiography are modern methods and give better resolution than ordinary X-ray films.
5. Micro radiography is being used to study very small defects and to identify thin sections having minute voids.
6. Electron radiography uses fast electrons and is based on the principle of surface electron emission due to secondary photoelectrons. This technique is more useful for metals having high atomic number.
7. In neutron radiography, neutrons are used instead of X-rays. Neutron radiography is very useful for light materials like plastic, rubber etc. where X-rays and gamma rays cannot be used.
8. Gamma radiography being more economical than X-rays is being used in many fields. However, it has lower contrast as compared to X-ray radiography. Radium, Cs, Radon, Cobalt-60 are some of the gamma-ray sources. Cs is suitable to study the radiography of steel.
(d) Ultrasonic Methods
Ultrasonic waves find useful in the detection of cracks or cavities in metal castings due to the property of reflection of sound waves when a change of medium takes place.
Method consists of sending a strong ultrasonic beam through an object. Due to a crack or cavity there is a change of medium and so the beam gets reflected. The reflected beam is detected on a CRO (Cathode Ray Oscilloscope) and time of transit of beam is noted. Knowing the velocity of sound waves in the medium of solid, position of crack can be detected. An important advantage of ultrasonic method is that it can test very large size castings without destroying the material. This method of locating flaws is more sensitive and less costly than the radiography method. In sections of large casings, rails, heavy axles as in locomotives etc. ultrasonic methods are used.
(e) Thermal Methods
Basic principle in thermal tests is to apply heat to the specimen and measure the heat distribution. In case, there are flaws, heat distribution and consequently temperature measurement will differ as flaws change the heat distribution. Heat may be supplied directly or by electrical heating or by infra red sources. Temperatures may be measured by thermocouples, Bolometers or photoconductive devices.
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