Nondestructive TestingNondestructive testing: …..is an examination of an object in anymanner which will not impair the future usefulness of the object.

Although in most cases nondestructive tests do not provide a directmeasurement of mechanical properties, they are very valuable inlocating material defects.

Such a test used to detect faulty materials before it is formed ormachined into component parts, to detect faulty components beforeassembly, to measure the thickness of metals, to identify and sortmaterials etc.

Nondestructive TestingThere are five basic elements of nondestructive testing:1. Source2. Modification3. Detection4. Indication5. Interpretation

The most common methods of nondestructive testing or inspection are:

Radiography

Magnetic-particle inspection

Fluorescent-penetrant inspection

Ultrasonic inspection

Eddy current inspection

Nondestructive TestingRadiography of metals: The X-ray darkenthe film so that regions of lower densityappear more darken compared withregions higher density. Because, higherdensity absorbs more of the radiation.Thus a hole or crack appears as a darkenarea, whereas copper inclusions inaluminum alloy appear as lighter areas.

The radiography of metals are usedpreliminary for the inspection of castingand welded products, it may also be usedto determine the thickness of materials.

Nondestructive Testing

Nondestructive TestingMagnetic-particle Inspection: This isa method of detecting the presenceof cracks, laps, tears, seams,inclusions, and similardiscontinuities in ferromagneticmaterials such as iron and steel.

The method will also detect surfacediscontinuities too fine to be seen bythe naked eye and will also detectdiscontinuities which lie slightlybelow the surface. It is notapplicable to nonmagnetic materials.

When the discontinuity is open tothe surface, the magnetic field leaksout to the surface and forms smallnorth and south poles that attract themagnetic particles. (See fig. 1.31)

Nondestructive TestingFor this method, it is necessary to magnetize the materials to beinspected.

There are two methods for the magnetization (a) longitudinalmagnetization (b) Circular magnetization

Nondestructive Testing

Nondestructive TestingFluorescent-Penetrant Inspection: This is sensitive nondestructive methodof detecting minute discontinuities such as cracks, shrinkage and porositythat are open to the surface, while this method may be applied to magneticand nonmagnetic materials. Its primary application is for nonmagneticmaterials.

Nondestructive Testing

Ultrasonic Inspection: Ultrasonic is a fast, reliable nondestructivetesting method. Ultrasonic waves for nondestructive testing areusually produced by piezoelectric materials. These materialsundergo a change in physical dimension when subjected to anelectric field.

The conversion of electrical energy to mechanical energy is knownas the piezoelectric effects.

If an alternating electric field is applied to a piezoelectric crystal,the crystal will expand during the first half of the cycle andcontrast when the electric field is reversed.

Nondestructive Testing

Nondestructive TestingBy varying the frequency of the alternating electric field, we canvary the frequency of the mechanical vibration (sound wave)produced in the crystal.

Quartz is widely used ultrasonic transducer. Two common ultrasonictest method-the through transmission method & pulse-echo method.

If an electric pulse of the desired frequency is applied to thetransmitting crystal, the ultrasonic waves produced will travelthrough the specimen to the other side. The receiving transducer onthe opposite side receives the vibrations and converted them into anelectrical signals that can be amplified and observed on the cathode-ray tube of an oscilloscope.

Nondestructive TestingIf the ultrasonic waves travels through the specimen withoutencountering any flaw, the signal received is relatively large. If thereis a flaw in the path of the ultrasonic wave, part of the energy willbe reflected, and the signal received by the receiving transducer willbe reduced.