INTRODUCTION TO NON- DESTRUCTIVE TESTING (NDT)

INTRODUCTION TO NON- DESTRUCTIVE TESTING

INTRODUCTION

Non-destructive testing (NDT) Is a  method of quality control of materials without disturbing their aimed use.  The material under Inspection does not undergo any change In Its size, shape, physical and chemical properties. It is commonly used for:

Detection of flaws such as cracks, shrinkages, gas porosities, inclusions, and another weld, forging and casting defects.
Determining the material characteristics such as density, conductivity, hardness, grain structures, etc. to give clues about the strength of materials.
Dimensional measurements such as wall thickness, internal and outside dimensions, corrosion assessment coating/plating thickness, etc.
On-line monitoring of plants/ industrial systems.


There are six major NDT methods such as visual and optical aids, radiography, ultrasonics, eddy currents, magnetic particles, and dye penetrants, which are regularly used by industry in the various diagnostic applications. Besides these, there are. other NDT methods such as leak testing, thermal imaging, electrical resistivity, laser holography, etc. which are employed for special applications in material product testing.

Each of these methods has its own flaw detection characteristics and therefore no individual detection method can replace the other one. All these methods are discussed herein brief to bring out their basic principles and a few main features.

PRINCIPLE AND GROWTH

All NDT methods work on a simple principle that a suitable beam of energy, which is capable of interacting with the discontinuity to be detected, is launched into the specimens under test.  The emerging beam from the specimen - either transmitted, reflected or scattered is detected with a  suitable detection system, processed and analyzed for its information content.  The data thus obtained is evaluated to determine the soundness of the material for
acceptance/rejection.

History of  NDT methods shows that the visual examination has been in use since the beginning  of civilization. In the olden days, people practiced NDT methods such as ringing or knocking on earthen vessels to check their soundness, an examination of the egg against candlelight, chalk and oil method to detect surface discontinuities 1n meta, etc. The first use of some of the NDT methods can be tracked back to 1850  - dye penetrants, 1860  - leak testing  1879 - eddy current testing, 1895- radiography, 1930- ultrasonic testing, 1948- holography and 1950- acoustic emission.

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