Increasing recognition is being given to NDT (Non-destructive Techniques) in various engineering disciplines at all stages. Associated with this is a growing concern about the reliability of the material, technique used and cost. Testing and evaluation includes not only the measurement and presentation of data, but also requires inference from such data based on comparison and interpretation.
Ineffective use of NDT can be disastrous in industries such as nuclear, space and defense, where quality demands are stringent. In other industries, rejection of acceptable components due to ineffective NDT can lead to financial losses. Acceptance of rejected components results in damage to the reputation of the company due to supply of products that do not meet the quality requirements.
The choice of the appropriate NDT method depends on the components/structures to be inspected, the type of defect and the suitability of the technique from access point of view. While selecting a particular NDT, the cost factor for each specific method must also be considered. In selecting a particular method of NDT for evaluation of a discontinuity it should be kept in mind that NDT techniques complement each other and therefore several NDT methods may be capable of performing the same task. Before going into selection of a suitable NDT method for detecting discontinuities, the user must know what types of defect are to be expected in any inspection. The selection of one method over another is based on the following factors:
- Type and origin of discontinuity
- Material manufacturing process
- Accessibility of the component to perform NDT.
- Type of equipment available.
- Time available
NDT depends on human judgment for the interpretation of results. The skill, competence and knowledge of the person performing the test are highly important in NDT. This is the reason why certain procedures specify the certification of personnel conducting the tests in addition to other requirements. Failure analysis provides invaluable inputs in identification of defects which caused the failure. Failure analysis helps in evolution of improvements and refinements in NDT techniques. The following table provides a comparison the various NDT techniques used:
Table 2: Frequently used NDT Methods
|Optical or visual inspection||One point of access is enough.||Very versatile, little skill required.|
|Liquid penetrant||Requires a clean surface. Flaw shall be exposed to the surface.||Applied on all except porous materials.|
|Magnetic particle inspection (MPT)||Requires a clean and reasonably smooth surface.||Applied only on magnetic materials. Surface breaking and subsurface defects are detected.|
|Electromagnetic or eddy current testing.||Surface must be reasonably clean and smooth.||Cracks, thickness measurement, comparison of materials are possible. Applicable to electrically conducting materials.|
|Radiography||Access to both sides is required.||Considerable skill is needed for interpretation.|
|Ultrasonic Testing||One or both sides of the specimen.||Requires point-by-point search. Skilled personnel required.|
|Acoustic emission||Remote testing possible||Detection and location of growing defects possible under stress.|
|Thermography||Direct or remote testing||Thermal mapping for troubleshooting. High capital cost.|
Each NDT technique is unique with its own advantages and disadvantages. Nothing can be substituted for visual inspection, nor can visual inspection replace other established NDT techniques.