NDT APPLICATION AND LIMITATIONS

NDT APPLICATION AND LIMITATIONS

         Non-destructive testing (NDT) are non-invasive techniques to determine the integrity of a material, component or structure or quantitatively measure some characteristic of an object. In contrast to destructive testing, NDT is an assessment without doing harm, stress or destroying the test object. The destruction of the test object usually makes destructive testing more costly and it is also inappropriate in many circumstances.

          NDT plays a crucial role in ensuring cost effective operation, safety and reliability of plant, with resultant benefit to the community. NDT is used in a wide range of industrial areas and is used at almost any stage in the production or life cycle of many components. The mainstream applications are in aerospace, power generation, automotive, railway, petrochemical and pipeline markets. NDT of welds is one of the most used applications. It is very difficult to weld or mould a solid object that has no risk of breaking in service, so testing at manufacture and during use is often essential.

    While originally NDT was applied only for safety reasons it is today widely accepted as cost saving technique in the quality assurance process. Unfortunately NDT is still not used in many areas where human life or ecology is in danger. Some may prefer to pay the lower costs of claims after an accident than applying of NDT. That is a form of unacceptable risk management. Disasters like the railway accident in Eschede Germany in 1998 is only one example, there are many others.

         For implementation of NDT it is important to describe what shall be found and what to reject. A completely flawless production is almost never possible. For this reason testing specifications are indispensable. Nowadays there exists a great number of standards and acceptance regulations. They describe the limit between good and bad conditions, but also often which specific NDT method has to be used.

      The reliability of an NDT Method is an essential issue. But a comparison of methods is only significant if it is referring to the same task. Each NDT method has its own set of advantages and disadvantages and, therefore, some are better suited than others for a particular application. By use of artificial flaws, the threshold of the sensitivity of a testing system has to be determined. If the sensitivity is to low defective test objects are not always recognized. If the sensitivity is too high parts with smaller flaws are rejected which would have been of no consequence to the serviceability of the component. With statistical methods it is possible to look closer into the field of uncertainly. Methods such as Probability of Detection (POD) or the ROC-method "Relative Operating Characteristics" are examples of the statistical analysis methods. Also the aspect of human errors has to be taken into account when determining the overall reliability.

         Personnel Qualification is an important aspect of non-destructive evaluation. NDT techniques rely heavily on human skill and knowledge for the correct assessment and interpretation of test results. Proper and adequate training and certification of NDT personnel is therefore a must to ensure that the capabilities of the techniques are fully exploited. There are a number of published international and regional standards covering the certification of competence of personnel. The EN 473 (Qualification and certification of NDT personnel - General Principles) was developed specifically for the European Union for which the SNT-TC-1A is the American equivalent.

      The nine most common NDT Methods are shown in the main index of this encyclopedia. In order of most used, they are: Ultrasonic Testing (UT), Radiographic Testing (RT), Electromagnetic Testing (ET) in which Eddy Current Testing (ECT) is well know and Acoustic Emission (AE or AET). Besides the main NDT methods a lot of other NDT techniques are available, such as Shearography Holography, Microwave and many more and new methods are being constantly researched and developed.

NDT Method	Applications	Limitations
Liquid Penetrant	used on nonporous materials can be applied to welds, tubing, brazing, castings, billets, forgings, aluminium parts, turbine blades and disks, gears	need access to test surface defects must be surface breaking decontamination & precleaning of test surface may be needed vapour hazard very tight and shallow defects difficult to find depth of flaw not indicated
Magnetic Particle	ferromagnetic materials surface and slightly subsurface flaws can be detected can be applied to welds, tubing, bars, castings, billets, forgings, extrusions, engine components, shafts and gears	detection of flaws limited by field strength and direction needs clean and relatively smooth surface some holding fixtures required for some magnetizing techniques test piece may need demagnetization which can be difficult for some shapes and magetizations depth of flaw not indicated
Eddy Current	metals, alloys and electroconductors sorting materials surface and slightly subsurface flaws can be detected used on tubing, wire, bearings, rails, nonmetal coatings, aircraft components, turbine blades and disks, automotive transmission shafts	requires customized probe although non-contacting it requires close proximity of probe to part low penetration (typically 5mm) false indications due to uncontrolled parametric variables
Ultrasonics	metals, nonmetals and composites surface and slightly subsurface flaws can be detected can be applied to welds, tubing, joints, castings, billets, forgings, shafts, structural components, concrete, pressure vessels, aircraft and engine components used to determine thickness and mechanical properties monitoring service wear and deterioration	usually contacting, either direct or with intervening medium required (e.g. immersion testing) special probes are required for applications sensitivity limited by frequency used and some materials cause significant scattering scattering by test material structure can cause false indications not easily applied to very thin materials
Radiography Neutron	metals, nonmetals, composites and mixed materials used on pyrotechnics, resins, plastics, organic material, honeycomb structures, radioactive material, high density materials, and materials containing hydrogen	access for placing test piece between source and detectors size of neutron source housing is very large (reactors) for reasonable source strengths collimating, filtering or otherwise modifying beam is difficult radiation hazards cracks must be oriented parallel to beam for detection sensitivity decreases with increasing thickness
Radiography X-ray	metals, nonmetals, composites and mixed materials used on all shapes and forms; castings, welds, electronic assemblies, aerospace, marine and automotive components	access to both sides of test piece needed voltage, focal spot size and exposure time critical radiation hazards cracks must be oriented parallel to beam for detection sensitivity decreases with increasing thickness
Radiography Gamma	usually used on dense or thick material used on all shapes and forms; castings, welds, electronic assemblies, aerospace, marine and automotive components used where thickness or access limits X-ray generators	radiation hazards cracks must be oriented parallel to beam for detection sensitivity decreases with increasing thickness access to both sides of test piece needed not as sensitive as X-rays