Definition of Non-Destructive Testing
Nondestructive testing (NDT) has been defined as comprising those test methods used to examine an object, material or system without impairing its future usefulness. The term is generally applied to nonmedical investigations of material integrity.
What Is Non Nondestructive Testing?
Nondestructive testing asks "Is there something wrong with this material?" Various performance and proof tests, in contrast, ask "Does this component work?" This is the reason that it is not considered nondestructive testing when an inspector checks a circuit by running electric current through it. Hydrostatic pressure testing is usually proof testing and intrinsically not nondestructive testing. Acoustic emission testing used to monitor changes in a pressure vessel's integrity during hydrostatic testing is nondestructive testing.
Classification of NDT Methods
Nondestructive testing is a branch of the materials sciences that is concerned with all aspects of the uniformity, quality and serviceability of materials and structures. The science of nondestructive testing incorporates all the technology for detection and measurement of significant properties, including discontinuities, in items ranging from research specimens to finished hardware and products. By definition, nondestructive techniques are the means by which materials and structures may be inspected without disruption or impairment of serviceability.
Nondestructive testing has become an increasingly vital factor in the effective conduct of research, development, design and manufacturing programs. Only with appropriate use of nondestructive testing techniques can the benefits of advanced materials science be fully realized. Each method can be completely characterized in terms of five principal factors:
- energy source or medium used to probe the test object (such as X-rays, ultrasonic waves or thermal radiation);
- nature of the signals, image or signature resulting from interaction with the test object (attenuation of X-rays or reflection of ultrasound, for example);
- means of detecting or sensing resulting signals (photo emulsion, piezoelectric crystal or inductance coil);
- method of indicating or recording signals (meter deflection, oscilloscope trace or radiograph);
- Basis for interpreting the results (direct or indirect indication, qualitative or quantitative, and pertinent dependencies).
The objective of each test method is to provide information about the following material parameters:
- discontinuities (such as cracks, voids, inclusions, delaminations);
- structure or malstructure (including crystalline structure, grain size, segregation, misalignment);
- dimensions and metrology (thickness, diameter, gap size, discontinuity size);
- physical and mechanical properties (reflectivity, conductivity, elastic modulus, sonic velocity);
- composition and chemical analysis (alloy identification, impurities, elemental distributions);
- stress and dynamic response (residual stress, crack growth, wear, vibration);
Nondestructive testing is not confined to crack detection. Other discontinuities include porosity, wall thinning from corrosion and many sorts of disbonds. Nondestructive material characterization is a growing field concerned with material properties including material identification and microstructural characteristics - such as resin curing, case hardening and stress - that have a direct influence on the service life of the test object. Nondestructive testing has also been defined by listing or classifying the various methods. This approach is practical in that it typically highlights methods in use by industry.
Ensuring the Integrity and Reliability of a Product
The user of a fabricated product buys it with every expectation that it will give trouble-free service for a reasonable period of usefulness. Few of today's products are expected to deliver decades of service but they are required to give reasonable unfailing value. Year by year the public has learned to expect better service and longer life, despite the increasing complexity of our everyday electrical and mechanical appliances.
Today our railroads, automobiles, buses, aircraft and ships carry people to more places faster than ever before. And people expect to get there without delays due to mechanical failure. Meanwhile factories turn out more products, better, faster and with more automatic machinery. Management expects machinery to operate continuously because profits depend on such sustained output. The complexity of present-day products and the machinery which makes and transports them requires greater reliability from every component.
Preventing Accidents and Saving Lives
Ensuring product reliability is necessary because of the general increase in performance expectancy of the public. A homeowner expects the refrigerator to remain in uninterrupted service, indefinitely protecting the food investment, or the power lawnmower to start with one pull of the rope and to keep cutting grass for years on end. The manufacturer expects the lathe, punch press or fork lift to stand up for years of continuous work even under severe loads.
But reliability merely for convenience and profit is not enough. Reliability to protect human lives is a valuable end in itself. The railroad axle must not fail at high speed. The front spindle of the intercity bus must not break on the curve. The aircraft landing gear must not collapse on touchdown. The mine hoist cable must not snap with people in the cab. Such critical failures are rare indeed. And this is most certainly not the result of mere good luck. In large part it is the direct result of the extensive use of nondestructive testing and of the high order of nondestructive testing ability now available.