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| Digitized film radiograph of magnesium tensile bars. |
Classification of NDT Methods (continued)
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); and
- 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); and
- signature analysis (image content, frequency spectrum, field configuration).
The terms used above are defined in Table 2. The limitations of a method include conditions required by that method: conditions to be met for technique application (access, physical contact, preparation) and requirements to adapt the probe or probe medium to the test object. Other factors limit the detection or characterization of discontinuities, properties and other attributes and limit interpretation of signals or generated images.
TABLE 2. Objectives of nondestructive testing methods Discontinuites Surface anomalies roughness, scratches, gouges, crazing, pitting, inclusions and imbedded foreign material Surface connected anomalies cracks, porosity, pinholes, laps, seams, folds, inclusions Internal anomalies cracks, separations, hot tears, cold shuts, shrinkage, voids, lack of fusion, pores, cavities, delaminations, disbonds, poor bonds, inclusions, segregations Structure Microstructure molecular structure, crystalline structure and/or strain, lattice structure, strain, dislocation, vacancy, deformation Matrix structure grain structure, size, orientation and phase, sinter and porosity, impregnation, filler and/or reinforcement distribution, anisotropy, heterogeneity, segregation Small structural anomalies leaks (lack of seal or through-holes), poor fit, poor contact, loose parts, loose particles, foreign objects Gross structural anomalies assembly errors, misalignment, poor spacing or ordering, deformation, malformation, missing parts Dimensions and metrology Displacement, position linear measurement, separation, gap size, discontinuity size, depth, location and orientation Dimensional variations unevenness, nonuniformity, eccentricity, shape and contour, size and mass variations Thickness, density film, coating, layer, plating, wall and sheet thickness, density or thickness variations Physical and mechanical properties Electrical properties resistivity, conductivity, dielectric constant and dissipation factor Magnetic properties polarization, permeability, ferromagnetism, cohesive force Thermal properties conductivity, thermal time constant and thermoelectric potential Mechanical properties compressive, shear and tensile strength (and moduli), Poisson's ratio, sonic velocity, hardness, temper and embrittlement Surface properties color, reflectivity, refraction index, emissivity Chemical composition and analysis Elemental analysis detection, identification, distribution and/or profile Impurity concentrations contamination, depletion, doping and diffusants Metallurgical content variation, alloy identification, verification and sorting Physiochemical state moisture content, degree of cure, ion concentrations and corrosion, reaction products Stress and dynamic response Stress, strain, fatigue heat-treatment, annealing and cold-work effects, residual stress and strain, fatigue damage and life (residual) Mechanical damage wear, spalling, erosion, friction effects Chemical damage corrosion, stress corrosion, phase transformation Other damage radiation damage and high frequency voltage breakdown Dynamic performance crack initiation and propagation, plastic deformation, creep, excessive motion, vibration, damping, timing of events, any anomalous behavior Signature analysis Electromagnetic field potential, strength, field distribution and pattern Thermal field isotherms, heat contours, temperatures, heat flow, temperature distribution, heat leaks, hot spots Acoustic signature noise, vibration characteristics, frequency amplitude, harmonic spectrum and/or analysis, sonic and/or ultrasonic emissions Radioactive signature distribution and diffusion of isotopes and tracers Signal or image analysis image enhancement and quantization, pattern recognition, densitometry, signal classification, separation and correlation, discontinuity identification, definition (size and shape) and distribution analysis, discontinuity mapping and display
- Wenk, S.A. and R.C. McMaster. Choosing NDT: Applications, Costs and Benefits of Nondestructive Testing in Your Quality Assurance Program. Columbus, OH: American Society for Nondestructive Testing (1987).
McMaster, R.C. and S.A. Wenk. A Basic Guide for Management's Choice of Nondestructive Tests. Special Technical Publication No. 112. Philadelphia, PA: American Society for Testing and Materials (1951).
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