 |
| 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).
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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).
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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 |
|
Objectives
|
Attributes
Measured or Detected
|
| 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 |
References
- 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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