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Ultrasonic waves
recorded as they pass through a concrete coated steel plate. |
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. However, the information required for appreciating the broad scope of nondestructive testing is widely scattered in a multitude of publications and reports. Tables 1 and 2 summarize information about nondestructive testing methods arranged to show their purposes and similarities.
The term method as used here refers to the body of specialized procedures, techniques and instruments associated with each nondestructive testing approach. There are usually many techniques or procedures associated with each method. The following text describes these methods without details on application or procedure, providing a resume of each method in a single place, for quick reference.
The National Materials Advisory Board (NMAB) Ad Hoc Committee on Nondestructive Evaluation adopted a system that classified methods into six major categories: visual, penetrating radiation, magnetic-electrical, mechanical vibration, thermal and chemical-electrochemical [see references 1 and 2]. A version of the classification system is presented in Table 1, with additional categories included to cover new methods. The first six categories involve basic physical processes that require transfer of matter or energy to the object being tested. Two auxiliary categories describe processes that provide for transfer and accumulation of information, and evaluation of the raw signals and images common to nondestructive testing methods.
| TABLE 1. Nondestructive testing method categories | |
|
Basic Categories
|
Objectives
|
| Mechanical and optical | color, cracks, dimensions, film thickness, gaging, reflectivity, strain distribution and magnitude, surface finish, surface flaws, through-cracks |
| Penetrating radiation | cracks, density and chemistry variations, elemental distribution, foreign objects, inclusions, microporosity, misalignment, missing parts, segregation, service degradation, shrinkage, thickness, voids |
| Electromagnetic and electronic | alloy content, anisotropy, cavities, cold work, local strain, hardness, composition, contamination, corrosion, cracks, crack depth, crystal structure, electrical and thermal conductivities, flakes, heat treatment, hot tears, inclusions, ion concentrations, laps, lattice strain, layer thickness, moisture content, polarization, seams, segregation, shrinkage, state of cure, tensile strength, thickness, disbonds |
| Sonic and ultrasonic | crack initiation and propagation, cracks, voids, damping factor, degree of cure, degree of impregnation, degree of sintering, delaminations, density, dimensions, elastic moduli, grain size, inclusions, mechanical degradation, misalignment, porosity, radiation degradation, structure of composites, surface stress, tensile, shear and compressive strength, disbonds, wear |
| Thermal and infrared | bonding, composition, emissivity, heat contours, plating thickness, porosity, reflectivity, stress, thermal conductivity, thickness, voids |
| Chemical and analytical | alloy identification, composition, cracks, elemental analysis and distribution, grain size, inclusions, macrostructure, porosity, segregation, surface anomalies |
|
Auxiliary
Categories
|
Objectives
|
| Image generation | dimensional variations, dynamic performance, anomaly characterization and definition, anomaly distribution, anomaly propagation, magnetic field configurations |
| Signal image analysis | data selection, processing and display, anomaly mapping, correlation and identification, image enhancement, separation of multiple variables, signature analysis |
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