OVERVIEW

Structural materials are composed of atoms and molecules that ideally have material continuity extending down into the microscopic scale. Uniformity of material and material properties is desired for most engineering applications. Design engineers assume some level of structural continuity, homogeneity, and definition of material properties. However, absolute homogeneity and continuity never exist in any engineering component.

Spatially sharp departures from material homogeneity and continuity inside a component at any level of magnification are called discontinuities. Engineering materials always possess some discontinuities, although they may be very small and they may or may not be acceptable. Examples of these discontinuities include voids, inclusions, laps, folds, cracks, chemical segregation, and local changes in microstructure.

Sharp transitions in surface homogeneity, continuity, and contour are also considered to be discontinuities on component surfaces. Geometric surface discontinuities include sharp angles, notches, gouges, scratches, galling, fretting, pitting, and welding undercut.

Discontinuities in engineering structures are unacceptable when they degrade the performance or durability of the structure below the expectations of design and when they challenge the operability, reliability, and life of a component. The primary goal of nondestructive examination for discontinuities in engineering materials is to determine whether or not the continuity and homogeneity of a component are adequate for its use.

Identified discontinuities are evaluated as either rejectable or nonrejectable conditions in a part. An evaluation usually is made in reference to a design basis and may include a code or rule-based criteria for acceptance and rejection. The evaluation of a discontinuity generally requires...