A linear indication is simply an indication that has a lenght that is greater than its width. There are two aspect ratios in common use when defining what is a linear indication; 3:1 or 4:1. The specific aspect ratio is dependent on the code being used. If the aspect ratio is 1:1 or 2:1 the discontinuity is called nonlinear.
By definition cracks are linear. Undercut, slag inclusions, elongated porosity, incomplete fusion, and overlap are a few additional examples of weld discontinuities that are typically linear. In general the severity of a discontinuity is influenced by: the location, i.e., surfaces versus subsurface, nature of load conditions, i.e., static versuis cyclic, end conditions, i.e., rounded versus sharp, linear versus nonlinear, and whether the dscontinuity is subject to tension or compression.
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