Stress Intensity Factor
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Stress Intensity Factor, K, is used in fracture mechanics to more accurately predict the stress state ("stress intensity") near the tip of a crack (crack tip) caused by a remote load or residual stresses.
Linear elastic theory predicts a variation, for mode I, near the crack tip of the form
This breaks down very close to the tip (small r), where plastic distortion typically occurs
Stress Intensity, K, is a parameter that amplifies the magnitude of the applied stress that includes the geometrical parameter Y (load type). These load types are categorized as Mode-I, -II, or -III. The Mode-I stress intensity factor, KIc is the most often used engineering design parameter in fracture mechanics and hence must be understood if we are to design fracture tolerant materials used in bridges, buildings, aircraft, or even bells. Polishing just won't do if we detect a crack. Typically for most materials if a crack can be seen it is very close to the critical stress state predicted by the "Stress Intensity Factor".
Mode I is opening or tensile mode where the crack surfaces move directly apart. Mode II is sliding or in-plane shear mode where the crack surfaces slide over one another in a direction perpendicular to the leading edge of the crack. Mode III is tearing and antiplane shear mode where the crack surfaces move relative to one another and parallel to the leading edge of the crack. Mode I is the most common load type encountered in engineering design.
Three different basic modes
The SIF for a through crack of length 2a, at right angles, in an infinite plane, to a uniform stress field σ is
