Stress Intensity Factor Approach to Fracture

Although Griffith put forward the original concept of linear elastic fracture mechanics (LEFM), it was Irwin who developed the technique for engineering materials. He examined the equations that had been developed for the stresses in the vicinity of an elliptical crack in a large plate as illustrated in Fig. 2.66. The equations for the elastic stress distribution at the crack tip are as follows. 

Irwin observed that the stresses are proportional to where a is the 

The stress intensity factor is a means of characterising the elastic stress distribution near the crack tip but in itself has no physical reality. It has units of MN and should not be confused with the elastic stress concentration factor (K,) referred to earlier. 

In order to extend the applicability of LEFM beyond the case of a central crack in an infinite plate, K is usually expressed in the more general form 

Thus the basis of the LEFM design approach is that 

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