Crack initiation geometry

The fracture behaviour of polymers, usually under conditions of mode I opening, considered the severest test of a material s resistance to crack initiation and propagation, is widely characterised using linear elastic fracture mechanics (LEFM) parameters, such as the plane strain critical stress intensity factor, Kic, or the critical strain energy release rate, Gic, for crack initiation (determined using standard geometries such as those in Fig. 1). LEFM... 

This geometry change can occur quite naturally as a bit of adhesive squeezes out from the joint. Alternatively, the joint can be chamfered or machined to smooth out the sharp corner. If the stress concentration is reduced, then the joint appears stronger at the crack initiation. However, the propagation of the crack which causes failure is still dominated by Equation (15.5), so such strengthening can give a false sense of security. In fact, it is easy to show that the sharp-edged... 

The results of experiments based on fracture mechanics are, in principle, independent of the particular specimen geometry. They therefore can be compared even if obtained with different experimental arrangements. On the other hand, precracked specimens do not allow for the study of crack initiation time, which, in practice, is often a determining factor for the lifetime of a structure. 

Fig. 2. Left crack initiation within the Heat Affected Zone (HAZ) during the so-called Erichsen test. 2nd picture shrinkage of the seam at the upper and lower side of two dissimilar sheet geometries. 3rd picture Transverse crack due to shrinkage. Right welding pore at the surface of the seam.

The U.S. NRC has also provided recommended IGSCC crack growth rates for use in dispositioning detected and sized flaws in piping (NUREG-0313, Revision 2 [7.5]). These data, in conjunction with geometry specific stress intensity solutions, can be used to determine any subsequent crack growth. This method does not consider the time to crack initiation it relies on inspection to detect cracks or on historical data to predict the time to initiation. 

Fatigue Wear Associated with cyclic stress variations and therefore, the lifetime of the material is dependent on the number of cycles. Cyclic deformation of the contacting surfaces leads to the initiation and propagation of microcracks (Rowe, 1980). Subsurface crack initiation generally occurs in the region of maximum shear stress which wilt depend upon the geometry of the materials... 

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