Crack Closure Concept and Superposition

Stress intensity factors, provided that they are all of the same mode, may be either added or superimposed. One commonly used method to raise the fracture toughness 

Half-penny crack of radius a at the edge of a spherical pore of radius r with an applied stress a perpendicular to crack direction  

Both of these represent a Mode I load, and hence can be added to Mode I stress intensity factors arising from other uniaxial stress perpendicular to the crack. The significance of this will subsequently become evident. 

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