Fundamentals of Specimen Design and Testing

The mode I (tensile-opening mode) stress intensity factor at every point along the crack border is given by Eqn. (4.7). 

In Eqn. (4.7), 2a is the effective crack diameter. Formally 2a represents the diameter of the physical crack and the associated plastic zone correction factor, namely. 

To conduct a satisfactory Kic measurement, it is necessary to provide for autographic recording of the applied stress (or load) versus the output of a transducer 

For this penny-shaped crack model specimen, the crack diameter would be the only dimension of concern the other dimensions would be taken to be very large. The crack size requirement may be considered by writing the effective crack diameter 2a in terms of the plane strain fracture toughness Kic and the yield strength ys of the material by using Eqn. (4.7) i.e., for the conceptual case where yielding and fracture occur concurrently. 

In essence, the physical crack size 2ao is being considered in relation to the crack-tip plastic zone size through the following three cases  

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