What R-Curves Can Tell Us About Specimen Size Effects in the KIc Test

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Abstract
Size effects in the KIc test were investigated by testing duplicate compact specimens in ten different combinations of five thicknesses and five widths. Curves of crack resistance as a function of crack extension were calculated in terms of strain energy release rate, G, rather than stress intensity factor, K. It was found that the entire set of data could be represented by a single linear equation of the form, R = Ro + ρΔa. The coefficient, ρ, was found to be an inverse function of specimen thickness. When this family of R-curves is compared with the driving force, G, curves for the various specimen sizes, it can be seen that the ratio of maximum load to the measuring point load (Pmax/PQ) in the record of a plane strain toughness test, depends primarily on the width/thickness ratio of the specimen. This ratio appears to bear no relation to the contribution of crack tip plasticity to the apparent crack extension at the measuring point. Accordingly, inclusion of a maximum value of this ratio in the validity requirements of ASTM Method E 399 for the purpose of restricting such a contribution is questioned.

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