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Abstract A series of new specimens were proposed for testing the toughness of mode I fractures：flattened ring with edge cracks，semicircular flattened ring with edge cracks，edge cracked semicircular flattened disc，and their counterparts without flattened ends. Finite element method was used to calibrate the dimensionless stress intensity factor of these specimens and analyse and curve fit the variation of the stress intensity factor. The results reveal that for the edge cracked(flattened) ring，the curve of dimensionless stress intensity factor versus dimensionless crack length appear to be ascending succeeded by descending；so the critical point can be identified directly from the corresponding load-displacement curve based on this unique property，bringing convenience for measuring the material?s fracture toughness. The edge cracked flattened semicircular disc also has such property，but wider flattened ends should be chosen. While the edge cracked semicircular disc and edge cracked(flattened) semicircular ring do not have this unique characteristics. In addition，in the preliminary experimental test，the fracture toughness of a sandstone determined with the edge cracked flattened semicircular disc and the edge cracked flattened semicircular ring coincides respectively with the results derived with the specimen suggested by the International Society for Rock Mechanics.
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Received: 22 August 2013
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2014, Vol. 33 
