Specimen Size Requirements for Fracture Toughness Testing in the Transition Region

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Abstract
The article utilizes plane strain elastic-plastic finite element analysis and a local criterion for cleavage fracture to establish specimen size requirements for the ductile-brittle transition region. Critical J and CTOD values, relative to the small-scale yielding value, were predicted as a function of specimen size, strain hardening exponent, and a/W. These analyses predict an increase in the apparent toughness with decreasing specimen size due to a loss in crack tip constraint; this effect is particularly pronounced in shallow notched specimens and low hardening materials. For deeply notched bend and compact specimens, the following size requirement must be met for critical J values for cleavage to be size independent: b,B,a>200JcσY where b is ligament length, B is thickness, a is crack length, and σγ is flow stress. This criterion is eight times more severe than the size requirements in ASTM E 813-87, but it is less stringent than the requirements of ASTM E 399-83. In order for a CTOD value to be nearly size independent, it must be less than 1/300 times the relevant specimen dimensions.The constraint loss in shallow notched specimens is usually far too rapid to obtain J-controlled cleavage fracture, but the analyses presented in this article provide a means for correcting fracture toughness data for constraint loss. Predictions of the effect of a/W on toughness in the transition region agree favorably with experimental data.Future work will consider the effects of specimen thickness and ductile tearing on transition region toughness.