Preferable Initial Crack Length for Fracture Toughness Evaluation Using sub-sized Bend Specimens

摘要

In order to guarantee maximum crack-tip constraints, fracture toughness testing standards require an initial crack length of about a_0 = W/2. When performing impact tests on specimens smaller than the required size the situation is somewhat different: Since the thickness is usually not sufficient to provide plane strain conditions, it is questionable whether the initial crack length should still be about a_0 = W/2 to provide maximum in plane constraints. On impact testing of small specimens, shorter cracks exhibit several experimental advantages, the main ones being, first, the lower influence of dynamic oscillations and second, the extended validity range of the measured toughness properties. For these reasons, we suggest to use shorter cracks and account for the insufficient in-plane constraints by correcting the measured (apparent) fracture toughness values. For this purpose the theoretical correction formula is derived in this paper. Based on this correction and the standard size requirements one obtains a lower-bound fracture toughness that can be used as a conservative design value. The latter is shown to be maximum for a relative crack length of about a_0/W = 0.25 - 0.3.