Determination of valid mode-II fracture toughnesses K_{llc

From a discussion of the sizes of the plastic zones at the tip of a crack under shear (mode–II) and tensile (mode–I) conditions of loading, hypotheses on specimen size requirements are derived for determining valid values of the shear fracture toughness Kllc. The following conclusions are drawn: The minimum specimen thickness for a Kllc test can be smaller, but the minimum in–plane specimen dimensions should be larger than for a Kllc test. For verification of these hypotheses, mode–II and. additionally, mode–I fracture toughnesses are determined for the aluminium alloy 7075 and the tool steel 90 MnCrV 8. Measurements are performed with specimens of different sizes with respect to [he size of the crack lip plastic zones. The obtained data are in good agreement with the derived criteria for measuring mode–II fracture toughnesses Kllc and confirm their validily.