Initiation of ductile fracture in mixed-mode I and II aluminum alloy specimens

This study investigates the initiation of mixed-mode crack extensions in four-point bend and shear specimens fabricated from aluminum alloys. The determination of fracture initiation utilizes a new strain detection method, which provides a uniform criterion for specimens over the complete mixed-mode I–II loading range. The 0.2 mm offset method for mode I specimens and the physical features on the fracture surface of mode II dominant specimens validate this method. The total measured fracture toughness shows a minimum value for the pure mode I case, a maximum for the pure mode II case and an oscillated trend over the mixed-mode I–II loads.

► We propose and validate a new strain method to determine fracture initiation.
► This method provides a uniform criterion for the entire mixed-mode range.
► The critical fracture resistance follows an “S” shape versus mode mixity.