THREE DIMENSIONAL FINITE ELEMENT CALCULATIONS OF CRACK TIP PLASTIC ZONES AND KIC SPECIMEN SIZE REQUIREMENTS

Three dimensional finite element calculations are performed of crack tip plastic zones in CT-specimens of the aluminum alloy Al 7075. With specimens of in-plane dimensions sufficient for measuring valid fracture toughnesses KIc the specimen thickness is varied over a large range. Compared to the “dog-bone ” model the results show characteristic differences: The crack tip plastic zones near to or at the specimen surface do in general not agree with the usual results of 2D plane stress calculations. Furthermore, for specimen thicknesses well above the usual minimum thickness the crack tip plastic zones have not decayed yet to the size of 2D plane strain calculations. The conventional minimum size specimen requirements for determining valid fracture toughnesses are addressed in view of the found results