Application of the Finite Element Method to mixed-mode cyclic crack propagation calculations in specimens

Cracks in real applications frequently exhibit mixed-mode characteristics. At MTU a software tool CRACKTRACER3D was developed to predict mixed-mode cyclic crack propagation in a fully automatic way. It consists of a preprocessor, inserting the actual crack into a finite element input deck for the uncracked structure, a call to the free software finite element program CalculiX and a postprocessor evaluating the stress intensity factors and calculating the new crack front. This loop is repeated until some predefined criterion has been reached. In order to verify the code, mixed-mode fracture in several specimens has been simulated and compared with the experimental results. This includes the propagation of a slanted crack in a 4-point bending specimen, a Compact Tension Shear Rotation Specimen under nominal Mode-III loading and a biaxial test of a square specimen with holes. Qualitatively the crack shapes are well predicted, quantitatively the numerical results are frequently on the conservative side due to the neglect of friction and interlocking in between the crack faces.