3-D Finite Element Modeling of Ductile Crack Growth in Thin Aluminum Materials

This work describes the development and verification of a 3-D model to predict stable, Mode I crack growth in thin, ductile aluminum alloys. The model extends the standard 2-D form of the crack tip opening angle (CTOA) methodology, which determines crack extension based on obtaining a critical angle at the crack tip. When the CTOA reaches the critical value, all the nodes along the current, 3-D crack front are released simultaneously, thereby growing the crack in a self-similar manner. Evaluation of the CTOA occurs at a specified distance behind the crack tip; this decouples CTOA evaluation from mesh refinement. The CTOA-based model also includes adaptive load control strategies to minimize the effects of discrete load increments on the growth response. To evaluate the effectiveness of the described approach, this work describes a validation study using load-crack extension data from 2.3-mm-thick Al 2024-T3 specimens tested at NASA-Langley. The test matrix includes C(T) and M(T) specimens, with varying widths (50 to 600 mm), a/W ratios, and levels of constraint to suppress out-of-plane bending. Comparisons of load-crack extension curves from experiments and analyses of a constrained 150-mm C(T) specimen provide a calibrated critical CTOA value of 5.1°. Analyses using the calibrated CTOA value for constrained and unconstrained specimens provide predictions of peak load in good agreement with the experimental values.