Simulation of ductile crack growth in thin aluminum panels using 3-D surface cohesive elements

This work describes the formulation and application of a 3-D, interface-coh esive finite element model to predict quasi-static. ductile crack extension in thin aluminum panels for mode I loading and growth. The fracture model comprises an initially zero thickness. interface element with constitutive response described by a nonlinear traction-separation relationship. Convent ional volumetric finite elements model the nonlinear (elastic-plastic) resp onse of background (bulk) material. The interface-cohesive elements undergo gradual decohesion between faces of the volumetric elements to create new traction free crack faces. The paper describes applications of the computat ional model to simulate crack extension in C(T) and M(T) panels made of a 2 .3 mm thick, Al 2024-T3 alloy tested as part of the NASA-Langley Aging Airc raft program. Parameters of the cohesive fracture model (peak opening tract ion and local work of separation) are calibrated using measured load vs. ou tside surface crack extensions of high constraint (T-stress > 0) C(T) speci mens. Analyses of low constraint M(T) specimens. having widths of 300 and 6 00 mm and various a/W ratios, demonstrate the capabilities of the calibrate d model to predict measured loads and outside surface crack extensions. The models capture accurately the strong 3-D effects leading to various degree s of crack front tunneling in the C(T) and M(T) specimens. The predicted cr ack growth response shows rapid convergence with through-thickness mesh ref inement. Adaptive load increment procedures to control the rate of decohesi on in the interface elements leads to stable, rapidly converging iterations in the globally implicit solution procedures.