Validation of Class of Applications Using Progressive Failure and Discrete Cohesive Zone Model for Line And Surface Cracks

Abstract

Combination of multi-scale and multi-physics Progressive Failure Analysis (PFA) methodology and linear elastic fracture mechanics is introduced. The approach combined approach detects crack-path or multi-site crack path and predicts the complete load-displacement curve correctly. Though more accurate, LEFM based methodologies of Discrete Cohesive Zone Modeling (DCZM) and Virtual Crack Closure Technique (VCCT) alone seem to be test duplication methodologies rather than test prediction for several complex problems. This is primarily because the structural finite element (FE) models need pre-defined crack path information from the test, in addition, to fracture toughness and cohesive material properties. This seriously limits the capability of both DCZM and VCCT approaches. The proposed approach in this paper focuses mainly to use DCZM and VCCT approach as test prediction methods rather than test duplication methodology. The methodology works in two stages: first, analyzing an FE model with the strength-/strain-based PFA; second, performing crack propagation analysis with either DCZM or VCCT approach. The approach is designed as a test prediction/reduction strategy for analyzing crack initiation and propagation problems. The first step of simulating a problem with PFA helps determine the expected crack-path initiation and propagation information that both DCZM and VCCT methodologies require solely based on material stiffness and strength properties. The second step helps improve the simulation results and overcome the stress singularity issues predominate in crack propagation fracture analysis problems (associated with removal of elements in FE simulations). The methodology was validated by simulating the delamination process (initiation and propagation) in composite structural components such as joints and z-pinned reinforced composite beam structures. The two step strategy has been applied to a skin/stringer structural joint debonding analysis subjected to a tension and a threepoint bending load. The FE models are a three dimensional surface crack problem analyzed using PFA and DCZM approach consisting predominantly of Mode II (tension) and Modes I and II (3 point bending) failure. All simulation results are in close agreement with the test data. It is noted that strength-based PFA approach alone gave results which are in good agreement with the test data and was able to show secondary failure mechanisms not visible when DCZM alone was used to analyze the joint. On the other hand, the usefulness of the DCZM methodology is apparent for simulating the crack propagation in z-pinned unidirectional composite double cantilever beam (here crack path is well defined). The results are within reasonable agreement with the test data.