A Rate-Dependent, Elasto-Plastic Cohesive Zone Mixed-Mode Model for Crash Analysis of Adhesively Bonded Joints

Presently, there are various cohesive zone models implemented in LS-DYNA. The simplest one consists of a bi-linear traction separation-law in both modes I and II. Further models allow more complicated shapes of the traction-separation law, such as the material model of Tvergaard and Hutchinson or the General Cohesive Zone Model. However, none of these implemented models consider rate-dependency or effects of plasticity. Crash-optimized structural adhesives used in automotive structures, as for example Henkel Terokal 5077, often show a rate-dependent elastic-plastic material behaviour. An extended mixed-mode co- hesive zone model is proposed in this paper. The model considers the effects of rate-dependency and plasticity, and therefore is able to predict the failure of adhesively bonded joints more precisely than the common models. The material parameters describing the rate-dependency of yield strengths or critical energy release rates can be identified directly by (fracture) mechanical tests. The new model is validated by simulations of single lap-shear, T-peel, End-Loaded Shear Joint (ELSJ) and Tapered Double Cantilever Beam (TDCB) tests. A comparison of numerical and experimental results shows the benefits and the limitations of the new model, which will be available from one of the next versions of LS-DYNA. Its official name will be MAT COHESIVE MIXED MODE ELASTOPLASTIC RATEDEPENDENT, or in short MAT 240. The tests were proceeded at velocities ranging over several orders of magnitude. The results, which depend strongly on the test velocity, are predicted well by the new model. Further advantages are seen, when simulating a specimen unloading during a TDCB test. The irreversible displacement after unloading, which is caused by the adhesive’s plasticity, is obtained also in simulations when using the new model. Finally, a side-impact test on a floor pan is simulated, using the new model to predict the failure of adhesive bond lines connecting a cross beam to the structure. The crash tests were performed by Adam Opel GmbH. First simulations of such impact tests, using MAT 138 to model the adhesive layer, were already presented at the recent German LS-DYNA-Forum in Bamberg. The new results obtained with the elastic-plastic, rate-dependent MAT 240 show a good agreement with the experimentally observed behaviour. Thus, the model has been successfully employed in the crash simulation of a large, bonded vehicle structure.