Adhesive joining for crashworthiness : material data and explicit FE-methods

Sammanfattning: Today, crash simulations replace crash testing in the product development phase in theautomotive industry. High quality simulations enable shorter product development time andhigher competitiveness. However, increasing requirements regarding emissions andcrashworthiness are demanding optimised material choice in the parts constituting the carbody structure. Lightweight materials are becoming frequently used. Joining dissimilarmaterials is difficult using common joining techniques like spot welding. To this end,adhesive joining is currently gaining popularity not only due to the ability to join dissimilarmaterials, joint integrity and structural stiffness both increase by the use of adhesive joining.Moreover, the number of spot welds may be reduced in hybrid joints.In this thesis, adhesive joints are studied with respect to crashworthiness of automotivestructures. The main task for the adhesive is not to dissipate the impact energy, but to keep thejoint integrity so that the impact energy can be consumed by plastic work of the basematerials. Fracture of adhesives can be accurately modelled by cohesive zones. The dynamicbehaviour of finite element structures containing cohesive zones is studied using a simplifiedstructure. An amplified strain rate is found in the adhesive as compared to the base material.The cohesive zone concept is used in the development of a 2D interphase element. Theaccuracy and time step influence of the interphase element is compared to solutions based oncontinuum element representation of the adhesive. The interphase element is found to predictfracture of the adhesive joint with engineering accuracy and has a small effect on the timestep of the explicit FE method.The cohesive laws for use in the material models of the adhesive have been determined usingdedicated test methods. The double cantilever beam specimen and the end notched flexurespecimen are used with inverse methods to determine cohesive laws in peel and shear,respectively. The cohesive laws are determined for varying temperature, strain rate andadhesive layer thickness. A built up bimaterial beam is designed for testing and simulation ofjoints consisting of bolts, adhesives and combinations of bolts and adhesives, i.e. hybridjoints. The model of the hybrid beam developed was found to be able to predict results fromimpact tests, quantified as maximum load and deformed shape of the beam.