Adhesive Joining for Crashworthiness -Material Data and Explicit FE-methods

Doktorsavhandling, 2008

Today, crash simulations replace crash testing in the product development phase in the automotive industry. High quality simulations enable shorter product development time and higher competitiveness. However, increasing requirements regarding emissions and crashworthiness are demanding optimised material choice in the parts constituting the car body structure. Lightweight materials are becoming frequently used. Joining dissimilar materials 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 dissimilar materials, 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 automotive structures. The main task for the adhesive is not to dissipate the impact energy, but to keep the joint integrity so that the impact energy can be consumed by plastic work of the base materials. Fracture of adhesives can be accurately modelled by cohesive zones. The dynamic behaviour of finite element structures containing cohesive zones is studied using a simplified structure. 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. The accuracy and time step influence of the interphase element is compared to solutions based on continuum element representation of the adhesive. The interphase element is found to predict fracture of the adhesive joint with engineering accuracy and has a small effect on the time step of the explicit FE method. The cohesive laws for use in the material models of the adhesive have been determined using dedicated test methods. The double cantilever beam specimen and the end notched flexure specimen are used with inverse methods to determine cohesive laws in peel and shear, respectively. The cohesive laws are determined for varying temperature, strain rate and adhesive layer thickness. A built up bimaterial beam is designed for testing and simulation of joints consisting of bolts, adhesives and combinations of bolts and adhesives, i.e. hybrid joints. The model of the hybrid beam developed was found to be able to predict results from impact tests, quantified as maximum load and deformed shape of the beam.