Impact Simulation of Adhesively Joined Structures

Abstract: The development of competitive and crashworthy automotive car bodies has reached so far that the manufacturers no longer rely on mono-material steel structures. Improved strength/weight performance may be achieved by using optimal material in each part of the car structure, leading to bi-material joints and ruling out spot welding; the common joining method during the past half century of automotive history. Adhesive bonding is an attractive joining method, not only capable of managing dissimilar materials, but also capable of improving stiffness and strength in monomaterial structures. Moreover, the joints do not need additional sealing and there may be cost savings using adhesive bonding. Impact simulation of car structures is mostly performed using an explicit FE-method. This method has an inherent stability criterion: the time step used may not exceed the stable time step, or critical time step, ?tc. In this thesis, a simplified cohesive zone model is studied. It is implemented into an explicit FE-code and compared to a closed form solution. The FE-solutions agree with the closed form solutions. It is found that the evolution of damage in the adhesive layer may stop under certain conditions that are likely to occur in a real structure. It is shown that an explicit FE-analysis with a “large” time step is more prone to give immediate rupture. Thus, the method is conservative. An interphase element formulation is derived for a 2D-adhesive joint model, joining beam adherends. It is shown that the mass matrix of the interphase element gives a small contribution to the mass matrix of the structure. However, this contribution is positive for the numerical stability of the explicit FE-method and it is recommended to keep this matrix in the analysis. Moreover, it is concluded that the contribution of material damping of the adhesive layer can be neglected as compared to the effects of plasticity of the adherends. The interphase element formulation is used to analyse the Double Cantilever Beam specimen. The results are compared to an alternative model using continuum elements. The comparison shows substantially faster convergence and shorter execution time for the interphase formulation. A rough estimate indicates fifteen times shorter execution time using the interphase elements in a realistic structure.