Integrative calculation of the weld strength of plastics parts based on an interdiffusion model presented for laser transmission welding

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Independent of the considered welding process the bond strengths is an important characteristic value for the mechanical design of welded parts, because the weld seam often is a weak point and therefore a potential place of failure in a structure. In addition to the mechanical and thermal properties of the welded materials the height of the bond strength is crucially dependent on the welding parameters. Currently the design of the welding zone is found in an iterative process, where both the welding parameters as well as the actual geometry of the welding zone are adjusted following to empirical guidelines. At this time there is no prediction of the resulting bond strengths based on physical models.Therefore in this present project a multi scale simulation for the calculation of the bond strength through the example of laser transmission welding shall be established and validated. The laser transmission welding is an important method for the welding of thermoplastics, so that the established approach can be validated using the laser welding. In addition the transferability of the developed general approach shall be verified. Besides the low heat input into the welded parts the main reason for the use of this process is the high welding speed as well as the high bond strength in the weld. The bond strength in the weld seam is a local size, which is both position-dependent as well as process parameter-dependent. The bond strength in the weld seam can be determined through a simulation of the interdiffusion processes, which take place on a molecular level. This simulation should be based on a reptation model. Therefore a close to reality calculation of the local temperature distribution in the weld seam is necessary. A suitable concept has already been developed by the petitioner, which should be developed further in the course of this research project. This concept can be used to calculate the interdiffusion itself as well as the bond strength distribution in the weld seam caused by the interdiffusion. In addition to the distribution of the bond strength in the weld seam thermal induced residual stresses have a huge impact on the tensile strength of welded parts. Based on the thermally calculations thermo-mechanical simulations for the determination of the residual stress distribution in the weld seam shall be executed. This distribution is dependent on the welding parameters. Through a simulative determination of both the bond strength as well as the residual stress distribution a clearly improved prediction of the structural mechanical behaviour of welded parts is possible.