Fundamentals of thermoplastic sandwich structure fabrication using laser-based in-situ thermoplastic Automated Fiber Placement. (TheSaLab)

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Structures made of carbon fiber reinforced plastic (CFRP) offer enormous lightweight potential due to their high specific strengths. In recent years, the trend has been moving from thermoset to thermoplastic materials in order to exploit the advantages of recyclability as well as the joining of two components by local melting of the matrix. A particularly good ratio of mechanical properties to weight is achieved by sandwich structures. The manufacture of thermoplastic sandwich structures using conventional processes is currently limited to flat sandwich structures. A few approaches for the production of three-dimensional structures require highly complex molds as well as multi-step manufacturing processes and are also limited to a single shape to be defined. Thermoplastic Automated Fiber Placement (TAFP), on the other hand, offers a flexible fiber composite manufacturing process. A comprehensive understanding of the process already exists for processes in which similar semi-finished products are used. However, there is a lack of knowledge about the mechanisms and the characteristics of the optical-thermal-mechanical interactions during the deposition of anisotropic tapes on isotropic foam structures. Preliminary investigations have shown that the formation of a cohesive bond between deposited tapes and foam core is possible in principle under TAFP process conditions. However, if unsuitable process parameters are selected, local collapse of the foam structure or insufficient cohesive bonding of the two joining partners occurs. The aim of this project is therefore to develop a fundamental understanding of the relationships between the optical and thermomechanical interactions in the heating and joining zones during the deposition of carbon fiber reinforced thermoplastic tapes on thermoplastic foam cores using laser-based TAFP. For this purpose, in a first step, the optical interaction of the laser radiation with the foam material and the prepreg tapes is investigated and the power distribution in the heating zone resulting from the reflection, absorption and transmission characteristics of the two joining partners is determined in a model-based manner as a function of the laser settings. Furthermore, the behavior of the foam and consolidation roller is mechanically characterized under process parameters and transferred to material models. The results of the investigations will then be combined and validated in a thermomechanical process model. By coupling the thermomechanical process model with a bond strength model, the influence of the process parameters on the bond quality between deposited tape layers and foam core will be explored. By generating a fundamental understanding, the basis for the fabrication of individually curved thermoplastic sandwich structures using laser-based TAFP is established.