Modeling and validation of gripper induced membrane forces in finite element forming simulation of continuously reinforced composites

Thermoforming of multilayered, thermoplastic tape-laminates into lightweight structural vehicle components has become a considerably important process during the past years due to its large-scale production potential. However, depending on process conditions and material behavior, macroscopic defects such as fiber fracture, gapping or wrinkling are feasible. To counteract such defects, blank holders or grippers, which introduce membrane forces in the laminate, can be employed in the forming process. Usually, the number and location of grippers, as well as direction and magnitude of gripping forces are determined by a cost and time consuming "trial and error" process design. Therefore, an advanced gripper system for online monitoring of gripper forces, elongations and rotations during thermoforming is presented in this work. Along with the kinematics of the grippers, the measured forces supply the beforehand mostly unknown boundary conditions for FE forming simulation. B ased on a modeling approach for FE forming simulation of CFRPs implemented in the commercially available FE solver Abaqus, appropriate modeling techniques for gripper-assisted forming are outlined. These modeling techniques are applied to two different generic geometries and the simulation results are compared with a good agreement to experimental tests.