Spring-in of curved CFRP/foam-core sandwich structures

This paper reports on a comprehensive study on process-induced distortions (PID) of curved CFRP/foam-core sandwich panels. The study’s aim is to validate the prediction quality of two existing analytical models and a novel straightforward simulation technique, which utilizes measured spring-in distortions of representative monolithic samples as input. Sandwich specimens with a radius of approximately 350 mm are manufactured with three different core thicknesses and two different core run-out geometries.The manufactured parts show a significant radius reduction which declines for increasing core thickness which is satisfactorily predicted by both, the analytical models and the simulation. The effect of the run-out shape turned out to be of inferior relevance for the investigated specimen configurations. However, it is found to be essential to use plane-stress assumptions and to capture the stiffness ratio between the skins and the core. Therefore, an extension of the Fernlund model is proposed whose predictions match excellently with the measured distortion of the specimens. Given the simplicity of the simulation approach in combination with the limited number of inputs makes it a tool of great value, particularly early on in the design cycle.