Investigation of Crystallization Kinetics of Thermoplastic Composites

Abstract

Semicrystalline thermoplastics as a matrix material of fiber-reinforced composites have become increasingly common in the aerospace industry. The crystallization of the polymer matrix heavily influences part quality and performance and is also a common source of internal stress and warping when not accurately predicted. A detailed material model coupled with robust process simulations can help to alleviate uncertainties and produce high-quality parts. In order to understand the crystallization mechanics of the composite material, experimental techniques such as differential scanning calorimetry are used to develop digital material models and determine the influence of thermal gradients and different material parameters on the final degree of crystallization. In a first study at the DLR, a simple method has been developed for adapting existing material models, developed by the UBC and Convergent Inc., to a new material. Recent works at the DLR, UBC and Convergent showed that the crystallization process is influenced both by the matrix material itself and by the fibers. The fiber volume content and fiber diameter, among other material properties, change the crystallization process and must be considered when creating material models. These influences on the crystallization temperature and rate increase with higher rates of cooling. Future works will focus on understanding the influences on crystallization and in turn the influence of crystallinity on material parameters even further. Process simulations rely on detailed material models, and understanding all influencing parameters is crucial in predicting material behavior during processing and more firmly establishes processes like automated fiber placement.