Forming evaluation metrics and methods for complex stretch broken and continuous carbon fiber parts

Abstract

Carbon fiber composites are an ideal material for aircraft structures due to their light weight and high strength and stiffness. Traditional continuous carbon fiber composites are limited by their ability to form into complex geometric features as they have minimal abilities to stretch into a mold. Stretch broken carbon fiber is discontinuous which allows for the different tows of carbon fiber to slip relative to each other allowing for laminate displacements unattainable to continuous carbon fiber. In order to develop and improve stretch broken carbon fiber manufacturing processes, a method is developed herein to analyze the forming quality of a complex carbon fiber part. This method quantifies common qualitative forming metrics by measuring, evaluating, and scoring carbon fiber defects including: bridging, wrinkling, variation in thickness, forming depth, corner thinning and thickening, resin pooling, and additional texture defects. The scores produced from evaluating the various forming metrics on formed carbon fiber parts are assembled in a weighted matrix to produce an overall forming score. This forming evaluation method is demonstrated on three different geometric mold designs to assess that it is quantitative, repeatable, accounts for many carbon fiber defects, and accounts for discrepancies between the formed part and the designed part. This method was also tested on different carbon fiber prepreg materials: MSU SBCF/Cycom 977-3 resin from Solvay, MSU SBCF/Hexcel 8552, and Hexcel IM7/8552. Through testing, the method is found to have achieved the method design goals as well as ascertaining areas of improvement for MSU SBCF prepreg which is still under development. Results show that stretch broken carbon fiber and traditional continuous carbon fiber achieve similar overarching forming scores, but outperform one another in different category metrics. MSU SBCF has higher scores in corner thickening and resin pooling; continuous carbon fiber has higher scores in wrinkling and bridging. The method forms the foundation to evaluate layup practices and autoclave cure cycles when systematically applied in a process development setting. This method is also extendable to dissimilar geometric part types with modifications in suitable metrics.