Fiber Steering, Waviness and Strength: Correlating out-of-plane-deviation with tensile strain fields in automated fiber placement

acm7 Tracking Number 3

Automated Fiber Placement (AFP) represents a cutting-edge manufacturing process that enables the precise and automated production of fiber-reinforced plastic composite structures. The possibility of curvilinear layup paths, designated as "fiber steering", facilitate the placement of fibers along load-optimized trajectories. This gives the opportunity for the fabrication of mechanically demanding components that are difficult or impossible to achieve using conventional manufacturing methods as hand layup. However, the introduction of steering introduces challenges within the layup quality, most notably in-plane and out-of-plane effects leading to fiber waviness, which form microscopic angular deviations functioning as a mechanical weak point. Out-of-plane defects have been shown to induce interlaminar effects, thereby resulting in z-deviation. Conversely, in-plane defects have been observed to modify the fiber orientation exclusively within the plane. The present study investigates the effects of steering on the mechanical behavior of the laminate. First the steering behavior across a range of machine and tooling configurations was evaluated. This aimed to generate a qualitative value indicating the impact of process parameters employed on the used thermoset material. With this information two parameter windows are defined for the manufacturing of steering integrated coupon laminates used for tensile testing: one with induced out-of-plane effects and the other with in-plane effects. The objective was to evaluate the impact of varying degrees of fiber waviness. The evaluation was conducted using a novel methodology, which was specifically developed for this purpose. The method employs 3D scanning, using a machine integrated laser light section sensor, to detect steering induced surface deviations, identified with a defined z-threshold from the laminate plane. The resulting defects are then correlated with strain and failure patterns obtained through Digital Image Correlation (DIC) during the tensile testing of the specimens. This enables the establishment of relationships and conclusions about their mechanical impact.