MICRO-MECHANICAL INVESTIGATION OF PROCESS-INDUCED FIBER WAVINESS IN THERMOPLASTIC COMPOSITESThermoplastic carbon fiber reinforced polymer (CFRPs) composite materials are attractive for their higher throughput, chemical resistance, and higher toughness compared to their thermoset counterparts. One challenge of processing these materials is controlling the process induced wrinkle/waviness formation arising from the high temperature change from process to room temperature. The process induced waviness is a significant microstructural defect that negatively affects the static and fatigue properties of the CFRP. This study offers a micromechanical approach to isolate, visualize, and monitor the development of fiber waviness as a function of process temperature, temperature dependent viscosity, and polymer crystallization. The approach utilizes a single AS4 carbon fibers (7 μm nominal diameter) in a thin film. Three different matrix polymers are studied: amorphous polyetherimide (PEI) and semi- crystalline low-melt poly-aryl-ether-ketone (LM-PAEK) and polypropylene (PP). A key observation is that fiber waviness initiates during cooling from the melt processing temperature (where all polymers are amorphous) and ceases at a temperature associated with a critical viscosity level (40,000 P) for all three matrix resins (PEI, PP, LM-PAEK) having very different ranges of temperature dependent viscosity. The magnitude of the fiber waviness was measured in a hot-stage microscope by observing fiber waviness upon cooling the micro-composites from various starting temperatures. In the case of LM-PAEK and PP, the critical viscosity (40,000 P) occurs during the onset of crystallization (cooling rate dependent). The amplitude of the fiber waviness does not increase further due to crystallization shrinkage. Amorphous PEI polymer at the recommended process temperature of 335 °C exhibits a viscosity approximately equal to the critical value of 40,000 P and the fiber remains straight through the entire cooling process to room temperature confirming the important role of viscosity on waviness The results indicate that one route to reduce fiber waviness is to reduce the melt processing temperature to achieve the critical viscosity level that stabilizes the fiber.
Authors: CHEN BRANNDON R. , PARAMBIL NITHIN K. , DEITZEL JOSEPH M. , GILLESPIE, JR., JOHN W. 
Published: November 2023

Thermoplastic carbon fiber reinforced polymer (CFRPs) composite materials are attractive for their higher throughput, chemical resistance, and higher toughness compared to their thermoset counterparts. One challenge of processing these materials is controlling the process induced wrinkle/waviness formation arising from the high temperature change from process to room temperature. The process induced waviness is a significant microstructural defect that negatively affects the static and fatigue properties of the CFRP. This study offers a micromechanical approach to isolate, visualize, and monitor the development of fiber waviness as a function of process temperature, temperature dependent viscosity, and polymer crystallization. The approach utilizes a single AS4 carbon fibers (7 μm nominal diameter) in a thin film. Three different matrix polymers are studied: amorphous polyetherimide (PEI) and semi- crystalline low-melt poly-aryl-ether-ketone (LM-PAEK) and polypropylene (PP). A key observation is that fiber waviness initiates during cooling from the melt processing temperature (where all polymers are amorphous) and ceases at a temperature associated with a critical viscosity level (40,000 P) for all three matrix resins (PEI, PP, LM-PAEK) having very different ranges of temperature dependent viscosity. The magnitude of the fiber waviness was measured in a hot-stage microscope by observing fiber waviness upon cooling the micro-composites from various starting temperatures. In the case of LM-PAEK and PP, the critical viscosity (40,000 P) occurs during the onset of crystallization (cooling rate dependent). The amplitude of the fiber waviness does not increase further due to crystallization shrinkage. Amorphous PEI polymer at the recommended process temperature of 335 °C exhibits a viscosity approximately equal to the critical value of 40,000 P and the fiber remains straight through the entire cooling process to room temperature confirming the important role of viscosity on waviness The results indicate that one route to reduce fiber waviness is to reduce the melt processing temperature to achieve the critical viscosity level that stabilizes the fiber.