Tensile fatigue characterization of polyamide 66/carbon fiber direct/in-line compounded long fiber thermoplastic composites

Abstract

Fiber reinforced polymer (FRP) composite materials can be advantageous alternatives to structural metals due to their high specific stiffness and strength. One potentially useful class of FRPs are Long-Fiber Thermoplastic composites (LFT) as these materials have significantly higher aspect ratios for the fibers. A possible lower cost variant of LFTs, Direct Long Fiber Thermoplastics (DLFT), where the long discontinuous fibers are created directly during the extrusion process from fiber rovings. The objective of this study was to determine the fatigue performance of DLFT composites. Therefore, tensile fatigue characterization was carried out for compression molded direct/in-line compounded (DLFT) 40% (by weight) carbon fiber/polyamide 66 composites. This characterization yielded fatigue life prediction curves (stress ratio of 0.1) for 0°, 45°, and 90° orientations with respect to flow. The evaluations showed a reasonable match to a power law approximation for the relationship of peak stress to number of cycles to failure for these materials under these test conditions (23 °C, dry as molded, R = 0.1, 3 Hz). Peak stresses at which the samples achieved 106 cycles were 105 MPa for samples oriented in the flow direction, 72 MPa for samples oriented 45° to the flow direction, and 53 MPa for samples oriented 90° to flow.