Mechanical Characterization of Direct/In-line Compounded, Compression Molded Polyamide / Carbon Fibre Long Fibre Thermoplastic
Date of Award
Mechanical, Automotive, and Materials Engineering
Long fibre thermoplastic
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This work is licensed under a Creative Commons Attribution 4.0 International License.
Direct compounded compression molded carbon fibre long fibre thermoplastic (LFT-D) combines the high strength and stiffness of carbon fibre with a mass production manufacturing process intended to maximize fibre length. However, this process is more commonly used in industry with glass fibre. Extensive characterization of mechanical properties, spanning fundamental tensile tests to impact characterization of standard specimens and a complex automotive component, was completed to understand the strengths and deficiencies of this novel material formulation for engineering applications: fundamental uniaxial tension (quasi-static and intermediate strain rate) and three-point bending tests, tensile stress-life fatigue characterization, ISO 6603-2 instrumented impact of standard specimens, and quasi-static/low velocity impact loading of an automotive seating component. These studies provide the data necessary to advance commercial adoption of direct compounded long carbon fibre thermoplastic.
Uniaxial tension and three-point bending characterization of 9% to 25% weight fraction compression molded carbon fibre LFT-D polyamide-6 was completed with orientations of 0°, ±45°, and 90°. A novel finding that has importance for process modelling was that uniaxial tension and flexural properties were higher in the +45° direction compared to –45° (tensile modulus: 20%, strength: 10%, flexural modulus: 8%). Correspondingly, engineering strain at failure for uniaxial tensile tests was 18% lower in the +45° direction. These observations are hypothesized to be the result of fibre orientation asymmetry in the compression molding charge due to the screw of the compounder.
Tensile fatigue characterization was carried out for 40% (by weight) carbon fiber/polyamide 66 LFT-D composites. This characterization yielded fatigue stress-life curves (23 °C, dry as molded, R = 0.1, 3 Hz) for 0°, 45°, and 90° orientations with respect to flow. Peak stresses at which the samples achieved 1E6 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. Poorly dispersed fibre with little to no wet-out were identified by SEM at the fracture surfaces for those specimens with fatigue properties near the stress-life lower bound. Further development of the direct compounding process is needed for carbon fibre.
Direct/in-line compounded PA6/CF long fibre thermoplastic was also characterized under low velocity impact consistent with ISO standard 6603-2. Additionally, a quasi-static variant of the ISO method was employed to assess rate sensitivity. At quasi-static loading rates, flow region specimens were notably more brittle considering the force-deflection response. However, the energy absorption did not differ significantly between charge and flow region specimens. In terms of rate sensitivity, puncture energy under low velocity impact decreased by 18% on average with respect to quasi-static loading.
Tensile specimens were extracted from an automotive seatback structure compression molded from PA66/CF. Additionally, quasi-static and low velocity impact loading of seatback components was completed. Under low velocity impact loading a local force maxima was observed for seatbacks produced with a longitudinal charge orientation. No local maxima were consistently observed for transverse charge seatbacks. In terms of rate effects: initial stiffness was 550% higher for low velocity impact with respect to quasi-static loading. Digital image correlation identified localized deformation at the hemispherical indenter for low-velocity impact indicating an inertial component to the rate sensitivity. Catastrophic failure occurred at larger deflections for low velocity impact (36% increase for longitudinal charge placement, 24% for transverse).
A study of specimen size effect for quasi-static uniaxial tension puncture test was completed for compression molded direct compounded carbon fibre LFT. No significant size effects were observed for the elastic modulus or tensile strength obtained from tensile specimens with four different gauge lengths (6.25 mm to 57 mm). The failure strain decreased by 27.5% and 29.9%, respectively, across the gauge length range for the 0°/90° directions. This material was also characterized at intermediate strain rates (10 s–1 to 100 s–1) through uniaxial tension tests on a novel apparatus and ISO 6603-2 puncture tests. Intermediate strain rate tensile tests showed little to no strain rate sensitivity for the 0° and 90° directions. However, initial stiffness was approximately 50% higher for ISO 6603-2 impact tests compared to quasi-static puncture tests.
Bondy, Matthew, "Mechanical Characterization of Direct/In-line Compounded, Compression Molded Polyamide / Carbon Fibre Long Fibre Thermoplastic" (2022). Electronic Theses and Dissertations. 8988.