Accelerated fatigue characterization of additively manufactured continuous carbon fiber reinforced thermoplastic: A thermodynamic approach

Abstract

A thermodynamic approach for accelerated fatigue characterization of additively manufactured continuous carbon fiber (CCF)-reinforced thermoplastics produced via fused filament fabrication (FFF) is presented. Specifically, we applied the concept of fracture fatigue entropy (FFE) to run-stop-cooldown (RSC) cyclic tests to efficiently predict fatigue life across both low- and high-cycle regions (10⁴ – 10⁷ cycles) while minimizing experimental workload. Results are presented for two fiber orientations: unidirectional (0°) and [0°/90°/±45°]_s specimens. Elastic properties are established via tensile tests, and RSC tests are performed to assess the cyclic plastic strain energy and its associated temperature variations via thermographic measurements, leading to fatigue limit prediction. Through extensive tension–tension fatigue test accounting for internal friction, the study revealed average FFE values of 3.10 MJ/m³K and 3.67 MJ/m³K for 0° and [0°/90°/±45°]_s specimens, respectively. These values are valid for low- and high-cycle fatigue regimes. A comparison between the experimental results and analytical predictions confirmed FFE’s capability for S-N curve prediction while highlighting the significant role of fiber orientation in cyclic response. Additionally, the steady-state temperature rise (ΔT_ss) was found to be significantly affected by fiber orientation, ranging from 0.3 °C in unidirectional to 14.6 °C in multidirectional specimens under the same applied load.