On the fatigue assessment of 3D-printed quasi-isotropic short-carbon-fiber composites

Document Type

Article

Publication Date

10-1-2026

Abstract

This study presents a comprehensive experimental investigation of the fatigue behavior of additively manufactured (AM) quasi-isotropic short-carbon-fiber-reinforced PA12 composites, emphasizing the coupling between mechanical degradation, thermal signature, and entropy generation. A combination of single-cycle, constant amplitude, stepwise, and multi-interrupted fatigue tests was conducted to characterize the fatigue response under different loading frequencies and stress ratios. The thermomechanical response is shown to be governed by the interaction between rate-dependent viscoelastic dissipation of the polymer matrix and limited irreversible damage mechanisms, including matrix micro-cracking and fiber–matrix debonding. The results demonstrate that the composites exhibit excellent stiffness stability with less than 5 % reduction and low accumulated permanent strain ('0.25 %) up to failure and limited temperature rise (4–10 °C). The Fracture Fatigue Entropy (FFE) was determined to remain nearly constant in the low-cycle fatigue (LCF) regime, independent of frequency and mean stress. To mitigate the effects of non-damageable, viscoelastic parts of entropy, a method based on the ratio of hysteresis-loop areas obtained from single-cycle frequency tests relative to quasi-static reference tests is proposed. This approach excludes the non-damageable entropy and yields consistent FFE values across all LCF conditions. Additionally, a stepwise thermographic method was applied to estimate the fatigue-limit stress from the bilinear correlation between the temperature-rise rate Rθ and applied stress. This rapid, non-destructive approach accurately identified the fatigue limit between 48–54 MPa, demonstrating excellent agreement through comparison with conventional S–N data at all loading frequencies of 5, 10, 15, 25 Hz. The apparent fatigue limit increases with loading frequency, exhibiting a systematic upward trend up to 15 Hz, followed by a reduction at 25 Hz. Scanning electron microscopy (SEM) observations reveal that fatigue damage is governed by matrix-dominated mechanisms, including fiber–matrix debonding, fiber pull-out, and distributed micro-damage, with minimal evidence of extensive fiber breakage. Overall, the findings highlight the strong potential of combining energy- and entropy-based analyses with thermographic monitoring for reliable and cost-efficient fatigue assessment of short-fiber AM composites.

Publication Source (Journal or Book title)

International Journal of Fatigue

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