Flexural-tensile fatigue resistance of CFRP composites: quantifying residual strength for infrastructure design

Carbon Fiber Reinforced Polymer (CFRP) composites are increasingly used in civil infrastructure. However, their long-term performance under realistic flexural-tensile fatigue loading remains poorly understood. Notably, this is among the first studies to link residual strength degradation to a critical displacement threshold under complex flexural-tensile fatigue, moving beyond conventional tensile–tensile life prediction. Commercially representative pultruded CFRP reinforcements with a 50% fiber volume fraction were utilized. To ensure methodological rigor and reproducibility, specimen preparation incorporated standardized tab bonding per ASTM D3039, and Digital Image Correlation (DIC) was employed for precise strain monitoring. The experimental program evaluated residual mechanical properties after subjecting the specimens to flexural-tensile fatigue loading at various mid-span displacement levels (3, 5, 7, and 9 mm). The results revealed a gradual stiffness reduction of approximately 40% across all load levels due to matrix degradation. While lower displacement levels (3–5 mm) resulted in a minimal residual tensile strength reduction of 3–4%, a critical inflection point was identified at the 7 mm condition. At this threshold, accumulated matrix damage induced severe delamination, causing a sharp, statistically significant decrease in residual strength by 23.30% (p<0.05). Consequently, within the specific geometric boundaries of this experiment, the 7 mm displacement serves as an illustrative benchmark for the effective loading level. The authors cautiously note that this absolute displacement value is highly system-specific; thus, rather than serving as a direct universal engineering guideline, it demonstrates a methodological protocol to identify the inflection point. To be practically applicable in broader infrastructure design, this threshold must be normalized into intrinsic structural parameters – such as strain, curvature, or dimensionless geometric ratios – in subsequent research.