PSI - Issue 64

A. Cagnoni et al. / Procedia Structural Integrity 64 (2024) 951–958 955 Alessandro Cagnoni, Pierluigi Colombi, Marco A. Pisani, Tommaso D’Antino / Structural Integrity Procedia 00 (2019) 000–000 5

the typical stress-time curve expected and predicted the value of the stress relaxation coefficient, defined as the ratio between the decrease of the stress and the stress initially applied. With an initial applied stress of 40% of the tensile strength, the relaxation coefficient predicted was equal to 6.3% after one million hours. Zou (2003) conducted similar relaxation tests on CFRP rods and observed a relaxation of less than 1% after 1,000 hours of loading with an initial stress level equal to 50% of the tendon tensile strength. Grace et al. (2023) performed relaxation tests on CFCC strands with initial stress levels equal to 70% and 80% of the tendon tensile strength. For both initial stress levels, the authors observed strand relaxation of less than 1% after 1,000 hours and estimated a load loss of 1.9% after one million hours of applied load.

Fig 1. Typical curves of (a) creep deformation and (b) relaxation

3.3. Fatigue Fatigue phenomenon is the accumulation of damage caused by the repetitive application of a load over time. This phenomenon may lead to premature rupture of the material, even with an applied load significantly lower than the material strength. For FRPs, the damage typically manifests as microcracks, delamination between layers, fiber breakage, and matrix degradation (Dyer and Isaac (1998)). Fatigue depends on a several parameters, including the magnitude and frequency of applied load, the environmental condition, and the properties of FRP components. However, a limited number of studies were conducted to investigate the fatigue behavior of CFRP products. Barron (2001) stated that the effect of the increase of the temperature inside the material is negligible when the loading frequency is less than 10 Hz. Consequently, the effect of load frequency can be neglected under this condition. Saadatmanesh and Tannous (1999) discovered that the fatigue life of CFCC strands depends on the applied stress range and stress level. Wu et al. (2010) conducted tensile fatigue tests on CFRP coupons with a load range of 5% and maximum stress between 55% and 93% of their tensile strength. The results indicated that the maximum applied load that allowed for achieving at least 2 million cycles was approximately 84% of the corresponding tensile capacity. Song et al. (2019) conducted fatigue tensile tests on CFRP rods and predicted the maximum stress limit to achieve at least 2 million cycles for various stress ranges. Grace et al. (2023) performed similar tests on CFCC and steel strands. Results confirmed the higher fatigue resistance of composite strands compared to traditional steel strands. The results of the cited studies are summarized in Table 2.

Table 2 . Fatigue strength of CFRP tendons in the literature. Authors Fatigue life [cycles]

Stress range [MPa]

Stress level [%]

Saadatmanesh and Tannous (1999)

> 3,000,000

100 200 400 200 400 600 534

83 61 44 64 53 37 81

Song et al. (2019)

> 2,000,000

Grace et al. (2023)

> 2,000,000

Based on the reported results, predicting the ultimate lifespan of CFRP tendons subjected to fatigue loading is