ABSTRACT
Closed-form analytical solutions were developed to determine the fatigue resistance of metallic members retrofitted by bonded carbon fiber-reinforced polymer (CFRP) laminates. Different fatigue failure criteria based on the constant life diagram (CLD) approach were introduced to predict the strengthening properties that would prevent fatigue cracks in metallic members. The CLD methodology is a method that uses the combined effect of alternating stress, mean stress and material properties to predict the material life-time under high-cycle fatigue. For a given maximum stress (σ max) and minimum stress (σ min) in a sample stress history, the stress amplitude (σ a) and the mean stress (σ m) can be demonstrated in a half-plane CLD and can be partitioned into three regions as shown in Figure 1. Stress regions of tension–tension (T-T) with 0 ˂ R ˂ 1, tension-compression (T-C) with R ˂ 0 and compression-compression (C-C) with R ˃ 1 (<xref ref-type="bibr" rid="ref112_3">Ghafoori et al. 2015</xref>). https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315207681/cd556cd4-4dcf-4efe-8e29-56fc67b8bfbd/content/fig112_1.tif"/>
The level of the fatigue failure probability has also been schematically illustrated by different markers in Figure 1. The green triangle marker shows the safe zone, in which no cracks will form. The yellow square marker indicates the risky zone, where cracks may form with approximately a 50% probability. The red circular marker shows the unsafe region, where cracks will form with a probability of more than 50%. In this paper, the use of normal modulus (NM), high modulus (HM) and ultra-high modulus (UHM) CFRP laminates with different pre-stress levels was considered in the modeling. To validate the model, a series of fatigue experiments were performed in a four-point bending set-up, as shown in Figure 2, on four steel beams strengthened with bonded NM, HM and UHM CFRP laminates (Ghafoori and Motavalli 2015a, Ghafoori and Motavalli 2015b). The results have shown that there are two main mechanisms that can transition the detail from a risky finite-life regime to a safe infinite-life regime (Ghafoori et al. 2015). Although the major focus of the paper is on strengthening of steel beams, the method can be also applied for fatigue retrofitting of metallic plates. Fatigue test set-up (<xref ref-type="bibr" rid="ref112_3">Ghafoori et al. 2015</xref>). https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315207681/cd556cd4-4dcf-4efe-8e29-56fc67b8bfbd/content/fig112_2.jpg"/>
