Fatigue behavior of CFRP-strengthened butt-welded high-strength steel connections with surface cracks

Abstract

This paper presents an experimental and numerical study on the fatigue behavior of butt-welded high-strength steels (HSS) Q960, Q890, and HG785, strengthened with a one-layer, double-sided carbon fiber-reinforced polymer (CFRP) sheet in the presence of surface cracks. Two tensile fatigue loading spectrums were designed: one with a maximum stress at half the yield strength and another at 200 MPa, both maintaining a stress ratio (R) of 0.1. The results indicated that CFRP strengthening increased fatigue life by 1.06–1.36 times at half yield strength and by 1.86–2.59 times at 200 MPa. The primary failure modes were a combination of CFRP fracture, fiber separation, and delamination at half yield strength and delamination and fiber separation at 200 MPa. A finite element method (FEM) model was developed, showing high accuracy for strengthened specimens using a bilinear bond-slip model to account for adhesive damage. Based on the FEM results, a fatigue life prediction model was proposed using Paris Law. The study also examined the effects of adhesive damage, initial crack size, and plate dimensions. Considering adhesive damage in fatigue life predictions is crucial, especially when high modulus CFRPs are applied. The initial crack shape had some influence on the elliptic crack propagation and debonding development. CFRP was more effective in restricting crack growth near the surface than in deeper regions. However, the fatigue life and strengthening effect of CFRP were largely dependent on the initial crack area rather than its shape. Increasing the width of the steel plate had a limited effect on extending fatigue life, as surface crack propagation (SCP), which is primarily influenced by thickness, represented a significant proportion of the total fatigue life.