Fatigue tests on UHM-CFRP strengthened steel plates with central inclined cracks under different damage levels

Ageing structures and metallic bridges, in particular, are vulnerable to fatigue failure due to having sensitive fatigue details. The literature on upgrading these structures highlights the efficiency of tensile (mode I) fatigue strengthening using carbon fibre reinforced polymer (CFRP) composites. However, cracks or defects are often oriented to the loading angle, which develops mixed-mode (I + II) conditions that govern crack propagation. Therefore, the efficacy of mode-I fatigue strengthening needs to be evaluated under such conditions and at different stages of the fatigue service life. This paper extends the current understanding of fatigue strengthening with CFRP from mode I loading to the case of mixed-mode (I + II) with different initial damage level. CFRP-strengthened steel plates were produced with six loading angles between 90° and 10° and two damage levels. Test results revealed that the mixed-mode crack propagation curves of steel plates strengthened at different stages of their fatigue life were approximated by the same crack growth curve in pure tensile mode by utilizing the shifting concept. Mixed mode modification factor was derived from test results of 36 specimens to predict the fatigue life of CFRP-strengthened steel plates initially inclined cracked with various degrees of damage.