Crack propagation and debonding development of CFRP laminate strengthened high-strength steel plates under fatigue loadings

Carbon fiber reinforced polymer (CFRP) has been used to strengthen steel structures to enhance fatigue life. A crack growth induced debonding region in the adhesive-steel interface has been observed in failure modes. However, limited studies focus on its gradual development with fatigue loadings, especially for CFRP laminate strengthened cracked high strength steel plates. The purpose of this paper is to obtain the corresponding relationship of crack propagation and debonding development. Thus fatigue tests with measuring real-time change of CFRP strain distribution by Digital Image Correlation (DIC) system were conducted for Q345, Q460, Q690 steel under fatigue loading at the same stress level 50% but difference stress ranges. Although debonding at the adhesive-steel interface is invisible, through calculation of strain gradient, it is found that as the increase of crack length, the debonding region around the crack tip occurs and gradually becomes larger. Furthermore, the shape of debonding region is always similar to an ellipse from the beginning of loading to failure.