Fatigue performance of metallic beam strengthened with a bonded CFRP plate

This paper gives details of a fatigue test programme of a series of small-scale steel beams bonded with a carbon fibre reinforced polymer (CFRP) plate. Backface-strain technique was used to detect crack initiation and monitor crack growth. Results show that this technique could detect crack initiation and track the deterioration of the adhesive layer. The strains recorded show that crack initiates and grows in Mode I earlier than in Mode II in the bonded joints, and this finding agrees with the observations on double-lap joints published in literature. The strains recorded also show that the adhesive spew fillet at the end of the CFRP plate is beneficial to the fatigue performance, although the improvement is not significant. An S–N curve was developed from the test results. The curve correlates the maximum principal interfacial stress at the plate end to the number of cycles to crack initiation. This maximum stress can be calculated from the authors’ previous analytical work. The fatigue limit of the S–N curve was found to be about 30% of the ultimate static failure stress.