Terminal distances for CFRP plates adhesively bonded to steel girders

Abstract

This research studies the flexural behaviors of steel beams strengthened with partial-length adhesive-bonded CFRP plates under static and constant amplitude fatigue loadings. Three main behaviors related to the determination of a terminal distance, a distance between the theoretical plates cut-off point and the actual plates cut-off point for the strengthening purposes, are : 1) the premature plate separation failure mode, which is an unfavorable local failure mode, under static loading 2) the development of tensile stresses in the bonded plate to conform to the conventional beam theory, and 3) the occurrence of debond cracks at the end of the bonded plate under fatigue loading. The terminal distance is the maximum distance from the consideration of these three behaviors. To predict the first behavior, a fracture criterion is proposed using results from the double strap joint testing. The fracture criterion is based on linear elastic fracture mechanics concepts. To study the second behavior, tensile stresses in bonded CFRP plates were measured during the static test. It is found that a shear lag analysis provides reasonable agreements with the test data. So it may be used for determining a distance that the bonded plate should extend beyond the theoretical cut-off point to achieve flexural conformance. The third behavior, on the other hand, is investigated by conducting fatigue tests at a stress ratio of 0.2 and frequency 2 Hz. From the fatigue study, the stress intensity factor range is found to be suitable for evaluating debond initiation life. One advantage found from all such composite beams tested is that no fatigue crack occurred in the flanges of the beams. From the study, interface characteristics of an adhesive/steel bi-material are the important factors affecting the first and the third behaviors. Finally, effects of the plate thickness, the adhesive layer thickness, the modulus of the plate, and the modulus of the adhesive on the terminal distance are discussed. From a parametric study, a longer plate is required when all parameters, except the adhesive thickness, increase regarding the first and the third behaviors. For the second behavior, a distance that the plate requires to achieve the flexural conformance increases when all parameters, except the adhesive modulus, increase.