Shear Strength Model for FRP-Strengthened RC Beams with Adverse FRP-Steel Interaction

RC beams shear strengthened with externally bonded fiber-reinforced polymer (FRP) U strips or side strips usually fail owing to debonding of the bonded FRP shear reinforcement. Because such debonding usually occurs in a brittle manner at relatively small shear crack widths, some of the internal steel stirrups intersected by the critical shear crack may not have reached yielding at beam shear failure. Consequently, the yield stress of internal steel stirrups in such a strengthened RC beam cannot be fully utilized. This adverse shear interaction between the internal steel shear reinforcement and the external FRP shear reinforcement may significantly reduce the benefit of the shear strengthening FRP but has not been considered explicitly by any of the shear strength models in the existing design guidelines. This paper presents a new shear strength model considering this adverse shear interaction through the introduction of a shear interaction factor. A comprehensive evaluation of the proposed model, as well as three other shear strength models, is conducted using a large test database. It is shown that the proposed shear strength model performs the best among the models compared, and the performance of the other shear strength models can be significantly improved by including the proposed shear interaction factor. Finally, a design recommendation is presented.

Numerical investigation of the effects of drill geometry on drilling induced delamination of carbon fiber reinforced composites

Drilling is a major process in the manufacturing of holes required for the assemblies of composite laminates in aerospace industry. Simulation of drilling process is an effective method in optimizing the drill geometry and process parameters in order to improve hole quality and to reduce the drill wear. In this research we have developed three-dimensional (3D) FE model for drilling CFRP. A 3D progressive intra-laminar failure model based on the Hashin's theory is considered. Also an inter-laminar delamination model which includes the onset and growth of delamination by using cohesive contact zone is developed. The developed model with inclusion of the improved delamination model and real drill geometry is used to make comparison between the step drill of different stage ratio and twist drill. Thrust force, torque and work piece stress distributions are estimated to decrease by the use of step drill with high stage ratio. The model indicates that delamination and other workpiece defects could be controlled by selection of suitable step drill geometry. Hence the 3D model could be used as a design tool for drill geometry for minimization of delamination in CFRP drilling. © 2013 Elsevier Ltd.