Role of the resin matrix properties in the tensile failure of FRP bars

Fiber reinforced polymer (FRP) bars are increasingly used in concrete structures because of their corrosion resistance, non-magnetic properties, and low weight. However, tensile strengths measured in standard tests are typically considerably lower than values predicted from the properties of the constituent materials. Although this discrepancy is often attributed to shear lag, the combined effects of matrix cracking, fiber–matrix bond behavior, and anchorage-induced stress concentrations on tensile failure remain insufficiently understood. To investigate these mechanisms, tensile tests were conducted on 12- and 16-mm glass FRP bars and 8- and 12-mm basalt FRP bars, with six specimens of each type and 24 specimens in total. Axial deformation was monitored using cross-head displacement, an extensometer, and digital image correlation. Based on the experimental results, nonlinear finite element models were developed in which the fibers and resin matrix were represented separately, the anchorage region was explicitly included, and the limited tensile strength of the matrix was considered through a smeared-crack formulation. The validated model was then used in a parametric study of matrix modulus, matrix tensile strength, and fiber–matrix bond behavior. The results showed that the matrix modulus controls the intensity of shear lag, the fiber–matrix bond influences the extent of stress non-uniformity, and matrix cracking reduces local load-transfer capacity, causing fiber overloading and progressive rupture. The novelty of the study lies in showing that tensile-strength reduction cannot be explained by elastic shear lag alone, but must also be related to matrix cracking and the resulting degradation of stress transfer.