FRP rupture strains in the split-disk test

The strengthening of concrete and concrete-filled steel columns by applying externally-bonded fibre-reinforced polymer (FRP) composite jackets has become a popular retrofit technique. Failure of such FRP-wrapped columns is usually governed by rupture of the FRP in the hoop direction. Two common material test methods have been used to obtain the hoop strength and rupture strain of FRP composites used in these applications: tensile tests of flat coupons and split-disk tests. The FRP rupture strain obtained from a split-disk test is usually less than that obtained from a flat coupon test, but there is neither a well-defined relationship between the two nor a comprehensive explanation for why they are different. This paper presents an investigation, using finite element (FE) analysis, into FRP hoop strains in the split-disk test with the aim of explaining possible reasons of the reduction of the FRP rupture strain in the test. The FE results show that the local strains in an FRP ring in a split-disk test are increased by the geometric discontinuities at the ends of the FRP and by circumferential bending of the FRP ring at the gap due to change of curvature there caused by the relative moment of the two half disks. The FRP ruptures once the strain at one of these locations reaches the FRP coupon rupture strain, leading to a lower apparent tensile strength than that obtained from flat coupon tests. The influence of parameters such as adhesive properties, FRP stiffness, geometry of the overlapping zone and FRP-to-split disk friction are examined.