PSI - Issue 47

Jaroslav Václavík et al. / Procedia Structural Integrity 47 (2023) 282–289 Author name / Structural Integrity Procedia 00 (2019) 000–000

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was realized using pultruded CFRP plate Sika ® CarboDur ® S512 with Young’ modulus E strength equal to 2800 MPa. For all tests a polyurethane structural adhesive SikaPower® 1277 was used, which has reasonable mechanical strength as well as high ductility. The mechanical and fracture properties in tension and shear of this adhesive were investigated by M.G.Cardoso at al. (2020). Some characteristics, not given in the adhesive datasheet, were obtained, like tensile (G IC ) and shear fracture energies (G IIC ) and some other were not in agreement with data sheets (sheer strength τ f = 20.35 MPa). Based on above mentioned theory the distribution of sheer stress τ a and peeling stresses σ a (relations are not presented here) at the CFRP end for one side reinforced (OSR) sample were calculated for maximum allowable adhesive sheer strength and are given in Fig. 1. f = 170,000 MPa, tensile

Fig. 1. Shear and peeling stress distribution in the OSR adhesive sample ( P = 32 kN), corresponding to tensile stress σ = 213 MPa

Fig. 2. Relation between sheer stress in the adhesive and the tensile stress with allowable steel and adhesive stress limits

According the linear relation between the structure tensile stress and sheer stress in adhesive at the reinforced ends (presented in Fig. 2) it is obvious, that the maximum allowable tensile stress at the CFRP end without interface delamination can reach 213 MPa; this value is lower than the yield point of the steel material. Using double side reinforcement, the maximum allowable tensile stress increases to 370 MPa; this value is comparable with the yield limit. For this second case is obvious, that the CFRP reinforcement is able to accommodate the development of plastic deformation in the steel. In any case the debonding process to start at the CFRP plate ends after yielding of the steel. 2.2. FEM simulations of debonding growth The simplified 2D FEM model can be used to simulate debonding growth. In each step of calculation, a part of the modeled glue of 1 mm length was removed up to a total length of 25 mm. This access is based on strain measurement; the location of strain gauges T3, T4, T5 and T6 is in the middle of the CFRP plate at distances 4, 10, 20 and 70 mm from the edge of the CFRP plate, strain gauges T1 and T2 are on the steel plate at distances 4 and 20 mm and strain gauge T9 is on the opposite side in the center of the sample see Fig. 3. The debonding growth was simulated by elements death in 25 steps, when area with removed glue is larger by one mm in each step. The calculated strain profiles at the strain gauges are shown in Fig. 4a. The presented diagram is calculated for level of the loading 300 MPa used for debonding growth study in this article. The ratio r defined as the ratio between the initial strain divided by the strain at strain gauge T4 represent debonding 5 mm an is equal r = 1.61. Evaluated ratio r on strain-gauge T4 is less sensitive to level of loading and nonlinear behaviour of test specimen and it is possible to use it for identification of 5 mm debonding length. During the test it is impossible to identify moment of deboning initiation, only the beginning of cyclic loading is assumed as that moment. But the initiated debonding doesn’t mean the complete losing of its influence in structural part with the CFRP reinforcement. It was stated that the debonding of 5 mm is appropriate distance to be identified by strain-gauge T4, which position is 10 mm from the CFRP plate edge. Scheme of the test specimen and placing of strain gauges are in Fig. 5.The determination of the CFRP plate debonding is shown in Fig. 4b, where the maximum