PSI - Issue 54

Jaroslav Václavík et al. / Procedia Structural Integrity 54 (2024) 294–299 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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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. 3. Experiment 3.1. Bending tests of rectangular beams The set-up of 3-point bending test is given in Fig. 3a. The width of the steel specimen was 50 mm, thickness 4 mm and the length 450 mm. The adhesive layer thickness was 0.3 mm. The CFRP plate has the length 140 mm, the thickness 1.2 mm and 50 mm width. The required minimum length of CFRP plate was calculated for our sample of investigation l min = 39 mm.

a) …………………………………………………………………………b) Fig. 3 Three-point bending test of steel plate with glued CFRP plate (a). Plastic joints were formed on both reinforced ends followed with debonding of the CFRP plate (b). In the elastic state the sample stiffness with CFRP plate has increased of about 65 %. Due to the shift of the beam neutral axis the strain on the beam upper part drops of about 14 %. With increased deflection, plastic joints formed in the steel in the region behind the end of the CFRP strips. Plastic deformation was also observed on the specimen upper middle steel side. With increased deflection cracks appears in the glue on both transversal ends of the CFRP plate followed with the change of glue colour from red to hell pink. After increasing the deflection to the maximum possible value of 129 mm, it occurred the delamination off the end of the CFRP strip, but the strip did Separation occurred at the steel × adhesive interface. The beginning of adhesive failure occurred at a relative strain of ε = 3000 μm·m -1 , which corresponds to apparent stress of approx. 600 MPa. When comparing the strains of the specimens without end with the CFRP plate: the highest strain in the middle of the specimen moved beyond the end of the CFRP plate, both strain magnitudes were approximately identical. 3.2. Bending tests of hollow beams The 3-point bending test was made on the hydraulic loading frame. Closed profiles 100×40-1200 with wall thickness t = 3 mm were tested; steel only, steel with CFRP plate Sika ® CarboDur ® S812 length 800 mm, steel + CFRP plate + polyurethane foam, steel + CFRP + epoxy foam. The distance between the supports was 1000 mm. The test was displacement-controlled with a stroke speed of 30 mm·s -1 . Also double hollow beams welded to each other by intermittent welding, 50×40 - 800, thickness 3 mm were the samples under investigations.