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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strain time history during loading by cyclic force at strain gauges is given. Using the defined ration r of strain decreasing at strain gauge T4, the estimated number of cycles for 5 mm debonding can be red out from the chart and is N = 908 000.

Fig. 3 2D model of test specimens and corresponding places of strain gauges

Fig. 4a) Comparison of strain histories at strain gauge positions during the debonding growth evaluated using nonlinear calculation for nominal stress 300 MPa, b) Strain time history at 10 mm (T4) and 20 mm (T5) distance from the CFRP edge. The number of cycles at 5 mm debonding is given with the totted line as the strain drop from the initial value (500 μ m·m -1 ) by the value r = 1.61 3. Experiment 3.1. Fatigue tests of rectangular specimen The fatigue tests were performed on standard flat OSR specimens as well as on large structural samples. The goal was to find the limit stress amplitude at which no or small (x < 5 mm) delamination at CFRP ends occurs for N = 5 E6 cycles. This condition is necessary to ensure the functionality of the CFRP reinforcement in the event of a car rollover. The specimen surface of the steel plate was first ground to create a clean, rough, and chemically active surface by using an electric angle grinder. Next, the bonded surfaces of steel were further cleaned by using a large amount of acetone. The CFRP surface was first ground with abrasive cloth and then cleaned with isopropyl alcohol. After bonding the CFRP sheets, the specimens were mechanically rolled to remove air bubbles and excess the adhesive to create a uniform, thin layer of adhesive. Then it was loaded with the weight. All the specimens were cured in an indoor environment for 21 days before testing. The basic fatigue sample is given in Fig. 5 including selected strain gauges. The width of the steel specimen was b s = 30 mm and the thickness 3 mm. The CFRP plate reinforcement of the width b f = 25 mm and the length 140 mm was glued from one side. The fatigue test was performed on hydraulic testing frame Schenck 250 kN at R = 0.03 and several loading amplitudes. The test was performed also for pure steel sample. A constant-amplitude sinusoidal loading was applied to each cyclic specimen at a frequency of 15 Hz. The interfacial shear stress in the adhesive was calculated from the strains ε i at several strain gauges, glued in the mutual distance x = 5 mm from each other according the relation (4) and is plotted in the Fig. 6a together with normal stress, measured at CFRP surface. These stresses correspond to the maximum value of oscillating stress 220 MPa and the stress amplitude 107 MPa just for