PSI - Issue 51

Tereza Juhászová et al. / Procedia Structural Integrity 51 (2023) 213–218 Author name / Structural Integrity Procedia 00 (2022) 000–000

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1.2. Experimental study This test primarily aims to determine the fatigue crack growth rate of AISI 304 and AISI 316 under different stress ratios ( R = 0.1, R = 0.3, and R = 0.5). Experimental study took place on six IPE 80 cross-section beams loaded in

Fig. 1. Experimental set-up of three-point bending with extensometer.

three-point bending, three of each grades. As the focus of the study was to study fatigue behavior, the initial notch of the size of 0.5 mm was created in the middle of the span of the supports. As for the ratio of the support span to with of the specimen, S/W = 3 , due to the requirement of the experimental set-up and maximal deformation, further described in Braet et al. (2021). Each of the specimens in the grade set was tested in different stress ratio R , 0.1, 0.3 and 0.5, which was obtained as a ratio of minimal to maximal force applied during loading cycle. For the specimen of R = 0.1 maximal force used was P max =75 kN for R= 0.3 P max =100kN, and for the R= 0.5, P max =120 kN, as the results of previous testing for this value with the same stress applied didn’t lead to crack propagation. In the vicinity of the crack resistance extensometer was applied, which measured maximal and minimal value of CMOD (crack mouth opening displacement) for each 100 cycles. Experimental set-up is visible in Fig.1. The loading frequency varied in the range from 80 Hz to 120 Hz, depending on the specimen stiffness (given by specimen thickness and crack length). The experiments were done in a laboratory with controlled temperature and humidity. The temperature was set to 23 °C and the absolute humidity was kept at 10 g/m 3 (the corresponding relative humidity is 50%). 1.3. Numerical modelling Numerical modelling took place in FEM environment Ansys Mechanical APDL. With use of symmetry of the profile, only quarter of the specimen was modelled, replacing the rest with boundary conditions, as is plotted in Fig.2. Material parameters input was chosen as Young’s modulus of 190 GPa and Poisson‘s ratio of 0.3 corresponding to stainless steel. Crack was modelled as area without boundary conditions with straight crack front continuous through the width of flange. The growth was simplified in plane of original crack, only in the vertical direction. As the brittle behavior was observed after reaching the specimen web, the crack was modelled only to 90 percent of the flange thickness. The size of crack was considered as vertical dimension, which grew in ratio to thickness in range from 0.1 to 0.9 with step of 0.05. Stress intensity factor was calculated using direct method, see Juhaszova et al. (2022). To the best correspondence to actual experimental testing, additional elements were added in the bottom of flange of the size of plates glued to the specimen to attach extensometer. For each step of the growth, CMOD was measured in the lowest level of elements, to obtain relation to stress intensity factor, K I , and also to crack length.