PSI - Issue 57

J. Torggler et al. / Procedia Structural Integrity 57 (2024) 152–160

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Author name / Structural Integrity Procedia 00 (2019) 000 – 000

4. Results and discussion Experiments have been carried out with different longitudinal preloads F y (causing fibre strains from 3.5 up to 6 percent) and different lateral displacement amplitudes U . These tests result in an appropriate number of load-cycles at different load situations. The results will be presented and discussed in the following sections.

4.1. Global analysis based on lateral displacements

For a global evaluation, like in case of the component tests, the test parameter lateral displacement U was used as a comparative value. As mentioned before, fibre angles φ f of ±15, ±25 and ±35 degrees in respect to the longitudinal direction Y were tested. Figure 5 on the left shows the fatigue test results for the different fibre angles.

5 25 30 ateral d s la ement am l t de n mm 13. 9.9 .9 10 15 20

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tests 15 deg P s 50 tests 25 deg P s 50 tests 35 deg P s 50

es gn o nts sym tot t y a a .51 41.0 0. 2 1

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15 ateral d s la ement am l t de n mm 9 10 11 12 13

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35

10 2

10 3

10 4

10 5

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F re angle

n deg

N m ero load y lesN

Figure 5: Global analysis, Comparison of S/N-curves (left) and data fit for N d = 50 000 load-cycles (right)

The S/N-curves are presented for a survival probability of P s = 50 %, whereas the fatigue test data points are statistically evaluated applying the procedure according to German Standard DIN 50100 (2022). Based on European Standard EN 13597:2003 E (2008) , a load-cycle number of N d = 50 000 is defined as the design point. In Figure 5 (right), the values for U at the design number of load-cycles are plotted against the fibre angle. Based on the results, a distinctive decrease of the tolerable lateral displacement amplitude U over increasing fibre angle can be observed. Estimating the endurable displacement at N d = 50 000 load-cycles by a linear relationship, a decrease of the amplitude of around U = 2 mm for an increase of 10 degrees in fibre angle φ f can be assessed. However, this approach leads to a comparable low coefficient of determination with R 2 = 0.895. Furthermore, an asymptotic fit and the according parameters are presented in Figure 5 (right), which shows a sound applicability leading to a value of R 2 = 1. The flattening of the function in the direction of higher fibre angles can be explained by the fact that, due to the specimen geometry, fewer load carrying fibres are involved. Finally, over about 55 degrees fibre angle the rubber matrix takes over the load carrying function. The given functions can be used for an estimation in case of the given material and loading conditions, but will be verified by additional fatigue test data points in the future. The nonlinear behaviour may be explainable via geometric relationships. The higher the fibre angle, the more the fibre gets loaded by applying the same lateral displacement. Therefore, in the next section a local assessment of the fibre strain with the help of the measured parameters lateral displacement amplitude U and longitudinal displacement amplitude V is shown.