PSI - Issue 39

Deborah Weiß et al. / Procedia Structural Integrity 39 (2022) 139–147 Author name / Structural Integrity Procedia 00 (2019) 000–000

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3.2. Modified CTS specimen Besides the additional spacers in the loading device, other modifications have to be applied to the CTS specimen due to the low sheet thickness. For measuring the crack length during the experiments, as explained in section 3.1, the standard CTS specimen has to be modified, see Fig. 3a. The standard CTS specimen is equipped with holes for the current input and the potential measurement points on the side face. The metal sheet used in the clinching process is only 1.5 mm thick which makes holes on the side face impossible. That is why the current input and the potential measurement points are moved from the side face to the front face of the specimen.

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Fig. 3. (a) Comparison of the geometry of standard (left) and modified (right) CTS-specimen; (b) comparison of the calibration function of standard (orange) and modified (blue) CTS-specimen.

Due to this modification, a new calibration function of the CTS specimen has to be determined numerically to measure the crack length during the experiment. The calibration curve describes the relationship between the potential difference and the crack length. To define the calibration function numerically, the simulation program A BAQUS TM was used. There, a direct current is fed in and the potential drop is measured between defined nodes for different states of crack length. A compensating curve is laid through these points and leads to the calibration function (blue color) in Fig. 3b. This function is stored in the execution program of the testing machine before the experiment. 3.3. Evaluation of the kinking angle In the following, the experimental results are discussed in more detail. At first, the experimentally determined kinking angle (orange) is compared with the theoretically determined kinking angles according to the MTS-criterion (grey line), see Fig. 4a. For illustration, the highest percentage deviation of 12.6 % of the measured angle compared to the theoretical kinking angle is at a loading angle of 30°, where the measured kinking angle is 26.5° and the kinking angle determined according to the MTS-criterion is only 23.53°. In summary, however, it can be stated that the deviations of the kinking angles measured under a microscope (see Fig. 4b) are within the usual range to the analytically calculated. Accordingly, it can be stated that the MTS-criterion is also valid for thin sheet metals of only 1.5 mm thickness.