PSI - Issue 39

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

143

5

3.1. Experimental setup The experimental setup with a uniaxial testing machine as well as the required loading device for the determination of the kinking angle are illustrated in Fig. 2. By rotating the loading device through angle α, pure Mode I (α = 0°), pure Mode II (α = 90°) and different mixed-mode loadings (0° < α < 90°) can be applied to the crack tip of the specimen. Besides the loading device, there are spacers in blue color which prevent the specimen from bulging and from sliding back and forth.

Fig. 2. Loading device and experimental test setup.

A tested specimen, a reference specimen, a measuring device and a PC with evaluation software are utilized in the experimental setup. The reference specimen is required to compare the initial crack length with the crack length of the tested specimen during the experiment with the help of the direct current potential drop method, Richard and Sander (2016). This method is used to measure the crack length a within a specimen of a metallic material. In the procedure, the specimen is fed with a constant electric current I while the potential difference U is measured. With this potential difference and the help of a numerically determined calibration curve, the crack length is calculated. The principle is based on the Ohmic law where the electrical resistance increases with crack growth, because the residual cross section is reduced. It is necessary to measure the crack length because in each CTS specimen a pre-crack with a length of 9 mm is initiated by Mode I loading to reduce the influence of the starter V-notch and to begin each test series at a defined crack length. Afterwards the specimens are tested at differ ent loading angles α and are loaded equally to the initiation phase under a general cyclic load of 2.5 kN with an R -ratio of 0.1 and a frequency of 20 Hz.