PSI - Issue 28

D. Weiß et al. / Procedia Structural Integrity 28 (2020) 2335–2341 D. Weiß et al. / Structural Integrity Procedia 00 (2019) 000–000

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determined numerically with the finite element program A BAQUS TM . There, a constant direct current is introduced into the specimen and the potential drop is calculated at defined nodes for different stages of crack length. The numerically determined calibration function with indication of some selected simulation results is illustrated in Fig. 3.

Fig. 3. Numerically determined calibration curve and selected simulation results.

3. Experimental details and results The analysis of the fracture mechanical properties of the base material with the new specimen is executed using an electro-mechanical tensile testing machine called E LECTROPULS E10000 by I NSTRON . The analyzed characteristics are determined under mode-I-loading for the same environmental conditions. The experimental setup consists of a test specimen with growing crack length, a reference specimen with constant initial crack length, a PC with evaluation software and a measuring device called DCM-2 by M ATELECT LDT. The reference specimen is used to monitor the initial potential drop U 0 for the initial crack length for instance to eliminate influences on the potential drop due to temperature changes during the experiment, Richard and Sander (2012). The identification of fracture mechanical parameters is conducted by using the system F AM Control, Sander and Richard (2004). A complete crack growth rate curve is determined with the combination of two different tests under a periodic sinusoidal loading. To evaluate the lower branch of the crack growth rate curve and the near threshold behavior as well as the threshold Δ K I,th , tests with an exponentially decreasing cyclic stress intensity factor �� � �� � � � ������ � � at an R -ratio of 0.1 and a test frequency of 40 Hz are executed (down test). Besides tests with an increasing cyclic stress intensity factor are carried out to identify the upper branch of the crack growth rate curve up to the cyclic fracture toughness Δ K IC (up test). Thereby, the specimen is loaded under a general periodic load with a test frequency of 10 Hz. In both cases the tests start with a ΔK resulting in a moderate crack growth rate. For the evaluation of the crack growth rate curve the applied forces, the load cycles and the potential drop are continuously recorded.