PSI - Issue 28

Anja Gosch et al. / Procedia Structural Integrity 28 (2020) 1184–1192 Anja Gosch/ Structural Integrity Procedia 00 (2019) 000–000

1188

5

deformation field around the crack tip. In this work, the crack length evaluation was based on two parallel sections on each side of the initial crack. Due to the crack opening during the crack growth, the distances between the sections increased. To determine the position of the crack tip, a threshold value for the critical crack opening had to be defined. Experimental results displayed a good correlation of the actual crack tip position (measured with the travelling microscope) with the inflection point of the measured displacements along the sections. Therefore, the displacement at the inflection point was defined as threshold value and was evaluated for every frame taken in the experiment, as shown in Figure 3.

2.4. Evaluation of crack growth kinetics curves To determine the crack growth kinetics curve, two methods were used within this study: the secant method (ASTM International E 647-11, 2011; International Standard ISO 15850, 2002a) for the PVC-U and PMMA specimens and the compliance calibration method proposed by Novotny (Novotny, 1997) in combination with a flexible fitting according to (Berer and Pinter, 2013; Berer et al. , 2014) for the POM specimens. 3. Results and Discussion The subsequent chapter shows the results of the current study. To compare the three testing set-ups, the difference between the investigated methods was determined and averaged over several samples. The average differences are displayed in Table 1. It was not possible to determine an average difference for the microscope vs. DIC set-up for PMMA, due to insufficiently available data. Figure 3. Automated crack length measurement via DIC: (1) determination of the increasing distance between two sections parallel to the crack growth and the inflection point of this curve, which represents the crack tip position and (2) determination of increasing crack length over time via the inflection point of several DIC frames during the experiment (Frame 16 to Frame 36).