PSI - Issue 28

Levke Wiehler et al. / Procedia Structural Integrity 28 (2020) 925–932 Author name / Structural Integrity Procedia 00 (2019) 000–000

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Indeed, the fracture surface of specimen 2 shows two crack initiations sites (fig. 6). The starting point of the first crack is in the center of the specimen’s width. The second crack becomes visible close to the backside of the specimen. The overloads not only frame the center crack (less than 15,000 cycles) but also the secondary crack (less than 30,000 cycles). This confirms the statement that an initial crack occurred at about 11,250 cycles and the secondary crack between 15,000 and 30,000 cycles. 3.4. Determination of the shape of the crack front Besides the determination of time and location of crack initiation, reliable statements about the shape of the crack front can be made by analyzing the quotients. In case of a straight crack front running parallel to the notch (example 1 in fig. 7), all relative potentials are equal and therefore Q front = Q back = 1. When the crack front is inclined (example 2), both quotients are different and Q front > 1, whereas Q back < 1. When the crack front is curved and has the same length on both sides of the specimen (example 3), both quotients are equal but less than 1.

Fig. 7. Visualization of crack fronts on the fracture surface.

3.5. Centered crack In figure 8 the theoretical considerations are applied to a real specimen. For simplification only continuous crack fronts are analyzed which became visible on the fracture surface after the third overload. The diagram on the left hand side shows that at the third overload (green line) both quotients are below 1, while Q back > Q front , clearly indicating a curved crack front with a slightly greater crack length on the backside of the specimen. In case of the fourth overload (pink line), the quotient Q back has visibly approached the value 1 and the quotient Q front has decreased. In addition, the distance between Q front and Q back has increased compared to the previous overload. The slightly convex crack front remains. At the fifth overload (purple line), the quotient Q back clearly exceeds the value 1, while the quotient Q front remains below 1. The crack front is nearly straight but inclined in regard to the notch root. The crack front of the fifth to seventh overload have almost identical shapes. According to the model described above, the ratio of the quotients to one another should only change moderately and Q back should remain below 1. In the diagram, however, a steep increase in both quotients can be observed, while both quotients are getting closer. This behavior is probably caused by a distortion of the relative potential difference on the narrow side or by the lack of calibration of the two lateral potential differences.