13th International Conference on Fracture June 16–21, 2013, Beijing, China -9- Generally, the F1-F4 and F2-F3 signals do not instantly show sharp drops at unstable crack initiation as it is known from Type I, II or III brittle behaviour of small PCVN specimens as per Annex 17 of [2] but keep rising until the crack has reached the crack sensor in the ligament, Fig. 10. This is not of concern when stable crack growth is absent and the test is only analyzed until unstable crack initiation. Nevertheless, this underlines the necessity of an adequate experimental method for detection of unstable crack initiation. In this study, a method was elaborated where the initiation of unstable cleavage crack extension is indicated by fracture of the first strand of a crack sensor (Fig. 9) close to the crack tip (first steep signal rise in Fig. 10). However, since this crack sensor is not positioned at the crack tip directly but some millimeters in front of, a correction of the initiation time is necesssary. This is done by means of the crack velocity which is calculated from the known distance between the two crack sensors and the time measured between their responses. Since the distance between the crack tip and the first crack sensor is known too, the corrected time of unstable cleavage crack initiation can then be calculated. Due to the very limited plasticity in the ligament no problems related to sensors stripping away from the specimen surface were observed. Since tactile displacement measurement devices are much too slow, the load line displacement was measured using an electro-optical camera, Fig. 12, as practiced with the small scale dynamic tests. Figure 12. Electro-optical camera for measurement of load line displacement in dynamic SE(B)140-specimen tests. Figure 11b, Examples of force-displacement records of dynamic SE(B)140 fracture mechanics tests.. Figure 11c, Examples of force-displacement records of dynamic SE(B)140 fracture mechanics tests.
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