13th International Conference on Fracture June 16–21, 2013, Beijing, China -8- loading. As expected, the machine load cell only provides a damped and less sensitive force signal which is delayed in time. Figure 9. Strain gage instrumentation and crack sensors in and near the ligament of a SE(B)140-specimen. Finite element simulations of the SE(B)140 impact tests in [10] showed a good agreement between time dependent F1–F4 and F2–F3 force signals and numerically determined force-time data. The calculated crack tip loading in terms of K at the experimentally provided time of cleavage crack initiation corresponds very well to the experimental KId value. Common feature of all test series is that the ASTM strain gage positions F1-F4 and F2-F3 show smallest dynamic effects, best sensitivity and reproducibility up to unstable crack initiation. Fig. 11 exemplarily shows the responses of different ASTM as well as BS force strain gage instrumentations for several specimens. As Fig. 11a reveals, the signals of ASTM and BS strain gage positions may nearly coincide. But in most of the cases the BS signals are below ASTM and show much more dynamic effects, Figs. 11b and 11c. Mostly, the ASTM half bridges F1-F4 and F2-F3 provide comparable signals to the ASTM full bridge FO1-FO2-FU1-FU2 up to unstable crack initiation. Nevertheless, the F1-F4 and F2-F3 signals show slight advantages in reproducibility and, very important, they offer valuable redundancy of measurement and simultaneously information on loading symmetry. Crack sensor ligament Force F1-F4, F2-F3 (related to ASTM [2]) Crack sensor crack tip Force F5-F7, F6-F8 (as per BS [1]) Force FO1-FO2-FU1-FU2 (as per ASTM [2]) Force FO1-FO2-FU1-FU2 (as per ASTM [2]) Figure 10, Signals from a dynamic SE(B)140 fracture mechanics test. Figure 11a, Examples of force-displacement records of dynamic SE(B)140 fracture mechanics tests.
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