13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- Figure 4. Details of force-time record from Fig. 3, DCI, SE(B)25-specimen, a0/W = 0.5, -40 °C As already indicated above, strain gage instrumentations according to ASTM and BS provide different results independent from temperature, Fig. 5, left. To clarify this, reference tests under quasistatic loading had been performed with the same instrumentation, Fig. 5, right. They revealed that the ASTM signal widely conforms to the reference while BS significantly differs and displays a remaining tensile force at the end of the test when the specimen is fully unloaded. In order to investigate if plasticity at the BS W/2 positions causes these differences (note: W = B and not W = 2B), a test with BS strain gages at a distance of W from the ligament was performed. A significant improvement could not be achieved. Therefore it is concluded for low blow tests that strain gages at ASTM positions work well with the investigated SE(B)25-specimens while BS positions cannot be recommended. Figure 5. Comparison of ASTM and BS strain gage force measurements on SE(B)25-specimens in low blow tests (left) and quasistatic tests (right). Basically, the load line displacement in tests using instrumented pendulum impact machines or drop towers can be determined by double integration of the force-time record. This is considered an attractive way to provide displacement data when expensive non-contact measuring equipment is not available or not applicable. However, as can be seen from Fig. 6, left, tremendous differences/errors may occur between the calculated values and the reference measured by an
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