-1 0 1 2 3 4 5 6 7 -2 0 2 4 6 8 10 12 DISPLACEMENT f, mm FORCE F, kN -0.002 -0.001 0 0.001 0.002 0) P9V -1 0 1 2 3 4 5 6 7 -2 0 2 4 6 8 10 12 DISPLACEMENT f, mm FORCE F, kN -0.0015 -0.001 -0.0005 0 0.0005 0.001 0.0015 0.002 0.0025 PD, V S WUHQGOLQH Figure 2. F(t)-f(t) diagrams with MF and PD signals for specimen C11 (Eo=50 J; ao/W=0.48; PD input 50 A; insulated) -1 0 1 2 3 4 5 6 7 0 0.5 1 1.5 2 TIME t, ms FORCE F, kN -0.0014 -0.0012 -0.001 -0.0008 -0.0006 -0.0004 -0.0002 0 0.0002 0) P9V -1 0 1 2 3 4 5 6 7 0 0.5 1 1.5 2 TIME t, ms FORCE F, kN -0.0005 0 0.0005 0.001 0.0015 0.002 0.0025 PD, V S WUHQGOLQLH Figure 3. F(t) diagrams with MF and PD signals for specimen C11, indicating time-to-fracture (tF) The relative difference between critical values of dynamic J-integral is very low, with an absolute average |∆(Jc d)| aver.=7.3%, and standard deviation |∆(Jc d)| st.dev.=6.9%. Results are even better for the relative difference and standard deviation in crack initiation time: ∆(ti)aver.=3.6%, ∆(ti)st.dev.=6.5%. Indeed, both techniques were successful in evaluating crack initiation, although some major difficulties in performing the tests were: - stability of the specimen, because of the massive PD-input wire connections; - selection of the optimal PD-input electric current and voltage, because it affects the output signal quality; - specimen insulation from direct contact of the anvil and tup, in order to reduce amplitude noise probably developing from the power source and eddy currents; - special attention should be made to the quality of the acquisition device, since the adequate selection of the coupling impedance of the input signal and proper grounding on all electric devices decreases noise. The curves in Figs. 4 and 5 show J-integral vs. loading rate, and J-integral vs. initial crack length, respectfully. tF tF
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