The J-integral at the initiation of the stable crack growth under dynamic conditions (JI d) is determined from Eq. (1), given below J U B W m pd I n i d i a i = − = 2 , where , with results from ME tests , with results from PD tests (1) The released energy – Um or Upd, is integrated from the load–displacement curve F(t)-f(t) by using U F f df fc i = 0 (2) where the displacement at stable crack growth initiation fc(t) is determined at a time-to-stable crack initiation interval (ti) or by time-to-fracture (tF), whose determination is sometimes a difficult task, especially in the case of complete ductile fracture, when recorded diagrams, ME(t) and PD(t), do not always show a clear discontinuity. In these circumstances, it is necessary to analyze and compare all other diagrams as well: F(t), integrated-ME(t) (or MF(t)), or several multi-point linear or polynomial PD(t)-trendlines. This is a crucial moment when simultaneous implementation of two independent techniques is a benefit. In cases when diagrams acquired from a single technique cannot be distinguished, as an alternative, the other testing technique gave complementary diagrams. As an example of good agreement, specimen C11 results are shown on diagrams F(t)-MF(t) and F(t)-PD(t) in Figs. 2 and 3. Table 2 contains relevant data for some HSLA steel specimens, tested at 20°C. Columns include: pre-crack length–ao; impact load rate–vo; maximum impact load–Fmax; time to stable crack initiation –ti (or time-to-fracture–tF), and calculated relative time difference between ME and PD tests–∆(ti); released energy–U; calculated dynamic J-integral from ME and PD tests–Jcm d, J cpd d, and the relative absolute difference between dynamic J-integrals calculated from ME and PD tests–|∆(Jc d)|. Table 2 - Measured and calculated results from ME and PD impact tests ao bm vo Fmax ti , µs ∆(ti) U, J Jc d, kJ· m-2 |∆(J c d)| Specimen id. mm mm m· s-1 kN ME PD % ME PD ME PD % A4 Ni * 5.60 7.03 1.69 4.59 920 940 -2.2 3.68 3.81 167.3 173.2 7.7 B1 5.37 6.77 1.81 5.62 1160 1190 -2.6 4.59 4.84 198.2 208.9 14.1 B3 Ni 5.70 7.65 1.82 4.96 540 500 7.4 3.56 3.27 165.4 152.0 5.5 B14 5.30 6.83 1.82 5.84 610 530 13.1 4.78 4.08 203.2 173.6 4.5 A7 NiCr 5.57 7.75 1.82 5.16 800 790 1.3 4.35 4.28 196.4 192.9 3.5 C10 Ni 4.70 6.62 2.09 6.17 1160 1040 10.3 6.77 5.45 255.4 205.7 0.0 C15 5.12 7.93 2.22 5.67 640 640 0.0 4.51 4.51 184.8 184.8 0.0 C11 4.83 7.72 2.33 6.54 800 770 3.8 4.88 4.51 188.9 174.6 0.0 B16 5.27 9.40 2.55 5.60 460 452 1.7 4.71 4.61 198.9 194.7 2.1 B17 5.37 9.40 2.55 5.59 530 480 9.4 5.03 4.42 217.1 190.8 10.9 A5 Ni 5.47 9.35 2.56 5.37 528 440 16.7 4.95 3.93 218.4 173.3 2.0 A6 NiCr 5.38 9.37 2.56 5.14 460 458 0.4 4.39 4.36 190.1 189.1 0.5 B20 5.30 9.17 2.56 5.43 640 684 -6.9 4.85 5.37 206.5 228.7 4.1 B19 5.37 9.27 2.75 5.66 490 490 0.0 4.48 4.48 193.4 193.4 5.5 C2 5.22 9.35 2.75 5.77 560 550 1.8 5.17 5.03 216.0 210.2 2.7 *) Some specimen identification also notes the material type of the PD output wire connections Some diagrams, F(t)-ME(t) or F(t)-PD(t), may not provide exact information about stable crack initiation time, and so they were averaged, Figs. 2 and 3. Usually, the change of slope in the F(t)-MF(t) diagram, and the local extreme point in the F(t)-PD(t) diagram (normal or average) is used instead (Fig. 3). This is also in agreement with the physical meaning of ME and MF quantities [2,3]. In the case of PD signal, stable crack initiation time is evaluated directly [9].
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