wall surface when gradual specimen bending commences during impact. However, this appeared much later after crack initiation, and did not produce any unwanted effects within the investigated time window. The magnetic emission probe (ME) on the hammer tup recorded changes in the external magnetic field in the vicinity of the propagating crack, Fig. 1. The calibrated strain gauge on the tup acted as a load transducer. Sophisticated data acquisition equipment consisted of a DC signal amplifier and voltage supply, both tied to a TEKTRONIX TDS 420A digital oscilloscope. The transient recorder, with an interior circuit amplifier, was tied to the remaining channel on the oscilloscope. The load was measured by strain gauges and emission probes on the hammer, and both magnetic and electric potential drop signals were monitored and recorded in real sampling time intervals of 2, 4, 10 and 40 microseconds. All acquired data was then handled by PC spreadsheet procedures, including the evaluation of absorbed energy (U) and the critical dynamic J-integral. In order to check whether the ME instrumentation influenced PD measurements, and vice versa, the first two specimens were tested by applying techniques separately. The other specimens were tested simultaneously, with no record of signal interference, or other disturbing effects on the recorded signals. The high noise ratio in the PD signal is probably due to the poor grounding or it may have originated from nearby installations, but did not influence the character of the signal. This was minimized to a certain degree by toggling the input coupling impedance on the PD channel from 1 MΩ to 50 Ω, and also by analysing the diagrams and several multi-point linear trendlines in spreadsheet analyses. On the other hand, the ME signal retained good quality, having much less noise. Figure 1. The Charpy pendulum instrumentation RESULTS AND DISCUSSION In the case of complete ductile fracture, the fracture toughness is calculated from the J-integral values under dynamic conditions (Jc d), and is denoted by KId, whereas in the case of mixed fracture, especially when the ratio of elastic strain energy is not negligible, calculations should then include brittle and ductile fracture, separately (Jel + Jpl). In these tests fracture was generally ductile (Jel < 0.03· Jpl). The obtained results shown in Table 2 and in Figs. 2 and 3 clearly indicate a satisfying agreement between ME and PD tests on the evaluation of ductile crack growth initiation for this type of HSLA steel.
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