13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- The specimens were loaded under displacement control in an universal testing machine with a 50 kN load capacity. Load was applied to the specimen through the use of shafts that are pushed into holes drilled through the top and bottom cantilevers. A total of 4 specimens were tested at constant displacement-rate of 0.5 mm/min. Synchronized with the testing machine, an 8-bit Charge-Coupled Device camera measured the displacement fields. Thanks to this full-field optical method, the displacement evolution on the specimen surface could be recorded throughout the test. Also note that the image rate of the camera was set at 2 frames/s. Figure 2. Photography of a specimen instrumented with AE sensors During the test, AE event signals were monitored and recorded using a Euro Physical Acoustics (EPA) system: • Four piezoelectric transducers (Nano30), with a band characteristic from 125 to 750 kHz, and 140 (and 300) kHz resonant frequency, were mounted on the specimen (Fig. 2). The transducers were coupled to the specimen with a silicon grease to avoid loss of acoustic signal at the transducer-sample interface; • Pre-amplification of the AE signals was provided by four preamplifiers (IL40S model) with a gain set for 40 dB; • AE signals were sampled at 20 MHz and filtered with amplitude threshold about 40 dB. It is clear that the detected events depend on the value of this threshold. Before the loading test, the effective propagation velocity of the longitudinal waves was determined by generating an elastic wave using the conventional pencil lead breaking. • A signal conditioner and software that allow recording the AE features in a computer for further analysis.
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