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Results from the sensors assumed that the crack front was located at one of the sensor locations. The charts in Figure 8 show this experiment's output compared to the actual measured crack length. The results show an agreement between the sensors and actual crack length. Since the sensors take into account the length of the plastic zone ahead of the crack front, the sensors were able to effectively predict the actual crack length.

The results from the sensors also indicate the amount of crack growth can be predicted based on the amount of energy in the waveforms. The energy values from the detected acoustic emission signals were placed into a 10 bin histogram to determine if the energy recorded for each waveform corresponded to a crack growth event from the initial crack length. Each sensor uses the energy values of the trained empirical database to detect and acknowledge the presence of a crack growth event. The energy threshold was determined for each sensor based on a standard deviation from the training dataset to ensure that sensor output does not trigger false signal events. Since acoustic emission sensors can be easily affected by the magnitude of the applied load (see Figure 9), different energy thresholds were required for each sensor.

Figure 10 shows the result of the sensors from the crack length experiment when the load was increased until crack growth occurred. Crack growth occurs when the crack mouth enlarges beyond a particular length that represents the beginning of an increase in load required to extend the crack length. The presence of only two sensors was confirmed in the figure with the 20 s between crack growth events. The sensors (a) and (b) detected the presence of crack growth when the crack length was approximately 200 mm. The sensors (c) and (d) were located further away than the sensors to the left side of the crack growth for the specific load level. For these sensors, the crack did not grow beyond 200 mm, and thus they did not detect the crack growth. The sensors (e) and (f) were located at the right edge of the crack as compared to the sensors (a) and (b) used for the predetermined crack length experiment. They were able to detect the crack growth when it reached 200 mm. d2c66b5586