when the test is done. However, it was soon realized that this is not a simple task as the structure is too big and it would be not possible to identify real initial crack with its location and actual fatigue cycles with visual inspection. Therefore, AE technology is introduced in the test, it tries to identify in what fatigue cycle that a crack is initiated? How earlier that a crack can be detected by AE than by visual inspection? Can AE locate the crack sources? What is the behavior of crack growth during the fatigue process? Where are the tangible crack locations in the orthotropic steel bridge deck etc.. Experimental results have shown that AE is a very effective tool for monitoring the dynamic process of fatigue crack growth of larger structures. 2. Cracking monitoring with acoustic emission AE is a phenomenon of rapid energy release of stress concentration in the material. Cracking is associated with stress relief and therefore generation of burst AE signal. AE does not depend on the length and width of the existing crack, but on if the crack is actively opening or growing, no AE exists if there is no active cracking. As such, AE can be effectively used for monitoring crack growth processes. The frequency range of crack induced AE is usually in between 20 – 1000 KHz which is above the frequency range of auditable sound range. The detectable fracture related AE signal level varies considerably in accordance with materials and cracking status. In a very quiet lab environment, AE induced by material crystal dislocation can be detected with a signal level in 20 dB. On the other hand, AE can also be detected in macro fracture or sudden break of a structure with a signal level in 120 dB with the same AE system without gain adjustment. Figure 1 depicts an active cracking process with AE monitoring. When a crack is growing microscopically or macroscopically, burst type AE is generated at the crack tip with acoustic wave energy travels along the structure with a velocity that is inherent with particular type of material. AE sensors installed in surrounding areas would pick up the acoustic wave energy in a time order depending on the distance between the crack source to the sensors. AE system does the data acquisition and signal processing with filtering, AE feature extraction, crack identification and source location to determine if a crack is in active growing and where is the crack located. 1 AE Sensor Cable AE System Active Crack Figure 1. Illustration of active crack monitoring with AE Burst AE Amplitude
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