crack stresses which are certain distance away from the strain gauge. On the other hand, it is quite common that cracks were visually inspected in fatigue tests. However, disadvantages of visual inspection are also obvious. First of all, a fatigue test process might need to be stopped in order to have a reliable visual inspection. Secondly, crack initiation is not possible to be visualized due to its microscopic size. Thirdly, if a structure size is big or cracks are located in blocked or inaccessible areas of the structure, the cracks are not able to be picked up by visual inspection. Actually, it is relatively feasible to visually check crack development during fatigue test of small samples, but it is very difficult to know overall crack growing process from a fatigue test of a big structure, e.g. how many active cracks exist in one time? when is each crack initiated? where are cracks located? what are the behaviors of crack growth? This is particular true for a big steel bridge structure test that lasts for days and months. Although, in some instances, NDT technology, e.g. dye penetration, ultrasonic, eddy current and radiology methods is also used to inspect fatigue cracks, they are actually not real time monitoring technologies, the fatigue testing process has to be stopped in order to perform the NDT tests. Not only it interrupts the fatigue testing process, but also it takes a lot extra time to prepare the NDT test. If the tested part is not a small object, but a very big object like the full size orthotropic steel deck, it would be impossible to inspect the complete structure or to inspect all potential crack areas within a limited time. In the recent years, acoustic emission (AE) technology is getting more and more widely used in not only fatigue tests in the laboratories, but also active crack detection of bridge structures[3-7]. There are quite a few unique advantages in using AE for crack detection and structure health monitoring, such as • Real time on-line structure health monitoring • Very sensitive to crack development or active crack growth • Global area monitoring with sensors away from exact crack locations • Location of one or more crack sources • Suitable for long term Ethernet or wireless remote monitoring With these advantages, it is possible to answer the questions regarding crack initiation and development of full size steel structures during a fatigue test or actual structure health monitoring. Wang et al[2] has used AE to test a full size orthotropic steel bridge deck, but it only monitored very short time when a crack has already been found before AE applied, there was no crack development process studied. In this paper, a fatigue test process of a full size orthotropic steel bridge deck with dimension 12.54x2.99x0.6 (LxWxH) meters is studied. Initially, the objective of this test is pretty straight forward, i.e. to find out all fatigue cracks with visual inspection and their corresponding fatigue cycles
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