The fatigue crack growth characteristics of structural steel and weld connections are analyzed using quantitative acoustic emission (AE) technique. This was experimentally investigated by three-point bending testing of specimens under low cycle constant amplitude loading using the wavelet packet analysis. The crack growth sequence, that is, initiation, crack propagation, and fracture, is extracted from their corresponding frequency feature bands, respectively. The results obtained proved to be superior to qualitative AE analysis and the traditional linear elastic fracture mechanics for fatigue crack characterization in structural steel and welds. 1. Introduction Paris and Erdogan  demonstrated that linear elastic fracture mechanics (LEFM) is a useful tool for characterizing crack growth by fatigue. Since that time, application of fracture mechanics to fatigue problems has become a fair routine. Acoustic emission technology is the most appropriate nondestructive testing (NDT) method for studying fatigue crack growth in civil engineering structure because it can monitor its health in real time . Effective crack detection may lead to an early warning. The AE technique can be used to continuously detect slight deformation and damage in the interior of materials. In other words, sampling AE signals and analyzing their characteristics may contribute to the understanding of the real-time failure behavior of materials . The AE parametric analyses have been commonly employed during fatigue crack growth characterization. Ohtsu and Tomoda  reported that the AE waveform shape depends on the cracking mode, enabling the classification of cracks in different materials. Shear cracks generally follow tensile as the material approaches to final failure. Yoneda and Ye  report that failure phenomena in metals can be interpreted by evaluating the amplitude distribution, AE event count, and total AE energy. Aggelis et al.  discuss the application of other AE parameters, such as rise angle (RA) value, rise time (RT), AE hit rate, and duration damage characterization of metal. They realized that as the duration and RT increase, there is a shift of cracking mode from tensile to shear. Boinet et al.  correlated the AE parameters like rise time and duration with corrosive processes in aluminium. A good correlation between AE parameters and fracture mechanics principles during fatigue has been reported by [8, 9]. Grosse et al.  reported the pros and cons of the parametric AE analysis. They postulated that in practical applications it can be difficult to
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