Study On Acoustic Emission Of Welding Cracks

Acoustic emission (AE) testing technique is increasingly used in real-time and dynamic monitoring and analyzing of the deformation and rupture behavior of materials. The use of advanced Acoustic emission (AE) technology in welding can help not only assessment of the welding structure's integrality, but also the study of the crack initiation and propogation mechanism of welding cracks. Based on a extensive vewing work on AE researhes activities from both home and abroad, fundamental principals of AE technology summarized, and a variety of effect factors on AE are discussed. As for the particularity of welding process, a system of AE welding crack detecting system is developed. Using this system, AE signals of different materials'weld joints are collected, displayed and analyzed under different welding process conditions,The hardware part of the system is composed of an AE sensor, an A/D data acquisition card, a filter circuit and a mirocomputer. Application program is developed upon LabVIEW 7.0, which is a powerful graphical programming language for virtual instrumentation development. The developed system is has many merits such as powerful analysis function and simple to use. One of the most important features of the system is that the noisy signals of the system are filtered out by using both hardware circuit and software methods. Of the latter method, wavelet transform is applied because it can analyze signals at time domain and frequency simultaneously and can therefore reduce the noise effect significantly.Through collecting the AE signals of welding cracks of cast iron and 45# steels under the different restraint welding conditions, the frequency of AE signals of welding cracks are found to be in the range of 110k～200k, and the AE waveform density is related to crack initiation rather than to the size of the welding cracks.Besides, the shape and distribution of the AE signal waveforms of cast rion welding crcks are deferent from that of hydrogen embrittlement cracks, with the former occurring in a of sudden and separate manner while latter occurring in a transive and continuous and manner.