Acoustic Emission Studies On Fracture Process Of Q245R Steel Welded Joint

In this paper, the acoustic emission(AE) technique was utilized to record signals generated during the unidirectional static tensile process of the weld, heat affected zone (HAZ) and base metal in Q245R compact tension specimens. Characteristic parameters analysis methods (experienced figure analysis, distribution analysis and correlation diagram analysis methods) were used to study the parameter characteristics of weld, HAZ and base metal in the dislocation slip, micro-crack initiation and crack propagation stages. Meanwhile the FFT and wavelet transform were applied to process AE signals in order to obtain both time and frequency informations on the different failure stages of the three specimens. The fracture morphology of weld, HAZ and base material were analyzed by the optical microscopy and scanning electron microscopy(SEM).The characteristic parameters analysis method was used to analyze the AE signals produced in the tensile deformation process of weld, HAZ and base metal. The distribution rule of AE signal energy, ringing count, duration and other parameters which produced in the various damage sources was summarized. The researches find that ringing counts with energy is the most relevant, which is approximately linear correlated. Followed by the amplitude with energy and the ringing counts with amplitude. It is the least relevant for rising time with energy, the dispersibility of the points in rising time with energy graph is very large. Meanwhile the AE cumulative ringing counts rate and cumulative amplitude rate were applied to achieve the effective division about weld, HAZ and base metal, the rates of cumulative AE parameter changes in these three samples show a decreasing trend in turn, that is weld> HAZ> base metal.The FFT technique was used to the AE signals of different injury stages in weld, HAZ and base metal for spectrum analysis. The frequency distribution and amplitude variation regular pattern corresponding to the maximum peak of AE signals, which were produced in the stages of dislocation slip, micro-crack initiation and crack propagation of the various welded regions. In the dislocation slip stage, the signal largest peak frequency of weld is less than HAZ. But in the stages of micro-crack initiation and crack propagation, the result is on the contrary. Compared with the weld and HAZ, the main peak frequency of base metal is the biggest in the micro-crack initiation and crack propagation stages, the frequency value changes in the band of 350-370 kHz. The largest peak amplitude value of AE signals in weld, HAZ and base metal shows an increasing trend.We use sym5 wavelet to make decomposition and reconstruction of the AE signal waveforms. The waveform characteristics were obtained with respect to the different failure stages for weld, HAZ and base metal, and it realized the effective identification for various zones of welded joint. The AE signals waveform of weld decomposed by wavelet transform were concentrated in d2, d3 and d4 layers, the waveform amplitude is the largest compared with other two samples. The signal waveform of base metal is concentrated in d2, d3 and d4 layers, the waveform amplitude is the least. The signal waveform of HAZ is concentrated in d3, d4 and d5 layers, the amplitude is between the two. d3 and d4 layers contain the main damage AE informations, it accounts for over 80% of the total energy for original signal. Meanwhile wavelet analysis achieves the separation of effective AE signals and noise signals, we obtain damage AE informations which have high signal to noise ratio.