Research On Weld Defect Monitoring Based On Coda Wave Interferometry Of Piezoelectric Signal

Welding is widely used in various industries such as aerospace engineering,civil engineering,mechanical engineering,among others because of the characteristic of high production efficiency and conventional processing,etc.However,due to the influence of welding materials,crafts and other factors in the welding process,there will be inevitably small defects such as incomplete penetration and cracks.When the weld is subjected to fatigue,corrosion,high temperature and high pressure and other factors,these defects will develop further,which may lead to the failure of the entire structure.Therefore,it is important to monitor weld defects timely and accurately.In this paper,we proposed a Coda Wave Interferometry(CWI)-enabled monitoring method using piezoceramic transducers to monitor the damage development of the two kinds of defective welds with the incomplete penetration and micro cracks,and finally verified the feasibility of the CWI method through the theoretical research,the numerical simulation and experimental verifications.The major research works of this paper are concluded as follows:(1)The monitoring implementation method and principle of weld defects based on the CWI method are introduced from three aspects:the technical scheme,theoretical level and numerical simulation.The technical scheme of the proposed method is based on piezoelectric transducers,which are used to emit and receive signals.On the theoretical level,the formation mechanism of coda waves in defective welds is explained,which is the multiple scattering of ultrasonic waves in the medium;and then the interferometry principle for the coda wave analysis and the calculation principle of the stretching method are introduced in detail;Moreover,the relationship between weld defects and coda wave signals is revealed by combining the CWI theory with the acoustoelastic effect.In terms of the numerical simulation,a two-dimensional butt weld finite element model was established using COMSOL software to verify the wave interference phenomenon in the welding test piece and the influence of small weld defects of different sizes on the coda wave.(2)When there is an incomplete penetration defect occurring in the weld,the test piece will have slight plastic deformation under the cyclic loading.Aiming at the defective weld with the incomplete penetration,a cyclic loading experiment was designed to generate the plastic deformation and then the small damage change of the welded specimen under the cyclic loading was monitored using the CWI method.Experimental results show that the slight plastic deformation was occurring in the weld and developed gradually under the cyclic loading.According to the monitoring results of the CWI method,the coda wave is more sensitive to the small deformation than the direct wave.Then calculated using the stretching method,the stretching coefficient?_max was obtained,which is equal to the relative velocity variation of coda wave,and increases gradually with the increasing of cycles and the overall trend is consistent.(3)The weld crack is the most serious defect in the weld and therefore it is essential to monitor the weld crack in the early stage.In this paper,we used the CWI method to monitor the crack width change in terms of the small growth of the weld crack under the monotonic loading.Firstly,the theoretical model between the crack width change and the coda wave relative velocity variation is developed by combining the CWI theory with the acoustoelastic effect.Then through the experimental verification,the CWI technique is applied to the monitoring of the slight crack change of three butt welded specimens.Experimental results show that the relative velocity variation of coda wave increases linearly with the increase of the width of welding crack.In addition,the energy-based active sensing(EAS)approach was also used as a comparison.Compared with the EAS method,the CWI technique is more sensitive to slight changes of welding cracks and therefore holds a great potential for the monitoring of the micro crack in the early stage.