Study On Non-Destructive Characterization For Porosity In CFRP With Recurrence Quantification Analysis On Ultrasonic Back-Scatter Signal

The ultrasonic attenuation method is commonly used to analyze back-wall echo for characterizing porosity of CFRP now.However,back-wall echo decreases when high porosity,large-sized or extremely clustering voids in CFRP exist,this method is ineffective.Ultrasonic back-scatter signal caused by scattering of void always exists,which can essentially be considered as a nonlinear dynamic system consisting of n particles.A non-linear dynamic system analysis method,recurrence quantification analysis?RQA?method,is used to analyze ultrasonic back-scatter signal for characterizing porosity of CFRP in this paper.Based on definition of RQA basic parameters and significance of quantitative indicators,void CFRP models with porosity range of 0.2%to 5.1%were simulated to establish the correlation between recurrence rate RR and porosity P.The effect of characterizing porosity was influenced by void morphology to a certain extent.The relationship between RR and void morphology?e.g.void size and position distribution?was established.CFRP laminates with porosity range of 0.6%to 4.2%was experimented to verify above conclusions.The main research contents are as follows:1.Based on 61 sampling points of CFRP laminates with thicknesses of 3 mm,3.6 mm and 4 mm,contact pulse reflection method was used to collect ultrasonic signals on sampling points.The metallographic method was used to obtain CFRP void metallographs on sampling points,then the porosity was counted.Void position distribution and size in models were quantitatively described bythe average width of voids W_mean,the maximum width of void W_max and the fractal dimension F,respectively.2.Based on ultrasonic detection signals of CFRP with porosity range from 0.2%to 3.6%,the parameter k was used to describe amplitude variation of ultrasonic back-scatter signal quantitatively,which was defined as the ratio between the maximum amplitude A₂ of ultrasonic back-scatter signal and the amplitude A₁ of the surface echo signal on sample in CFRP.The RQA method was used to analyze ultrasonic back-scatter signals,and then recurrence plots?RP?which can describe the change of signal amplitude were obtained.The characteristic structures were observed in RPs,such as uniform structure,periodic structure,diagonal line,horizontal/vertical line and so on.The characteristic structures determine whether the back-scatter signal was suitable for RQA analysis or not.3.Regular spherical void CFRP models with porosity range of 0.7%to 5.1%were established and simulated by the time-domain finite difference method.The RQA method was used to analyze ultrasonic back-scatter signals,and a monotonous decreasing relationship between P and RR was obtained.Based on the established models,the two other groups of CFRP models were established.Analysis results obtained from RQA method were compared.Results show that RR is declined with void diameter D increasing when void position distribution is the same.RR is inversely proportional to distance d between centers of voids when the void size is the same.4.Based on real morphology void models?RMVM?with porosity range of 0.6%to 4.2%,simulation results show that RR decreases with P increasing.And the model of RR_minin corresponds the smallest F and W_mean while the largest W_max at the same porosity.The result indicates that the model has the most abnormal void size and position distribution.Experimental results verify the availability of the results obtained from RMVM simulation with porosity range of 0.6%to 4.2%.The accuracy of P was characterized by Euclidean distance ED which measure distance of the upper-lower limit line in the P-RR scatter-plot.ED was normalized.Then ED of RR was compared with the results of ultrasonic attenuation coefficient?.The results demonstrate that ED_RR is higher than ED?as P is lower,the accuracy of?characterization is higher.The accuracy of RR characterization is higher with P increasing.