Research On The Ultrasonic Stress Nondestructive Testing Technology Of Composite Structures

It is vitally important to obtain the stress state information of large-scale composite structures accurately for the assembly quality assessment and structural health monitoring of large aircraft.The ultrasonic method based on the acoustoelasticity relationship is one of the most promising stress measurement techniques.However,because of the anisotropic properties of carbon fiber reinforced polymer(CFRP)composites which commonly used for body material,the acousticstress relationship will be more complex.Currently,there are no solutions for stress measurement of composites.Centering on the key issue of “Stress nondestructive testing technology for composite structures”,this paper focuses on the work of “Law of elastic wave propagation of CFRP composites”,“Analysis and compensation of influencing factors in ultrasonic stress testing”,“Inversion of elastic constants and temperature coefficients of CFRP composites”.An ultrasonic stress detection method for anisotropic composites is proposed,which can be used to measure the plane stress field and gradient stress of composites.Then the accuracy and reliability of the method are also verified by experiments.The specific research contents of this paper are as follows:For the stress measurement of anisotropic media,an ultrasonic stress measurement method based on the acoustoelasticity principle is proposed.According to the theory of acoustic elasticity,the anisotropic wave equation in stressed medium is deduced,and the propagation law of elastic wave in the anisotropic medium is obtained.The propagation rule of critical refracted longitudinal waves(LCR)in composites is analyzed by finite element method,and the optimal incident angle of LCR is obtained.On the basis of the above work,the detection principle of plane stress and gradient stress is deduced.The accuracy of the ultrasonic stress detection method proposed in this paper is verified by experiments.The ultrasonic stress detection system is designed and built,which can measure ultrasonic velocity and time of flight.An inversion method for determining the stiffness coefficient of composites by longitudinal wave velocity is proposed.The verification experiment of plane biaxial stress state is carried out by cruciform sample method,and the results show the accuracy of the method.The theoretical derivation of the penetration depth of LCR wave was validated by experiment.The gradient stress detection method is verified by four-point bending experiment.The good results show the accuracy and reliability of the method.For the ultrasonic stress detection method proposed in this paper,the influence of ambient temperature,lamination period and surface curvature on the stress measurement results is analyzed,and the corresponding compensation scheme is given.Through the experimental analysis of temperature effect of ultrasonic wave in composites,the law of influence of ambient temperature on stress measurement is clarified,and the targeted temperature compensation scheme is given.The homogenization condition of the layer structure was determined by finite element simulation and experimental characterization,and the selection strategy of incident frequency is also proposed.The propagation law of ultrasonic wave on curvature component surface is studied by finite element method,and the effect of curvature on amplitude and phase of LCR wave is clarified,and a compensation scheme was proposed for the stress detection of curvature component.Based on the previous research,the stress field detection of typical composite structures in engineering is carried out.The non-uniform stress measurement of typical tensile plate with variable section is carried out.By comparing the results of ultrasonic stress method,finite element method and strain gauge method,the reliability of this method is demonstrated.For the front beam of composite wing,this paper tests the assembly stress and plots the residual stress field caused by the assembly,then the attention area of the front beam is pointed out.For the scaled-down model of a composite wing box segment,this paper measures the residual stress fields caused by machining and assembly which indicate the concentration areas of residual stress in the process of machining and assembly.These results provide data support for the assembly quality assessment of the box segment.