| Fiber reinforced polymer（FRP）has been widely used in lightweight fabrication of complex structures due to its high specific strength,high specific stiffness and strong corrosion resistance.In the fixing and assembling of FRP structures,adhesive bonding technology is a common operation means.For the FRP bonded structures,affected by the surface states of substrates,the machining precision and the bonding technology,FRP bonded structures with thick adhesive layers are very common,such as in the fields of wind turbine blade,new automobile and others.In addition,similar to FRP bonded structures with thin adhesive layers,due to various material composition types and complex bonding states and other reasons,weak bonding will also appear in FRP bonded structures with thick adhesive layers in the manufacturing and service process.Moreover,the types of weak bonding involved in FRP bonded structures with thick adhesive layers are more complex,including degradations on bonding interface and in cohesion layer.Therefore,it is necessary to study the bonding quality testing method for FRP bonded structures with thick adhesive layers.To solve this problem,this paper will propose a bonding quality detection method based on ultrasonic reflection/transmission coefficient spectrums.The main content include the derivation of ultrasonic reflection/transmission coefficient expressions,analysis of ultrasonic reflection/transmission characteristics,finite element simulation and experimental detection,as in the follws:（1）Derivation of ultrasonic reflection/transmission coefficient expressions applied to FRP bonded structures with thick adhesive layers.Aiming at bonded structures with thick adhesive layers,a "five-layer model" is established and the spring model is applied to the bonding interfaces.Based on this,combined with the wavelet expansion method,the expressions of ultrasonic reflection/transmission coefficients in isotropic and anisotropic bonded structures with thick adhesive layers are derived,respectively.（2）Analysis of ultrasonic reflection/transmission characteristics of FRP bonded structures with thick adhesive layers under different bonding states.Numerical examples are established to study different weake bonding problems.The shift characteristics of ultrasonic reflection/transmission coefficient frequency and angular spectrums are analyzed when the single bonding interface is weakened,the dual bonding interfaces are weakened and the cohesive layer is weakened for FRP bonded structures with thick adhesive layers.At the same time,by comparing the shift characteristics of ultrasonic reflection/transmission coefficient frequency and angular spectrums for dual bonding interfaces degraded at same degree or not,the influence of the symmetry for material type of bonded substrates,geometric thickness and bonding states about adhesive layer is studied.（3）Simulation study of ultrasonic reflection/transmission characteristics in FRP plate and FRP bonded structures with thick adhesive layers.In this paper,the finite element method is used to study the simulation calculation method of transmission coefficient spectrums in FRP single layer plate under water immersion in time domain,and the necessity of using large size sensor as signal receiver is pointed out.Based on this,combined with the finite element simulation of bonding interfaces,the ultrasonic transmission coefficient spectrums of FRP bonded structures with thick adhesive layers are simulated.In addition,the simulation model of ultrasonic reflection/transmission characteristics of FRP bonded structures with thick adhesive layers in frequency domain is established,then the simulation calculation and characteristic analysis of ultrasonic reflection/transmission coefficient frequency and angular spectrums under different bonding states are carried out.（4）Study on the method of measuring complex elastic constants for CFRP plate and epoxy adhesive block.The phase velocity distribution and ultrasonic transmission coefficient spectrum in the isotropic plane and anisotropic plane for the uniaxial CFRP plate with different acoustic incident angles are measured by water-immersion ultrasonic transmission method.Then complex elastic constants of CFRP plate are inversed by particle swarm optimization algorithm based on simulated annealing.Similarly,the phase velocity distribution and transmission coefficient spectrum of epoxy adhesive blocks at different acoustic incidence angles are measured,then the complex elastic constants are inversely deduced by using the correlation algorithm.（5）Measurement of ultrasonic reflection/transmission coefficient spectrums for FRP bonded structures with thick adhesive layers and evaluation of bonding quality.CFRP bonded specimens with degradation on single bonding interface,degradation on double bonding interfaces at same degree are manufactured.Then ultrasonic reflection and transmission coefficient spectrums of perfectly bonded specimens are measured,the results are in good agreement with the theoretical calculation results.According to different types of weakly bonded specimens,the corresponding ultrasonic transmission coefficient spectrums are measured and the difference of bonding quality between different specimens is predicted according to theoretical analysis.At the same time,according to the different shift characteristics of ultrasonic transmission coefficient spectrums when single bonding interface is weakened and dual bonding interfaces are weakened at the same degree,the influence of the symmetry of CFRP bonded structures with thick adhesive layers on the ultrasonic transmission coefficient spectrums is explored experimentally.Finally,by comparing and analyzing the time-domain characteristics and coefficient spectrum characteristics of transmitted waves for weakly bonded specimens,the application of coefficient spectrum measurement method in bonding quality detection is expounded. |
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