Research On Cohesive Damage Monitoring Of CFRP Adhesive Joints Based On Non-uniform Strain Distribution Reconstruction

The adhesive bonded joint of Carbon Fiber Reinforced Plastic(CFRP)is one of the core components in aircraft structure design.However,due to the new bonding interface in the CFRP adhesive joints and it's more complex damage modes the correct understanding of the failure mode of the CFRP single-lap adhesive joint and the effective monitoring and early-warning of internal failure of the adhesive layer are effective guarantee for the safety and reliability of the aerospace aircraft structure.Because of its advantages of small volume,light weight,good compatibility with composite materials,high sensitivity,and capability of multipoint measurement,FBG sensor was considered very suitable to monitor the failure of the CFRP single-lap adhesive joint.However,the strain of CFRP adhesive joint under load is non-uniform,especially when the damage occurs,the local stress concentration is serious and the gradient is large.Traditional wavelength demodulation was to identify strain through the detection of the central wavelength shifts of the reflected spectrum,which can only measure the average strain of the sensing area.This would certainly cause such loss of strain gradient information regarding to strain field in the damage area that may inevitably lead to a big error of the strain identification,which would make it very difficult to accurately judge and predict the failure of the adhesive joint.Therefore,in order to monitor the failure of CFRP adhesive bonded joints more accurately and effectively,it is very necessary to study the identification method of non-uniform strain in the damage area.To this end,in this paper,the typical adhesive joint structure of aircraft-CFRP single lap adhesive joint is taken as the research object,this paper named “Research on Cohesive Damage Monitoring of CFRP Adhesive Joints Based on Non-uniform Strain Distribution Reconstruction” is proposed,aiming to establish the mapping relation model between the strain profiles of the adherend of the CFRP adhesive joint and the damage evolution of adhesive layer,and puts forward the reconstruction of the strain profile of FBG sensing area.These studies establish the theoretical foundation for the application of FBG sensors on damage monitoring of CFRP adhesive joints.The research work mainly includes the following aspects:Firstly,in this paper,the CFRP single lap adhesive joint is taken as the researchobject,the stress analysis of CFRP single lap joint was carried out based on the mechanics of composite materials and damage mechanics.And this paper applied the finite element analysis software to obtain the mapping relationship model between the evolution of adherend strain profile of CFRP single-lap joint and the stiffness degradation evolution of adhesive layer,and accordingly determined the distribution of FBG which was then embedded in the adherend between the first and second layer at the end of the adhesive layer instead of the adhesive layer.Secondly,the tensile test of CFRP adhesive joint was carried out.The reliability of CFRP laminates embedded with fiber gratings was verified.Simultaneously,it also proved that traditional wavelength demodulation used to identify strain through the detection of the central wavelength shifts of the reflected spectrum leads to a big error of the strain identification,which makes it very difficult to accurately judge and predict the failure of the adhesive joint.Thirdly,it calculated the reflection spectrum of FBG sensor with the improved transfer matrix method,and used the calculated spectra in the reconstruction of the adherend strain profile of FBG sensing area with the method of genetic algorithm(GA).According to the reconstruction results,the maximum error between the ideal and reconstructed strain profile under different tension loads did not exceed 7.43%,showing a good coincidence degree.The monitoring method of the damage evolution of adhesive layer of the CFRP single-lap joint based on the reconstruction of the adherend strain profile of FBG sensing area thus was figured out.