Prediction And Fiber Bragg Grating Detection Research On Matrix Cracks Of Composite Materials

Matrix cracking is one of the basic failure modes of composite materials.Among the failure modes,usually matrix cracking happens first.Generally matrix cracking will not cause the failure of composite materials directly,but degrades their properties.Meanwhile matrix cracking triggers other failure modes,such as delamination,which will lead to the ultimate failure.Thus it has great potential harm.Therefore,it is meaningful to study the nature of matrix cracks for the design and application of composite structures.In this paper,matrix cracks in E-glass fiber/2511 epoxy resin laminate were taken as the research object.The research was conducted to solve the problems related to matrix cracking like difficulty in detection,hard for precise prediction,coupling effect with other failure modes.Based on FBG sensors,micromechanics,damage mechanics and fracture mechanics theory,meanwhile with the aid of computational mechanics,the study was conducted on monitoring of matrix cracking,crack initiation,propagation and distribution and the influence on the properties of composite materials.On-line monitoring method of matrix cracks and general simulation strategy and models were obtained.The main contents are as follows:First of all,the on-line monitoring technology based on FBG sensors is proposed to solve the problem of matrix cracks detection.There is no effective detection method because the cracks are small in size and hidden in the material.The technology was preliminarily demonstrated.The crack initiation and propagation of?angle plies of[0₁/±?_n/0₁](?=30°,45°,60°,90°,n=1,2,3)laminates were observed under uniaxial tensile tests.The validity of the spectrum reconstruction method was verified by the observed spectra.The analysis were conducted from spectra and central wavelength.We pointed out the general variation law of the spectrum ditortion,the quantitative relationship between crack density and peak width and the relationship between the central wavelength and the stiffness of the laminates.The theoretical basis of FBG sensor for monitoring matrix cracks of composite material is then established.Secondly,aiming at solving the complex procedures of calculating the strain field of cracking laminates,the stress strain field and stiffness degradation caused by matrix cracks in composite laminates are deduced by variational method based on the minimum complementary energy principle.The stiffness degradation of[0₁/90₁]_slaminate calculated here is in good agreement with the FEM results,which verified the correctness of the theory proposed in here.Meanwhile,the stiffness degradation and stress strain distribution of[0₁/?₂]_s(?=45°,60°,75°,90°)laminates are analyzed.The calculation process shows the high efficiency of the analytical method.The results showed that when the crack density was low,the stiffness of the laminated plate decreased rapidly,which means the presence of a few matrix cracks has a great influence on the laminate plate.Then,a multi-scale model based on continuum damage mechanics was used to study the crack initiation and propagation behavior of[0₁/90_n]_s(n=1,2,3)orthogonal laminates and the influence of matrix cracks on the properties of composites.In the multi-scale model,the macro model was used for 0°layer and micro model was used for 90°layer.The fibers were discrete distributed in the resin.A second order damage tensor was used to describe the damage behavior of resin and the damaged constitutive relation of resin was deduced.In the numerical model,the fiber-resin interface debonding was ignored in order to improve the calculation efficiency.We increased the residual stress for the compensation of neglecting fiber-resin interface debonding.By using this strategy,we successfully simulated the matrix cracking of orthogonal laminates under uniaxial tensile load and verified the method with experiment.The results showed that the crack initiation was sensitive to residual stress and a higher residual stress would cause the crack in advance.In the early stage of crack initiation,there were several microcracks,but only one main crack was formed in a small area at last.Finally,an XFEM/CZM intergrated way was proposed to solve the problems of the difficulty to accurately predict the dispersion distribution of matrix cracks and the coupling effect between matrix cracks and delamination taken the open-hole[90_n/±45_n/90_n](n=1,2)laminates as examples.Many technical means were integrated as the computing strategies like setting the XFEM element sets as the possible crack initiation and propagation area,using CZM to simulate interlayer delamination,writing UMAT of the orthogonal anisotropic elastoplastic constitutive relationship for the plastic behavior of the polymer matrix composites,Gaussian distributed element strength,adjusting the fracture energy release rate to ensure the consistency of stiffness degradation rate of the elements.The accuracy of the calculation strategy was verified by the comparison the cracks initiation,propagation,distribution,delamination region,fracture profile and fracture strength with the experimental results.The results showed that only the earliest initiated crack can extend to the boundary of the sample and formed the final fracture profile.Delamination occured at the interface where the matrix crack initiated early.