Equivalent Modeling And Strength Analysis Of C/SiC Composite Thin-walled Joint Structures

C/SiC composites have the characteristics of high specific strength,high specific stiffness,high temperature resistance and excellent thermal stability,and have been widely used in large area thermal protection structures of aerospace vehicles.For complex C/SiC composite thin-walled structures,there are a large number of joint structures.Therefore,characterization,analysis and evaluation of C/SiC composite joint structures are the key to overall structural design,integrity analysis and reliability evaluation.Different from traditional metal materials,the hundred-micron magnitude surface roughness of C/SiC composite directly affects the performance of the C/SiC composites joint structures.In the C/SiC composite joint structures,in order to avoid the thermal stress problem caused by the difference in thermal expansion coefficient of different material fasteners,C/SiC composites are also adopted in fasteners.The hybrid joint form of mechanical joint and adhesive coexistence is often formed through secondary deposition,its failure mechanism is more complex.Experimental and numerical simulation methods are usually used to study the joint structures.For the complex C/SiC composite joint structures,the cost hinders the development of tests,the large number of meshes and contact pairs also hinder the computational efficiency of the 3D solid model.The development of rapid modeling and analysis method is an effective way to reduce the computational cost and computational cycle of the connection structure.Based on C/SiC composite thin-walled joint structures,equivalent models of joint stiffness are set up.The shell-fasteners equivalent model is developed.The influence of the bonding strength on hybrid joint specimens is studied.The strength of the shell-fastener model analysis method is determined.For the C/SiC composite thinwalled joint structures,it is laid a foundation for the fast modeling,analysis and evaluation.Firstly,the research ideas of equivalent stiffness modeling,quantitative analysis of hybrid joint structures,fast modeling and strength analysis of complex joint structures by shell-fastener model are summarized and analyzed from the four aspects of equivalent modeling method,contact behavior,stress and strength analysis method and C/SiC composites joint structures.The axial and shear stiffness tests of the C/SiC composite bolted joints are carried out.Nevertheless,it is found that there is a large error in the joint stiffness obtained by the ideal contact model compared with the test results.It is indicated that the surface of C/SiC composite specimen cannot be assumed to be the ideal surface.By analyzing the surface roughness of the material,fractal contact mechanics is used to modify the interface contact stiffness.The equivalent stiffness model of bolted joint structures was established,which is embedded into the shellfastener,hybrid model and solid model.The calculated response results are consistent with the test results.Secondly,the tensile test of C/SiC z-pinned-bonded single-lap joint is carried out.It is found that the interlamellar shear strength of the joint plate would affect the failure mode of bonded area.When the interlaminar shear strength of the joint plate is less than the in-plane shear strength of the bonded layer,instead of the failure of bonded layer,the failure mode is the delamination failure of joint plate.Based on the cohesive contact analysis model and the material strength theories,the shell-fastener model is used to simulate the tensile failure behavior of C/SiC z-pinned-bonded single-lap joint.The failure mode and response of the calculated C/SiC z-pinned-bonded single-lap joint are consistent with the test results.When the interlaminar shear strength is large enough,the effect of the bonded area on the shear strength of C/SiC z-pinned-bonded single-lap joint is analyzed.It is found that the bonded degree could affect the shear strength of the C/SiC z-pinned-bonded single-lap joint.Then,the contact behavior of the interface of C/SiC composite bolted joints/adhesive hybrid joint specimen is analyzed.To distinguish the hybrid contact with other bonded zones,it is combined with the equivalent stiffness model modified by fractal contact mechanics.The contact stiffness calculation method of mixed contact area is established and embedded into the shell-fasteners model to simulate the response of hybrid joint structures.Using the phenomenological nonlinear constitutive model of C/SiC composites,the strength envelope of the corresponding bolted joints structures is drawn by the finite element method,and the strength of the C/SiC composite single-row three-bolts single-lap joint structures are calculated based on the bolt load analysis.The effectiveness of the strength analysis method is verified by the comparison with the test results.Finally,for the complex thin-wall joints structures,a method which can quickly transform the solid model of thin-wall joint structures into the shell-fastener model is proposed to realize the fast modeling and analysis of complex joint structures through the shell-fastener model.In addition,the tensile failure tests of C/SiC composite threerow,single-lap,three-bolt joint structures are carried out.There are two typical failure modes,tensile failure of the joint plate and bolts shear failure.The tensile failure mode of the jointing plate is obtained by the strength analysis model of shell-fastener.The bolt shear failure mode is attributed to the actual low shear strength of bolt.It is combined with the equivalent stiffness model and the contact stiffness algorithm.The shear strength of bolt is given by the shell-fastener model,and the bolt shear failure process of C/SiC composite three-row,single-lap,three-bolt joint structure is further reproduced.In this paper,the shell-fastener model which considers nonlinear contact stiffness is established.It can be further applied to the performance analysis and evaluation of large and complex C/SiC composite joint structures to guide the optimal design of the overall joint structures.