Multi-Scale Analysis Method Of Composite Tanks At Cryogenic Temperatures Considering Fiber/Matrix Interface

With the demand of lightweight and low cost,continuous fiber reinforced composites have been gradually increased in the application of space launch vehicles due to their advantages of high specific modulus,high specific strength,fatigue resistance and strong designability.As the cryogenic propellant tank accounts for the largest proportion of weight and volume in the propulsion system of space launch vehicle,the development of linerless composite tank structure has become an important direction of lightweight rocket structure.Since the tank structure is working in extremely cryogenic temperature environment,the component phases of composite materials will produce micro stress due to thermal mismatch under thermal load,resulting in matrix cracking,fiber/matrix interface debonding and other failure modes,which jointly cause micro cracks through the composite winding layer and propellant medium leakage.Therefore,multiscale failure analysis of composite structures at cryogenic temperatures considering fiber/matrix interface is of great significance for lightweight design of linerless composite tanks.Based on the hexagonal representative volume model,a microscopic matrix failure criterion considering matrix in-situ effect is proposed to accurately predict the matrix cracking failure mode.Furthermore,the mechanical test and microstructure observation of T700/TDE86 composite system,which commonly used in composite structure manufacturing,is carried out at cryogenic temperatures.The input data of material properties and geometric structure form are provided for the multi-scale failure analysis of composite considering fiber/matrix interface.Subsequently,the representative volume element of adjacent fiber crack propagation is established.Through the relationship between the statistical distribution characteristics of constituent materials and the tensile fracture morphology of composite unidirectional plates,an evaluation method of interfacial shear strength at cryogenic temperatures based on statistical inversion is proposed to measure the interfacial shear strength at cryogenic temperatures.A three-phase fiber/matrix damage model is proposed to represent the fiber/matrix interface degradation.The main contents of this thesis are as follows:(1)Considering the discontinuity of the resin matrix in the direction perpendicular to the fiber caused by fiber intervention,the matrix stress concentration factor is determined by calculating the matrix cracking angle of hexagonal representative volume element under tensile,compressive and shear loads,and the modified microscopic maximum stress criterion considering the matrix in-situ effect is established.Compared with the strength prediction of composite laminates under transverse tension and biaxial tension,the proposed criterion in this thesis has higher prediction accuracy than other macroscopic and microscopic strength criteria.(2)Aiming at T700/TDE-86 composite system which is commonly used in composite structure manufacturing,the mechanical test of composite and its component materials is carried out at cryogenic temperatures.Based on the scanning electron microscope(SEM)images,the characterization method of the microstructure is studied.The volume fraction of the fiber is calculated by using the trainable Weka gray segmentation algorithm,and the element content near the fiber/matrix interface is measured by using the energy dispersive spectrometer function,so as to obtain the thickness of the fiber/matrix interface.Based on the test standard of composite materials and their component phases,the mechanical tests of resin and typical composite laminates at 300K,248k,198k,173k and 77K are carried out.The mechanical properties and strength distribution at cryogenic temperatures are studied,the failure mechanism of laminates is studied by observing the fracture morphology.(3)Considering the statistical distribution characteristics of component materials,the representative volume element of adjacent fiber crack propagation is constructed,and a MCP model is established to simulate the crack propagation behavior nearby the fiber/matrix interface.At the same time,image recognition technology and gray value analysis are used to process the SEM images of the fracture surface of the unidirectional composite plate,and the fracture surface morphology is characterized by the probability of different crack propagation modes in the fracture surface.Based on the relationship between the statistical distribution characteristics of structural components and the tensile fracture morphology of unidirectional composite plates,an evaluation method interfacial shear strength at cryogenic temperatures based on statistical inversion is proposed.Compared with the experimental results of droplet debonding and resistance fragmentation at cryogenic temperatures,this method exhibits good prediction accuracy.(4)Based on the geometric characteristics of fiber/matrix interface and the test results of mechanical properties,a three-phase representative volume element model of fiber/matrix interface is established.The mapping relationship between macro stress and micro stress is constructed.The microscopic failure criterion and macroscopic stiffness degradation model are developed.Considering the influence of random strength of component materials,a multi-scale progressive damage analysis method composite at cryogenic temperatures considering fiber/matrix interface is proposed.Compared with the test results of typical laminates at cryogenic temperatures,this method has good prediction accuracy.Based on this method,the macro-micro stress mapping relationship is constructed,and a method to determine the allowable value of composite structure at cryogenic temperatures is proposed.