Influence Of Quartz Fabric Structures On The Mechanical Properties Of Silica Matrix Composites

Quartz fiber reinforced silica (SiO2f/SiO2) composites are widely used in aircraft radomesbecause of their good thermal protective performance and wave permeability. During high-speedflights, the radome endures aerodynamic and inertial loads. Hence, the radome needs to have goodmechanical properties, while maintaining its dielectric properties. Previous studies on the mechanicalproperties of SiO2f/SiO2composites mostly focused on the influence of the preparation process anddifferent matrix compositions on performance, with a few on the influence of three-dimensional (3D)integrated quartz fabric structures on the mechanical properties of SiO2f/SiO2composites. This thesisaims at studying SiO2f/SiO2composites using experimental and theoretical prediction models tounderstand the influence of quartz fabric structures on the mechanical properties of composites.In this thesis, four kinds of SiO2f/SiO2composites were prepared by silicasol-infiltration-sintering method, namely:2.5D shallow bend-joint and straight joint (same density ofwarp and weft threads, similar fiber volume fraction),3D four-directional and five-directional (similarbraiding parameters and fiber volume fraction) SiO2f/SiO2composites. The density and porosity of thefour kinds of SiO2f/SiO2composites were measured by the Archimedes displacement method. Thephase components of the SiO2f/SiO2composites were analyzed by XRD. The tensile properties,flexural properties, shear properties and fracture toughness of the four kinds of SiO2f/SiO2compositeswere measured using the tensile test, three-point-bending method, Iosipescu shear testing method andsingle edge precracked beam test respectively. The fracture morphology characteristics of theSiO2f/SiO2samples were observed by SEM, and the failure modes of the damage evolution process ofcomposites were analyzed using SEM results and mechanical properties testing curves. Based onmesoscopic unit cell model, criteria to predict the elastic properties of composites was established interms of the stiffness volume average method, and the effective elastic constants of the four kinds ofSiO2f/SiO2composites was obtained. The specific research contents and the main researchachievements are as follows:(1)The mechanical properties of2.5D shallow straight-joint SiO2f/SiO2composites along warpyarns and weft yarns were studied contrastively. The experimental results showed that the mechanicalproperties of2.5D composites along warp yarns degraded due to the undulation angle of warp yarnsresulting in a mismatch between the ratio of mechanical properties of2.5D SiO2f/SiO2alongwarp/weft yarns and the ratio of warp/weft yarns content. The testing curves of SiO2f/SiO2composites showed distinct nonlinear characteristics due to the weak bonding interface, voids and microcracks inthe composites. The differences in mechanical properties of2.5D shallow straight-joint and bend-jointSiO2f/SiO2composites along warp yarns were studied. The experimental results showed that thedegree of the undulation angle is the key factor affecting the tensile properties of2.5D SiO2f/SiO2composites; the smaller the undulation angle, the greater the tensile strength and fracture strain.However, the influence of composite density on the flexural properties, shear properties and fracturetoughness of2.5D shallow straight-joint and bend-joint SiO2f/SiO2composites was more distinct thanthe influence of the undulation angle.(2)According to the basic assumptions of the yarns cross section, small unit cell models of2.5Dshallow straight-joint and bend-joint SiO2f/SiO2composites were obtained. Models to predict theelastic properties of2.5D SiO2f/SiO2composites were established by using the stiffness volumeaverage method. The prediction results were in good aggreement with experimental results. Hence,the prediction models were precisely adequate. The influence of fiber volume fraction and weft yarnsdensity on elastic properties of2.5D SiO2f/SiO2were analyzed.(3)The differences in the mechanical properties of3D four-directional and five-directionalSiO2f/SiO2composites along the braiding direction were studied contrastively. The experimentalresults showed that the tensile strength, tensile modulus, flexural strength, shear strength and fracturetoughness of the3D five-directional SiO2f/SiO2composite were superior to that of the3D four-directional SiO2f/SiO2composite, but the flexural modulus and shear modulus of the3Dfive-directional SiO2f/SiO2composite were inferior to that of the3D four-directional SiO2f/SiO2composite. The influence of the coupling of flexure and shear of3D five-directional SiO2f/SiO2composite was more distinct than3D four-directional SiO2f/SiO2composite. Under flexural load, thestress of axial yarns in3D five-directional SiO2f/SiO2composite was uniform, having a distinctflexural behavior from that of3D four-directional SiO2f/SiO2composite. The flexuralstress–deflection curve showed a steep drop and then quickly restored to the quasi-linear stage. Thefailure mode of both3D four-directional and five-directional SiO2f/SiO2composites in Iosipescu sheartesting was a mixed mechanism: in addition to fiber/matrix interface debonding and inter-laminardamage, fiber bundle splitting in the notch root.(4)The interior unit cell models of3D four-directional and five-directional SiO2f/SiO2compositeswere obtained based on the basic assumptions of the yarns cross-section. By using the stiffnessvolume average method, models to predict the elastic properties of3D four-directional andfive-directional SiO2f/SiO2composites were established. It was discovered that "cutting-edge effect" existed in3D four-directional and five-directional SiO2f/SiO2composites and an explanation wasgiven on the impact degree of "cutting-edge effect" in3D four-directional and five-directionalSiO2f/SiO2composites. The prediction results agree well with experiment results of3Dfour-directional and five directional net shape samples, hence, the prediction models were preciselyadequate. The influence of fiber volume fraction and interior braiding angle on elastic properties of3D four-directional and five-directional SiO2f/SiO2were also analyzed.