Multi-scale Analysis Of The Mechanical Behavior For Braided Ceramic Matrix Composites

Due to their high temperature resistance,high specific strength and specific modulus,notch insensitivity and outstanding designability,braided ceramic matrix composites are potential materials for use in the high-temperature parts of the next generation aero-engines.The multi-scale method can establish a quantitative relation between the macro properties of braided ceramic matrix composites and the component performance and the micro structure.In engineering,the multi-scale method can take advantage of the composite's performance sufficiently,obtain the optimal structural parts and decrease the design cost and the development cycle.Therefore,the multi-scale method is requisite for the engineering application of the braided ceramic matrix composites.However,multi-scale analysis of the mechanical behavior of braided ceramic matrix composites is not impeccable at present.For example,the nonlinear response,strength and failure mode cannot be predicted precisely.There are three main reasons for this.Firstly,the damage mechanisms is complicated which makes their mechanical model incomplete.Secondly,the mechanical properties of the components,e.g.,fiber and matrix,are hard to obtain.Lastly,research on the yarns is not sufficient enough.Accordingly,a multi-scale method was developed for the analysis of the mechanical behavior of braided ceramic matrix composite in the present study.The mechanical behavior and damage mechanisms of braided ceramic matrix composites were researched in the micro scale,the meso scale and the unit cell scale.Through the multi-scale analysis,relationships between the macro mechanical behavior of the composites and the component properties and the microstructure were established.In addition,the macro mechanical behavior of the composites was explained.The micro-mechanical models were developed for multiple damage mechanisms.For fiber broken,a fiber flaw distribution model was developed.In this model,the strength and probability of each level of flaws were derived from the experimental results.For matrix cracking,a strength model of the matrix element was developed to predict the matrix cracking process and the strength formula of the matrix element was presented.For fiber/matrix interfacial slipping,a distribution model of slip regions was developed to describe the slipping process of the fiber/matrix interface under arbitrary loading and unloading.The appearance,disappearance and extension of forward and reverse slip regions were discussed and the length and stress distribution of each slip region when there are any number of forward and reverse slip regions were also provided.The test methods for the in situ properties of multiple components were developed.For the in situ properties of fibers,fibers were heat-treated according to the fabrication process of ceramic matrix composites.The heat-treated fibers were counted as the in situ fibers in the composites.Then,tensile tests were performed on the heat-treated single fibers and fiber bundles to obtain the in situ strength distribution and modulus.For the density and distribution of matrix cracks,a real-time matrix crack detection system was developed which consists of a digital microscope and a specific loading apparatus.The test system can hold the sample and measure the load.Meanwhile,the matrix cracks can be observed using the digital microscope.The locations and amounts of matrix cracks under different stress levels can be obtained through this method.Minicomposites were used to research the mechanical behavior of yarns in braided ceramic matrix composites.The minicomposites were fabricated by the same process of braided ceramic matrix composites and therefore the mechanical property of minicomposites are the same as that of yarns in braided composites.The test system of minicomposites was developed and the mechanical responses under monotonic and cyclic loading were obtained.In addition,a constitutive model of yarns which considering the load carrying capacity of broken fibers was developed.The predicted strength,stress-strain response and failure mode of minicomposites were in good agreement with the experimental values.The unit cell model of braided ceramic matrix composites was built according to the microstructure.The mechanical behavior of yarns in the unit cell model was determined using the constitutive model of yarns.The stress-strain response and stress distribution of the unit cell were then predicted using the progressive damage analysis.For both the macro stress-strain response and the damage patterns,the predicted results were in good agreement with the experimental results.At last,the effects of components' properties and micro structure on the mechanical properties of braided ceramic matrix composites were analyzed.Different macro mechanical behaviors were obtained by changing the components' properties and micro structure.The numerical results indicated that micromechanical multi scale analysis can link the composites' macro mechanical behavior to the components' properties and micro structure and therefore realize the designability of composites' mechanical behavior.