| High temperature-resistant carbon fiber composites have been provided with high specific strength,high specific modulus,corrosion resistance and fatigue resistance.Consequently,it has been used widely in various fields including aerospace.However,the failure mode of the above materials has been related closely to the load condition and even discrete because of complex service environment and different the manufacturing process.The key technology bottleneck for its further application has been produced.Therefore,it has theoretical research significance and engineering application value to qualitatively and quantitatively characterize the failure mode under typical working conditions.The failure mechanisms and failure modes under different load conditions in published literature were evaluated and compared.A method of failure modes characterization involving macroscopic,mesoscopic and microscopic scales was proposed.In the meantime,the related experiments under different material systems,layup structures were carried out,which included plane biaxial compression and high temperature fatigue.The above method was validated and supported through the test results.The specific work was as follows:Firstly,correspond damage evolution and mechanical properties to fracture characteristics respectively during the failure process of fiber-reinforced composites through literature research.On this basis,failure mechanism under basic tensile and compress was reviewed.And then,a method was proposed to characterize the failure modes under typical conditions such as tension and compression qualitatively and quantitatively.Secondly,the plane biaxial compression test of C/C satin weave composites were designed and carried out.The failure mechanism of biaxial compression was analyzed by combining the load-displacement curves and load-strain curves in different directions with the above-mentioned basic compression failure mechanism.Afterwards,the uniaxial and biaxial compression failure modes were evaluated and compared by the proposed failure mode characterization method,which combined with the failure mechanism of biaxial compression.The conclusion showed that the compressive failure modes have been divided into three types according to the interface strength.The three types contained strong interface,medium interface and weak interface failure.The strong interface failure mainly manifested as fiber bundle shear fracture,while the moderate interface and weak interface failure mainly displaced fiber bundle buckling failure.Finally,the high temperature fatigue test of C/Si C plain weave composites was designed and carried.And the high temperature fatigue failure mechanism was studied through specific analysis,which was finished by incorporating the change of fatigue life,stiffness and other mechanical properties with the above-mentioned basic tensile failure mechanism.The proposed failure mode characterization method integrated with the high temperature fatigue failure mechanism was used for the evaluation and comparison of five failure modes,which included uniaxial tensile,high temperature tensile,normal temperature fatigue,and high temperature fatigue with and without antioxidant coating materials.The conclusions showed as follows: 1)The fatigue life of the coated material is shorter than that of the uncoated material interestingly,and the degree of reduction is more than 55%.2)For the uncoated material,the residual stiffness decreased with the increase of the fatigue cycles number.And the decline rate improved with the increase of and fatigue stress.For the coated material,changes in residual stiffness remain stable at lower stresses.And the residual stiffness under higher fatigue stress decreased much more than the lower stress.3)The pull-out length of fiber bundles at high temperature fatigue fracture of coated and uncoated materials decreased with the increase of fatigue stress.There are significant differences in fracture flatness and fiber pull-out length of specimens under the five loads conditions mentioned above. |
