Characterization Of Intrinsic Compressive Strength Of Carbon Fiber By Bundle Method And Research On Its Correlation With Structure

With the development of the third generation advanced composites materials,which are characterized as the combination of high strength,high modulus,high toughness and blance of tensile and compressive strength,more attention is focused to the comprssive performance.The high tensile properties of carbon fiber contribucted the excellent tensile properties,the compressive strength of carbon fiber is somehow not satisfied.Carbon fiber,one of the most important reinforcement,determines the application of the composites.Efforts are given to improve compressive performance of carbon fiber and its composite materials,Lack of effective way to characterize compressive properties and little know of the relationship between structure and compressive strength of carbon fiber hinders the development of new type of carbon fiber with high compressive strength.In this thesis,a new fixture for compression test using bundled of carbon fiber was designed and manufactured,and the test method using the specially designed fixture was established by investigation of the effect of testing parameters on the test results.The effects of diameter and modulus on intrinsic compressive strength were studied.By investigation the microstructure of carbon fiber using XRD,SEM,TEM and Raman as the main Characterization,main structural factors determined the intrinsic compressive strength of carbon fiber were analyzed.Failure mechanisms of carbon fiber under compression loading were proposed for carbon fiber with high tensile strength and high tensile modulus.It provides theoretical guidance for the development of a new type of carbon fiber with high compressive strength,and the research results obtained are as follows:(1)Five pricinples was summarized for designing compression test fixture using bundled carbon fiber,and a fixture suitable for 1K to 24K carbon fiber was designed and manufacted.(2)Average compressive strength of 2744MPa with dispersion coefficient of 8.01%was obtained for T800HB-12K carbon fiber at test length of 2.00mm?3.00mm and adhesive thickness of about 0.101mm?0.110mm.Typical compressive failure mode in the cross-section of the fiber comfirmed effectiveness of the established test method.(3)The crystal width La increased with increasing diameter of carbon fiber,while the crystal thickness Lc decrease with increased diameter for carbon fibers with different diameters but aproaimaly the same tensile strength and modulus,which was obtained by adjusting the drawing ratio of PAN fiber during stabization.(4)The intrinsic compressive strength of carbon fibers decreased with increasing diameter in the range of 4.86?m to 6.03?m at this study.The crystallite volume Vg and its orientation were main structural factors affecting the intrinsic compressive strength of carbon fibers with high tensile strength.(5)The tensile modulus increased linearly with increased crystal width La,crystal thinckness Lc for carbon fiber with approximately the same tensile strength and dimeters.(6)Lower intrinsic compressive strength was found for carbon fiber with high tensile modulus in the range of 371GPa to 433GPa.No obvious corelationships was found betwen crystal structures and intrinsic compressive strength for carbon fibers with high tensile modulus.The unevenly distribution of graphitzation degree determined the compressive strength.(7)Two compressive failure mechanisms wrer proposed.For carbon fibers with high tensile strength,compressive stress was evenly distributed in the direction of fiber diameter,and shear failure accured in the fiber,resulting high compressive strength.For the compression failure of high tensile modulus carbon fibers,compressive failure was dominated by kinking of the fiber,which resulted low compressive strength.