Relation of Structure to Properties in Graphite Fibers

• Main Authors: Diefendorf,R J, Chen,K J, Seagroves,W P, Wang,J J-H
• Format: Report
• Language: English
• Subjects: Axial properties; CARBON FIBERS; COMPRESSIVE PROPERTIES; DEFECTS(MATERIALS); GRAPHITE; MECHANICAL PROPERTIES; Mechanics; MODULUS OF ELASTICITY; ORIENTATION(DIRECTION); PE61102F; Preferred orientation; STRENGTH(MECHANICS); Textiles; WUAFOSR2303A3; X RAY DIFFRACTION

The axial properties of carbon fibers were found to correlate well with preferred orientations determined by either x-ray diffraction or optical techniques. High modulus carbon fibers were found to have significant gradients in preferred orientation. For HMS fiber, the modulus of the surface layers is about twice the average fibers modulus, while the interior modulus is only about one-half the average. This modulus gradient suggests that higher modulus fibers, with good strength, could be produced, if the modulus at the interior of these fibers could be increased. The gradient in preferred orientation produces high residual stresses in HMS fibers. While the high surface compressive stresses minimize the effect of surface flaws, the high axial tensile stresses in the interior may decrease strength by causing fracture to initiate at flaws in the interior rather than at the surface. Similarly, the high axial compressive stressed outer layers of a fiber may initiate buckling when the fiber is compressively loaded. Modifications of the residual stress pattern might allow increased tensile and/or compressive strengths to be obtained in high modulus carbon fibers. Many correlations between properties and fiber diameter, reported in the past, may be in error because it is very difficult to determine the fiber diameter precisely and accurately. (Author)