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A method of measuring the longitudinal residual stress distribution in boron fibers is presented. The residual stresses in commercial chemical vapor deposited boron on tungsten fibers of 102, 142, and 203 µm (4, 5.6, and 8 mil) diameters were determined. Results for the three sizes show a compressive stress at the surface, [−800 to −1400 MN/m2 (−120 to −200 ksi)], changing monotonically to a region of tensile stress within the boron. At ∼25 percent of the original radius, this tensile stress reaches a maximum [600 to 1000 MN/m2 (90 to 150 ksi)], and then decreases and becomes compressive near the tungsten-boride core. The core itself is under a compressive stress of ∼−1300 MN/m2 (−190 ksi). The effects of surface removal on core residual stress and core-initiated fracture are discussed.
composite materials, boron, fibers, residual stress, tungsten borides
Physicist, Materials Science Branch, Lewis Research Center, National Aeronautics and Space Administration, Cleveland, Ohio