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Flexural stress relaxation (FSR) and flexural internal friction (FIF) techniques were employed to measure the time-dependent deformation of boron fibers from −190 to 800°C. The principal specimens were 203-μm (8 mil) diameter fibers commercially produced by chemical vapor deposition (CVD) on a 13-μm tungsten substrate. The observation of complete creep strain recovery with time and temperature indicated that CVD boron fibers deform flexurally as anelastic solids with no plastic component. The mechanical relaxation processes responsible for the anelasticity were found to be controlled by a continuous distribution of thermally activated relaxation times τ = τ0 exp(Q/kT). Excellent fit of the FSR data with FIF data was obtainable across more than seven time decades by assuming that all processes have the same lnτ0 (= −33.1) but are continuously and broadly distributed in energy Q. The incorporation of the derived distribution parameters into anelastic theory yielded simple analytical equations to describe the time-temperature-stress dependence of CVD boron fiber deformation at low strains. In order to extend the flexural results to high strain and conditions of axially applied stress, an analysis of fiber axial deformation was made using data from the literature and our own fracture measurements. For the latter study, etched fibers were employed to ensure coreinitiated fracture. A model of a “composite” fiber with an anelastic sheath and elastic core was assumed in order to convert fracture stress versus time-temperature data to anelastic relations descriptive of axial deformation. These results in combination with literature data confirm the flexural result that up to at least 800°C all non-elastic axial behavior of CVD boron fibers can be explained solely by anelastic mechanisms. Diameter, core, coating, etching, and manufacturing source appear to affect negligibly boron sheath anelasticity. With a simple empirical model to account for observed stress effects, final analytical equations and curves are presented to describe total axial deformation strain. These relations are further developed to demonstrate the significant effects of anelasticity on such fiber/composite properties as modulus, creep, creep recovery, stress relaxation, and damping capacity. Finally, under the elastic-core/anelastic-sheath model, modern boron fibers on tungsten substrates are shown to have predictable fracture stresses for time-temperature conditions ranging from impact to long time stress rupture. Possible techniques for altering these stresses are discussed.
boron, composite materials, anelasticity, flexing, creep recovery, stress relaxation, damping capacity, tungsten borides, thermal expansion
Solid state physicist, Lewis Research Center, National Aeronautics and Space Administration, Cleveland, Ohio