Strain modulation measurements of stiffening effects in carbon fibers

An experimental technique for directly measuring strain-induced stiffening effects in individual carbon fibers has been developed. The technique involves the superposition of a small amplitude oscillating strain (Δε∼10−4) on a slow linearly increasing strain (∼1.1×10−3 s−1). Synchronous detection of the resultant ac component of the measured stress yields a signal proportional to the slope of the stress versus strain curve provided the modulation amplitude is small compared to the ultimate strain at failure (∼0.005). The experimental arrangement has been designed for use in conjunction with an Instron tensile test apparatus. A novel design for a fiber gripping apparatus which permits direct measurement of the strain is also described. The technique has been applied to carbon fibers produced from two different precursor materials: polyacrylonitrile and mesophase pitch. Fibers from both precursors exhibited strain-induced stiffening. The modulus of the pitch-based fibers increased in proportion to the square root of the strain, whereas that of the PAN-based fibers showed an almost linear increase.