Stress concentration in carbon fiber revealed by the quantitative analysis of X-ray crystallite modulus and Raman peak shift evaluated for the variously-treated monofilaments under constant tensile forces

The stress distribution at the various parts of PAN-based carbon fiber (CF) monofilament has been estimated using a complex mechanical model on the basis of the quantitative evaluation of crystallite modulus through the micro-beam wide-angle X-ray diffraction measurement under tension as well as the micro-Raman spectral measurement under tension. When a CF monofilament was tensioned along the fiber axis, the outer-surface part of nanometer-order depth was found to owe a tensile stress about 1.5 times higher than the inner-surface and core parts, indicating an easier start of fiber breakage from the outer surface part. This knowledge gives us an important hint to the development of mechanically-tough carbon fiber without any such local stress concentration on the outer surface part.