Strength–Modulus Relation for Carbonized Acrylic Fibres

Abstract

THE Young's modulus of carbon fibres based on a single commercial acrylic steadily increases with increasing process temperature^1–3, and reaches 30×10⁶ pounds/inch² at a final processing temperature of 1,000° C. A corresponding relation for the breaking strength of the fibres shows a peak in strength at either 1,500° C (ref. 1) or 1,200° C (refs. 2 and 3), giving the high breaking strain (1.3 per cent) type of fibre, followed by a decrease in strength at higher processing temperatures. Tests of strength versus gauge length^1,4 have shown that weak spots or flaws influence the breaking strength of the fibres, and this is illustrated, at least for the high strain type of fibres, by microscopy and fractography^2,3,5. Any relationship between strength and modulus, which has a positive slope, seems to be limited to the processing temperature range 500°–1,000° C. In this temperature range particularly, mechanical properties of the fibres are influenced by the processing conditions and by the nature of the starting material. Any material or processing condition which allows fusion or gross disruption of the pyrolysing polymer will lead to incoherent carbon fibre with poor mechanical properties.