The effect of gauge length on the tensile strength of R.A.E. carbon fibres

Abstract

Tensile tests using 0·5, 5·0 and 10·0 cm lengths of R.A.E. carbon fibre after heat-treatments to 1000, 1500, 2500 and 3000°C have shown that the length tested has a significant effect on the strengths obtained. With fibres heat-treated to 2500°C, the average strength of 0·5 cm lengths was 400 × 103 lb in-2, this being about 30% higher than the corresponding result obtained with 5 cm lengths. The highest strengths were found after the heat-treatment to 1500°C, the average for the 0·5 cm lengths being 450 × 103 lb in-2, again about 30% higher than the value for 5 cm lengths. Similar gauge length effects were found after each heat-treatment temperature, and the ratios of the standard deviations of the strengths to the average strengths in general remained constants at about 25%. It appears therefore that the type of defects limiting the strengths and their distribution have not been influenced by the different heat treatments. The strengths of the 0·5 cm lengths have been used to predict values for the longer lengths by means of weak link theory and these values have been compared with the experimental results. © 1969.