Long-term behaviour of aramid fibre
An investigation of the creep rupture of Kevlar@ 29, Twaron@ 1000, and Technora T2000 aramid fibre yams was carried out in the temperature range 25-120T in air and three other environments (Saudi/Iranian crude oil, pure water, and low pH water) at 65T with the objective of characterising the long term failure behaviour of these fibres and determine if they can be used to replace the stress rupture test conducted on RTP for qualification purposes. The standard loglo-loglo ISO 9080 extrapolation procedure, the modified lin-logio ISO 9080 extrapolation procedure and the time temperature superposition method were used to model and interpret the results. When tested in air, the different yams showed similar but not identical regression parameters. The results of the standard logio-logio ISO 9080 and the modified lin-logio methods correlated well. The standard logio-logio form, however, gave slightly higher values for the 20 year mean stress and LPL. Its use is recommended because it is consistent with the model assumed in the regression of RTP. There was no evidence of any 'knee' in the relationships after long periods or at high temperatures. This suggests that there is no change in failure mechanism, and lends confidence to both the long term use of aramid fibre in air and the recommended qualification procedures. RTP regression curve constants and those of tested aramid fibre were similar indicating the validity of aramid fibre stress rupture tests as a replacement for RTP tests. Crude oil immersion produced a small but significant reduction in behaviour. At 650C a suitable crude oil reduction factor applied to the long ten-n LPL would be 0.9. Pure water produces a marked reduction in strength after short periods, at 65C. Finally, water of low pH resulted in a large reduction in performance and considerable scatter with one fibre type. With the other, the effect appeared to be similar to that of pure water. Based on the understanding of the failure mechanism and the creep process in aramid fibres, a creep strain model is proposed for estimating the strain to failure based on the material's properties.