Lifetime statistics for single Kevlar 49 filaments in creep-rupture

Experimental data are presented for the lifetime of single Kevlar 49 filaments under moderate to high stress levels at standard ambient conditions (21°C, 65% r.h.). Filaments were drawn from two spools, A and B, taken from the same production lot. Previously we found that filaments from spool A were 7% lower in mean strength but much less variable in diameter than filaments from spool B; however, the respective variabilities in failure stress were equivalent. The lifetime data were interpreted in light of a previously developed kinetic model embodying Weibull failure statistics and power law dependence of lifetime on stress level. As predicted, lifetime data at each stress level generally followed a two-parameter Weibull distribution with a shape parameter value near 0.2. Based on absolute stress levels, the filaments drawn from spool B had a Weibull scale parameter for lifetime about ten times greater than those from spool A; however, when the stress-levels were normalized by the respective Weibull scale parameters for short-term strength, these differences disappeared. With respect to power law dependence of lifetime on stress level, three distinct time domains emerged, each marked by a different power law exponent. Similar behaviour was observed earlier for preproduction Kevlar 49/epoxy strands, and the values for the power law exponents for the filaments agree closely with those for the strands.