STP813

    Prediction of Long-Term Failure in Kevlar 49 Composites

    Published: Jan 1983


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    Abstract

    Creep rupture data in Kevlar 49 epoxy usually exhibit considerable scatter: the coefficient of variation (CV) about the mean failure time at a given stress often exceeds 100%. In this paper, analysis of existing creep rupture data on Kevlar epoxy vessels at four storage pressures has produced an interesting and useful result. It was found that a significant portion of the scatter in failure times for pressure vessels is due to spool-to-spool variation in the eight spools of Kevlar fibers used to wind the vessels. The rank order of mean times to failure was consistent over a pressure range from 23.4 to 29.7 MPa (3.4 to 4.3 ksi), 68 to 86% of short-term burst. Also, the coefficient of variation about the mean failure time for each spool was less than that for the total sample. The statistical inference that the sample is nonhomogeneous was supported by a nonparametric check using the Kruskal-Wallis test and by a parametric analysis of variance. The rank order found in long-term tests did not unequivocally agree with static strength ranks; several spool sets were distinctly high or low. The implication is that, while static strengths are not valid predictors of long-term behavior, short-term creep-rupture tests at high stress definitely are.

    The material difference that causes the spool-to-spool variations has not yet been identified for all eight spools. However, it appears that Kevlar behavior at lower pressures may be predicted through the use of curves fitted to the data for each spool. A power law relating failure time to pressure, t = t0(p/p0)m, was found to fit the data reasonably well. From these fits were constructed design-allowable life values with an expected reliability of 0.999999; these predictions are supported by data on vessels at 17.2 MPa (2.5 ksi), where no failures have occurred after 4½ years. The power law fits also demonstrated the extreme sensitivity of failure time to pressure (m ≈ −15 to −25). This implies that, both in composite vessel design and in creep-rupture experiments, the pressure (or stress) level must be carefully controlled.

    Keywords:

    composite pressure vessels, Kevlar 49 epoxy, creep rupture, spool-to-spool variability, reduced scatter, power law, design allowables, reliability, pressure sensitivity, composites


    Author Information:

    Gerstle, FP
    Supervisor and member of the technical staff, Composites and Polymer Mechanics Division, Sandia National Laboratories, Albuquerque, NM

    Kunz, SC
    Supervisor and member of the technical staff, Composites and Polymer Mechanics Division, Sandia National Laboratories, Albuquerque, NM


    Paper ID: STP31827S

    Committee/Subcommittee: D30.91

    DOI: 10.1520/STP31827S


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