Failure of Braided Composite Cylinders under Biaxal Tension

Abstract

Braided preforms of advanced fiber composites are being considered for structural applications with potential cost and through-the-thickness length advantages relative to conventional laminates. However, the in-plane strength properties of braided preforms are relatively unknown. The present study reports on biaxial tension tests on AS4 braided reforms that were resin transfer molded with 1895 resin, in four different sets of braiding parameters. The fiber direction figure strain was observed to be independent of the biaxial stress state in the failure tests. Accordingly a maxim fiber direction strain lure criterion was used to correlate the results of this study. Failure properties of these materials appear to be sensitive to the material processes from which they ar produced, and scatter of the current results was observed to be relatively large. Examinaton of the specimen surfces as well as the braiding parameters has provided clues regarding the inconsistent material performance. The specimens were observed to be sensitive to parameters which influence yarn straightness. Braider coverage and preform bulk are believed to be primary among these parameters. Scatter of the experimental data make comparison difficult, but in general the data are in good agreement with the failure envelopes.