Testing of Hygrothermally Aged E-Glass/Epoxy Cylindrical Laminates Using a Novel Fixture for Simulating Internal Pressure

This paper discusses the development of a novel test fixture for applying a uniform radial stress to cylindrical composite laminates in such a way as to eliminate bending moments and axial loads. This fixture requires the use of easily fabricated poly-tetrafluoroethylene (PTFE) ring seals. These seals, which fit just inside the composite sample, are designed to be internally pressurized in the fixture and expand outwards, applying a uniform radial load to the sample. The results of mechanics analyses of the PTFE ring seal and composite sample are presented, which estimate the radial stress applied to the composite sample as a fraction of the applied internal hydraulic pressure. Further, results are presented from a prolonged hygrothermal aging study, in which the E-glass/epoxy, cylindrically wound composite samples are aged under varying hygrothermal conditions (ambient dry; and 30, 40, and 50 degrees C submerged in water). At various points during the aging, a subset of samples are removed and destructively tested to measure the reduction in strength from the initial values. Annular specimens are tested using the fixture, and hoop tensile strengths are recorded. In addition, samples cut axially from the cylindrical composite tubes are used to evaluate tensile and compressive axial strengths. Arrhenius fits are used to estimate activation energies for the strength loss in the axial (compression) and hoop (tension) directions.