Strain Measurement in the Wavy-Ply Region of an Externally Pressurized Cross-Ply Composite Ring

Abstract

Ply-level strains are determined in the cross-section of an externally pressurized cross-ply (3:1 circumferential to axial fiber ratio) graphite-epoxy ring containing an isolated circumferential wavy region. A special test fixture was used which permitted measuring orthogonal displacement components in the wavy area using moire´ interferometry as the pressure was increased. Strain components were determined at selected locations in the wavy area up to approximately 90 percent of failure pressure. The study shows: (1) Large interlaminar shear strains, which are non-existent in the “perfect” ring, are present near the wave inflection points; (2) The wavy plies generate increased interlaminar normal compressive strains in both circumferential and axial plies along a radial line coinciding with maximum wave amplitude; and (3) Nonlinear strain response begins at approximately 60 percent of failure pressure.