Strain Measurement in Thick-Walled Composite Rocket Motor Case Coupons Using Moire Interferometry

Abstract

This paper describes an experimental program which used moire interferometry to measure deformations in the through-the-thickness meridional plane in coupons cut from the transition region near the aft section of thick-walled composite motor cases proposed for the space shuttle solid rocket booster. These coupons were loaded in compression to simulate bending introduced in the casing caused by prelaunch requirements. The design of the transition region incorporates axially graded reinforcement broadgood plies and helical ply drop-offs. The deformation in this region is highly nonuniform, especially near the innermost broadgood termination where the adjacent helical ply experiences localized shear and compression. An understanding of the state of strain in this region is required to understand eventual delamination failures observed in an associated strength test program. Strains are determined by moire interferometry and compared with results using photoelastic coatings and miniature strain gages. Nonlinear strain response was observed in the helical ply adjacent to the innermost broadgood termination at approximately 20% of the coupon failure load. Well defined cracks were also observed to form in the moire grating over the helical ply in the vicinity of the adjacent innermost broadgood termination at approximately 50% of the coupon failure load.