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Compression design allowables for carbon/epoxy laminates could be increased by taking advantage of hole-filling effects, if reliable design data were available. Previous research had demonstrated that filled hole compression strengths range between 5% to 35% higher than open hole strengths, due to variability in fastener-hole clearance and clamp-up torque. The importance of measuring these parameters for proper data interpretation resulted in the development of ASTM Practice for Filled Hole Tension and Compression Testing of Polymer Matrix Composite Materials (D 6742/D 6742M). Specific instructions regarding measurement, recording, and calculation of critical parameters for filled hole specimens are provided. The standard was implemented at Boeing- Philadelphia in the development of compression allowables for high-temperature materials used on the RAH-66 airframe. Enhanced compression strengths were demonstrated for several material systems tested in a variety of environments. A semi-empirical analysis technique permitted strain allowables to be generated using fewer laminate configurations and specimens than in previous programs.
composites, bolted joints, compression loading, hole tolerance, finite-element analysis, strength prediction, design allowables
Associate Technical Fellow, Structural Technology & Prototyping, The Boeing Company, Philadelphia, PA