Elastic properties and fracture behavior of graphite/polyimide composites at extreme temperatures

Abstract

The influence of elevated and cryogenic temperatures on the elastic moduli and fracture strengths of several Celion-6000/PMR-15 laminates was measured. Tests were run at -157,24 (room temperature), and 316°C (-250, 75, and 600°F). Both unnotched and notched laminates were tested. Several failure criteria, developed to predict the uniaxial fracture strength of epoxy laminates, were used to predict the fracture strength of polyimide laminates.

Lamina elastic moduli were measured at each temperature by testing unnotched [0]₈ , [90]₈, and [±45]_2S laminates. The measured values were used with classical laminate theory to predict the elastic constants in [0/45/90/-45]_S, [0/45/90/-45]_2S, [45/0/-45/0Js, and [45/90/-45/90]_S laminates. With few exceptions, the predictions agreed with the moduli measured experimentally. As for the ultimate tensile strength, although the 8-ply and 16-ply quasi-isotropic laminates were about equally strong at elevated temperature, their respective strengths diverged at the lower temperatures. The 8-ply laminates lost strength as the temperature decreased, whereas the 16-ply laminates became stronger.

The notched laminates had layups of [±45]_2S, [0/45/90/-45]_S, and [45/0/-45/0]_S. The measured moduli, the ultimate strengths, and the point stress or average stress criterion of Nuismer and Whitney were combined to calculate the characteristic lengths associated with each criterion. Characteristic lengths were compared to determine the effect of temperature.