HIGH-TEMPERATURE MECHANICAL CHARACTERIZATION OF POLYIMIDE DIAPHRAGM FORMING FILMS

During diaphragm forming of complex curvature thermoplastic composite parts, the diaphragm material imparts necessary in-plane tension to th e component. The resultant quality of the formed part will depend on t he mechanical properties of the film used. The present work examines t he mechanical properties of the diaphragm films, at temperatures from 300 to 400 degrees C, using uniaxial tensile tests. Previous data for these materials were limited to a maximum temperature of 300 degrees C . The purposely designed wave-face tensile grips allow determination o f the ultimate strength and strain of the polymer film. The effects of temperature and dwell time at temperature are presented. The maximum strain to failure of the film is 225% at 380 degrees C. By holding the film at 380 degrees C for 50 min the strain to failure was reduced to 125%. High strain cycling tests were conducted to demonstrate the ela sticity of the material at large deformations. A large strain time dep endent model, based on the network theory of polymer elasticity is pre sented. The model requires values for three shear moduli and three ass ociated relaxation time constants. The constants for this model are ob tained using both stress relaxation and tensile data. By providing a c onstitutive model for the diaphragm material, an important building bl ock has been added to future diaphragm forming models. (C) 1998 Elsevi er Science Limited. All rights reserved.