High temperature mechanical characterisation of polyimide diaphragm forming films

Abstract

During diaphragm forming of complex curvature thermoplastic composite parts, the diaphragm material imparts necessary in-plane tension to the component. The resultant quality of the formed part will depend on the mechanical properties of the film used. The present work examines the mechanical properties of the diaphragm films, at temperatures from 300 to 400°C, using uniaxial tensile tests. Previous data for these materials were limited to a maximum temperature of 300°C. The purposely designed wave-face tensile grips allow determination of 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°C. By holding the film at 380°C for 50 min the strain to failure was reduced to 125%. High strain cycling tests were conducted to demonstrate the elasticity of the material at large deformations. A large strain time dependent model, based on the network theory of polymer elasticity is presented. The model requires values for three shear moduli and three associated relaxation time constants. The constants for this model are obtained using both stress relaxation and tensile data. By providing a constitutive model for the diaphragm material, an important building block has been added to future diaphragm forming models.