Characterization of the viscoelastic behavior of bismaleimide resin before and after exposure to high temperatures

Creep and high strain rate mechanical properties, shrinkage strain, and thermal properties of a bismaleimide neat resin after exposure to a high temperature in air were evaluated and compared with the corresponding properties for a pristine resin. Under tension at a strain rate of 6x10(-4) s(-1), the Young's modulus decreases and Poisson's ratio increases with temperature, measured up to 310 A degrees C. The tensile creep behavior was determined at stress levels of 12, 24, and 33 MPa at elevated temperatures. At each stress level, the creep compliance curves at different temperatures were shifted horizontally to form a master curve. These creep compliance master curves are nearly identical, indicating a linearly viscoelastic behavior up to 33 MPa. The bismaleimide resin was also exposed to air at other temperatures of 245, 260, and 280 A degrees C for 1500 hours. After exposure to a high temperature, three regimes were observed in the resin through optical micrographs: an outer layer showing darker color, an interior that nearly maintained its original color, and a transition (or reacting) region in between. The average shrinkage on surface was determined as 3.4 % strain after 1500 hours of exposure to 260 A degrees C in air. Compression at a high strain rate using a long split Hopkinson pressure bar shows that the bulk bismaleimide resin is rather insensitive to the exposure to a high temperature, exhibiting only a slight reduction in mechanical properties after 1500 hours of exposure to 245 A degrees C. The uniaxial creep compliance of the neat resin was converted into the Young's relaxation modulus, which was then used to calculate the Young's modulus under tension at the strain rate and temperatures involved, and a good agreement was achieved between the calculated results and the experimental data, indicating that the rate-dependent Young's modulus is the representation of viscoelastic properties.