LOW-TEMPERATURE FLEXURAL DYNAMIC MEASUREMENTS ON PEEK, HTA AND SOME OF THEIR CARBON-FIBER COMPOSITES

Polymers and polymer matrix composites undergo mechanical relaxations which are dependent on the temperature and frequency. A major glass-rubber (alpha) transition occurs for most structural polymers above 100-degrees-C. The work reported here concerns the low-temperature beta and gamma transitions and their effect on the modulus and mechanical damping of some structural thermoplastic materials. To this end, the flexural moduli and specific damping capacities of unreinforced PEEK, HTA and some unidirectional PEEK/AS4 and HTAIAS4 specimens have been measured between -196-degrees-C and room temperature in vacuo. Specimens containing PEEK show a linear decrease of modulus with temperature whereas those containing HTA show a decrease which incorporates a pronounced drop (as a result of a peak in the damping). This occurs at -73-degrees-C however, an additional drop at -190-degrees-C is shown by the unreinforced HTA specimen. The effect of thermal expansion on the results is emphasised, and thermal expansion data were required accurately to calculate the flexural moduli presented here. Specimens containing PEEK show beta and 7 damping peaks at -63-degrees-C and -143-degrees-C. Those containing HTA show a more intense beta peak at -73-degrees-C, and a gamma peak is implied at or just below the lower end of the temperature range. Damping results are compared with theoretical predictions which were derived by the use of the viscoelastic correspondence principle. In particular, the derivation of a predictive formula for the transverse damping of a unidirectional composite is presented, and this is shown to produce good results in view of experimental scatter. Longitudinal shear values of modulus and specific damping capacity for unidirectional PEEK/AS4 have been estimated between -196-degrees-C and room temperature.