A Rapid Temperature Superposition Method to Determine Creep Behaviour of Thermoplastics for Automotive Electronics Applications

Recent developments in polymer technology have gained automotive industry's attention as a viable alternative for ubiquitous metals. Apart from the design loads envisaged, an automotive part is subjected to multiple environmental loads including variation in ambient temperature. In particular, several automotive electronics applications feature product placements in zones of high temperature (on-engine being the most severe with temperatures exceeding 125°C), which drive the material close to melting points. Exposure to such a harsh environment will have adverse effects on the components (particularly those made of plastics) whose life span is expected to be 20 years.

This calls for better characterization of polymers to help predict their behavior during usage. The challenge lies in devising an effective experimental procedure backed with an analytical method to present the material property in a rapidly usable format for technical analysis. Traditional long-term creep test procedures are too time-intensive to suit the current scheme. The focus of this study is on developing a methodology using short time (less than 24 hours) creep tests wherein the interrelationships between Young's Modulus, time and temperature are presented and analyzed. The creep testing technique applied in such an environment has evolved as a promising tool to be used in the design of thermoplastic components. The plots at each temperature are superposed by log aT the temperature dependent shift factor, to form a master curve of sigmoid shape. William-Lendel- Ferry (WLF) relation [1] is used to compute the shift factor. Polycarbonate (LEXAN) is the polymer highlighted in this study.

For a viscoelastic material, each loading step makes an independent contribution to the final deformation or extension. A new approach known as Temperature Superposition Method is devised to obtain the temperature dependency of strain at various load levels.