CREEP DESIGN ANALYSIS FOR A THERMOPLASTIC FROM STRESS-RELAXATION MEASUREMENTS

Innovative designs using thermoplastics have been limited by available mechanical property data which often do not span appropriate ranges of stress, time, strain and temperature. In an effort to improve the design process, a methodology using short-time (less than 24-hour) high-precision stress relaxation tests has been developed to provide comprehensive design information. This study uses stress relaxation tests to investigate the creep properties of VALOX, a polybutylene terephthalate, or PET, developed by General Electric. The approach is extremely efficient, and the data generated can be presented in several convenient formats. For example, pseudo-stress-strain curves may be used to generate strain versus time (at a constant stress) or stress versus time (at a constant strain). Secant modulus curves as a function of strain, time and temperature can also be produced. The results are in good agreement with those generated by traditional test methods. The stress relaxation testing technique thus shows great promise as a tool to be used in the design of thermoplastic components.