Viscoelastic Behavior of Highly Aligned Discontinuous Fiber Thermoplastic Melts

• Charles Whealton, Mechanical Engineering, University of Delaware

• Thomas Cender, Mechanical Engineering, University of Delaware

Aligned discontinuous fiber (ADF) composites offer multiple advantages over traditional continuous fiber composites, particularly in terms of formability. To better understand how to form this material, the fundamental material behavior must be understood. This work establishes a method to conduct repeatable stress relaxation tests on thermoplastic ADF composites above the glass transition temperature and discusses the findings and results in terms of the implications of the material behavior. Data was collected using a static loading frame and implementing a stress relaxation method. The method was modified to include a preload/unload which was actuated on a force condition, to reduce slack of the sample and utilize the range of the load cell. The test was found to be very repeatable in the tested range. The results show that the stress relaxation behavior was nonlinear, where the relaxation time was directly proportional to the applied elastic strain. A master curve was generated by shifting reference curves by their temperatures. Overall, this leads to the idea that stress relaxation data can be generated at any initial strain and temperature above the glass transition temperature.