Session: 03-11-01: Thermoplastic Composites

Paper Number: 137556

137556 - Stamp Forming of Highly Aligned Discontinuous Fiber Thermoplastic Matrix Composite Laminates 

Stamp forming of thermoplastic composite laminates for production of composite airframe structures can achieve excellent consolidation quality with the benefit of rapid (3 minute) cycle times. The major challenge is that such processes have severe limitations on part complexity. Due to the inextensibility of continuous fiber reinforcement, material which is drawn into complex features is prone to wrinkling—which is more prevalent for unidirectional laminates. Highly aligned (unidirectional) discontinuous fiber composite laminates have been shown to retain properties comparable to those of continuous fiber with 55-60% fiber volume fraction and medium aspect ratio (L/d~600) fibers. With the processing benefit of lamina extensibility in the fiber direction, complex geometry can be stretch formed from a flat blank to mitigate the formation of compression wrinkles. To understand how deformation during forming influences material performance and reliability, a study is designed to evaluate formability of a thermoplastic matrix ADF composite with respect to process conditions: temperature, strain rate, and strain mode. Digital image correction was used to measure local surface strain during forming. Unidirectional samples were stretched in the longitudinal fiber direction above the melt temperature using an Instron test frame and environmental. Local strain variability from DIC and the observed strain softening response to elongation were used to define the material formability to optimize the strain rate and temperature process conditions. Multiaxial laminates were fabricated to study the effect of material orientation, laminate configuration, and biaxial strain modes on formability. A customized material forming characterization press is utilized to directly observe the material behavior in controlled process conditions. Blanks are clamped with a binder ring and heated until isothermal at 380C. DIC cameras observe the sample through a window in a view port from above, while a hemispherical Nakajima punch deforms the blank with a controlled displacement rates of 1 to 50 mm/s. Biaxial strain on the outer ply surface is computed for each test. The results establish the effects of process conditions (temperature and strain rate) on peak extensional viscosity, strain softening, and forming limits of the material.

Presenting Author: Thomas Cender University of Delaware

Presenting Author Biography: Dr. Cender is a Scientist with the Center for Composite Materials at University of Delaware. He has over 10 years experience in composites manufacturing process characterization. Currently, he is focused on material characterization and process development for highly aligned discontinuous fiber composites with applications in Automated Fiber Placement and stamp forming processes.