Model reduces stress and warpage in polymer composite structures

August 31, 2000

CHAMPAIGN, Ill. - Fiber-reinforced composites are strong and lightweight, but suffer from hidden stresses that can warp the final product or degrade its performance. Modifying the process variables through trial and error is expensive and time consuming. Now, a model developed at the University of Illinois promises to improve both the quality and reliability of these polymeric parts.

"The warpage of composite structures during the manufacturing process is a direct consequence of residual stress development,² said Philippe Geubelle, a professor of aeronautical and astronautical engineering at the UI. "These stresses arise because of thermal expansion, chemical shrinkage and non-uniform curing. The ability to predict the residual stresses and their effects is crucial to the manufacture of dimensionally accurate composite structures."

Geubelle and his colleagues - professors Charles Tucker and Scott White, and graduate students Qi Zhu, Min Li and Daniel O'Brien - have assembled a model that simulates the heat transfer, pressure, curing and residual stress development that occurs during the manufacturing cycle of thermoset composite parts. "When working with metals, you can carve, bend or stamp the material into the desired shape; but with composites, you actually make the material as you make the part," said Tucker, the W. Grafton and Lillian B. Wilkins Professor of Mechanical and Industrial Engineering at the UI. "The manufacturing process is complicated, with many interacting physical phenomena that can affect the final form. Our model allows us to explore those phenomena and to perform our 'trial and error' on a computer instead of on a factory floor."

Often, improving one processing variable only makes the overall problem worse because another variable that had previously been offset becomes more prominent, Geubelle said. "This points out the need for a thorough and fundamental approach to the issues that control the manufacturing process." By simulating the mechanical effects of process variables, the model allows engineers to predict accurately the final dimensions and residual stresses in polymer-matrix components - including the tendency of parts to change shape and "spring forward" when removed from their molds.

"Combining the simulation with special optimization methods creates a powerful and versatile analytical tool that can help reduce product defects and improve dimensional accuracy," Tucker said. "We can tell the software which parameters we are willing to change, and the program runs multiple simulations to find the best manufacturing solution that satisfies all of our requirements."
Zhu presented the model at the International Congress of Theoretical and Applied Mechanics, held Aug. 27 to Sept. 2 in Chicago. A paper describing the model will appear in the Journal of Composite Materials. Another paper in which the model is used to optimize the curing process will appear in the journal Polymer Composites. Funding was provided by the National Science Foundation.

University of Illinois at Urbana-Champaign

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