Comparing Characterization Methods for Investigating High-Temperature Bending Behavior of Melted Thermoplastic Prepreg Using DMA and Rheometer

Abstract

With the growing popularity of thermoplastic composites, controlling and optimizing processing methods, such as in-situ Automated Fiber Placement (AFP) consolidation or continuous compression molding (CCM), has become increasingly important. These methods involve melting and deforming materials at extreme conditions, followed by consolidation and autohesion. To control these processes, the thermo-mechanical behavior of viscoplastic materials at high temperatures, such as bending response, must be well understood. This understanding enables the use of process simulation to explore optimization of processing parameters. In this work, a new characterization technique for examining the bending behavior of thin-ply thermoplastic prepreg using Dynamic Mechanical Analysis (DMA) is developed and compared with a test method that uses a rheometer and was previously developed by Convergent. Toray TC1225 LM PAEK thermoplastic prepreg was used in both methods to study the bending-relaxation behavior of the material at different temperatures and loading rates. A standard 3-point bend fixture and a custom-built 4-point bend fixture were used for DMA testing, while a custom-built pure bending fixture was used for rheometer testing. Although the initial testing with the 3-point bend fixture was promising, limitations were found due to the variable bending moment across the beam and highest bending under the loading contact point, leading to micro-buckling and formation of plastic hinges in early deformations. This was mitigated by employing a custom-built 4-point bending fixture, which enabled pure bending in a larger section of the beam. For data reduction and analysis of viscoplastic moment-curvature of the prepreg, a combination of pseudo-viscoelastic constitutive analysis and/or additional deformation measurement using a laser profiler were used. In comparison, the custom-built pure bending fixture in the rheometer was capable of directly measuring the moment-curvature at high temperatures for larger specimens to higher levels of curvature. It was shown that the 4-point bending method is an approximation as the new fixture cannot apply pure beam bending. Nevertheless, the results were found to be reliable and repeatable. Given the availability of DMA or rheometer, and the limitations of different test methods, the results of this study can be used as a guideline to characterize the bending behavior of thermoplastic composites.