Stress relaxation in poly(methyl methacrylate) (PMMA) during large-strain compression testing near the glass transition temperature
Creators:Vogtmann, Dana E.
Advisor:Dupaix, Rebecca B.
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Publisher:The Ohio State University
Series/Report no.:The Ohio State University. Department of Mechanical Engineering Honors Theses; 2009
Poly(methyl methacrylate) (PMMA) is a common polymer with useful applications in micro-scale compression-molding. Better understanding of this polymer’s behavior when held in compression during molding could greatly improve accuracy when imprinting micro-scale features on the material’s surface. Previous work has been done to record the behavior of PMMA under complex loading situations, and from this research a material model has been developed. However, the current model inaccurately predicts how stress in the material will naturally decrease in a sample held in compression at elevated temperatures. This behavior is referred to as “stress relaxation.” The purpose of my work is to help improve the current material model by collecting data on PMMA’s stress relaxation behavior. To do this, I tested about 50 small samples of PMMA within an environmental chamber heated to near the material’s glass transition temperature, Tg, or the temperature at which a rigid polymer transitions to a more rubbery, deformable state. These tests consisted of compressing the samples between two plates at a constant rate and then holding at a constant compression level. During the holding period, I recorded the stress in the material by measuring the force exerted by the sample on the compression plate. Between tests, I varied temperature, compression rate, and the compression level applied during the holding period. I observed differences in the stress relaxation behavior based on changes to each of these variables. I explored these behavior differences further through several data manipulation techniques and through comparisons of the results against a general viscoelastic material stress relaxation model. I have also compared my results with simulations based on the current material model to determine the model’s accuracy, which proved to be low for this behavior. Ultimately, I would like to help improve the model’s simulation capabilities using the data I have collected. When it is able to accurately predict material behavior under complex loading, this material model will be a valuable aid in creating inexpensive and precisely molded PMMA parts with micro-scale features.
National Science Foundation