Continuum characterization of flexible foams for medical applications and computational evaluation of pressure profile

Rebecca C Gunn, Purdue University

Abstract

Every year over 1 million hospital patients develop bedsores due to poor mattress support. Bedsores can be avoided with the use of pressure relieving mattress support. This project investigated and characterized the material properties of five types of flexible foam and applied results to a computational evaluation of foam pressure profiles under given loading conditions to find the optimal foam. Characterizing the material properties of flexible foam, a hyperelastic material, was not a straightforward task. Initially, mechanical testing was performed on small samples of each foam to determine compression stress, strain, force, deflection and stress relaxation characteristics. Next, Neo-Hookean, Mooney Rivlin and Ogden Constitutive models were fit to the experimental data to quantify the hyperelastic behavior of each foam. The Neo-Hookean model fit the data for the first 25% nominal strain, while the Ogden model fit the entire range of nominal strain. The Mooney Rivlin model did not correlate well with the experimental data. Then, a finite element model was developed to simulate a person lying on a mattress. While the sacral region is the most common location for pressure sores, a model of a human buttock was included to simulate the pressure profiles. The material properties and loads obtained from experimental testing and constitutive model curve fitting were applied to the simulated foam block. Finally, the finite element model was validated through a series of mechanical tests matching the simulation conditions. An anatomical buttocks form was provided to duplicate the simulation buttocks model. The results showed that the simulations accurately represent the loading conditions up to 25% nominal strain. Additional test conditions were used to compare stress relaxation results with the small sample mechanical tests and demonstrate cyclic loading conditions. The only substantial difference in stress relaxation values was with the memory foam sample. The small sample testing setup may have inhibited the memory foam from giving the proper results. Cyclic loading revealed the relative degrees of nonlinearity for each foam. Overall, this project was successful in characterizing material behavior of five different types of flexible foam, applying constitutive behavior models and integrating into simulation tools for further analysis. A recommendation was made to use the memory foam for medical applications, as it possesses excellent durability, support and pressure relief characteristics.

Degree

M.S.M.E.

Advisors

Nauman, Purdue University.

Subject Area

Mechanical engineering

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