Cyclic Compression Testing of Three Elastomer Types-A Thermoplastic Vulcanizate Elastomer, a Liquid Silicone Rubber and Two Ethylene-Propylene-Diene Rubbers

High-quality 3D printing of ethylene vinyl acetate with direct pellet-based FDM for medical applications: Mechanical analysis, energy absorption and recovery evaluation

Ethylene Vinyl Acetate (EVA) is a biocompatible and non-toxic copolymer known for its applications in medicine, drug delivery systems, and energy absorption. This paper presents a novel method for 3D printing EVA using the Fused Deposition Modeling (FDM) technique. By directly utilizing pellet-form EVA material and employing pneumatic pressure for extrusion, we addressed the issue of filament buckling commonly encountered in traditional filament-based printing. A custom-made pellet printer enabled direct feeding of polymer pellets. The mechanical properties, microstructure of the printed EVA parts was analyzed to evaluate the suitability of the direct pellet printing technique for producing high-quality EVA components. The results showed acceptable printing quality and favorable mechanical properties (about 1000% elongation at break and 6.59 MPa tensile strength). Additionally, a cyclic compression test was conducted, subjecting specimens to various strains (30%,80% and 120%) and assessing their compressive and energy absorption properties undergoing significant deformations. Observations reveal a noticeable decline in the mechanical properties after the first compression cycle. The mechanical properties tend to stabilize and reach a steady state from the second cycle onwards until the sixth cycle. Furthermore, a compression cyclic test with various strain rates conducted, investigated the functional performance of the printed EVA components in engineering and biomedical applications, considering the impact of strain rate. Observations indicate that lower strain rates tend to enhance the material's ability to withstand deformation before yielding. This study contributes to expanding the potential of EVA in additive manufacturing, particularly in biomedical and energy absorption systems.

Evolution of Shore Hardness under Uniaxial Tension/Compression in Body-Temperature Programmable Elastic Shape Memory Hybrids

Body-temperature programmable elastic shape memory hybrids (SMHs) have great potential for the comfortable fitting of wearable devices. Traditionally, shore hardness is commonly used in the characterization of elastic materials. In this paper, the evolution of shore hardness in body-temperature programmable elastic SMHs upon cyclic loading, and during the shape memory cycle, is systematically investigated. Upon cyclic loading, similar to the Mullins effect, significant softening appears, when the applied strain is over a certain value. On the other hand, after programming, in general, the measured hardness increases with increase in programming strain. However, for certain surfaces, the hardness decreases slightly and then increases rapidly. The underlying mechanism for this phenomenon is explained by the formation of micro-gaps between the inclusion and the matrix after programming. After heating, to melt the inclusions, all samples (both cyclically loaded and programmed) largely recover their original hardness.