Browsing by Subject "Coextrusion"
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Item Coextrusion : a feasible method to manufacture negative stiffness inclusions(2013-08) Hook, Daniel Taylor; Kovar, DesiderioThis work demonstrates the effectiveness of coextrusion as a method to manufacture negative stiffness inclusions for use in vibrational damping applications. The theory and mechanics of negative stiffness and coextrusion are introduced and the process of creating and extruding a feed rod with negative stiffness architecture explained. Coextrusion is shown to be a viable method to create negative stiffness inclusionsItem Energy dissipation and stiffness of polymeric matrix composites with negative stiffness inclusions(2016-08) Cortes, Sergio Andres; Kovar, Desiderio; Seepersad, Carolyn C.; Haberman, Michael R.; Bourell, David L.Typical structural materials have high stiffness to support a static load but offer low damping capacity. These materials easily transmit vibrations that can propagate through the structure, inducing fatigue and premature failure. Thus, structural materials with enhanced damping would increase the operating life of the structure and improve its performance. Here, we study a new class of metamaterials that exhibits simultaneously high damping and stiffness through the use of negative stiffness structures (NSS) embedded into a polymer matrix. Traditional materials have positive stiffness behavior, meaning that the stress increases monotonically with the strain. Similarly, structures made from traditional materials exhibit a positive stiffness, so that the load increases monotonically with displacement applied. NSS structures, however, exhibit a region of negative slope in the force versus displacement response. It has been predicted that the incorporation of these mechanically activated NSS into a polymer matrix would improve the damping behavior, but this has not previously been demonstrated experimentally. A significant part of this work was aimed at determining the geometry of the NSS and the material properties of the NSS and matrix required to achieve high damping. Thus several combinations of NSS geometries, matrix stiffnesses and NSS properties were considered. Analytical and numerical models were developed to guide the design of specimens. Experiments were aimed at producing specimens where damping performance was measured for NSS embedded in a polymer matrix. To conduct these experiments, macro-scale NSS were produced from stainless steel 17-4PH and the properties of the NSS and the NSS embedded in matrices were measured. Results showed that both the design of the NSS and the ratio of the stiffness of the NSS to that of the matrix are important for producing composites that offer simultaneously high damping capacity and high stiffness. Another key challenge is producing NSS at a fine enough scale so that they can be incorporated into a polymer matrix to produce a composite damping material. Amongst potential manufacturing techniques, the multi-filament co-extrusion (MFCX) was selected because it has the potential to produce ceramic, metal or polymer micro-configured geometries in large quantities, quickly and at low cost. This process uses combinations of ceramic-polymer or metal-polymer compounds to reduce an initially macroscopic structure to the microscale while preserving the geometry of the cross-section. When the viscosities of the compounds are ideally matched, co-extrusion is capable of reducing the cross-section by a factor of up to 1000 times (e.g. well into the microscale). However, extensive characterization of the rheology of the compounds is required to achieve very large reductions for complex cross-section such as these. Preliminary results with co-extruded materials were presented to demonstrate the feasibility of this approach.Item Simulation and analysis of the multiphase flow and stability of co-extruded layered polymeric films(2011-08) Chabert, Erwan; Bonnecaze, R. T. (Roger T.); Paul, Don R.The flow and stability of co-extruded layers of different polymers in a forced assembly process is studied computationally to determine the extent of the stable process window and the types of instabilities that occur. Recent advances in layer-multiplying co-extrusion of incompatible polymers have made possible the fabrication of multilayered nanostructures with improved barrier, thermal and mechanical behavior. However, existing layering techniques are very sensitive to mismatches in viscosity and elasticity of the co-extruded polymers which often give rise to layer non-uniformity and flow instabilities, such as encapsulation. Simulations of the flows inside the feedblock and the successive multiplier dies of the multi-layering system are used to track the interface and predict instabilities and degrees of encapsulation as a function of process parameters, primarily the flow rates and rheology of the polymers. Encapsulation is found to be negligible in practice in the feedblock even for large viscosity contrasts and differences in elasticity between the two co-extruded polymers. Encapsulation or pinch-off of interfaces is more severe in the multiplier dies when there the rheologies of the polymers differ. A secondary flow due to the second normal stress differences for non-Newtonian fluids is primarily responsible for the encapsulation. A new multiplier design is proposed and simulated. The pressure drop in the proposed design is half that of the current design, which is useful for extruding highly elastic materials. Further, the degree of encapsulation is also reduced. The results of the simulations are validated with experimental measurements of pressure drop and flow visualization provided by research collaborators.