Energy dissipation and stiffness of polymeric matrix composites with negative stiffness inclusions
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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.