Experimental and Numerical Study of Ductile Metal Auxetic Tubular Structures
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Methods to mitigate the risk posed by seismic and blast loads to structures are of high interest to researchers. Auxetic structures are a new class of metamaterials that exhibit counterintuitive negative Poisson's ratio (NPR) behavior based on their geometric configuration. Cellular auxetics are light-weight and cost-effective materials that have the potential to demonstrate high strength and resilience under axial forces. Existing research on metallic auxetics is scarce and based mostly on analytical studies. Apparent NPR behavior of auxetics has also been linked to enhanced energy absorbing potential. A pilot study was undertaken to investigate and understand auxetic behavior in tubes constructed using ductile metals commonly found in structural applications i.e. steel and aluminum. The main objective was to establish whether performance enhancements could be obtained through auxetic behavior in ductile metal tubes. In addition, any potential benefits to auxetic performance due to base material plasticity were studied. These objectives were fulfilled by conducting an experimental and analytical investigation, the results of which are presented in this thesis. The experimental program consisted of establishing a design methodology, manufacturing, and laboratory testing for tubular metallic specimens. A total of eight specimens were designed and manufactured comprising five steel and three aluminum. For each base metal, three different geometric configurations of cells were designed: one with a rectangular array of circular voids and two with void geometries based on the collapsed shape of circular cells in a design tube under uniaxial compressive stress. A parameter called the Deformation Ratio (DR) was introduced to quantify cell geometry. Designed tubes were manufactured via a six-axis laser cutting process. A custom-made test assembly was constructed and specimens were tested under reverse-cyclic uniaxial loading, with one exception. Digital Image Correlation (DIC) was used to acquire experimental strain data. The performance of the auxetic and non-auxetic tubular structures was evaluated based on the axial load-deformation characteristics, global deformations, and the specific energy absorption of the test specimens. The experimental test results confirmed that ductile metal tubes with special collapsed cell geometries were capable of demonstrating auxetic behavior under the applied elastic and inelastic uniaxial strains; both tensile and compressive. Base material plasticity was observed to have an insignificant effect on the auxetic response. Experimental results suggested that the unique deformation mechanism precipitated by the auxetic cell geometries resulted in more stable deformed shapes. Stability in global deformed shapes was observed to increase with an increase in DR value. In addition, the unique auxetic mechanism demonstrated an ability to distribute radial plastic strains uniformly over the height of the auxetic pattern. As a result, plastic strains were experienced by a greater fraction of auxetic tubes; this enhanced the energy-dissipating properties of auxetic specimens in comparison to the tested non-auxetic tubes. Tubes with cell geometries associated with higher DR values exhibited greater energy absorption relative to the non-auxetic specimen. For the same base metal, auxetic specimens exhibited greater axial strength and effective strain range, when compared to their non-auxetic counterparts. The increased strength was partially attributed to the increased cell wall thickness of the auxetic specimens. However, the increased strain range was attributed to the rotation in unit cells induced by the unique auxetic geometry. Experimental test data was used to validate the finite element (FE) and simplified macromechanical modeling approaches. These methods were adopted to develop design tools capable of replicating material performance and behavior as well as accurately predicting failure loads. Load-deformation response and effective Poisson's ratio behavior was established using FE models of as-built specimens, while simplified macromechanical equations were derived based on the equilibrium of forces to compute failure loads in tension. These equations relied on pattern geometry and measured experimental unit cell deformations. It was established that the manufacturing process had a detrimental effect on the properties of the aluminum specimens. Accordingly, empirical modifications were applied to the aluminum material model to capture this effect. FE models accurately replicated load-deformation behavior for both non-auxetic and auxetic specimens. Hence, the FE modeling approach was shown to be an effective tool for predicting material properties and response in ductile metal tubes without the need for experimental testing. The simplified strength equations also described material failure with reasonable accuracy, supporting their implementation as effective design tools to gauge tube strength. It is recommended that FE models be refined further through the addition of failure criteria and damage accumulation in material models. The result of this study established that auxetic behavior could be induced in ductile metal tubes through the introduction of unique cell geometry, thereby making them highly tunable and capable of exhibiting variable mechanical properties. Owing to their deformation mechanism and NPR behavior, auxetic tubes demonstrated geometric stability at greater deformations, which highlighted their potential for use as structural elements in systems designed to deform while bearing extreme loads e.g earthquakes and blast events. Additionally, the capability of auxetic geometries to distribute strains uniformly along their length was linked to the potential development of energy-dissipating structural components. It was suggested that new knowledge acquired in this study about auxetic behavior in ductile metals could support the development of new structural systems or methods of structural control based on NPR behavior. Finally, recommendations for future research were presented, based on the expansion of research to study the effects of multiple loading regimes and parametric changes on auxeticity as well as additional mechanical characteristics e.g shear resistance.
General Audience Abstract
Special structures known as Auxetics have been studied that exhibit counterintuitive behavior based on their geometric configuration. The novel shapes and architecture of these structures allow them to deform such that they expand laterally in tension and contract laterally in compression; a property known as negative Poisson's ratio (NPR) which is rarely observed in naturally-occurring materials. Auxetic materials demonstrate mechanical properties such as high resilience, indentation resistance, and energy-absorption. An experimental and analytical study was undertaken to explore the beneficial properties of auxetic behavior, along with the effect of inelastic deformations in ductile metal auxetics. To this end, tubular test specimens, made with steel and aluminum, were designed and manufactured. To achieve auxetic behavior, a unique array of collapsed cells was cut out from metal tubes using a laser cutting process. Subsequently, specimens were tested in the laboratory under cyclic and monotonic loads. Experimental results indicate that tubes with auxetic geometries exhibited NPR behavior and a unique deformation mechanism based on the rotation of the unit cells. Owing to this mechanism, auxetic specimens possessed greater geometric stability under applied axial deformations, when compared to the tested non-auxetic specimens. The deformation mechanism was also responsible for a uniform distribution of strains along the length of the auxetic geometry which was linked to relatively better energy absorbing capacity than the non-auxetic tubes. Developed finite element (FE) models captured the response and behavior of all specimens with good accuracy. Derived simplified strength equations were also able to calculate the ultimate tensile failure loads for all specimens accurately. Both numerical methods demonstrated the potential to be utilized as design and evaluation tools for predicting material properties. Finally, recommendations to expand research, based on metal auxetic structures, were presented to further our understanding of auxetic behavior in ductile metals and to explore its benefits under varying loading regimes. Results from this research can be used to support the design of new structural systems or methods to control existing structures by exploiting NPR properties of ductile metal auxetics. Furthermore, energy-dissipating properties of metal auxetic materials may prove to be beneficial for structural applications under extreme loading conditions such as earthquakes and blasts.
- Masters Theses