Impact Testing of Polymer-filled Auxetics Using Split Hopkinson Pressure Bar (original) (raw)
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Influence of Auxetic Structure Paramaters on Dynamic Impact Energy Absorption
HAL (Le Centre pour la Communication Scientifique Directe), 2021
The present work focuses on the dynamic crushing response of 2D re-entrant auxetic honeycomb by extending previous published models in order to include more design parameters (specific geometrical ratios and/or material properties). If the crushing velocity is constant, the energy absorption occurs at a constant plateau stress up to densification of the structure. An analytical equation based on shock waves propagation analogy in a rigid, perfectly plastic, locking material model is deduced from the study of periodic collapse of the structure. Our analysis enables to theoretically predict the dynamic crushing strength. The formulation depends on the geometric and the material characteristics of the auxetic but also on the impact velocity. Two series of Finite Element simulations of quasi-static and dynamic compressive test of an auxetic structure were carried out using the RADIOSS TM explicit solver. The first simulation series consist of a crushing plate loading the structure at a constant imposed velocity (0.5 m/s up to 100 m/s). Results show good accordance between analytical and Finite Element results. The time history of the cellstrain (ratio of deformed cell height to initial cell height) in the crushing direction shows that the peaks observed in the stress-strain curve of the entire structure are linked to complete crushing of each unit cell within a row. The second simulation series replicates the impact of the plate on the structure (initial plate velocities 50 m/s up to 100 m/s). The numerical simulations presented in this study, make it possible to relate the cell-strain, energy absorption and geometrical/material parameters of the auxetic structure.
2018
Foams are commonly used for cushioning in protective sporting equipment. Volumetrically compressing open-cell polyurethane foam buckles cell ribs creating a re-entrant structure—set by heating then cooling—which can impart auxetic behaviour. Theoretically, auxetic materials improve impact protection by increasing indentation resistance and energy absorption, potentially reducing sporting injuries and burdens on individuals, health services and national economies. In previous work, auxetic foam exhibited ~3 to ~8 times lower peak force (compared to its conventional counterpart) under impacts adopted from tests used to certify protective sporting equipment. Increases to the foam’s density and changes to stress/strain relationships (from fabrication) mean Poisson’s ratio’s contribution to reduced peak forces under impact is unclear. This work presents a simple fabrication method for foam samples with comparable density and linear stress/strain relationship, but different Poisson’s rati...
Validation of a Finite Element Modeling Process for Auxetic Structures under Impact
physica status solidi (b), 2020
Auxetic materials behave unconventionally under deformation, which enhances material properties such as resistance to indentation and energy absorption. Auxetics, therefore, have the potential to enhance sporting protective equipment. Herein, finite element modeling, additive manufacturing and impact testing of three auxetic lattices, and a conventional equivalent, with a view to advance auxetic implementation within sports equipment, are explored. The lattices are modeled and impacts are simulated between 1 and 5 J, for flat and hemispherical drop hammers. Simulation outputs including peak impact force, impact duration, maximum axial strain, and Poisson's ratio are compared with experimental results from equivalent impact energies on additively manufactured lattices, using an instrumented drop tower and a high‐speed camera. The simulation and experimental results show broad agreement for all lattices and scenarios, demonstrated by comparative force versus time plots and maximum...
Impact testing of cellular materials with field measurement-A review
International Journal of Protective …, 2011
This paper presents a study of the strength enhancement under impact loading of metallic cellular materials as well as sandwich panels with cellular core. It begins with a review of likely causes responsible for the strength enhancement of cellular materials. A testing method using 60mm diameter Nylon Hopkinson pressure bars is used to investigate the rate sensitivity of various metallic cellular materials. In order to identify the factor responsible for the strength enhancement of those materials, a multiscale analysis is performed on a model structure which is a square tube made of rate insensitive materials. At the macroscopic level, significant enhancement is experimentally observed under impact loading, whereas the crushing mode is nearly the same under both static and impact loading. Numeric simulations and theoretical models give a satisfactory explanation of the role of the lateral inertia. An inversed perforation test on sandwich panels with an instrumented pressure bar is also presented. Such a new testing setup provides piercing force time history measurement, generally inaccessible. Testing results show a notable enhancement of piercing forces, even though the skin aluminum plates and the foam cores are nearly rate insensitive.
Cellular Structures under Impact Loading
THERMEC 2006, 2007
This paper presents a study of the strength enhancement under impact loading of metallic cellular materials as well as sandwich panels with cellular core. It begins with a review of likely causes responsible for the strength enhancement of cellular materials. A testing method using 60mm diameter Nylon Hopkinson pressure bars is used to investigate the rate sensitivity of various metallic cellular materials. In order to identify the factor responsible for the strength enhancement of those materials, an experimental analysis is performed on a model structure which is a square tube made of rate insensitive materials. Significant enhancement is experimentally observed under impact loading, whereas the crushing mode is nearly the same under both static and impact loading. Finally, an inversed perforation test on sandwich panels with an instrumented pressure bar is also presented. Such a new testing setup provides piercing force time history measurement, generally inaccessible. Testing results show a notable enhancement of piercing forces, even though the skin aluminum plates and the foam cores are nearly rate insensitive.
Effect of Compressive Strain Rate on Auxetic Foam
2021
Auxetic foams have previously been shown to have benefits including higher indentation resistance than their conventional counterparts, due to their negative Poisson’s ratio, making them better at resisting penetration by concentrated loads. The Poisson’s ratio and Young’s modulus of auxetic open cell foams have rarely been measured at the high compressive strain rates typical during impacts of energy absorbing material in sporting protective equipment. Auxetic closed cell foams are less common than their open cell counterparts, and only their quasi-static characteristics have been previously reported. It is, therefore, unclear how the Poisson’s ratio of auxetic foam, and associated benefits such as increased indentation resistance shown at low strain rates, would transfer to the high strain rates expected under impact. The aim of this study was to measure the effect of strain rate on the stiffness and Poisson’s ratio of auxetic and conventional foam. Auxetic open cell and closed ce...
Experimental study of open-cell cellular structures with elastic filler material
Experimental mechanics, 2009
Open-cell cellular structures have a high potential for use in automotive, railway, ship and aerospace industry as crash energy absorbers. This paper focuses on the influence of the second phase filler material as a way to further increase the capability of cellular material energy absorption. The behaviour of ductile (aluminium alloy) and brittle (polymer) cellular structures with regular topology with and without the pore filler (silicon rubber) under quasi-static and dynamic compressive loading conditions has been ...
Dynamic properties of high structural integrity auxetic open cell foam
Smart Materials & Structures, 2004
This paper illustrates various dynamic characteristics of open cell compliant polyurethane foam with auxetic (negative Poisson's ratio) behaviour. The foam is obtained from off-the-shelf open cell polyurethane grey foam with a manufacturing process based on mechanical deformation on a mould in a temperature-controlled oven. The Poisson's ratio is measured with an image processing technique based on edge detection with wavelet methods. Foam samples have been tested in a viscoelastic analyser tensile test machine to determine the Young's modulus and loss factor for small dynamic strains. The same samples have also been tested in an acoustic impedance tube to measure acoustic absorption and specific acoustic resistance and reactance with a transmissibility technique. Another set of tests has been set up on a cam plastometer machine for constant strain rate dynamic crushing analysis. All the tests have been carried out on auxetic and normal foam samples to provide a comparison between the two types of cellular solids. The results from the experimental tests are discussed and interpreted using microstructure models for cellular materials existing in the literature. The negative Poisson's ratio foam presented in this paper shows an overall superiority regarding damping and acoustic properties compared to the original conventional foam. Its dynamic crushing performance is also significantly superior to the normal foam, suggesting a possible use in structural integrity compliant elements.
Analytical solution and finite element approach to the 3D re-entrant structures of auxetic materials
Mechanics of Materials, 2014
Chiral, star honeycomb, and re-entrant structures are among the most important structures of auxetic materials. In this study, a dense re-entrant unit cell is introduced for making a 3D auxetic structure to be used in high stiffness applications. A re-entrant structure is chosen due to its fundamental characteristics underlying the main characteristics of auxetic structures. The energy methods of solid mechanics along with numerical methods are used to study the fundamental concept of auxetic structures. Understanding the characteristics of the re-entrant structure will lead to the better comprehension of other structures of auxetic materials, which will eventually contribute to the advance of research in this new class of materials. 1 Introduction Poisson's ratio is defined as a negative ratio of transverse strain to longitudinal strain in the tension or compression test. It is usually obtained by computing the ratio of lateral contraction and longitudinal extension in a tensile test. It is so named after Simeon Poisson [1]. A Poisson's ratio value of most metals is between 0.25 and 0.3 [2,3]. From the solid mechanics point of view, all materials used in design and manufacturing can be classified into two types: regular materials with positive Poisson's ratio and irregular ones with negative or zero Poisson's ratio, known as auxetic materials. They have been known conceptually since 1944, even though the experimental sample was only available when Lakes [4] presented a material with negative Poisson's ratio in 1987. Such study was the pioneer to introduce an auxetic foam and to produce it by converting standard polyurethane foam into an auxetic one [4-6]. Despite being introduced more than five decades ago, the engineering usage of these materials has been quite limited. Meanwhile, many important mechanical properties such as indentation resistance, energy absorption, fracture toughness, fatigue toughness, and shear modulus are mainly dependent on Poisson's ratio [7]. Due to the fact that negative Poisson's ratio results in an improvement of these properties, auxetic materials have great potential for many applications. For instance, they can be