Jaehyung Ju

Design of Cellular Shear Bands of a Non-Pneumatic Tire -Investigation of Contact Pressure

In an effort to build a shear band of a lunar rover wheel which operates at lunar surface temperatures (40 to 400K), the design of a metallic cellular shear band is suggested. Six representative honeycombs with aluminum alloy (7075-T6) are tailored to have a shear modulus of 6.5MPa which is a shear modulus of an elastomer by changing cell wall thickness, cell angles, cell heights and cell lengths at mesoscale. The designed cellular solids are used for a ring typed shear band of a wheel structure at macro-scale. A structural performance such as contact pressure at the outer layer of the wheel is investigated with the honeycomb shear bands when a vertical force is applied at the center of the wheel. Cellular Materials Theory (CMT) is used to obtain in-plane effective properties of a honeycomb structure at meso-scale. Finite Element Analysis (FEA) with commercial software ABAQUS is employed to investigate the structural behavior of a wheel at macro-scale. A honeycomb shear band designed with a higher negative cell angle provides a lower contact pressure along the contact patch associated with an in-plane shear flexible property.

Design of Honeycomb Meta-Materials for High Shear Flexure

A numerical study for a functional design of honeycomb meta-materials targeting flexible shear properties (about 6.5MPa effective shear modulus and 15% maximum effective shear strain) is conducted with two material selections — polycarbonate (PC) and mild-steel (MS), and five honeycomb configurations. Cell wall thicknesses are found for each material to reach the target shear modulus for available cell heights with five honeycomb configurations. PC honeycomb structures can be tailored with 0.4 to 1.3mm cell wall thicknesses to attain the 6.5MPa shear modulus. MS honeycombs can be built with 0.2mm or lower wall thicknesses to reach the target shear modulus. Sensitivity of wall thickness on effective properties may be a hurdle to overcome when designing metallic honeycombs. The sensitivity appears to be more significant with an increased number of unit cells in the vertical direction. PC auxetic honeycombs having 0.4 to 1.9 mm cell wall thicknesses show 15% maximum effective shear strain without local cell damage. Auxetic honeycombs having negative Poisson’s ratio show lower effective shear moduli and higher maximum effective shear strains than the regular counterparts, implying that auxetic honeycombs are candidate geometries for a shear flexure design.Copyright

Design of Honeycomb Mesostructures for Crushing Energy Absorption

This paper presents the energy absorption properties of hexagonal honeycomb structures of varying cellular geometries under high speed in-plane crushing. While the crushing responses in terms of energy absorption and densification strains have been extensively researched and reported, a gap is identified in the generalization of honeycombs with contr’olled and varying geometric parameters. This paper addresses this gap through a series of finite element (FE) simulations where the cell angle and the inclined wall thickness, are varied while maintaining a constant mass of the honeycomb structure. A randomly filled, nonrepeating design of experiments (DOEs) is generated to determine the effects of these geometric parameters on the output of energy absorbed and a statistical sensitivity analysis is used to determine the parameters significant for the crushing energy absorption of honeycombs. It is found that while an increase in the inclined wall thickness enhances the energy absorption of the structure, increases in either the cell angle or ratio of cell angle to inclined wall thickness have adverse effects on the output. Finally, the optimization results suggest that a cellular geometry with a positive cell angle and a high inclined wall thickness provides for maximum energy absorption, which is verified with a 6% error when compared to a FE simulation. [DOI: 10.1115/1.4006739]

Shear compliant hexagonal meso-structures having high shear strength and high shear strain

A shear layer for a shear band that is used in a tire is provided that has multiple cells or units having an auxetic configuration and that are constructed from aluminum or titanium alloys. The cells may have an angle of −10°.

Design of Chiral Honeycomb Meso-Structures for High Shear Flexure

Chiral honeycombs are auxetic cellular structures that exhibits negative Poison’s ratio. Chiral honeycombs are structures arranged in an array of cylinders connected by ligaments. Four different configurations of these geometries with 4- and 6- ligaments attached are investigated for its use in shear layer of non-pneumatic wheel. The objective of the study is to find an ideal geometry for the shear layer while meeting its requirements of shear properties (about 6.5 MPa effective shear modulus and 0.15 maximum effective shear strain) with polycarbonate as base material. Finite Element (FE) based numerical tests are carried out and optimum chiral meso-structures are found for the target shear properties. Parametric studies on geometries are also conducted to find the effect of geometries on the target properties. The effect of cell wall thickness is studied and the optimum thickness is suggested to meet the target requirements. Effect of direction of shear loading has been studied on each different configuration in order to minimize the effect of direction of loading.Copyright

DESIGN OF SINUSOIDAL AUXETIC STRUCTURES FOR HIGH SHEAR FLEXURE

This paper presents the analytical model to predict the effective in-plane shear modulus G12* for auxetic honeycomb mesostructure with sinusoidal re-entrant wall. Also, a comparative study is conducted on the ability of the sinusoidal mesostructure over auxetic mesostructure for high shear flexure. In an effort to design components with high shear flexure, the re-entrant wall of the auxetic honeycomb is replaced with a sinusoidal wall. Existing analytical models that predict the effective in-plane elastic properties for auxetic honeycomb mesostructure are limited to straight re-entrant wall. In order to predict the effective in plane shear modulus, G12*, for conceptual design study, an analytical model is needed. The principle of energy methods is used to determine the effective in-plane shear modulus and is verified with the solution in ABAQUS. The analytical model is in agreement with the computational model with a 10% maximum error over a wide range of cell wall thickness and shear strain. The two structures are designed to possess the same equivalent shear modulus and the degree of shear flexure is measured computationally in terms of yield shear strain. The sinusoidal structure introduces nonlinearity with increase in cell wall thickness and shear strain. This nonlinearity causes the sinusoidal auxetic mesostructure to have low shear flexure at a high shear modulus which is higher than about 10MPa. However, it is marginally better than auxetic mesostructure at a low shear modulus which is 10MPa and less.

Porous materials with high negative Poisson’s ratios—a mechanism based material design

In an effort to tailor functional materials with customized anisotropic properties—stiffness and yield strain, we propose porous materials consisting of flexible mesostructures designed from the deformation of a re-entrant auxetic honeycomb and compliant mechanisms. Using an analogy between compliant mechanisms and a cellular material’s deformation, we can tailor the in-plane properties of mesostructures; low stiffness and high strain in one direction and high stiffness and low strain in the other direction. An analytical model is developed to obtain the effective moduli and yield strains of the porous materials by combining the kinematics of a rigid link mechanism and deformation of flexure hinges. A numerical technique is implemented with the analytical model for the nonlinear constitutive relations of the mesostructures and their strain-dependent Poisson’s ratios. A finite element analysis (FEA) is used to validate the analytical and numerical models. The designed moduli and yield strain of porous materials with an aluminum alloy are 2 GPa and 0.28% in one direction and 0.2 MPa and 28% in the other direction. These porous materials with mesostructures have high negative Poisson’s ratios, xy down to 82 due to the large rotation of the link member in the transverse direction caused by the input displacement in the longitudinal direction. The porous materials also show higher moduli for compressive loading due to the contact of flexure hinges. This paper demonstrates that compliant mesostructures can be used for next-generation material design in terms of customized mechanical properties; modulus, strength, strain, and Poisson’s ratio. The proposed mesostructures can also be easily manufactured using a conventional cutting method. (Some figures may appear in colour only in the online journal)

Shear Compliant Hexagonal Cellular Solids With a Shape Memory Alloy

In this study, hexagonal honeycombs with a shape memory alloy (SMA) are explored for super-compliant meso-structural design. A nitianol (NiTi) SMA based shear compliant hexagonal cellular materials are introduced and their elastic properties in shear are investigated. The constitutive relation of SMA and Cellular Materials Theory (CMT) are used to develop analytical constitutive equations of SMA honeycombs under isothermal shear loading. A fixed volume based SMA honeycombs are designed with a target shear modulus, (GA*)12 , of 10MPa and minimum uni-axial moduli (E11* and E22*) of 10MPa. About 27 to 70% of elastic shear strains are obtained with NiTi SMA honeycombs when they are designed with a G12* of 10MPa.Copyright

Experimental Damage Characterization of Hexagonal Honeycombs Subjected to In-Plane Shear Loading

Experimental study on the damage of hexagonal honeycombs under in–plane shear loading does not appear to be available in the literature. In this paper, shear damage behaviors of five different hexagonal mesostructures are investigated with rapid prototyped polycarbonate (PC) honeycomb coupon samples and proper design of a fixture for shear loading. Effective shear stress-strain curves of PC honeycomb coupons are generated for each shear test and the corresponding local cell wall failure is investigated. Two different failure modes of PC honeycombs were observed primarily depending on the cell wall thickness: The PC honeycombs having a lower cell wall thickness induce the plastic post buckling, resulting in preventing propagation of initial cracks through the cell wall end up with higher plastic load bearing. On the other hand, the failure mode of the honeycombs having a high cell wall thickness is the cell wall fracture by crack propagation through wall without severe buckling.Copyright

Optimisation of geometry and material properties of a non-pneumatic tyre for reducing rolling resistance

The effect of geometric and material parameters of a non-pneumatic tyre (NPT) on overall performance of the NPT is investigated. Parametric studies, design of experiments (DOE), and sensitivity analyses are conducted with a hyper-viscoelastic finite element model to determine effects of design variables: i) the thickness of spokes; ii) the shear band thickness and iii) shear modulus of polyurethane (PU), on rolling resistance, vertical stiffness, and contact pressure. A response surface model is generated from DOE and is used to find optimum design values for minimising rolling resistance of an NPT under constraints on vertical deflection and contact pressure. Shear modulus of PU and the shear band thickness are the most important design parameters to affect rolling resistance, vertical stiffness, and contact pressure. The optimised values show that the NPT has low rolling resistance with a higher shear modulus of PU and a higher shear band thickness associated with a lower shear deformation while rolling.