Mandel and Cryer problems of fluid-saturated foams with negative Poisson’s ratio

Summary. Professor Lakes created foams with negative Poissons ratios in 1987. Since then he has examined interesting and fantastic characteristics of such foams and proposed their realistic and future possible applications. The present study was motivated by our interest in how such a foam behaves for a sudden loading if it is saturated with fluid. To highlight mechanical interactions between elastic deformation of such a foam and diffusion of the saturating fluid, we solved four problems in a unified manner: 2D and 3D, Cryer and Mandel problems. It is found that for all these problems the pore fluid pressure near the sample center climbs up from its initial pressure and then declines and vanishes; its peak is much higher for negative Poissons ratios, that is, much more remarkable Mandel-Cryer effect. Stress components such as tangential stress also show similar behavior, and the tangential stress has tension near the sample surface in spite of compressive loading. It is also found that for the step-like axial compression the Mandel cylinder sample laterally expands immediately after the loading, then shrinks and finally becomes thinner than its original thickness; a possible application is proposed.     

Tensile fatigue of conventional and negative Poisson’s ratio open cell PU foams

In this paper, we present a comparative analysis between the cyclic loading tensile behaviour of conventional and auxetic thermoplastic PU foams. While the two types of foam share the same base material (open cell PU–PE), one batch is transformed into an auxetic one (i.e., negative Poisson’s ratio) using a special manufacturing process involving moulding and exposure to particular temperature profiles to stabilise the transformation of the microstructure. The effect of the stiffness degradation and accumulation of energy dissipation versus the number of cycles are discussed for different loading levels r. The results show that the fatigue behaviour until failure, subjected to cyclic loading depends on the loading levels and occur in three stages. The results obtained shows also that the auxetic foam have enhanced characteristics under static loading and tensile fatigue compared to the conventional parent phase form.     

Young’s modulus and Poisson’s ratio of concrete at high temperatures: Experimental investigations

When exposed to fire, Young’s modulus of concrete degrades. Thus, exact knowledge of temperature-dependent reduction is important to determine the fire-resistance of concrete or composite members. Nevertheless, existing material properties for the Young’s and shear modulus of concrete are linked with some incertitudes.In addition, normative regulations lack information on the temperature-dependent Poisson’s ratio. In an attempt to overcome some of the existing uncertainties, experimental work was conducted to investigate elastic material properties of fire-exposed concrete. For this purpose, the Impulse Excitation Technique was used as an innovative testing technique. Based on experimental results, the authors propose new elastic material formulations for fire-exposed concrete.     

Role of Poisson’s ratio mismatch on the crack path in glass matrix particulate composites

The hydrogen-related fracture propagation process in martensitic steel was investigated through crystallographic orientation and fracture surface topography analyses. The hydrogen-related fracture surface consisted of three typical surfaces, namely smooth surfaces, surfaces with serrated markings, and surfaces with dimples. Crystallographic orientation analysis suggested that the smooth surface was generated by intergranular fracture at prior austenite grain boundaries, and the surface with serrated markings originated from quasi-cleavage fracture propagated along \(\{011\}\) planes. According to the reconstructed fracture propagation process by fracture surface topography analysis, the intergranular fracture at prior austenite grain boundaries initiated and propagated suddenly at the early stages of fracture. The quasi-cleavage fracture along \(\{011\}\) planes then gradually propagated within the prior austenite grains. At the final stages of fracture, ductile fracture accompanied by dimples occurred around the edge of the specimen. The results clearly indicated that the fracture propagation path changed with the proceeding fracture from the prior austenite grain boundaries to along \(\{011\}\) planes within the prior austenite grains.     

Definition of diagonal Poisson’s ratio and elastic modulus for infill masonry walls

The prediction of the response of infilled frames through the simplified approach of substituting the infill with an equivalent pin-jointed strut is treated. In this framework the results of an experimental study for the mechanical characterization of different types of masonry infills having the aim of estimating strength, Young modulus and Poisson’s ratio are presented. Four types of masonry were investigated and subjected to ordinary compressive tests orthogonally to the mortar beds and along the directions of the mortar beds. The experimental campaign confirmed the possibility of using an orthotropic plate model for prediction of the Poisson’s ratio and Young modulus along the diagonal direction of infills (these parameters are requested by a model already known in the literature for the identification of struts equivalent to masonry infills). The experimental campaign made it possible to recognise a correlation between the Poisson’s ratios and the strengths of masonries investigated along the orthotropic axes and to obtain the diagonal Poisson’s ratio without specific experimental tests. Finally, the experimental responses of some infilled frames were used to test the reliability of the model proposed here.     

Prediction of Young’s modulus and Poisson ratio for foamed metals using octahedron model

Abstract

The present paper deals with the mathematical–physical expression of Young’s modulus and Poisson ratio of foamed metals. As it is known that, Young’s modulus and Poisson ratio are two basic mechanical parameters of engineering materials. Foamed metal is a class of excellent engineering materials with dual attributes of structural and functional characteristics; therefore, these two parameters are investigated for these materials, and the relevant mathematical–physical expressions are derived from the ‘octahedron model’ of porous materials in the present paper. The results show that the apparent Young’s modulus displays a quite complicated mathematical relationship to porosity of the porous body, and the apparent Poisson ratio is just a characteristic of the material constant almost not relative to porosity of the foamed metal.     

Evaluation of Poisson’s ratio for thin soft material by multiplex procedure of spherical indentation mechanics

International Journal of Mechanics and Materials in Design - The Poisson’s ratio plays a significant role in material science and manufacturing, and it should be effectively identified for...     

A novel butterfly-shaped auxetic structure with negative Poisson’s ratio and enhanced stiffness

Journal of Materials Science - Based on the butterfly pattern structure and the star-shaped honeycomb structure, a novel auxetic butterfly-shaped honeycomb structure (BSH) was constructed, which...     

Real time synchrotron X-ray observations of solidification in hypoeutectic Al–Si alloys

This paper demonstrates how recent advances in synchrotron technology have allowed for the real-time X-ray imaging of solidification in Al–Si alloys, despite the small difference in atomic number of these elements. The experiments performed at the SPring-8 synchrotron, involved imaging the solidification of Al–1wt.%Si and Al–4wt.%Si alloys under a low-temperature gradient and a cooling rate of around 0.3 °C/s. The nucleation and growth of the primary aluminum grains as well as the onset of eutectic solidification were clearly observed. In the alloys containing Al–4wt.%Si, contrast was sufficient to characterize the nucleation rate and growth velocity of the aluminum grains. The importance of improving observation of solidification in the Al–Si system by increasing the time resolution during critical events is discussed.     

Thermoreversible mucoadhesive in situ nasal gel for treatment of Parkinson’s disease

Abstract

Parkinson’s disease is a degenerative disorder of the central nervous system (CNS). The most obvious symptoms are movement-related such as shaking, rigidity, slowness of movement and difficulty with walking, rigid muscular movements and difficulty in chewing and swallowing especially solid dosage forms. Ropinirole is an anti-Parkinson drug that has low oral bioavailability which is primarily due to first-pass metabolism. The objective of proposed work was to increase bioavailability of ropinirole and avoid patient discomfort by formulating thermoreversible in situ nasal gel. Thermoreversible nasal gels were prepared by cold method using Pluronic F-127 and hydroxy methyl propyl cellulose (HPMC K4M) as gelling agents. Formulations were evaluated for various parameters such as drug content, pH, gelling time, gelling temperature, gel strength, mucoadhesive force, ex vivo diffusion, histological studies and in vivo bioavailability. Formulations displayed gelation at nasal temperature and the gelation time was found to be less than mucociliary clearance time. The nasal residence time was seen to be increased due to mucoadhesion and increased gel strength. The nasal gel formulations showed ex vivo drug release between 56–100% in 5?h. Histological study of sheep nasal mucosa revealed that the gel had a protective effect on the mucosa unlike plain ropinirole which showed evidence of moderate cellular damage. A fivefold increase in bioavailability in brain was observed on nasal administration as compared to IV route. Thermoreversible in situ nasal gel was found to a promising drug delivery for Parkinsonian patients.     

Design of auxetic sandwich panel faceplates comprising cellular networks with high stiffness and negative Poisson’s ratio

In the present work, a novel design is presented for producing auxetic laminated faceplates for structures that contain auxetic cores in order to produce fully auxetic sandwich structures. The design is based on the use of periodic cellular networks that are embedded in a fibre-reinforced polymer matrix. These networks have a high in-plane negative Poisson’s ratio and a high in-plane stiffness. Two auxetic networks were chosen for this purpose: a classical re-entrant hexagonal network and an anti-tetra chiral auxetic network. The finite elements method was used to model the auxetic network and the faceplate. For the auxetic network, the relative modulus (E_s/E_m) was investigated to determine its effect on the behaviour of the faceplate. The auxetic behaviour of the faceplate occurs when the auxetic network has a high relative modulus. For example, for a classical re-entrant hexagonal network faceplate, the auxetic behaviour starts to appear when E_s/E_m > 33, while for an anti-tetra chiral auxetic network faceplate, the auxetic behaviour starts to appear when E_s/E_m > 24. Analytical expressions for the elastic constants of the faceplates were developed using the representative volume element model (RVE) and a semi-empirical formula of the rule of mixtures (ROMs). The results of the analytical expressions were compared with the finite elements results for various values of the relative density parameter ρ^*. The relative density had a significant effect on the elastic constants of the faceplate. The model produced with the representative volume element method had higher values for the elastic constants than did that created with the finite elements model, but the semi-empirical rule of mixtures gave more accurate results for both types of faceplates. A modified design for the classical re-entrant hexagonal network that increases the in-plane stiffness of the auxetic network is presented since it has been found that it plays a major role in producing a high negative Poisson’s ratio. The modified networks are an oval re-entered and a stiffened re-entered network. Experiments were carried out on the oval re-entered faceplates to obtain the in-plane moduli and the Poisson’s ratios. The tested faceplate samples clearly showed auxetic behaviour. A good agreement between the finite elements results and the experimental data was obtained.     

Development of an image analysis technique for measurement of Poisson’s ratio for viscoelastic materials: application to leather

The application of a computerised image analysis system to simultaneously measure longitudinal and axial strain in naturally available anisotropic-viscoelastic material like leather is described. The results are used to calculate Poisson’s ratio and indicate that this parameter varies with position and orientation on the hide and depends on the degree of pre-existing fibre orientation. The origins of this dependency can be explained by a simple microstructural model. The practical implication of the findings for detecting lines of tightness (maximum fibre’s orientation) is discussed.     

Study of antisite defects in hydrothermally prepared LiFePO₄ by in situ X-ray diffraction

Hydrothermal synthesis has proven to be a cost-effective, energy-efficient approach for the manufacture of lithium iron phosphate (LiFePO?) and its related materials. However, hydrothermally prepared LiFePO? typically suffers from antisite defects, where some of the iron resides on lithium sites and restricts lithium-ion mobility. A post-heat-treatment temperature of around 700 °C is generally used to eliminate cation disorder, but little is known about these antisite defects or their concentration as a function of the post-heat-treatment temperature. In this study, time-resolved, synchrotron X-ray diffraction reveals that antisite defects are completely eliminated above 500 °C, suggesting that the electrochemical performance may be significantly enhanced by a milder postsynthesis heat treatment. The preliminary electrochemical results show a significant enhancement in the electrochemical capacity with the defect-free material, with the specific capacity increasing by approximately 60% at a C/20 rate.     

Correlation between ultrasonic shear wave velocity and Poisson’s ratio for isotropic porous materials

A new correlation between ultrasonic shear wave velocity and Poisson’s ratio has been established for isotropic porous material based on physical acoustic theory. Poisson’s ratio may decrease, increase or remain unchanged with decrease in shear wave velocity depending on pore-shape and Poisson’s ratio of the bulk solid. In case of decreasing Poisson’s ratio with decreasing shear wave velocity, it passes through a minimum and then increases again to reach a limiting value of 0.5. It has been further demonstrated that the Poisson’s ratio versus porosity relation deduced from the proposed correlation agrees with the experimental data extremely well.

---

A mixed integer programming approach to designing periodic frame structures with negative Poisson’s ratio

Materials and microstructures with specific configurations are able to have negative Poisson’s ratio. This paper proposes a topology optimization methodology of frame structures to design a planar periodic structure that exhibits negative Poisson’s ratio. Provided that beam section of each existing member is chosen from a set of some given candidates, we can reduce the topology optimization problem to a mixed-integer linear programming problem. Since the proposed approach treats frame structures and stress constraints are rigorously addressed, the optimal solution contains no hinge region. A heuristic method with local search is used to solve large-scale problems. Numerical examples and fabrication test demonstrate that planar periodic frame structures exhibiting negative Poisson’s ratio can be successfully obtained by the proposed method.