Rubber-like Materials Research Articles

The Valanis–Landel strain energy function Elasticity of incompressible and compressible rubber-like materials

The paper revisits the original article by Valanis and Landel on the strain energy density function W = w(λ1) + w(λ2) + w(λ3), where λi are the stretch ratios. The main new results and findings are: (i) An extension of the above algebraic form of W to compressible rubber-like materials (ii) Proof that the Cauchy-Green stress tensor t, as a function of the Left Stretch tensor V, has the same analytical form as the function λw′(λ), within a hydrostatic pressure. (iii) The determination of the form of the constitutive equation t(V) from a single experiment, the plane strain test. (iv) Comparison with experimental data of several empirical algebraic forms of w′(λ) in the literature. (v) Findings of a study of the Mooney-Rivlin and Ogden models of the Valanis- Landel function, in axial compression.

ASSESSMENT OF A NEW ISOTROPIC HYPERELASTIC CONSTITUTIVE MODEL FOR A RANGE OF RUBBERLIKE MATERIALS AND DEFORMATIONS

ABSTRACT The choice of an appropriate strain energy function W is key to accurate modeling and computational finite element analysis of the mechanical behavior of unfilled non-crystalizing rubberlike materials. Despite the existing variety of models, finding a suitable model that can capture many deformation modes of a rubber specimen with a single set of parameter values and satisfy the a priori mathematical and structural requirements remains a formidable task. Previous work proposed a new generalized neo-Hookean W (I1) function (doi: 10.1016/j.ijnonlinmec.2020.103626), showing a promising fitting capability and enjoying a structural basis. In this work we use two extended forms of that model which include an I2 term adjunct, W(I1,I2), for application to various boundary value problems commonly encountered in rubber mechanics applications. Specifically, two functional forms of the I2 invariant are considered: a linear function and a logarithmic function. The boundary value problems of interest include the in-plane uniaxial, equi-biaxial, and pure shear deformations and simple shear, inflation, and nonhomogeneous deformations such as torsion. By simultaneous fitting of each model to various deformation modes of rubber specimens, it is demonstrated that a single set of model parameter values favorably captures the mechanical response for all the considered deformations of each specimen. It is further shown that the model with a logarithmic I2 function provides better fits than the linear function. Given the functional simplicity of the considered W (I1, I2) models, the low number of model parameters (three in total), the structurally motivated bases of the models, and their capability to capture the mechanical response for various deformations of rubber specimens, the considered models are recommended as a powerful tool for practical applications and analysis of rubber elasticity.

A Generalized Strain Energy-Based Homogenization Method for 2-D and 3-D Cellular Materials with and without Periodicity Constraints

A generalized strain energy-based homogenization method for 2-D and 3-D cellular materials with and without periodicity constraints is proposed using Hill’s Lemma and the matrix method for spatial frames. In this new approach, the equilibrium equations are enforced at all boundary and interior nodes and each interior node is allowed to translate and rotate freely, which differ from existing methods where the equilibrium conditions are imposed only at the boundary nodes. The newly formulated homogenization method can be applied to cellular materials with or without symmetry. To illustrate the new method, four examples are studied: two for a 2-D cellular material and two for a 3-D pentamode metamaterial, with and without periodic constraints in each group. For the 2-D cellular material, an asymmetric microstructure with or without periodicity constraints is analyzed, and closed-form expressions of the effective stiffness components are obtained in both cases. For the 3-D pentamode metamaterial, a primitive diamond-shaped unit cell with or without periodicity constraints is considered. In each of these 3-D cases, two different representative cells in two orientations are examined. The homogenization analysis reveals that the pentamode metamaterial exhibits the cubic symmetry based on one representative cell, with the effective Poisson’s ratio v¯ being nearly 0.5. Moreover, it is revealed that the pentamode metamaterial with the cubic symmetry can be tailored to be a rubber-like material (with v¯ ≅0.5) or an auxetic material (with v¯ < 0).

Modelling adhesively-bonded T-joints by a meshless method

Bonding of non-parallel substrates has many applications in the transport industry. Adhesively-bonded T-joints are employed for that purpose. However, geometrical dimensions, substrates’ shape and material choices are broad. The effect that varying upper substrate thickness has on the joint strength (P max ) was investigated in this work through numerical analyses. The numerical analysis was performed using a meshless method, the natural neighbour radial point interpolation method (NNRPIM), which has been proven accurate and robust in another adhesive joint configuration. Materials were considered elastic-plastic. A yield criterion developed for rubber-like materials, the Exponent Drucker-Prager, was used for the adhesive layer, while the metallic substrates were analysed with the von Mises yield criterion. P max was determined numerically using a strain-based continuum mechanics failure criterion. Normalised peel and shear stress distributions along the bond-line are presented. Effective strains, both elastic and plastic, were also obtained. The estimated P max was compared with experimental data; a good agreement was found. The stress distribution along the bond line becomes asymmetric in joints with unbalanced substrate thicknesses. At P max , from 10 to 25% of the bond-line has entered into plastic regime. The results indicate that the proposed methodology is suitable to analyse adhesively-bonded joints under different load solicitations.

Energy based fracture initiation criterion for strain-crystallizing rubber-like materials with pre-existing cracks

Fracture prediction is indispensable for polymers, like rubbers, which have a broad range of applications mainly due to their high extensibility. The phenomenon known as strain-induced crystallization further contributes to the fracture toughness of certain rubbers. In this study, a criterion based on internal bond energy, incorporating the effects of crystallization, is proposed to predict fracture initiation in rubber-like materials with pre-existing cracks. First, a multi-scale mechanical model is developed for characterizing the behavior of rubber when subjected to both uniaxial and biaxial deformation states. At the microscale, both the amorphous and crystalline chain segments are modeled as elastic in order to consider the energy contribution by the molecular bond distortions. This internal energy is considered along with the entropic and crystalline free energy for each chain. In the chain model, the effects of loading condition and the relative orientation of a chain on its crystallinity are taken into account. At the macroscopic scale, an existing crystallinity distribution function is adapted and a mixed finite element formulation with an augmented Lagrangian multiplier is utilized to impose the incompressibility constraint. A non-affine maximal advance path constraint based homogenization model is utilized for bridging the two scales. Its potential to account for anisotropy in the stretched network compels the model to be preferable due to its physical significance, for the purpose of fracture modeling. The rigidity of the crystallites is accounted for by proposing a crystallite distortion energy in addition to the critical bond dissociation energy, for fracture initiation to occur. The model is validated by comparison with existing experimental results for both crystallizing and non-crystallizing rubbers. In addition to its potential to predict the material behavior when subjected to uniaxial and biaxial loading, the capability of the model to quantitatively estimate the effect of crystallization on fracture initiation is also verified.

Severe Complications Following Maxillary Sinus Augmentation Using Poly L-lactide-co-ε-caprolactone-Coated Bovine Bone: A Retrospective Study.

To describe the postoperative complications following lateral wall sinus augmentation using (poly L-lactideco-ε-caprolactone; PLCL) and natural polysaccharides polymers-coated bovine bone (PBB). The secondary aims were to examine histologic findings and to propose complication management alternatives. This retrospective study included 61 subjects who underwent 67 lateral wall sinus augmentation procedures using PBB in the standard protocol. In cases that presented complications, treatment included additional antibiotic therapy, implant removal, or sinus reentry and total removal of the grafting material. In three cases, biopsy specimens were taken from the sinuses, and histologic analyses were performed. The prevalence of postoperative complications was 32.8% (22 of 67 cases) in 18 of the patients (29.5%). The most prevalent symptoms were persistent pain (68.2%), swelling (63.6%), and oroantral fistula (54.5%). Radiographic signs appeared in 45.5% of the complications. A total of 24 implants failed; thus, an overall 80.3% survival rate was established at 19 months. The vast majority of complications (86.4%) were treated eventually with reentry surgery and revealed that the sinus was full with granulation tissue surrounding pieces of a nonossified rubber-like material. In cases where implants were placed, nonosseointegrated implants were surrounded by soft tissue. The sinus was cleaned thoroughly; the graft material remnants were removed together with inflamed parts of the sinus membrane, followed by chlorhexidine and saline lavages. In the biopsy specimens taken from the sinus cavity, there were no histologic features of new bone formation around the grafted material. Lateral wall maxillary sinus augmentation using PBB was associated with an acute sinus infection histologic appearance and with a 7-times-higher failure rate compared with previous reports. This serious adverse event suggests that PBB cannot be recommended for maxillary sinus augmentations.

Three-dimensional printed model of impacted third molar for surgical extraction training.

Extraction of impacted mandibular third molars is one of the most common surgical procedures performed at dental clinics; however effective training models for teaching oral surgery to dental students are limited. This study aimed to use three-dimension (3D) printing technology to develop an effective training model forimpacted third molar extraction. The data for the 3D model were digitally processed using high-resolution computed tomography, and two common, but different patterns of impacted third molars were simulated using computer-aided design. Thereafter, the model was printed using the 3D-printing technology, and the efficiency of the 3D-printed model and an animal model (pig mandible) were compared using a five-point Likert scale by 35 oral surgeons in the oral surgery department and 208 students of stomatology in the internship stage. The 3D-printed model consisted of three parts: a non-replaceable part (i.e., the body of the mandible and the teeth from the left first molar to the right first molar) and two replaceable parts (i.e., the part of the ascending ramus of the mandible, as well as the second and third molars). It was covered with a layer of rubber-like material to simulate the gingiva. For the comparison between the 3D-printed and animal models, a total of 205 questionnaires were collected. Both oral surgeons and students agreed that the 3D-printed model was better than the animal model in terms of total value and the anatomy of the bone and teeth, simulating the surgical procedure (p<0.05), while the two models achieved similar results for haptic feedback of the soft tissue (p>0.05). The 3D-printed model is realistic and effective for learning impacted third molar extraction and received positive feedback from students and oral surgeons. This model can significantly improve the pre-clinical skill training of dental students.

An experimental method for estimating the tearing energy in rubber-like materials using the true stored energy

A method for determining the critical tearing energy in rubber-like materials is proposed. In this method, the energy required for crack propagation in a rubber-like material is determined by the change of recovered elastic energy which is obtained by deducting the dissipated energy due to different inelastic processes from the total strain energy applied to the system. Hence, the classical method proposed by Rivlin and Thomas using the pure shear tear test is modified using the actual stored elastic energy. The total dissipated energy is evaluated using cyclic pure shear and simple shear dynamic experiments at the critical stretch level. To accurately estimate the total dissipated energy, the unloading rate is determined from the time the crack takes to grow an increment. A carbon-black-filled natural rubber is examined in this study. In cyclic pure shear experiment, the specimens were cyclically loaded under quasi-static loading rate of 0.01~{rm {s}}^{-1} and for different unloading rates, i.e. 0.01, 0.1 and 1.0~{rm {s}}^{-1}. The simple shear dynamic experiment is used to obtain the total dissipated energy at higher frequencies, i.e. 0.5-18~{rm {Hz}} which corresponds to unloading rates 0.46-16.41~{rm {s}}^{-1}, using the similarities between simple and pure shear deformation. The relationship between dissipated energy and unloading stretch rate is found to follow a power-law such that cyclic pure shear and simple shear dynamic experiments yield similar result. At lower unloading rates (i.e. {dot{lambda }}_{rm {U}} < 1.0~{rm {s}}^{-1}), Mullins effect dominates and the viscous dissipation is minor, whereas at higher unloading rates, viscous dissipation becomes significant. At the crack propagation unloading rate 125.2~{rm {s}}^{-1}, the viscous dissipation is significant such that the amount of dissipated energy increases approximately by 125.4% from the lowest unloading rate. The critical tearing energy is obtained to be 7.04~{rm {kJ}}/{rm {m}}^{2} using classical method and 5.12~{rm {kJ}}/{rm {m}}^{2} using the proposed method. Hence, the classical method overestimates the critical tearing energy by approximately 37.5%.

A model for the dynamic friction behaviour of rubber-like materials

Rubber friction has received much attention because of its practical importance for modelling, for example, tyre-road friction. Many experiments were carried out and a number of theories were proposed to investigate and explain its behaviour and dependency on external conditions such as temperature and velocity. However, only a few published articles describe the observed dynamic friction behaviour using compact analytical models. Models for dry friction, such as the Dahl and LuGre models and their several variations, have been proposed to describe rubber friction. Unfortunately, they cannot reproduce many important and specific phenomena observed in experiments focused on rubber friction. In this paper, we propose a simple analytical friction model suitable to simulate the dynamic friction behaviour of rubber-like materials on rigid surface. Phenomena such as pre-sliding, sticking/sliding transitions, relaxation of the friction force at standstill and stick-slip effects can be accurately modelled. We validated the model with several experiments, obtaining a good agreement between simulation and measurements. In addition, we propose a two-dimensions (2D) extension of the model, to broaden the field of cases, in which it can be used. The capability of the model to simulate 2D anisotropic friction behaviour was validated with measurements of a vehicle tyre.

A Comparison of Manual Nylon Bristle Toothbrushes versus Thermoplastic Elastomer Toothbrushes in Terms of Cleaning Efficacy and the Biological Potential Role on Gingival Health

Background: It is well-known that toothbrushing might be associated with the development of oral soft tissue lesions. There is currently a continuing increase in the demand for new safety and performing materials in daily homecare oral hygiene including soft and extra-soft toothbrush bristles that tend to be safer. The aim of this study was to compare the efficacy of plaque control and the potential effects on gingival health of two different toothbrush bristle models. Methods: In a three-month period, a total of forty subjects were evaluated for Oral Hygiene Index (OHI), Gingival Index (GI) and Plaque Index (PI) scores as well the Gingival Abrasion Assessment (GAA) between a toothbrush entirely made from a rubber-like material called thermoplastic elastomer (TPE) and a soft toothbrush (standard control with nylon bristles) in a clinical, single-blind, controlled, parallel-group trial. Results: The use of the TPE toothbrush allows a reduction in the PI, improves the OHI and modifies the GAA in the TPE group over a period of three months compared with a conventional soft bristle toothbrush applied for the same period, leading in this way evidence for a good influence of the TPE bristles on overall oral hygiene conditions. Conclusions: From the comparison between our data and the literature studies, we can state that the material and shape of the bristles of the toothbrush affect the home practice of oral hygiene. The TPE bristles reduce the presence of plaque formation and gingival bleeding, oral soft tissue injuries acquired during homecare oral hygiene.

A new type of constitutive equation for nonlinear elastic bodies. Fitting with experimental data for rubber-like materials

In this article, we develop a new implicit constitutive relation, which is based on a thermodynamic foundation that relates the Hencky strain to the Cauchy stress, by assuming a structure for the Gibbs potential based on the Cauchy stress. We study the tension/compression of a cylinder, biaxial stretching of a thin plate and simple shear within the context of our constitutive relation. We then compare the predictions of the constitutive relation that we develop and that of Ogden’s constitutive relation with the experiments of Treloar concerning tension/compression of a cylinder, and we show that the predictions of our constitutive relation provide a better description than Ogden’s model, with fewer material moduli.

Investigation on Mullins effect of rubber materials by spherical indentation method>

Rubber-like soft materials usually experience load-induced softening behavior called the Mullins effect, in which the preloaded material exhibits a significantly more compliant response than the virgin material. However, rare quantitative and simple method of characterizing Mullins effect can be founded and corresponding accurate simulations in practice were severely restricted. In this paper, a simple measurement method was developed to characterize this mechanical response. A comprehensive theoretical, computational, and experimental study of the spherical indentation of rubber materials was undertaken to interpret parameters of the Mullins effect. Neo-Hookean model is used to describe nonlinear hyper-elastic deformation behavior and a pseudo-elastic model originally proposed by Ogden and Roxburgh is applied to interpret load-induced softening effects. Relationships between indentation loads, indentation depths, radius of the spherical indenter, and constitutive parameters of the material are obtained by dimensional analysis. Based on finite-element methods and parameter optimization procedure, a general method for determining the parameters of neo-Hookean and pseudo-elastic models was established. The approach was numerically and experimentally validated. This method provides a simple and accurate way of interpreting load-induced softening of rubber materials from spherical indentation tests, which will enrich the application of pseudo-elastic model using in vary cyclic loading conditions, capturing more new features in rubber-like soft materials and structures.

EXPLICITLY MODELING PERMANENT SET AND ANISOTROPY PROPERTY INDUCED BY STRESS SOFTENING FOR RUBBER-LIKE MATERIALS

Stress softening, known as the Mullins effect, is observed in rubber-like materials after initial loading cycles. Experimental observations have shown that the Mullins effect leads to a permanent set and induces anisotropic properties. A multi-axial, compressible strain energy function based on the Hencky strain is proposed to account for the stress softening and simulate the permanent set and anisotropic properties by introducing two variables, separately characterizing the permanent set and anisotropic properties, which are dependent on dissipation. A comprehensive, explicit shape function is proposed by using the spherical coordinate system to make the model effective in arbitrary direction. The new model exhibits isotropic properties when it not yet induced stress softening, and becomes anisotropy when the Mullins effect occurs. The residual strain increases and reach a fixed value as the loading-unloading cycles proceeds, and the anisotropy becomes more obvious. The simulation results are shown good accord with the classical experimental data and successfully forecast the permanent set and anisotropic properties induced by stress softening.

Fatigue-resistant polyurethane elastomer composites

Polyurethane (PU) elastomers are among the most used rubberlike materials due to their combined merits, including high abrasion resistance, excellent mechanical properties, biocompatibility, and good processing performance. A PU elastomer exhibits pronounced hysteresis, leading to a high toughness on the order of 104 J/m2. However, toughness gained from hysteresis is ineffective to resist crack growth under cyclic load, causing a fatigue threshold below 100 J/m2. Here we report a fatigue-resistant PU fiber–matrix composite, using commercially available Spandex as the fibers and PU elastomer as the matrix. The Spandex fibers are stiff, strong, and stretchable. The matrix is soft, tough, and stretchable. We describe a pullout test to measure the adhesion toughness between the fiber and matrix. The test is highly reproducible, showing an adhesion toughness of 3170 J/m2. The composite shows a maximum stretchability of 6.0, a toughness of 16.7 kJ/m2, and a fatigue threshold of 3900 J/m2. When a composite with a precut crack is stretched, the soft matrix causes the crack tip to blunt greatly, which distributes high stress over a long segment of the Spandex fiber ahead the crack tip. This deconcentration of stress makes the composite resist the growth of cracks under monotonic and cyclic loads. The PU elastomer composites open doors for realistic applications of fatigue-resistant elastomers.

Mechanical Mechanism Analysis of the Diseases of the Circular Rubber Bearing of the Bridge

Rubber is a super-elastic material. Super-elasticity can be used to analyze rubber-like materials. This material can withstand large strains and large displacements, but has minimal volume change (incompressible), and has two characteristics: material nonlinearity and geometric nonlinearity. In the research process, the positive pressure, shear force and void of the circular rubber bearing were analyzed. For the circular bearing, it can be found through research that the tensile stress of the rubber layer of the rubber bearing increases rapidly with the increase of the vacant area of the bearing, and the slope of the curve is also increasing. The maximum stress of the rubber layer generally appears between the upper first layer of steel plate and the second layer of steel plate. When the void area reaches 30%, the tensile stress of the rubber layer between the first layer of steel plate and the second layer of steel plate reaches 32.8MPa, which is greater than the specified 17MPa, which causes the cracking and deterioration of the inner rubber layer of the bearing, and finally leads to the rubber support. Because of the collapse of the seat, the vacancy of the seat must be paid attention to in the design and construction process.

Properties of polyurethanes derived from poly(diethylene glycol terephthalate)

The synthesis of linear and crosslinked polyurethanes based on α,ω-dihydroxypolydiethylene glycol terephthalate (PDET) is described. Prior to the preparation of these materials, a PDET polymer was obtained by conventional condensation reaction between dimethyl terephthalate and diethylene glycol, and a well-characterized fraction of PDET with number average molecular weight of 3,650 g mol−1 was chosen for the synthesis of the polyurethanes. Specifically, three polyurethanes were prepared: a pure linear polyurethane by reaction of PDET with 4,4′-diphenylmethane diisocyanate (MDI), a chain extended polyurethane obtained with MDI, PDET and 1,4-butanediol as chain extender, and a crosslinked polyurethane synthesized from PDET and a polyfunctional isocyanate. Thermal, mechanical, dielectric and gas transport properties of the polyurethanes were studied, showing excellent performance. In particular, rubber-like flexible materials were obtained with elongation values over 1000%, due to the low glass transition temperature of PDET. Furthermore, it was observed that these materials, in particular the crosslinked polyurethane, behave as gas barriers.

Development of a Magneto-Mechanical Bench and Experimental Characterization of Magneto-Rheological Elastomers

Magneto-rheological elastomers (MREs) belong to the class of smart materials whose mechanical properties can be controlled by an external magnetic field. These materials can be integrated into mechatronic systems and submitted to multiple loadings such as temperature, mechanical stress, and magnetic field. Thus, this work is dedicated to the development of a magneto-mechanical bench and on first experimental characterizations of hard MREs taking multiphysics coupling into account. Regarding the mechanical loading, the experimental setup is able to create a uniaxial tensile stress in case of low strain (< 1%) without friction effect. In regards to the magnetic loading, a magnetic circuit made of a strong permanent magnet has been designed to impose a variable and a homogeneous magnetic field strength up to 41 kA/m. Experimental analysis has been performed on silicone rubber filled with 36%vol. of NdFeB particles. The purpose was first to investigate the evolution of the Young modulus with or without magnetic field. Results obtained from measurements show that the developed test bench is able to depict the mechanical behavior and phenomena linked to rubber-like material.

Experimental evaluation of frequency response and firmness of apples based on an excitation technique using a dielectric elastomer actuator

A vibration experiment technique based on a dielectric elastomer actuator (DEA) excitation for apples is proposed. The characteristics of DEAs such as flexibility, lightweight, large deformation, and a fast response are conducive to vibration applications. Moreover, DEAs composed of soft rubber-like materials should be compatible with diverse objects. They can be applied to structures with curved surfaces and transfer excitation forces effectively. Herein, as an agricultural application, a DEA excitation technique is applied to assess the quality of apples, which have a spherical shape, using quality-related indices. Firmness is a function of the fruit's mass and its second-lowest resonance frequency. To evaluate the frequency responses and firmness of apples, we developed a vibration experiment system and the appropriate DEA composition. Then, the vibration responses of apples were measured by laser Doppler vibrometer (LDV) and evaluated by a spectrum analyzer to obtain the frequency responses. By assessing the firmness index of apples upon storing, we experimentally demonstrate the effectiveness of the proposed measurement technique based on the DEA excitation.

On the central role of the invariant I2 in nonlinear elasticity

The necessity of the inclusion of the second invariant of the left Cauchy-Green deformation tensor B, namely I2, in the strain energy function W of rubber-like materials is analysed. Universal relationships that underline such necessity are revisited, and experimental data are examined to establish the trends in the variation of ∂W/∂I2. Corroborated by the established experimental trends, we consider (meso)structural arguments to devise a plausible approach for incorporation of I2 into the W function. On the basis of the molecular theory of rubbers and considering the entanglements as a topological tube constraint, our analysis confers that a first approximation of W(I1,I2) is of the form W(I1,I2)=f(I1)+g(I2). The f(I1) contribution may be that of any classical generalised neo-Hookean model, and the functional form of g(I2) is directly deduced from the tube model of entangled molecules. An additional logarithmic functional form of I2 is also devised based on the rational approximation of the response function β−1. The ensuing additive-type W(I1,I2) models are then compared with experimental datasets. While this additive consideration may not be sufficient to account for all aspects of the mechanics of rubber-like materials, the fitting results demonstrate an eminent improvement in the predictions of the additive-type models compared with generalised neo-Hookean models having the same number of constitutive parameters. These analyses underline the central role of I2 in modelling the finite deformation of rubber-like materials.

Material parameter identification using finite elements with time-adaptive higher-order time integration and experimental full-field strain information

In this article, we follow a thorough matrix presentation of material parameter identification using a least-square approach, where the model is given by non-linear finite elements, and the experimental data is provided by both force data as well as full-field strain measurement data based on digital image correlation. First, the rigorous concept of semi-discretization for the direct problem is chosen, where—in the first step—the spatial discretization yields a large system of differential-algebraic equation (DAE-system). This is solved using a time-adaptive, high-order, singly diagonally-implicit Runge–Kutta method. Second, to study the fully analytical versus fully numerical determination of the sensitivities, required in a gradient-based optimization scheme, the force determination using the Lagrange-multiplier method and the strain computation must be provided explicitly. The consideration of the strains is necessary to circumvent the influence of rigid body motions occurring in the experimental data. This is done by applying an external strain determination tool which is based on the nodal displacements of the finite element program. Third, we apply the concept of local identifiability on the entire parameter identification procedure and show its influence on the choice of the parameters of the rate-type constitutive model. As a test example, a finite strain viscoelasticity model and biaxial tensile tests applied to a rubber-like material are chosen.