A variable-order viscoelastic constitutive model under constant strain rate

The traditional viscoelastic constitutive models encounter the problems of massive parameters and ambiguous physical meanings. A new concept of variable-order viscoelastic constitutive (called VOVC) model is put forward based on the constant fractional-order constitutive model and the viscoelastic theory. The determination methods of the two parameters in the VOVC model, including the material coefficient and the viscoelastic coefficient, are discussed both in the tensile and the resilient processes. The comparisons are made between the VOVC model and the traditional constitutive models i.e. the constant fractional-order Kelvin-Voigt (CFKV) model, the Zhu-wang-tang nonlinear thermo-viscoelastic constitutive (ZWT) model and the Ogden nonlinear hyper-elastic (Ogden) model. The results show that the VOVC model with the constant material coefficient and the variable viscoelastic coefficient predicts the whole evolution of the constitutive behavior of the viscoelastic material under the constant strain rate more precisely. The constant material coefficient in the VOVC model means the stiffness of the viscoelastic material. The variable viscoelastic coefficient in the model means the distribution of the elasticity and viscosity. The VOVC model contains a simpler structure, fewer parameters, clearer physical meanings and higher precision.

Quantification of degraded constitutive coefficients of composites in the presence of distributed defects

Effect of distributed defects on effective material properties of composites is required for the progressive failure models. Although the degradation of the effective material properties due to the presence of the lower scale damages is well investigated, how each material coefficient should be compromised in a progressive failure model is still a dilemma. Percentage of defects, the shape of the defects and their stochastic distribution may affect the individual material coefficients in a unique way and may not be uniform across the constitutive matrix. Hence, to find how the individual material coefficients in a constitutive matrix changes due to the presence of the voids and fiber breakage, all material coefficients in a constitutive matrix were studied herein. Representative volume element of a unidirectional fiber-matrix composite was studied with appropriate boundary conditions and respective material coefficients were calculated. It was found that the local gradients of the degradation curve obtained for each material coefficient are not linear with the increasing percentage of degradation and not uniform for all material coefficients. The shape and different locations of the defects with constant defect percentage were found to be inert towards affecting the material coefficients.

Modelling, simulation and experiment of the spherical flexible joint stiffness

The spherical flexible joint is extensively used in engineering. It is designed to provide flexibility in rotation while bearing vertical compression load. The linear rotational stiffness of the flexible joint is formulated. The rotational stiffness of the bonded rubber layer is related to inner radius, thickness and two edge angles. FEM is used to verify the analytical solution and analyze the stiffness. The Mooney–Rivlin, Neo Hooke and Yeoh constitutive models are used in the simulation. The experiment is taken to obtain the material coefficient and validate the analytical and FEM results. The Yeoh model can reflect the deformation trend more accurately, but the error in the nearly linear district is bigger than the Mooney–Rivlin model. The Mooney–Rivlin model can fit the test result very well and the analytical solution can also be used when the rubber deformation in the flexible joint is small. The increase of Poisson's ratio of the rubber layers will enhance the vertical compression stiffness but barely have effect on the rotational stiffness.

Using Statistical Algorithms for Image Reconstruction in EIT

The problem with image reconstruction from impedance tomography is an ill-posed inverse problem. To get quantitative information on the change in conductivity, it would be better to use a non-linear model in the differential imaging solution. Statistical methods such as PCR, PLRS, elastic net, Lars, SVR were used to reconstruct the image. The discussed techniques can be applied to the problem of electrical tomography. The algorithms used to identify unknown material coefficient.

Analysis of Physical-Mechanical and Surface Properties of Wood Plastic Composite Materials to Determine the Energy Balance

Wood plastic composite (WPC) materials represent modern materials that are attracting interest worldwide. WPC are composite materials and they have properties of both components – plastic and wood. WPC materials are formed by combining two substances – discontinuous reinforcements (wood particles or cellulose microfibers) and a continuous binder (plastic matrix), in a certain proportion. The authors describe WPC machined surfaces after turning. On the basis of a set of experimental data collected by surface and mechanical tests obtained from the WPC materials, the mechanical deformation work was evaluated, the value of which presents specific information about the material as a specific material coefficient.

Application of the Continuous and Discontinuous Fields of Plastic Deformations to the Evaluation of the Initial Thickness of Bent Tubes

The paper presents the derived equations for calculations of the initial wall thickness g0 of a tube bent to elbow. The expressions for calculating g0 are presented in a suitable measure of the “great active actual radius Rj” in the bending zone for an exact-generalized solution (continuous fields) and for three formal simplifications (discontinuous fields) of the first-, second-, and third-orders. The expressions to calculate the components of deformation for a generalized solution (continuous fields) are obtained on the basis of kinematically admissible fields of plastic deformations. In any case, a value of initial tube thickness depends on the radius and on the angle of bending αb on the external diameter of the tube, on the displacement of the neutral axis, and on the allowable (required) elbow thickness according to European, American, or other national technical standard or regulations. The initial thickness also depends on the coordinates of the point where the allowable thickness was determined and on the technological–material coefficient of the bending zone range k (defined during the tests). The obtained calculation results are presented in the form of graphs and in table.

Comparative Measurements of the Thermal Properties of Solid Materials on a New Device and Using a New Non-Stationary Method

The aim of the publication is the comparative measurements of changes in temperature of the significant material coefficient - thermal conductivity for newly developed construction materials (lightweight concrete). The aim is met by using a newly proposed method and a newly developed device by the approximation modelling of the temperature dependence of the thermal conductivity coefficient of the new composites and also the interpretation of measurement results in the context of optimally desired characteristics of thermal insulation concrete. Construction materials for residential buildings should have good thermal insulation properties, i.e. relatively low coefficients of thermal conductivity. With regard to the relatively most important property of concrete – strength, however, the reduction in thermal conductivity of concrete is limited. Thermal conductivity of concrete can be reduced very effectively by increasing its porosity; on the other hand, by increasing the porosity, the strength of concrete is significantly reduced. The publication, therefore, compares the results of temperature dependences of thermal conductivity for three newly designed concretes, namely in the context of their compressive strength.

Effect of Surface Texture Molded by a Stainless Steel Mesh on the Invisalign Material Friction Coefficient

Objective: This study aimed to evaluate the effect of surface texture mold by stainless steel (SS) mesh on Invisalign material friction coefficient. Materials and methods: Ten kinds of SS mesh were used to mold surface texture on Invisalign material. Experiments were conducted using a zirconia ball that slid against Invisalign material plates with different surface textures. In the experiments, the coefficients of friction under normal loads of 100, 200, and 300 g under dry and artificial saliva conditions were recorded. Results: The coefficient of friction was relative to the pore size and wire diameter of SS mesh. Conclusions: The Invisalign material coefficient can be effectively managed by the surface texture mold by SS mesh.