Search results

The frictional behavior of rubber materials on various contact surfaces is strongly affected by the contact pressure and the relative sliding velocity as well as the environmental temperature. Based on a great number of experiments of rubber blocks moving on concrete and ice surfaces, a friction law for 3D contact analyses is presented in this paper. It is characterized by the dependency on the contact pressure, sliding velocity and the environmental temperature. The identification and correction of the parameters of this friction law were done by means of a least‐square method followed by re‐analyses of the respective experiments. Several examples are given in a numerical investigation of the frictional behavior of rubber materials.

Contains a report on three‐dimensional finite element (FE) analyses of deformations and stresses resulting from the excavation of shallow underground railway tunnels. Multisurface elasto‐viscoplastic material models are employed for consideration of the mechanical behaviour of the soil and the shotcrete shell supporting the excavation. Both are formulated within the framework of closest point projection algorithms. For soil a cap model is used, consisting of a curved failure surface, a tension cut‐off and an elliptical cap. The latter allows consideration of the evolution of plastic strains even for the limiting case of a purely volumetric stress state. The movement of the cap is governed by a hardening law, describing the relation between the hydrostatic pressure and void ratio. The shotcrete model is a rotating crack model, taking ageing of the maturing concrete into account. It consists of a strain‐hardening Drucker‐Prager cone and three Rankine (crack) surfaces. Demonstrates the usefulness of the cap model to predict the mechanical behaviour of the soil by means of tests on remoulded, saturated clay. The model parameters of the clayey silt of Vienna, where the analysed tunnel is located, are fit to standard test results. The parameters of the shotcrete model are fit to test results published in the literature. Compares the analysis of a single‐track tunnel with the results of field measurements from sliding micrometers. Furthermore, the stresses in the shotcrete lining are examined. In view of the inhomogeneity of the material and of unavoidable deficiencies of the measurements it is fair to say that the mechanical effects resulting from the excavation of tunnels are modelled reasonably well.

The purpose of this paper is to propose a progressive inverse identification algorithm to characterize flow stress of tubular materials from the material response, independent of choosing an a priori hardening constitutive model.

In contrast to the conventional forward flow stress identification methods, the flow stress is characterized by a multi-linear curve rather than a limited number of hardening model parameters. The proposed algorithm optimizes the slopes and lengths of the curve increments simultaneously. The objective of the optimization is that the finite element (FE) simulation response of the test estimates the material response within a predefined accuracy.

The authors employ the algorithm to identify flow stress of a 304 stainless steel tube in a tube bulge test as an example to illustrate application of the algorithm. Comparing response of the FE simulation using the obtained flow stress with the material response shows that the method can accurately determine the flow stress of the tube.

The obtained flow stress can be employed for more accurate FE simulation of the metal forming processes as the material behaviour can be characterized in a similar state of stress as the target metal forming process. Moreover, since there is no need for a priori choosing the hardening model, there is no risk for choosing an improper hardening model, which in turn facilitates solving the inverse problem.

The proposed algorithm is more efficient than the conventional inverse flow stress identification methods. In the latter, each attempt to select a more accurate hardening model, if it is available, result in constructing an entirely new inverse problem. However, this problem is avoided in the proposed algorithm.

This paper presents a comparison of various models for strain‐softening due to damage such as cracking or void growth, as proposed recently in the literature. Continuum‐based models expressed in terms of softening stress—strain relations, and fracture‐type models expressed in terms of softening stress—displacement relations are distinguished. From one‐dimensional wave propagation calculations, it is shown that strain‐localization into regions of finite size cannot be achieved. The previously well‐documented spurious convergence is obtained with continuum models, while stress—displacement relations cannot model well smeared‐crack situations. Continuum models may, however, be used in general if a localization limiter is implemented. Gradient‐type localization limiters appear to be rather complicated; they require solving higher‐order differential equations of equilibrium with additional bourdary conditions. Non‐local localization limiters, especially the non‐local continuum with local strain, in which only the energy dissipating variables are non‐local, is found to be very effective, and also seems to be physically realistic. This formulation can correctly model the transition between homogeneous damage states and situations in which damage localizes into small regions that can be viewed as cracks. The size effect observed in the experimental and numerical response of specimens in tension or compression is shown to be a consequence of this progressive transition from continuum‐type to fracture‐type formulations.

The purpose of this paper is to propose a path-finding algorithm to solve problems with an arbitrary load-displacement relationship which results from geometrical and material nonlinear models to simulate e.g. timber structures realistically.

A method using combined load and displacement control for the Newton method along with path-characterising measures and sub-incremention is introduced. A path-related stiffness measure is used to identify the situation when it is necessary to select the displacement control and chose the best degree of freedom as a parameter instead of the load factor. The nonlinearity index extracts information about the convergence behaviour during one incremental step. Together with the reduction of the load increments it avoids leaving the equilibrium path.

The method is discussed based on numerical examples with highly nonlinear behaviour. It is capable to solve systems with decreasing load capacity and snap-back effects.

The algorithm combines load and displacement control and adaptively choses the method and the corresponding degree of freedom and cares for reliable path following.

The purpose of this study is the numerical investigation of densification and molding processes of wood. Providing theoretical and numerical approaches with respect to a consistent multi-physical finite element method framework are further goals of this research.

Constitutive phenomenological descriptions of the thermo-mechanical and moisture-dependent material characteristics of wood are introduced. Special focus is given to a consistent hygro-thermo-mechanical modeling at finite deformations to capture the realistic material behavior of wood, especially when it is subjected to densification and molding processes.

Realistic theoretical formulations of different hygro-thermo-mechanical processes are provided. A successful numerical modeling is demonstrated for beech wood by validation at experimental findings.

The constitutive laws and numerical findings are new, as they govern a multi-physical large deformation framework and are applied to the advanced technology of densification and molding of wood.

This paper focuses on the development of a new class of eight‐node solid finite elements, suitable for the treatment of volumetric and transverse shear locking problems. Doing so, the proposed elements can be used efficiently for 3D and thin shell applications. The starting point of the work relies on the analysis of the subspace of incompressible deformations associated with the standard (displacement‐based) fully integrated and reduced integrated hexahedral elements. Prediction capabilities for both formulations are defined related to nearly‐incompressible problems and an enhanced strain approach is developed to improve the performance of the earlier formulation in this case. With the insight into volumetric locking gained and benefiting from a recently proposed enhanced transverse shear strain procedure for shell applications, a new element conjugating both the capabilities of efficient solid and shell formulations is obtained. Numerical results attest the robustness and efficiency of the proposed approach, when compared to solid and shell elements well‐established in the literature.

This paper describes the solution of a steady‐state natural convection problem in porous media by the radial basis function collocation method (RBFCM). This mesh‐free (polygon‐free) numerical method is for a coupled set of mass, momentum, and energy equations in two dimensions structured by the Hardy's multiquadrics with different shape parameter and different order of polynomial augmentation. The solution is formulated in primitive variables and involves iterative treatment of coupled pressure, velocity, pressure correction, velocity correction, and energy equations. Numerical examples include convergence studies with different collocation point density and arrangements for a two‐dimensional differentially heated rectangular cavity problem at filtration Rayleigh numbers Ra*=25, 50 and 100, and aspect ratios A=1/2, 1, and 2. The solution is assessed by comparison with reference results of the fine‐mesh finite volume method in terms of mid‐plane velocity components, mid‐plane and insulated surface temperatures, streamfunction minimum, and Nusselt number.

Due to the strong reliance on element quality, there exist some inherent shortcomings of the traditional finite element method (FEM). The model of FEM behaves overly stiff, and the solutions of automated generated linear elements are generally of poor accuracy about especially gradient results. The proposed cell-based smoothed point interpolation method (CS-PIM) aims to improve the results accuracy of the thermoelastic problems via properly softening the overly-stiff stiffness.

This novel approach is based on the newly developed G space and weakened weak (w²) formulation, and of which shape functions are created using the point interpolation method and the cell-based gradient smoothing operation is conducted based on the linear triangular background cells.

Owing to the property of softened stiffness, the present method can generally achieve better accuracy and higher convergence results (especially for the temperature gradient and thermal stress solutions) than the FEM does by using the simplest linear triangular background cells, which has been examined by extensive numerical studies.

The CS-PIM is capable of producing more accurate results of temperature gradients as well as thermal stresses with the automated generated and unstructured background cells, which make it a better candidate for solving practical thermoelastic problems.

It is the first time that the novel CS-PIM was further developed for solving thermoelastic problems, which shows its tremendous potential for practical implications.

The purpose of this paper is to present a micromechanical model based on a new truly local meshless method for analysis of heat transfer in composite materials.

The presented meshless method is based on the integral form of energy equation in the sub‐particles in the material. In the presented meshless method due to elimination of domain integration the computational efforts are decreased substantially.

Numerical results are presented for temperature distribution, heat flux and thermal conductivity. Numerical results show that the presented meshless method is simple, effective, accurate and less costly method in micromechanical modeling of heat conduction in heterogeneous materials.

A small area of the composite system called representative volume element is considered as the solution domain. The fully bonded fiber‐matrix interface is considered and contact thermal resistant is neglected from the fiber matrix interface and so the continuity of temperature and reciprocity of heat flux is satisfied in the fiber‐matrix interface.

For the first time a new truly local meshless method based on the integral form of energy equation for the sub‐particles in the materials is presented for analysis of heat transfer in composite materials.