Search results

The purpose of this study is to simultaneously determine the constitutive parameters and boundary conditions by solving inverse mechanical problems of power hardening elastoplastic materials in three-dimensional geometries.

The power hardening elastoplastic problem is solved by the complex variable finite element method in software ABAQUS, based on a three-dimensional complex stress element using user-defined element subroutine. The complex-variable-differentiation method is introduced and used to accurately calculate the sensitivity coefficients in the multiple parameters identification method, and the Levenberg–Marquardt algorithm is applied to carry out the inversion.

Numerical results indicate that the complex variable finite element method has good performance for solving elastoplastic problems of three-dimensional geometries. The inversion method is effective and accurate for simultaneously identifying multi-parameters of power hardening elastoplastic problems in three-dimensional geometries, which could be employed for solving inverse elastoplastic problems in engineering applications.

The constitutive parameters and boundary conditions are simultaneously identified for power hardening elastoplastic problems in three-dimensional geometries, which is much challenging in practical applications. The numerical results show that the inversion method has high accuracy, good stability, and fast convergence speed.

This paper shows a generalization of the classic isotropic plasticity theory to be applied to orthotropic or anisotropic materials. This approach assumes the existence of a real anisotropic space, and other fictitious isotropic space where a mapped fictitious problem is solved. Both spaces are related by means of a linear transformation using a fourth order transformation tensor that contains all the information concerning the real anisotropic material. The paper describes the basis of the spaces transformation proposed and the expressions of the resulting secant and tangent constitutive equations. Also details of the numerical integration of the constitutive equation are provided. Examples of application showing the good performance of the model for analysis of orthotropic materials and fibre‐reinforced composites are given.

The numerical simulation of metal forming processes follows a highly non‐linear analysis where general aspects such as elastoplasticity, finite deformation and contact mechanics are combined. Approximated solutions obtained by finite element techniques require strong computational effort, that contradicts the need of interactive industrial applications. The first part of the work deals with the description of the main elements of the formulation, with attention to mathematical modelling and the approximating algorithms in the incremental iterative frame of non‐linear analysis, ending with the results obtained in hot rolling simulation. The second part is dedicated to computational efficiency analysis and the presentation of the related methods and results obtained in this work.

Discusses an alternative formulation for the incremental determination of stresses in strain measures that can be used to replace the stress rates currently employed. The formulation is based on Doyle‐Hill generalized definition of strain, the corresponding conjugate stresses and an isotropic hyperelastic constitutive equation. When used to analyze the simple shear deformation, the proposed formulation avoids the pathologies usually observed (oscillations, pressure build up, path dependence). The origin and importance of these pathologies is then discussed in relation to different materials behavior. It is shown that the incremental procedure used together with the logarithmic definition of strain is the most convenient, but that other approximations may be used in well defined particular situations. The numerical algorithms proposed are detailed in an Appendix.

A finite element procedure is presented for refined transient analysis of two‐dimensional (plane or axisymmetric) non‐linear structures involving arbitrarily large displacements, rotations and strains. The finite element model is based on the biquadratic nine‐node element of the Lagrange family. The relevant points pertaining to the equations of motion and their integration and to the spatial description, including geometrical and material non‐linearities, are considered. In particular, stress and strain rates are discussed. Finally, significant numerical applications show the effectiveness of the proposed method.

Continua and discontinua coexist in natural rock materials. This paper aims to present an improved approach for addressing the mechanical response of rock masses based on the combined finite-discrete element method (FDEM) proposed by Munjiza.

Several algorithms have been programmed in the new approach. The algorithms include (1) a simpler and more efficient algorithm to calculate the contact force; (2) An algorithm for tangential contact force closer to the actual physical process; (3) a plastic yielding criterion (e.g. Mohr-Coulomb) to modify the elastic stress for fitting the mechanical behavior of elastoplastic materials; and (4) a complete code for the mechanical calculation to be implemented in Matrix Laboratory (MATLAB).

Three case studies, including two standard laboratory experiments (uniaxial compression and Brazilian split test) and one engineering-scale anti-dip slop model, are presented to illustrate the feasibility of the Y-Mat code and its ability to deal with multi-scale rock mechanics problems. The results, including the progressive failure process, failure mode and trajectory of each case, are acceptable compared to other corresponding studies. It is shown that, the code is capable of modeling geotechnical and geological engineering problems.

This article gives an improved FDEM-based numerical calculation code. And, feasibility of the code is verified through three cases. It can effectively solve the geotechnical and geological engineering problems.

Two kinds of time integration methods; the dynamic explicit method and the static implicit method, have been compared, especially with emphasis on the shell formulations and the stress integration methods. Two methods have been applied to the benchmark problem named the S‐rail stamping process, provided by NUMISHEET’96 committee, in order to compare their numerical results with each other and with the average values of the experiments as well. Based on the comparisons, it is shown that both time integration methods can be successfully applied to industrial sheet metal stamping simulations. In detail, the static implicit method is advantageous over the dynamic explicit method in terms of accuracy, while the latter is known to be more efficient than the former in terms of computation efficiency.

The purpose of this paper is to analyse, in a finite element simulation, the failure of a multilayer printed circuit board (PCB), exposed to an impact load, to better evaluate the reliability and lifetime. Thereby the focus was set on failures in the outermost epoxy layer.

The fracture behaviour of the affected material was characterized. The parameters of a cohesive zone law were determined by performing a double cantilever beam test and a corresponding simulation. The cohesive zone law was used in an enriched finite element local simulation model to predict the crack initiation and crack propagation. Using the determined location of the initial crack, the energy release rate at the crack tip was calculated, allowing an evaluation of the local loading situation.

A good concurrence between the simulated and the experimentally observed failure pattern was observed. Calculating the energy release rate of two example PCBs, the significant influence of the chosen type on the local failure behaviour was proven.

The work presented in this paper allows for the simulation and evaluation of failure in the outermost epoxy layers of printed circuit boards due to impact loads.

Investigates the effects of two‐phase instability on finite element (FE) solutions for porous hypoelastic solids saturated with an insterstitial fluid. Demonstrates that two‐phase instability creates definite problems to the FE computations of coupled solid‐fluid systems. The eigenvectors of the tangential finite element matrices which are responsible for problems are not artificial, but are the bifurcating modes of physical solutions. The investigation, although limited to the plane strain undrained compression of hypoelastic models, is relevant to the investigation of the two‐phase instability of other materials and boundary value problems, and may lead towards an explanation for numerical problems in soil liquefaction analysis.

Fatigue crack growth models based on elastic‐plastic stress‐strain histories at the crack tip region and strain‐life damage models have been proposed. The UniGrow model fits this particular class of fatigue crack propagation models. The residual stresses developed at the crack tip play a central role in these models, since they are applied to assess the actual crack driving force. This paper aims to assess the performance of the UniGrow model based on available experimental constant amplitude crack propagation data, derived for several metallic materials from representative Portuguese bridges. It also aims to discuss key issues in fatigue crack growth prediction, using the UniGrow model, in particular the residual stress computation and the suitability of fatigue damage rules.

The UniGrow model is assessed using data derived by the authors for materials from Portuguese riveted metallic bridges. Strain‐life data, from fatigue tests on smooth specimens, are used to propose a convenient fatigue damage model. Predicted crack growth rates are compared with experimental crack propagation data obtained by authors using fatigue tests on compact tension specimens. Since the UniGrow model is a residual stress‐based propagation model, elastoplastic finite element analysis is proposed for comparison with the analytical approach implemented in the original UniGrow model.

The use of the Smith‐Watson‐Topper damage parameter overestimates the stress R‐ratio effects on crack propagation rates, mainly if the material shows crack propagation rates with small to moderate sensitivity to stress R‐ratio, which is the case of the materials under investigation in this paper. Alternatively, the application of the Coffin‐Manson damage law leads to consistent fatigue crack growth predictions for the investigated range of positive stress R‐ratios. The stress R‐ratios effects may be solely attributed to the residual stresses. Their estimation, using an analytical approach, may lead to inconsistent results, which is demonstrated by an alternative elastoplastic finite element analysis.

Contributions for more accurate predictions of fatigue crack propagation rates, for several stress ratios, using a strain‐based approach is proposed. This approach is valuable since it may be used to reduce the time consuming and costly fatigue crack propagation tests. Furthermore, the proposed approach shows potential for an unified crack initiation and propagation approach.