Discrete-element analysis and constitutive model for coal with predefined voids

Investigation on the shear behavior of thin-layer grouted joints: discrete element analysis and analytical model

An anisotropic elastoplastic constitutive model for large strain analysis of fiber reinforced composite materials

The analysis of damage process and the characterization of damaged rock masses through numerical models are the most difficult and challenging tasks in geotechnical engineering. This review paper describes and collects information regarding the causes of damage in the rocks and damage models (constitutive and hybrid damage models) for rock damage analysis. The main objective of this review is to discuss the causes of damage process, characterization, constitutive modes, and impact of natural changes on the selection of damage model. The review suggests that releasable strain energy, crack propagation and coalescence, joints, natural changes, and engineering disturbance are the main causes of rock fracture and damage. Most studies showed that a wider range of rock mass characterization will be required to create an ideal numerical model due to the rock reality, inelasticity, fractures, anisotropy, and inhomogeneity. Hybrid models are more efficient computationally as compared with the constitutive models. The review concludes that numerical models are also applicable tools to understand damage scale, damage degree and type, damage location, and damage occurrence time in the rocks.

Recent developments in the offshore and deepwater industry have increased interest in the use of unbonded flexible risers for deep water applications, and in the development of improved large-scale models for global dynamic analysis. Flexible risers are slender composite structures consisting of multiple polymer layers and spiral wound steel layers with complex geometries. To better understand and capture the complex behavior of actual unbonded flexible risers, in which global response is influenced by the ability of layers to slide over each other, incorporating nonlinearity in global dynamic analysis of risers and employing a suitable and reliable constitutive model increase the accuracy of the riser motion and stresses predictions. In this work, a novel elasto-plastic constitutive model is developed to account for the macroscopic response of unbonded riser structures. Determination of the parameters of the constitutive model is achieved through calibration of riser's response plots obtained by numerical simulations of detailed Representative Volume Element model of the riser. Co-rotational hybrid beam elements are used to implement the developed constitutive law, created using a user-defined element subroutine in Abaqus. It is shown that the integration of hybrid beam elements and the proposed constitutive model effectively simulate the nonlinear response of unbonded flexible risers under various working conditions by accounting for effects of internal and external pressures, coupling of axial and twist deformations and slip between the different layers of the riser.

Development of a novel constitutive model for improved structural integrity analysis of piping components

The following article has been included in a multiple retraction: Karimi A, Navidbakhsh M, Beigzadeh B, et al. Hyperelastic mechanical behavior of rat brain infected by Plasmodium berghei ANKA – experimental testing and constitutive modelling. Int J Damage Mech 2014;23:857–871, DOI: 10.1177/1056789513514072. In 2015 SAGE became aware of author misconduct concerning the suspected fabrication of identities, as well as the impersonation of legitimate individuals, to manipulate the peer review process across three separate journal publications. SAGE and the journal Editors immediately launched an investigation and have decided to retract the following articles for reasons of author misconduct. Alireza Karimi was the submitting author on 12 of the articles. SAGE regrets the academic record was compromised by manipulation of the peer review process and apologises to readers. OnlineFirst articles (these articles will not be published in an issue) International Journal of Damage Mechanics Karimi A, Navidbakhsh M and Razaghi R. Dynamic finite element simulation of the human head damage mechanics protected by polyvinyl alcohol sponge. Int J Damage Mech, first published on 15 May 2014, DOI: 10.1177/1056789514535945. Journal of Thermoplastic Composite Materials Karimi A, Navidbakhsh M and Haghpanahi M. Constitutive model for numerical analysis of polyvinyl alcohol sponge under different strain rates. J Thermoplast Compos Mater, first published on 15 January 2014, DOI: 10.1177/0892705713520176. Karimi A, Navidbakhsh M, Yousefi H et al. An experimental study on the elastic modulus of gelatin hydrogels using different stress–strain definitions. J Thermoplast Compos Mater, first published on 29 April 2014, DOI: 10.1177/0892705714533377. Elhamian MM, Alizadeh M, Shokrieh MM, et al. An innovative three-dimensional biphasic-laminated composite model for articular cartilage tissue. J Thermoplast Compos Mater, first published on 1 February 2015, DOI: 10.1177/0892705715569821. Perfusion Karimi A, Navidbakhsh M, Yamada H, et al. A comparative study on the quasilinear viscoelastic mechanical properties of the umbilical artery and the umbilical vein. Perfusion, first published on 22 May 2014, DOI: 10.1177/0267659114536761. Tehrani P, Rahmani S, Karimi A, et al. A novel cardiac assist device (AVICENA): a numerical study to compute the generated power. Perfusion, first published on 19 August 2014, DOI: 10.1177/ 0267659114547943. Elhamian SMM, Alizadeh M, Shokrieh MM, et al. The effect of collagen fiber volume fraction on the mechanical properties of articular cartilage by micromechanics models. Perfusion, first published on 20 August 2014, DOI: 10.1177/0267659114547942. Razaghi R, Karimi A, Navidbakhsh M, et al. Determination of the vulnerable plaque in a stenotic human coronary artery – finite element modelling. Perfusion, first published on 28 October 2014, DOI: 10.1177/0267659114557720. Articles published in an issue International Journal of Damage Mechanics Karimi A, Navidbakhsh M, Beigzadeh B, et al. Hyperelastic mechanical behavior of rat brain infected by Plasmodium berghei ANKA – experimental testing and constitutive modelling. Int J Damage Mech 2014;23:857–871, DOI: 10.1177/1056789513514072. Perfusion Karimi A, Navidbakhsh M and Faghihi S. A comparative study on plaque vulnerability using constitutive equations. Perfusion, 2014;29:178–183, DOI: 10.1177/0267659113502835. Abdi M, Karimi A, Navidbakhsh M, et al. Modeling of cerebral aneurysm using equivalent electrical circuit (Lumped Model). Perfusion 2014;29:142–152, DOI: 10.1177/0267659113498617. Karimi A, Navidbakhsh M and Faghihi S. Fabrication and mechanical characterization of a polyvinyl alcohol sponge for tissue engineering applications. Perfusion 2014;29:231–237, DOI: 10.1177/ 0267659113513823. Karimi A, Navidbakhsh M, Yousefi H, et al. Experimental and numerical study on the mechanical behavior of rat brain tissue. Perfusion 2014;29:307–314, DOI: 10.1177/0267659114522088.

In the present work, the applicability of physics-augmented neural network (PANN) constitutive models for complex electro-elastic finite element analysis is demonstrated. For the investigations, PANN models for electro-elastic material behavior at finite deformations are calibrated to different synthetically generated datasets describing the constitutive response of dielectric elastomers. These include an analytical isotropic potential, a homogenised rank-one laminate, and a homogenised metamaterial with a spherical inclusion. Subsequently, boundary value problems inspired by engineering applications of composite electro-elastic materials are considered. Scenarios with large electrically induced deformations and instabilities are particularly challenging and thus necessitate extensive investigations of the PANN constitutive models in the context of finite element analyses. First of all, an excellent prediction quality of the model is required for very general load cases occurring in the simulation. Furthermore, simulation of large deformations and instabilities poses challenges on the stability of the numerical solver, which is closely related to the constitutive model. In all cases studied, the PANN models yield excellent prediction qualities and a stable numerical behavior even in highly nonlinear scenarios. This can be traced back to the PANN models excellent performance in learning both the first and second derivatives of the ground truth electro-elastic potentials, even though it is only calibrated on the first derivatives. Overall, this work demonstrates the applicability of PANN constitutive models for the efficient and robust simulation of engineering applications of composite electro-elastic materials.

ABSTRACTThe behavior of soft soils distributed in coastal areas usually exhibits obvious time‐dependent behavior after loading. To reasonably describe the stress‐strain relationship of soft soils, this paper establishes a viscoelastic‐viscoplastic small‐strain constitutive model based on the component model and the hardening soil model with small‐strain stiffness (HSS model). First, the Perzyna's viscoplastic flow rule and the modified Hardin–Drnevich model are introduced to derive a one‐dimensional incremental Nishihara constitutive equation. Next, the flexibility coefficient matrix is utilized to extend the one‐dimensional model to three‐dimensional conditions. Then, by combining the HSS elastoplastic theory with the component model, the viscoelastic‐viscoplastic small‐strain constitutive model is subsequently established. To implement the proposed model for numerical analysis, the corresponding UMAT subroutine is developed using Fortran. After comparing the results of numerical simulations with those of existing literature, the reliability of the constitutive model and the program written in this paper is verified. Finally, numerical examples are designed to further analyze the effects of small‐strain parameters and viscoelastic‐viscoplastic parameters on the time‐dependent behavior of soft soils.

A new integral constitutive model has been developed for short-term deformational analysis of flexural reinforced concrete members. The integral constitutive model consists of traditional constitutive relationships for reinforcement and compressive concrete and the integral constitutive relationship for cracked tensile concrete which accumulates cracking, tension stiffening, reinforcement slippage and shrinkage effects. A new method has been developed for determining average stress-strain relations for cracked tensile concrete from flexural tests of reinforced concrete members. For given experimental moment-curvature or moment-average strain curves, the material stress-strain relations (including the descending branch) are computed from the equilibrium equations for incrementally increasing moment assuming portions of the relations obtained from the previous increments. Using the method proposed, a number of average stress-strain relations for concrete in tension has been derived from beam tests reported by different authors. Analysis of the relations has shown that their shape mostly depend on the reinforcement ratio as well as diameter and surface of reinforcement bars. The length of the descending branch of the relations reflecting the tension stiffening effect was far more pronounced for lightly reinforced beams with deformed bars of small diameters. For the first time, a quantitative dependence (1) has been established for flexural members between the length of the descending branch and the reinforcement ratio. On a basis of the derived relations, a new stress-strain relationship, called the integral constitutive relationship (2) for cracked tensile concrete in flexure, has been proposed. Accuracy of the proposed integral constitutive model has been investigated by calculating deflections for a large number of experimental reinforced concrete beams (reported by several investigators) for a wide range of values of parameters such as the specimen dimensions, concrete strength, reinforcement ratio, reinforcement bar diameter and surface characteristics. Comparison has been carried out with the predictions made for well-known constitutive relationships of tensile concrete and design code methods. For beams with average and high reinforcement ratios (p > 0.7%), accurate predictions have been made by all the methods yielding standard deviations for relative deflections from 8.8 to 10.3%. However, predictions for lightly reinforced beams (p ≤ 0.7%) were far less accurate. These inaccuracies are related to the increased influence of the tensile concrete which characterised by is a highly dispersed value. For lightly reinforced beams, the most accurate predictions in terms of standard deviation (14.0%) have been achieved using the proposed integral constitutive model. Relatively accurate predictions were also made by the SNiP (former Soviet code) and the ACI methods yielding standard deviations of 20.1 and 22.0% respectively. The EC2 (Eurocode) method underestimates the cracking moment and often overestimates significantly the corresponding deflection, in some cases yielding an error of over 100%. Surprisingly, predictions made by the design code methods were superior than those based on the use of well-known constitutive relationships for cracked tensile concrete. An efficient combination of accuracy and simplicity has been achieved for the integral constitutive model. This allowed to incorporate the model into a simple engineering technique for deformational analysis of flexural reinforced concrete members based on classical principles of strength of materials extended to layered approach and use of full material diagrams. In the given form, the integral constitutive model can be readily used not only in the simple engineering technique, but also in the finite element analysis. These main directions are envisaged in further development of the integral constitutive model for deformational analysis of reinforced concrete structures: a) based on new experimental data, further quantitative investigation of the influence of such parameters as strength of tensile concrete, reinforcement ratio, diameter and surface of bars, section height, shape of the cross-section, etc on tension stiffening and possible inclusion of these parameters into both functional and neural network constitutive models; b) assessment of long-term deformations due to creep and shrinkage; c) application of the model for cases of a combined action of a bending moment and axial and shear forces.

In this article we formulate a stable computational nonlocal poromechanics model for dynamic analysis of saturated porous media. As a novelty, the stabilization formulation eliminates zero‐energy modes associated with the original multiphase correspondence constitutive models in the coupled nonlocal poromechanics model. The two‐phase stabilization scheme is formulated based on an energy method that incorporates inhomogeneous solid deformation and fluid flow. In this method, the nonlocal formulations of skeleton strain energy and fluid flow dissipation energy equate to their local formulations. The stable coupled nonlocal poromechanics model is solved for dynamic analysis by an implicit time integration scheme. As a new contribution, we validate the coupled stabilization formulation by comparing numerical results with analytical and finite element solutions for one‐dimensional and two‐dimensional dynamic problems in saturated porous media. Numerical examples of dynamic strain localization in saturated porous media are presented to demonstrate the efficacy of the stable coupled poromechanics framework for localized failure under dynamic loads.

Coupled hydro-mechanical (HM) processes are significant in geological engineering such as oil and gas extraction, geothermal energy, nuclear waste disposal and for the safety assessment of dam foundations and rock slopes, where the geological media usually consist of fractured rock masses. In this study, we developed a model for the analysis of coupled hydro-mechanical processes in porous rock containing dominant fractures, by using the numerical manifold method (NMM). In the current model, the fractures are regarded as different material domains from surrounding rock, i.e., finite-thickness fracture zones as porous media. Compared with the rock matrix, these fractured porous media are characterized with nonlinear behavior of hydraulic and mechanical properties, involving not only direct (poroelastic) coupling but also indirect (property change) coupling. By combining the potential energy associated with mechanical responses, fluid flow and solid–fluid interactions, a new formulation for direct HM coupling in porous media is established. For indirect coupling associated with fracture opening/closure, we developed a new approach implicitly considering the nonlinear properties by directly assembling the corresponding strain energy. Compared with traditional methods with approximation of the nonlinear constitutive equations, this new formulation achieves a more accurate representation of the nonlinear behavior. We implemented the new model for coupled HM analysis in NMM, which has fixed mathematical grid and accurate integration, and developed a new computer code. We tested the code for direct coupling on two classical poroelastic problems with coarse mesh and compared the results with the analytical solutions, achieving excellent agreement, respectively. Finally, we tested for indirect coupling on models with a single dominant fracture and obtained reasonable results. The current poroelastic NNM model with a continuous finite-thickness fracture zone will be further developed considering thin fractures in a discontinuous approach for a comprehensive model for HM analysis in fractured porous rock masses.

Application of plastic-damage multidirectional fixed smeared crack model in analysis of RC structures

Unified numerical model for performance analysis of various cross-sections of concrete-filled stainless-steel tubular stub columns under axial loading

The ultimate load-carrying capacity and deformation of laminated bamboo hollow decks: Experimental investigation and inelastic analysis

In this paper, the uni-axial tension of double-notched concrete specimens is analyzed by using a continuum plasticity model. As in a companion paper, the concept of minimization of the second-order energy is used as the criterion for judging a bifurcation. The energy computation is formulated in standard continuum plasticity. The analysis confirms that the unsymmetrical crack propagation (i.e. strains localizing on one side of the specimen) may occur either before or after the peak load. Influences on bifurcation of three factors, the notch depth to panel width, the local constitutive law, and the ratio of the panel width to panel length are investigated. A larger ratio of the notch depth to panel width, a steeper softening constitutive law, and a smaller ratio of the panel width to panel length, leads to an earlier bifurcation. These conclusions are consistent with those obtained from a simplified model.