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M. K. Mohd Nor - One of the best experts on this subject based on the ideXlab platform.

  • Modelling Dynamic Behaviour and Spall Failure of Aluminium Alloy AA7010
    Journal of Physics: Conference Series, 2017
    Co-Authors: N. Ma’at, M. K. Mohd Nor, Al Emran Ismail, K.a. Kamarudin, Saifulnizan Jamian, Mohd Norihan Ibrahim, M. K. Awang
    Abstract:

    A finite strain constitutive model to predict the dynamic deformation behaviour of Aluminium Alloy 7010 including shockwaves and spall failure is developed in this work. The important feature of this newly hyperelastic-plastic constitutive formulation is a new Mandel stress tensor formulated using new generalized orthotropic pressure. This tensor is combined with a shock equation of state (EOS) and Grady spall failure. The Hill's yield criterion is adopted to characterize plastic orthotropy by means of the evolving structural tensors that is defined in the isoclinic configuration. This material model was developed and integration into elastic and plastic parts. The elastic anisotropy is taken into account through the newly stress tensor decomposition of a generalized orthotropic pressure. Plastic anisotropy is considered through yield surface and an isotropic hardening defined in a unique alignment of Deviatoric Plane within the stress space. To test its ability to describe shockwave propagation and spall failure, the new material model was implemented into the LLNL-DYNA3D code of UTHM's. The capability of this newly constitutive model were compared against published experimental data of Plate Impact Test at 234m/s, 450m/s and 895m/s impact velocities. A good agreement is obtained between experimental and simulation in each test.

  • Implementation of Finite Strain-Based Constitutive Formulation in LLLNL-DYNA3D to Predict Shockwave Propagation in Commercial Aluminum Alloys AA7010
    IOP Conference Series: Materials Science and Engineering, 2016
    Co-Authors: M. K. Mohd Nor, N. Ma’at, K.a. Kamarudin, Al Emran Ismail
    Abstract:

    The constitutive models adopted to represent dynamic plastic behaviour are of great importance in the current design and analysis of forming processes. Many have studied this topic, leading to results in various technologies involving analytical, experimental and computational methods. Despite of this current status, it is generally agreed that there is still a need for improved constitutive models. There are still many issues relating to algorithm implementation of the proposed constitutive model in the selected code to represent the proposed formulation. Using this motivation, the implementation of a new constitutive model into the LLNL-DYNA3D code to predict the deformation behaviour of commercial aluminium alloys is discussed concisely in this paper. The paper initially explains the background and the basic structure of the LLNL-DYNA3D code. This is followed by a discussion on the constitutive models that have been chosen as the starting point for this work. The initial stage of this implementation work is then discussed in order to allow all the required material data and the deformation gradient tensor F to be read and initialised for the main analysis. Later, the key section of this implementation is discussed, which mainly relates to subroutine f3dm93 including equation of state (EOS) implementation. The implementation of the elastic-plastic part with isotropic plastic hardening, which establishes the relationship between stress and strain with respect to the isoclinic configuration Ω i in the new Deviatoric Plane, is then presented before the implemented algorithm is validated against Plate Impact test data of the Aluminium Alloy 7010. A good agreement is obtained in each test.

  • Modelling inelastic behaviour of orthotropic metals in a unique alignment of Deviatoric Plane within the stress space
    International Journal of Non-Linear Mechanics, 2016
    Co-Authors: M. K. Mohd Nor
    Abstract:

    A finite strain constitutive model to predict the deformation behaviour of orthotropic metals is developed in this paper. The important features of this constitutive model are the multiplicative decomposition of the deformation gradient and a new Mandel stress tensor combined with the new stress tensor decomposition generalized into Deviatoric and spherical parts. The elastic free energy function and the yield function are defined within an invariant theory by means of the structural tensors. The Hill’s yield criterion is adopted to characterize plastic orthotropy, and the thermally micromechanical-based model, Mechanical Threshold Model (MTS) is used as a referential curve to control the yield surface expansion using an isotropic plastic hardening assumption. The model complexity is further extended by coupling the formulation with the shock equation of state (EOS). The proposed formulation is integrated in the isoclinic configuration and allows for a unique treatment for elastic and plastic anisotropy. The effects of elastic anisotropy are taken into account through the stress tensor decomposition and plastic anisotropy through yield surface defined in the generalized Deviatoric Plane perpendicular to the generalized pressure. The proposed formulation of this work is implemented into the Lawrence Livermore National Laboratory-DYNA3D code by the modification of several subroutines in the code. The capability of the new constitutive model to capture strain rate and temperature sensitivity is then validated. The final part of this process is a comparison of the results generated by the proposed constitutive model against the available experimental data from both the Plate Impact test and Taylor Cylinder Impact test. A good agreement between experimental and simulation is obtained in each test.

  • Plane-Stress Analysis of the New Stress Tensor Decomposition
    Applied Mechanics and Materials, 2013
    Co-Authors: M. K. Mohd Nor, Rade Vignjevic, James C. Campbell
    Abstract:

    The accuracy and reliability of the new stress tensor decomposition to capture the plasticity behaviour of orthotropic materials under Plane-stress conditions was examined in this paper. No experiment was required to perform this work. Therefore, the suitable, published paper which provides a relevant test result and sufficient material properties to characterise the new stress tensor decomposition, was used. This new stress tensor decomposition was used to presents a new yield criterion for orthotropic sheet metals under Plane-stress conditions in this work. This was done by assuming the yield surface to be circular in the new Deviatoric Plane. The predictions of the new effectice stress expression were then compared with the experimental data of 6000 series aluminium alloy sheet (A6XXX-T4) and Al-killed cold-rolled steel sheet SPCE. The predicted new yield surfaces are in good agreement with respect to the experimental data for two materials (A6XXX-T4 and SPCE).

  • Modelling of Shockwave Propagation in Orthotropic Materials
    Applied Mechanics and Materials, 2013
    Co-Authors: M. K. Mohd Nor, Rade Vignjevic, James C. Campbell
    Abstract:

    Modelling of shockwave propagation in orthotropic materials requires an appropriate description of material behaviour within elastic and plastic regimes. To deal with this issues, a finite strain constitutive model for orthotropic materials was developed within a consistent thermodynamic framework of irreversible process in this paper. The important features of this material model are the multiplicative decomposition of the deformation gradient and a Mandel stress tensor combined with the new stress tensor decomposition generalised for orthotropic materials. The elastic free energy function and the yield function are defined within an invariant theory by means of the introduction of the structural tensors. The plastic behaviour is characterised within the associative plasticity framework using the Hills yield criterion. The complexity was further extended by coupling the formulation with the equation of state (EOS) to control the response of the material to shock loading. This material model which was developed and integrated in the isoclinic configuration provides a unique treatment for elastic and plastic anisotropy. The effects of elastic anisotropy are taken into account through the stress tensor decomposition and plastic anisotropy through yield surface defined in the generalized Deviatoric Plane perpendicular to the generalised pressure. To test its ability to describe shockwave propagation, the new material model was implemented into the LLNL-DYNA3D code. The results generated by the proposed material model were compared against the experimental Plate Impact test data of Aluminium Alloy 7010. A good agreement between experimental and simulation was obtained for two principal directions of material orthotropy.

Jiangu Qian - One of the best experts on this subject based on the ideXlab platform.

  • The Onset of Strain Localization in Cross-Anisotropic Soils Under True Triaxial Condition
    Soils and Foundations, 2011
    Co-Authors: Maosong Huang, Jiangu Qian
    Abstract:

    ABSTRACT The influences of inherent cross-anisotropy on soil strength on the homogeneous deformation and bifurcation characteristic of the stress-strain relationship are studied. By neglecting the cohesion term and employing an elliptical shape function in Mohr-Coulomb failure criterion, a 3D anisotropic failure criterion is proposed to describe the strength of cross-anisotropic sands under true triaxial conditions. Based on the proposed failure criterion, the influence of the anisotropic parameter on the failure curve on the Deviatoric Plane and the relationship between the peak friction angle and the intermediate principal stress ratio are obtained. The suitability of the criterion is justified by comparing with a series of true triaxial tests on sands without strain localization. The proposed failure criterion is adopted to build a three dimensional anisotropic elasto-plastic model, which allows the bifurcation analysis to be incorporated with the non-coaxial flow rule for the purpose of studying the onset of strain localization. Compared with the true triaxial tests under several intermediate principal stress ratios conditions in the literature, the proposed model and bifurcation analysis is shown to be capable of predicting the influence of inherent cross-anisotropy on the onset of strain localization.

Hua Jiang - One of the best experts on this subject based on the ideXlab platform.

  • A failure criterion for rocks and concrete based on the Hoek-Brown criterion
    International Journal of Rock Mechanics and Mining Sciences, 2017
    Co-Authors: Hua Jiang
    Abstract:

    Abstract There have been a number of attempts at developing three-dimensional Hoek-Brown (3D HB) failure criteria with non-circular cross sections to include the effect of the intermediate principal stress for rocks, which is neglected in the classical HB criterion. Those existing 3D HB criteria predict the same strength as the classical HB criterion in triaxial extension (TXE) and triaxial compression (TXC). This paper presents a new failure criterion for rocks and concrete based on a unified expression of the 3D HB failure criteria, which can account for the strength difference between TXE and TXC. The failure cone of the new criterion is convex and smooth everywhere and the Deviatoric cross-section of the failure cone can cover various shapes from a curved triangle as the lower bound to a circle as the upper bound. The Deviatoric Plane locus is controlled by a shape factor k, which can be easily identified from TXE tests. The proposed failure criterion has been validated by biaxial compression tests of concrete, TXC/TXE tests of intact rocks and concrete as well as polyaxial compression (PXC) tests of intact rocks, jointed rock masses and concrete. Results show that the new 3D criterion with an empirical parameter k=−0.99 performs well in characterizing the rock and concrete strength in PXC in the absence of TXE test data. The application of the new criterion to cross-anisotropic rock in PXC is also discussed to show the fabric effect on the rock failure.

  • Failure criteria for cohesive-frictional materials based on Mohr–Coulomb failure function
    International Journal for Numerical and Analytical Methods in Geomechanics, 2015
    Co-Authors: Hua Jiang
    Abstract:

    Summary This study presents two three-parameter failure criteria for cohesive-frictional materials based on the Mohr–Coulomb failure function. One proposed failure criterion can be linked to Mogi's empirical formula and incorporates the well-known Von-Mises, Drucker–Prager, and Linear Mogi criteria as special cases. Another one with smooth and convex cross sections contains a general Lode dependence in the Deviatoric Plane and includes the Matsuoka–Nakai and Lade–Duncan Lode dependences as special cases. The effect of the intermediate principal stress on the strength of the material can be taken into account in both criteria. The proposed criteria are numerically calibrated against polyaxial data sets of many different types of rocks and concrete. The comparison results show that the performance of the proposed criteria is excellent, and the failure criterion with a general Lode dependence performs better than the other one for concrete. Copyright © 2015 John Wiley & Sons, Ltd.

  • Numerical simulation of impact tests on reinforced concrete beams
    Materials & Design, 2012
    Co-Authors: Hua Jiang, Xiaowo Wang
    Abstract:

    This paper focuses on numerical simulation of impact tests of reinforced concrete (RC) beams by the LS-DYNA finite element (FE) code. In the FE model, the elasto-plastic damage cap (EPDC) model, which is based on continuum damage mechanics in combination with plasticity theory, is used for concrete, and the reinforcement is assumed to be elasto-plastic. The numerical results compares well with the experimental values reported in the literature, in terms of impact force history, mid-span deflection history and crack patterns of RC beams. By comparing the numerical and experimental results, several important behavior of concrete material is investigated, which includes: damage variable to describe the strain softening section of stress–strain curve; the cap surface to describe the plastic volume change; the shape of the meridian and Deviatoric Plane to describe the yield surface as well as two methods of incorporating rebar into concrete mesh. This study gives a good example of using EPDC model and can be utilized for the development new constitutive models for concrete in future.

Xiaowo Wang - One of the best experts on this subject based on the ideXlab platform.

  • Numerical simulation of impact tests on reinforced concrete beams
    Materials & Design, 2012
    Co-Authors: Hua Jiang, Xiaowo Wang
    Abstract:

    This paper focuses on numerical simulation of impact tests of reinforced concrete (RC) beams by the LS-DYNA finite element (FE) code. In the FE model, the elasto-plastic damage cap (EPDC) model, which is based on continuum damage mechanics in combination with plasticity theory, is used for concrete, and the reinforcement is assumed to be elasto-plastic. The numerical results compares well with the experimental values reported in the literature, in terms of impact force history, mid-span deflection history and crack patterns of RC beams. By comparing the numerical and experimental results, several important behavior of concrete material is investigated, which includes: damage variable to describe the strain softening section of stress–strain curve; the cap surface to describe the plastic volume change; the shape of the meridian and Deviatoric Plane to describe the yield surface as well as two methods of incorporating rebar into concrete mesh. This study gives a good example of using EPDC model and can be utilized for the development new constitutive models for concrete in future.

Yangping Yao - One of the best experts on this subject based on the ideXlab platform.

  • Three-dimensional dissipative stress space considering yield behavior in Deviatoric Plane
    Science China Technological Sciences, 2013
    Co-Authors: Yangping Yao, Wenjie Cui, Naidong Wang
    Abstract:

    Naturally deposited soils are always found in the complex three-dimensional stress state. Constitutive models developed for modeling the three-dimensional mechanical behavior of soils should obey the basic laws of thermo-mechanical principles. Based on the incremental dissipation function, a new Deviatoric shift stress is derived and then introduced into the existing constitutive models to describe the yield behavior in the Deviatoric Plane for geomaterials. By adopting the proposed shift stress, the relationship between dissipative stress tensors and true stress tensors can be established. Therefore, the three-dimensional plastic strain can be calculated reasonably through the associated flow rule in the three-dimensional dissipative stress space. At the same time, three methods that are conventionally adopted for generalizing constitutive models to model the three-dimensional stress-strain relationships are examined under the thermo-mechanical framework. The TS (transformed stress) method is shown to obey the thermo-mechanical rules and the TS space adopted in TS method is actually a translational three-dimensional dissipative stress space. However, it is illustrated that the other two approaches, the method of using failure criterion directly and the method of using g(θ) function, violate the basic rules of thermo-mechanical theories although they may bring convenience and simplicity to numerical analysis for geotechnical engineering. Comparison between model predictions and experimental data confirms the validity of the proposed three-dimensional dissipative stress space.

  • A generalized anisotropic failure criterion for geomaterials
    International Journal of Solids and Structures, 2010
    Co-Authors: Zhiwei Gao, Jidong Zhao, Yangping Yao
    Abstract:

    AbstractExperimental evidence shows that the strength of geomaterials, such as soils and rocks, is significantly influenced by inherent anisotropy and other factors such as shear banding and the intermediate principal stress, which cannot be properly described by an isotropic failure criterion. This paper presents a generalized failure criterion for geomaterials with cross-anisotropy. To account for the influence of cross-anisotropy, we introduce an anisotropic variable in terms of the invariants and joint invariants of the stress tensor and the fabric tensor into the frictional coefficient of the failure criterion. The anisotropic failure criterion is formulated in both the Deviatoric Plane and the meridian Plane which collectively offer a general three-dimensional description of strength anisotropy. All the parameters introduced in the criterion can be conveniently determined by conventional laboratory tests. We demonstrate that the new criterion is general and robust in describing the variation of strength with loading direction for a wide range of materials. The failure criterion has been applied to the prediction of strength for several clays, sands and rocks reported in the literature. The predictions compare favorably with available experimental data. Further discussion is made on possible improvement of the new criterion to address other materials with complex strength characteristics, as well as its potential usefulness for constitutive modeling of anisotropic geomaterials