User-defined Material Model Research Articles

A micro-mechanics-based elastoplastic friction-damage model for brittle rocks and its application in deformation analysis of the left bank slope of Jinping I hydropower station

This study develops a micro-mechanics-based elastoplastic damage model within the framework of irreversible thermodynamics. In the model, damage is related to growth of micro-cracks, while plastic deformation is induced by frictional sliding along those cracks. The damage criterion and functions of thermodynamic force are used to describe the damage evolution of rocks and determine whether they are damaged. Time-dependent deformation of rock is also taken into account by considering subcritical growth of micro-cracks. For engineering application, the proposed model is implemented in the standard finite element code Abaqus as a user-defined material model (UMAT) by using a specific local integration algorithm. The accuracy of the proposed model is assessed by comparing numerical results with experimental data in conventional triaxial compression tests and triaxial creep tests. The proposed model is then used for simulating the displacement field, damage and plastic zones of the left bank high slope. The predicted results are in a good consistency with field monitored data.

Plasticity based material model for concrete subjected to dynamic loadings

A new plasticity based material model has been proposed in the present work for concrete subjected to dynamic loads. The model has then been used for blast analysis of Reinforced Concrete slabs. The model has three parts; first the equation of state is described by a pressure versus volumetric strain relationship, second a method for defining damage and in the last part of the model we define a modified strength surface. Tensile and compressive strain rates are accounted separately. Three invariant failure surface is proposed by accounting for the effect of third invariant in the form of lode angle. Tensile and compressive damage are treated separately by considering strain rate effects and damage functions for tensile and compressive behavior. In order to account for the effect of shear dilation, a fully associative flow rule is considered. A user defined material model UMAT code has been developed for the proposed model and implemented in an FEM framework in LS-DYNA. The performance of the proposed model is then compared with in-built LS-DYNA concrete material models. The following inbuilt models namely : K&C Concrete Model (KCCM), Continuous Surface Cap Model (CSCM) and Winfrith Concrete Model (WCM) are considered for comparison. An unconfined and confined compressive and tensile test are done numerically and the advantages of proposed model over in built material model KCCM, WCM and CSCM has been brought out. A parametric study has been performed with the proposed model by varying the thickness of the slab, reinforcement ratio, scaled distance, and concrete strength. It seen that the slab thickness and scaled distance play a significant role in influencing the blast analysis results. The value of maximum deflection decreased by increasing the slab thickness and by increasing the scaled distance. But in case of increasing the strength of concrete, the slab shows a very small reduction in maximum deflection. From both the comparative and parametric studies the proposed model was found to be more efficient for blast analysis.

Wall stress analyses in patients with ≥5 cm versus

ObjectiveCurrent guidelines for elective surgery of ascending thoracic aortic aneurysms (aTAAs) use aneurysm size as primary determinant for risk stratification of adverse events. Biomechanically, dissection may occur when wall stress exceeds wall strength. Determining patient-specific aTAA wall stresses by finite element analysis can potentially predict patient-specific risk of dissection. This study compared peak wall stresses in patients with ≥5.0 cm versus <5.0 cm aTAAs to determine correlation between diameter and wall stress. MethodsPatients with aTAA ≥5.0 cm (n = 47) and <5.0 cm (n = 53) were studied. Patient-specific aneurysm geometries obtained from echocardiogram-gated computed tomography were meshed and prestress geometries determined. Peak wall stresses and stress distributions were determined using LS-DYNA finite element analysis software (LSTC Inc, Livermore, Calif), with user-defined fiber-embedded material models under systolic pressure. ResultsPeak circumferential stresses at systolic pressure were 530 ± 83 kPa for aTAA ≥5.0 cm versus 486 ± 87 kPa for aTAA <5.0 cm (P = .07), whereas peak longitudinal stresses were 331 ± 57 kPa versus 310 ± 54 kPa (P = .08), respectively. For aTAA ≥5.0 cm, correlation between peak circumferential stresses and size was 0.41, whereas correlation between peak longitudinal wall stresses and size was 0.33. However, for aTAA <5.0 cm, correlation between peak circumferential stresses and size was 0.23, whereas correlation between peak longitudinal stresses and size was 0.14. ConclusionsPeak patient-specific aTAA wall stresses overall were larger for ≥5.0 cm than aTAA <5.0 cm. Although some correlation between size and peak wall stresses was found in aTAA ≥5.0 cm, poor correlation existed between size and peak wall stresses in aTAA <5.0 cm. Patient-specific wall stresses are particularly important in determining patient-specific risk of dissection for aTAA <5.0 cm.

Damage Evolution Mechanisms of Rock Induced by Blasting with the Aid of Empty-Hole Effect

Blasting has been widely used in coal mining as a method of reducing the integrity of rock mass. Its low controllability often leads to the unsatisfactory fragmentation effect of rock mass. The empty borehole effect has great significance in avoiding the excessive breakage of rock mass and guiding the directional propagation of blasting-induced fractures. Nevertheless, the rock mass damage type evolution induced by the empty borehole has been rarely studied. A tension–compression constitutive model of rock mass damage is established in this paper. The model is incorporated into the numerical modeling code LS-DYNA as a user-defined material model. Then, LS-DYNA is used to investigate the evolution mechanism of rock mass damage under the empty borehole effect. The damage types of rock mass caused by the empty borehole effect are also studied. The Fortran language is utilized to monitor the number variation of the tensile damaged elements and the compressive damaged elements in the rock mass around the empty borehole. The results indicate that existence of the empty borehole significantly enhances the tensile stress and the stress concentration factor in the rock mass nearby the empty borehole. Meanwhile, the rock mass nearby the empty borehole mainly damages in tension. Both the number of the tensile damaged elements and the tensile stresses in the elements increases as the empty borehole diameter increases. The number of the compressive damaged elements decreases with increasing empty borehole diameter.

The influence of free water content on ballistic performances of concrete targets

During the last decades, the study of the influence of free water content on performances of concrete has been improved by numerous experimental works. It is believed that wet concretes exhibit much lower deviatoric strength capacity, higher volumetric stiffness, and more strain-rate sensitivity compared to dry concretes. In this paper, the Kong-Fang concrete model recently proposed (Int J Impact Eng 2018, 120: 60–78) is modified to capture the effect of free water content. Three major modifications are made respectively to the strength surface, dynamic increase factor (DIF), and equation of state of the original model to capture the influence of free water content mentioned above. The modified strength surface, DIF, and equation of state for wet and dry concretes are compared with experimental data and show good agreements. The modified Kong-Fang model is then implemented into the finite element code LS-DYNA through user-defined material model. Single element tests including biaxial compression and triaxial compression are firstly performed to validate the modified model at a material point level, where corresponding predictions are compared with experimental results and good agreements are obtained. The influence of free water content on ballistic performances of concrete is then systematically investigated with the modified model. The predicted time-histories of projectile velocity, deceleration, and penetration depth are well consistent with the experimental data. It is found that the wet concrete exhibits higher penetration resistance compared to a dry concrete reflected in a lower projectile residual velocity for perforation case and smaller penetration depth for penetration case.

Yield stress criteria to assess the buildability of 3D concrete printing

3D concrete printing (3DCP) using the extrusion method is a commonly used additive manufacturing technique to construct the structures layer by layer without using formwork. Therefore, the green (wet or fresh) strength of the printable concrete determines the possible printable height of the structure and the rate of printing to avoid collapse during the printing process. This paper aims to identify the buildability criteria based on the green strength of concrete and the effect of early age material properties on the stability of printed structures. The commonly available strength-based failure models based on the material yield stress and the vertical stresses induced were developed considering higher aspect ratios (height to width ratio). This may be not suitable for lower aspect ratios used in 3DCP layer geometries. Also, the frictional behaviour due to the applied vertical stresses should be considered in the material used for 3DCP applications. In this work, the Mohr-Coulomb based buildability criterion was developed and validated with laboratory experiments and numerical simulations. The laboratory experiments were carried out to establish the time-dependent material and rheological properties of 3D printable concrete mixes. Further, 3D printing of hollow circular sections was carried out to study the failure modes and to obtain the build height at the time of failure. The nonlinear finite difference model of the 3D printed sections was developed with the user-defined time-dependent material models to assess the accuracy of the proposed buildability criterion. Additionally, the proposed buildability criterion was further validated with the 3DCP experimental and numerical data presented in literature and all confirm that the proposed criterion shows higher accuracy over existing methods in assessing the buildability.

Capabilities of Macroscopic Forming Simulation for Large-Scale Forming Processes of Dry and Impregnated Textiles

Forming of continuously fibre-reinforced polymers (CoFRP) has a significant impact on the structural performance of composite components, underlining the importance of forming simulation for CoFRP product development processes. For an integrated development of industrial composite components, efficient forming simulation methods are in high demand. Application-oriented method development is particularly crucial for industrial needs, where large and complex multi-layer components are manufactured, commercial FE software is used, and yet high prediction accuracy is required. To meet industrial demands, this contribution gives an insight in macroscopic forming simulation approaches that utilize the FE software Abaqus in combination with user-defined material models and finite elements. Three CoFRP forming technologies are considered, which are in industrial focus due to their suitability for mass production: textile forming of dry unidirectional non-crimp fabrics (UD-NCF), thermoforming of pre-impregnated UD tapes and wet compression moulding (WCM). In addition to the highly anisotropic, large-strain material behaviour that composite forming processes have in common, the three process technologies face various process-specific modelling challenges. UD-NCFs require material models that capture the deformation behaviour and the slippage of the stitching. Thermoforming of UD tapes is highly rate- and temperature-dependent, calling for rheological membrane and bending modelling. Moreover, a thermomechanical approach including crystallisation kinetics enables the prediction of potential phase-transition during forming and resulting defects in the semi-crystalline thermoplastic matrix. For simultaneous forming and infiltration in wet compression moulding, a finite Darcy-Progression-Element is superimposed with the membrane and shell elements for forming simulation, capturing infiltration-dependent material properties. The three outlined technologies illustrate the complexity and importance of further simulation method development to support future process development.

Ballistic performance of polyurea-coated thin aluminium plates: numerical study

Polyurea elastomer exhibits desirable characteristics for impact mitigation, with varying stoichiometric-dependent properties that can be tailored for specific applications and applied to reinforce existing and new structural components. This numerical study aims to investigate the ballistic performance of polyurea-aluminium laminate targets, employing a user-defined material model for polyurea elastomer developed in a finite-element (FE) framework. The model consists of a rigid spherical projectile impacting the considered target plate. A linear increase in the ballistic performance with a growing thickness of polymer coating was observed and is consistent with previously conducted experimental work. The ballistic limit is increased by some 5% per millimetre of polymer coating thickness, when compared to the monolithic metallic plate. The presence of the polymer layer significantly affects the dynamic response mechanisms of the component during bending due to impact. The result is a more localised deformation compared to global bending of the target.

A constitutive model for concrete subjected to extreme dynamic loadings

A good physical-based constitutive model should not only properly reproduce the macroscopic dynamic response of concrete, but also capture the meso-damage behavior that actually occurs. This paper presents a constitutive model for concrete subjected to extreme dynamic loadings with the help of the meso-damage mechanics, Duvaut–Lions formulation and elasto-plastic theory. In the present model, a two-step homogenization scheme of concrete is proposed, where compressive damage and tensile damage are treated dividually. The tensile damage is controlled by the nucleation and growth of micro-cracks, while the compressive damage is related to the collapse of micro-voids. Mori-Tanka method is introduced to establish the relationship between the meso-structural parameters and the macro-physical quantities. What is more, a new cap model coupled to the compressive damage is presented to capture the irreversible deformation induced by the micro-void collapse. The present model, containing the erosion algorithm, was successfully implemented into finite element code LS-DYNA through user-defined material model. In order to validate the present model, dynamic tensile tests, quasi-static triaxial compressive tests, planar impact tests and projectile penetration tests were simulated in sequence. The results show that the present model has good consistency between the predicted results and the experimental data, thus verifying its rationality and practicability. Sensitivity analyses of two erosion criteria and mesh dependency were also carried out to understand the influence of these empirical parameters on the compressive damage and tensile damage predicted by the present model.

Experimental and Numerical Investigations of High-Speed Projectile Impacts on 7075-T651 Aluminum Plates

Simulation of the material failure under high strain rate conditions is one of the most difficult problems in the finite element analyses, and many researchers have tried to understand and reproduce dynamic material fracture. In this study, we investigate a failure criterion that minimizes the mesh dependency at high strain rates and incorporates the criterion into the Johnson-Cook constitutive relationship by developing a user-defined material model. Impact tests were performed using a gas-gun system in order to investigate the response of the 7075-T651 aluminum plate in high-speed collision. On the other hand, numerical simulations are carried out by considering various element sizes and the relationship between element size and failure strain is inversely obtained using numerical results. By accommodating the relationship into the damage model and implementing in the user-defined material model, mesh dependency is significantly reduced, and sufficient accuracy is achieved with alleviated computational cost than the existing damage model. This study suggests an element size-dependent damage criterion that is applicable for impact simulation and it is expected that the criterion is useful to obtain accurate impact responses with a small computational cost.

Computational analysis of tensile damage and failure of mineralized tissue assisted with experimental observations

In this study, deformation and failure mechanisms of mineralized tissue (bone) were investigated both experimentally and computationally by performing diametral compression tests on millimetric disk specimens and conducting finite element analysis in which a granular micromechanics-based nonlinear user-defined material model is implemented. The force-displacement relationship obtained in the simulation agreed well with the experimental results. The simulation was also able to capture location of the failure initiation observed in the experiment, which is inside out from the hole along the loading axis. Furthermore, propagation of micro-sized cracks into failure was observed both in the experiment using simultaneous slow-motion microscopy imaging and in the simulation analyzing the local distortion and local volume change within the specimen. The anisotropy evolution was found to be significant around the hole along the loading axis by evaluating the anisotropy index computed using finite element results. In conclusion, this work revealed that the prediction capability of granular micromechanics-based user-defined nonlinear material model (UMAT) is promising considering the match between the results and observations from the physical experiment and finite element analysis such as force-displacement relationship and failure initiation/pattern. This work has also shown that the tensile damage and failure of mineralized tissues can be characterized using diametral compression (split tension) test.

Compressive failure of fiber composites containing stress concentrations: Homogenization with fiber-matrix interfacial decohesion based on a total Lagrangian formulation

Compression failure by fiber kinking limits the structural applications of fiber composites. Fiber kinking is especially prevalent in laminates with holes and cutouts. The latter behavior is characterized by strain localization in the matrix material and fiber rotations. To study fiber kinking on the level of the individual constituents, a homogenization of fiber composites is presented. It is based on a total Lagrangian formulation, making it independent of fiber rotations. It accounts for the microstructure of the composite, including fiber-matrix interfacial decohesion, and enables all types of material behavior of the constituents. The response of each constituent of the composite is modeled separately and the global response is obtained by an assembly of all contributions. The model is implemented as a user-defined material model (UMAT) in ABAQUS and used for multiscale modeling of notched unidirectional plies subjected to compression. The model performs well in agreement with a finite element model of an explicit discretization of the microstructure and literature results. The simulations predict the formation of a kink band in near 0-degree plies and show that the open-hole compression strength is sensitive to fiber-matrix interfacial decohesion. The present work suggests a convenient and computationally efficient tool for simulating the elastic-plastic behavior of fiber composites on the fiber-matrix level and predicting the compressive strength of laminates.

Numerical–Experimental Correlation of Impact-Induced Damages in CFRP Laminates

Composite laminates are characterized by high mechanical in-plane properties and poor out-of-plane characteristics. This issue becomes even more relevant when dealing with impact phenomena occurring in the transverse direction. In aeronautics, Low Velocity Impacts (LVIs) may occur during the service life of the aircraft. LVI may produce damage inside the laminate, which are not easily detectable and can seriously degrade the mechanical properties of the structure. In this paper, a numerical-experimental investigation is carried out, in order to study the mechanical behavior of rectangular laminated specimens subjected to low velocity impacts. The numerical model that best represents the impact phenomenon has been chosen by numerical–analytical investigations. A user defined material model (VUMAT) has been developed in Abaqus/Explicit environment to simulate the composite intra-laminar damage behavior in solid elements. The analyses results were compared to experimental test data on a laminated specimen, performed according to ASTM D7136 standard, in order to verify the robustness of the adopted numerical model and the influence of modeling parameters on the accuracy of numerical results.

Material Characterization and Compression Molding Simulation of CF-SMC Materials in a Press Rheometry Test

The compression molding of sheet molding compounds (SMCs) is typically thought of as a fluid mechanics problem. The usage of CF-SMC with high fiber volume content (over 50%) and long fiber reinforcement structures (up to 50 mm) challenges the feasibility of this point of view. In this work a user-defined material model based on a solid mechanics formulation is developed in LS-DYNA®. The material model is built on a modular principle where the different influence factors caused by the material characteristics form building blocks. The idea is that these blocks are represented by simple mathematical models and interact in a way that forms the overall behavior of the SMC material. To analyze the behavior of the SMC material and create input parameters for the material model it is necessary to perform some kind of material characterization experiment. This paper presents the press rheometry test which can be perform in two variations, differing in terms of specimen size and shape and degree of coverage in the tool. Here the material response to the compression molding can be analyzed and by the visualization of the flow front development the anisotropy and homogeneity of the material can be assessed. For a comparison between the material model and reality the two variations of the press rheometry test are simulated. The simulation results show a good prediction of the experiments. The differences between experiment and simulation can be used to further improve the model in a later process.

Frequency domain modelling of transversely isotropic viscoelastic fibre-reinforced plastics

The subject of this work is the development of an approach describing the transversely isotropic viscoelastic material behaviour of carbon-fibre-reinforced plastics (CFRP) in the frequency domain. To identify the composite's macro-scopic transversely isotropic viscoelastic material behaviour, micro-scale numerical studies are performed on statistically representative volume elements (SRVE) under periodic boundary conditions to obtain master curves of five independent components of the homogenised complex stiffness tensor in the frequency domain. These homogenised properties are then depicted by a generalised and by a generalised fractional Maxwell model (GMM, GFMM) building the dataset for parametrising user-defined material models implemented in commercial finite element method (FEM) software. The material models are thus able to capture the material behaviour over multiple orders of magnitude in frequency with a single set of parameters each. Consequently, the conversion of time-dependent data is avoided. A comparison between experimental and numerical results is carried out and further studies on the influence of the fibre volume content are performed. The proposed method is found capable of characterising the viscoelastic behaviour of CFRP in the frequency domain and, thus, presents a viable tool to investigate e.g. the damping characteristics of fibre-reinforced plastics on component level.

Microstructure-based numerical simulation of the mechanical behaviour of ocular tissue

This paper aims to present a novel full-eye biomechanical material model that incorporates the characteristics of ocular tissues at microstructural level, and use the model to analyse the age-related stiffening in tissue behaviour. The collagen content in ocular tissues, as obtained using X-ray scattering measurements, was represented by sets of Zernike polynomials that covered both the cornea and sclera, then used to reconstruct maps of collagen fibril magnitude and orientation on the three-dimensional geometry of the eye globe. Fine-mesh finite-element (FE) models with eye-specific geometry were built and supported by a user-defined material model (UMAT), which considered the regional variation of fibril density and orientation. The models were then used in an iterative inverse modelling study to derive the material parameters that represent the experimental behaviour of ocular tissues from donors aged between 50 and 90 years obtained in earlier ex vivo studies. Sensitivity analysis showed that reducing the number of directions that represented the anisotropy of collagen fibril orientation at each X-ray scattering measurement point from 180 to 16 would have limited and insignificant effect on the FE solution (0.08%). Inverse analysis resulted in material parameters that provided a close match with experimental intraocular pressure-deformation behaviour with a root mean square of error between 3.6% and 4.3%. The results also demonstrated a steady increase in mechanical stiffness in all ocular regions with age. A constitutive material model based on distributions of collagen fibril density and orientation has been developed to enable the accurate representation of the biomechanical behaviour of ocular tissues. The model offers a high level of control of stiffness and anisotropy across ocular globe, and therefore has the potential for use in planning surgical and medical procedures.

Numerical Modeling of Frictional Contact Between a Blunt Tool and Quasi-Brittle Rock

Frictional contact is an important component of rock cutting with a blunt cutter. There has been a gap in knowledge of frictional contact between experimental tests conducted on quasi-brittle rocks and previous numerical and analytical analyses on elastoplastic rocks. The average contact stress between a sliding wear flat and a rock is generally overestimated in previous studies without considering material length scales. This study analyzes the contact stress using an elasto-plastic-damage model that accounts for the material length scales of quasi-brittle materials. The constitutive model is developed for quasi-brittle materials by incorporating fracture energy, and it is implemented in the commercial finite element software ABAQUS as a user-defined material model. The implemented constitutive model is verified under uniaxial tension and compression. The average contact stress in frictional contact is mainly governed by a dimensionless elastoplastic parameter $$ \eta $$ for an elastoplastic rock, while it is predominantly governed by two dimensionless parameters: an elastoplastic parameter $$ \eta $$ and a brittleness number $$ \xi $$ for a quasi-brittle rock. The dimensionless elastoplastic parameter $$ \eta $$ contrasts the magnitude of a characteristic elastic contact stress to the yield strength of a material, and the brittleness number $$ \xi $$ is the ratio of a geometrical length scale to a material length scale. The tool–rock interface can be approximately divided into an elastic zone and a damage process zone. The contact stress generally increases in the elastic zone and then decreases in the damage process zone towards the tip of the slider. The average contact stress generally increases with the dimensionless elastoplastic parameter $$ \eta $$ and then levels off at a limit value, and it generally decreases with the brittleness number $$ \xi $$ . This study reduces the gap in previous studies to some extent by introducing the brittleness number $$ \xi $$ in frictional contact. The current numerical results of the average contact stress are generally consistent with typical experimental results conducted on quasi-brittle rocks, despite limitations in the finite element modeling.

Implementation of the Tresca yield criterion in finite element analysis of burst capacity of pipelines

The Tresca and von Mises criteria are two classic yield criteria for metals. While the von Mises criterion has been widely implemented in commercial finite element analysis (FEA) software package, the Tresca yield criterion in the context of the associated-plastic flow rule is not available in any well-known FEA packages such as ABAQUS and ANSYS. In this study, the Tresca yield criterion as well as the associated plastic flow and hardening rules are implemented in a user-defined material model (UMAT) in commercial FEA package ABAQUS based on the return-mapping technique. The consistent tangent operator for the Tresca criterion is also implemented in the UMAT model to facilitate the computation of the tangent stiffness matrix during the Newton-Raphson iteration for solving the nonlinear global equilibrium equations in the plastic domain. The constitutive integration algorithm for the Tresca UMAT is written in the principal stress space, which markedly simplifies the implementation. The implemented Tresca UMAT model is validated by theoretical solution and full-scale burst tests of both pristine and defected pipes. Furthermore, based on the Tresca-based and von Mises-based FEA predictions, a new formula is proposed to accurately predict burst pressure of thin-walled corroded pipes.

Automated calibration of advanced cyclic plasticity model parameters with sensitivity analysis for aluminium alloy 2024-T351

The plasticity models in finite element codes are often not able to describe the cyclic plasticity phenomena satisfactorily. Developing a user-defined material model is a demanding process, challenging especially for industry. Open-source Code_Aster is a rapidly expanding and evolving software, capable of overcoming the above-mentioned problem with material model implementation. In this article, Chaboche-type material model with kinematic hardening evolution rules and non-proportional as well as strain memory effects was studied through the calibration of the aluminium alloy 2024-T351. The sensitivity analysis was performed prior to the model calibration to find out whether all the material model parameters were important. The utilization of built-in routines allows the calibration of material constants without the necessity to write the optimization scripts, which is time consuming. Obtaining the parameters using the built-in routines is therefore easier and allows using the advanced modelling for practical use. Three sets of material model parameters were obtained using the built-in routines and results were compared to experiments. Quality of the calibration was highlighted and drawbacks were described. Usage of material model implemented in Code_Aster provided good simulations in a relatively simple way through the use of an advanced cyclic plasticity model via built-in auxiliary functions.

An ice material model for assessment of strain rate, temperature and confining pressure effects using finite element method

ABSTRACTThis paper addresses an investigation of ice constitutive laws modeling with strain rate, temperature and confining pressure effects of interest in modeling ice compressive behaviour. For the proposed phenomenological model consisting of elastic, delayed elastic and viscous components, strain rate is taken into account by introducing a viscous term based on Glen’s law. The effects of temperature and confining pressure are also included in the ice model. With the consideration that the viscous term and delayed elastic term are affected by temperature, the pressure hardening and pressure softening phenomena are embedded in the constitutive model. The proposed three-dimensional constitutive model is implemented in explicit LS-DYNA as a user-defined material model, and the numerical simulations of constant strain rate and creep experiments are conducted to verify the proposed ice material model. Ice strength and strain-time curves at different strain rates, temperatures and confining pressures are obtained and compared with experimental results.