User Subroutine Research Articles

T-REX: Numerical tool for tungsten damage assessment for DEMO

One of the R&D focus in the European fusion energy program is to establish a physical and technological basis for reliable power exhaust during entire operational situations of a DEMOstrational power plant. Following the design adopted for ITER, the baseline Plasma Facing Component for DEMO divertor is made of tungsten as armor material. The tungsten divertor target are the most thermally loaded in-vessel components in fusion reactor. The PFCs lifetime is affected by material degradations under the different loadings including High Heat Flux (HHF) and neutron irradiation. It has been reported a loss of mechanical properties due to softening (restoration/recrystallization). Latest finite element modeling developments allow to estimate component lifetime under HHF loading taking into account the progressive mechanical properties change due to softening. We propose here to take into account the influence of thermal and neutron loadings on the tungsten mechanical properties change for PFC lifetime assessment. For this, up to date mechanical properties of tungsten, softened tungsten, and neutron-irradiated tungsten are used as input data in the tool named T-REX (for Tungsten-Response based on EXperimental data). This tool is implemented as a user subroutine in a finite elements code. The influence of the neutron irradiation is studied as function of the dpa dose, the incident heat flux intensity and the current defined divertor component geometries (ITER and DEMO). With the study performed, it is shown that the geometry may be optimized to reduce the damage induced by plasticity. The numerical results also highlights that the plastic strain increment obtained after each thermal cycle reduces with the dpa dose.

Experimental and Numerical Analysis of the Concrete Maturation Process with Additive of Phase Change Materials.

The article presents selected types of phase change materials (PCM) and their properties in terms of applications in various fields of science such as construction and concrete technology. The aim of the article is to present a comparative analysis between the results of the laboratory tests and numerical simulations. The analysis contains two types of PCM (powder and in liquid), which were dosed in a hybrid system to the concrete mix. The purpose of using PCM is to allow the technological barrier to be exceeded in hot and dry climate conditions, enabling the construction of non-cracking concrete structures. The paper presents a parametric analysis of the influence of various modeling elements on the obtained results. The procedure of generating and absorbing heat caused by the applied PCM was also implemented using user subroutine into finite element code (Abaqus/Standard). The numerically obtained results are consistent with the experimental results. The presented results demonstrate that the use of PCM improves the conditions of concrete maturation by reducing the average temperature of the mixture in its entire volume.

Numerical Simulations of Lightning Strikes to Carbon Fiber Reinforced Polymers Using a Multi-Variable Approach

Carbon fiber-reinforced polymer (CFRP) composite materials are being employed widely in the manufacturing of next-generation aircraft because of their superior mechanical properties compared to those of metallic-based materials. Owing to growing interest in CFRP materials from large commercial and military aircraft manufacturers, such as Boeing and Airbus, research that investigates lightning strikes and associated damage is of paramount importance, as the fiber-polymer composite is highly resistive to electrical current. The high resistivity of traditional CFRP materials means that they are susceptible to extreme damage caused by the Joule effect, also known as resistive heating. The aim of the study presented in this paper is to determine and quantify the effectiveness of a developed multi-variable pyrolysis approach to model the transient material degradation during a lightning strike event. The study employs the finite element method (FEM) in the form of a coupled thermal–electrical analysis in conjunction with a user subroutine available in Abaqus. A parametric study was conducted to compare three different peak current values (20[Formula: see text]kA, 30[Formula: see text]kA and 40[Formula: see text]kA) based on a model that utilized pyrolysis-dependent material constituents as well as a novel model for specific heat capacity. The study also included a heat transfer step for a cool-down process. The results obtained from the model developed have strong agreement with the surface damage area and depth of damage compared with those from experimental analysis in literature.

Research on fracture of steel structure welded joint based on micro-mechanism

Fracture failure of welding joints is one of the primary forms in steel structure, and fracture prediction can provide suggestions and guidance for engineering applications. To predict the ductile fracture of welded joints, the Void Growth Model (VGM) and the Stress Modified Critical Strain Model (SMCS) based on the microscopic mechanism were studied. The tensile tests of Q355 steel, Q235 steel, ER50-3 and ER49-1 consumables commonly used in engineering were carried out. The constitutive models of four materials were established through the tensile tests of standard round bar specimens. The fracture parameters ηmon and γ of base metal, weld metal, and heat affected zone were calculated through the tensile tests of notched round bar specimens and the UVARM (User subroutine to generate element output). Finally, the fracture prediction model was used to predict the fracture of the thin-walled steel tube beam-column joints, and the applicability of the VGM and SMCS models was discussed. The function fθ¯ on the Lode Angle parameter is introduced to modify the VGM model. The modified fracture prediction model was used to predict the initial fracture of beam-column joints.

Delamination Criteria for SiN/TiW Interfaces Tested by Cross-Sectional Nanoidentation and by Four Point Bending

Interfaces between different material layers are potentially weakest links of microelectronic devices. Therefore, reliable criteria for delamination of structures are required as prerequisite of design optimization. In the present article, delamination of adjacent material layers is studied experimentally and by FEM simulation. An interface between SiN and TiW was delaminated by Cross-Sectional Nanoindentation (CSN) and by Four Point Bending. The adhesion energy observed during CSN is determined by FEM simulation using a method of equivalent loads, which compares CSN with a peel test. In a second approach, delamination during Four Point Bending is simulated by Mixed Finite Elements based on Strain Gradient Elasticity. The Mixed Elements were implemented in ABAQUS with user subroutine UEL. In this approach, the delamination criterion can be defined with use of a critical elastic energy density. The accordance between experiments and theoretical models is demonstrated.

Explicit double-phase-field formulation and implementation for bending behavior of UHPC-NC composite beams

The Phase-field model emerges as a promising method since it can automatically capture the initiation, propagation, coalescence, and branching of cracks without any tracking algorithms. Despite its success in simulating brittle and quasi-brittle cracks, tensile (mode I) and shear (mode II) fractures cannot be distinguished effectively. Moreover, the phase-field model is rarely applied to specimens at the structural level (i.e., reinforced concrete beams), due to the demanding computational cost. To address these shortfalls, two sets of phase-field variables are formulated to describe the tensile and shear fractures of concrete, respectively, through crack-direction-based decomposition of strain energy density into tensile, shear and compressive parts. Computational burdens are greatly alleviated by implementing the model with an explicit finite element solver (Abaqus/Explicit), allowing for massive parallel computing. The sensitivity analysis is performed to investigate the effects of critical parameters with a 1D numerical tensile bar. The double-phase-field model is applied to study the bending behavior of composite beams consisting of normal-strength concrete (NC) overlayer and ultra-high performance concrete (UHPC) substrate. To this end, five UHPC-NC beams, with various thickness ratios of UHPC and NC layers, were prepared and loaded to failure. District bending strengths, cracking patterns, deformation characteristics and failure modes of the four beams were revealed and discussed. It shows that the proposed model is necessary to accurately capture the bending behavior of UHPC-NC composite beams. This model is also applied to discuss the effects of critical parameters on the bending behaviors of UHPC-NC composite beams. To benefit potential users, detailed explanations regarding the data structures of input files and user subroutines (VUEL, VUMAT, VUSDFLD in Abaqus/Explicit) are given and released on GitHub repository.

Plasticity damage self-consistent model incorporating stress triaxiality and shear controlled fracture mechanisms – Model verification and validation

The plasticity damage self-consistent (PDSC) model is a damage model developed in the context of continuum damage mechanics in which the damage dissipation potential is formulated to account for damage contributions due to intervoid necking, shearing, and sheeting for an arbitrary stress state. The synergic action of different micromechanisms contributing to ductile fracture is accounted for by their dependence on the plastic deformation, stress triaxiality, third invariant of the deviatoric of the stress tensor, and unilateral conditions. In this work, the PDSC has been implemented in the commercial finite element code MSC MARC via user subroutines and used to predict ductile failure response under different stress state conditions. Firstly, the model implementation has been verified by comparing calculated damage evolution as a function of plastic strain, for prescribed combinations of stress triaxiality and Lode parameter under proportional loading, with an analytical solution. Successively, the model has been validated in predicting ductile fracture for Al6061-T6 in different specimen geometries and comparing with experiment results. Model capability to anticipate fracture initiation conditions, together with specimen global response, and ductile crack propagation also for complex load paths, is demonstrated.

Analysis of Hydrogen-Assisted Brittle Fracture Using Phase-Field Damage Modelling Considering Hydrogen Enhanced Decohesion Mechanism

This study proposes a hydrogen-assisted fracture analysis methodology considering associated deformation and hydrogen transport inside a phase-field-based formulation. First, the hydrogen transport around a crack tip is calculated, and then the effect of hydrogen enhanced decohesion (HEDE) is modeled by defining the critical energy release rate as a function of hydrogen concentration. The proposed method is based on a coupled hydrogen mechanical damage under phase-field and implemented through a user subroutine in ABAQUS software. The test using compact tension (CT) sample is investigated numerically to study the hydrogen embrittlement on 45CrNiMoVA steel. Experimentally, the microstructural fracture presents a mixed brittle fracture mode, consisting of quasi-cleavage (QC) and intergranular (IG) fracture with hydrogen. This fracture mode is consistent with the suggested HEDE mechanism in the model. The simulation results show that hydrogen accumulates at the crack tip where positive hydrostatic stress is located. Moreover, the model estimates the initial hydrogen concentration through iterations. The simulated load-line displacement curves show good agreement with the experimental plots, demonstrating the predictive capabilities of the presented scheme.

Formulating delamination-fretting wear failure predictive equation in HAp coated hip arthroplasty using multiple linear regression model

Present paper addresses the formulation of delamination-fretting wear failure predictive equation in HAp-Ti-6Al-4V interface of hip arthroplasty femoral stem component using multiple linear regression model. A finite element computational model utilising adaptive meshing algorithm via ABAQUS/Standard user subroutine UMESHMOTION is developed. The developed FE model is employed to examine effect of different HAp-Ti-6Al-4V interface mechanical and tribological properties on delamination-fretting wear behaviour. The FE result is utilised to formulate predictive equations for different stress ratio conditions using multiple linear regression analysis. Delamination-fretting wear predictive equations are successfully formulated with significant goodness of fit and reliability as a fast failure prediction tool in HAp coated hip arthroplasty. The robustness of predictive equations is validated as good agreement is noted with actual delamination-fretting wear results. The influence of different mechanical and tribological properties such as delamination length, normal loading, fatigue loading, bone elastic modulus and cycle number under different stress ratio on delamination-fretting wear failure is analysed to formulate failure predictive equations. The formulated predictive equation can serve as a fast delamination-fretting wear failure prediction tool in hip arthroplasty femoral stem component. Limited attempt is done to explore the potential of utilizing multiple linear regression model to predict failures in hip arthroplasty. Thus, present study attempt to formulate delamination-fretting wear failure predictive equation in HAp -Ti-6Al-4V interface of hip arthroplasty femoral stem component using multiple linear regression model.

Developments in remeshing‐free fatigue crack growth simulation including a new adaptive virtual crack closure technique

AbstractThe present study aims to evaluate and improve the remeshing‐free fatigue crack growth (FCG) simulation and life estimation through two developing FEM‐VCCT and XFEMPN‐VCCT algorithms in Abaqus. First, new energy‐based forms of the Paris, Walker, and Elber FCG models are implemented by a user subroutine, a novel systematic meshing strategy is proposed, and some challenging features of these algorithms are investigated. Despite the success of this methodology in enhancing efficiency, decreasing the run‐time, and preventing potential errors, it is observed that the XFEMPN‐VCCT algorithm has some fundamental errors such as a serious overestimation in FCG life prediction. Thus, a novel “adaptive VCCT” is introduced and implemented by a Python script in Abaqus in the form of a new XFEMPN‐AVCCT algorithm. It is finally concluded that the adaptive VCCT can significantly enhance the accuracy of FCG simulation and life estimation.

A constitutive model of cyclic plasticity with Lode dependence for structural steels

A finite strain constitutive model is proposed to represent the cyclic elastoplastic response of structural steels. The model simulates the Lode angle dependence of plastic response, which has been recently observed in experimental studies. The model combines Frederick-Armstrong kinematic hardening rules with an exponential isotropic hardening rule, and a non-associated plastic flow rule. A Lode angle-dependent coefficient is defined to modify the isotropic hardening response and determine the shape of yield surface, with respect to the conventional von Mises surface. Stabilization of hysteresis for cyclic plasticity is achieved through a memory surface, defined in the plastic strain space. Analytical formulas for an implicit Return-Mapping algorithm are derived to realize the numerical implementation of this model via a UMAT user subroutine. The effect of Lode dependence of the plasticity response of structural steel Q235 is investigated through monotonic and cyclic experiments on flat shear specimens. A process for parameter identification is outlined, and model predictions are validated through comparison with experimentally-measured force–displacement curves on uniaxial and shear specimens.

Microplane model of cylindrical geometry for transversely isotropic polymer composites

In this paper, a microplane model based on cylindrical geometry where the cylinder axis coincides with the longitudinal direction is developed for a general 3D inelastic fracturing analysis by finite element method of transversely isotropic fiber reinforced polymer composites. The model bridges the mesoscale material behavior to macroscale one using the stress equilibrium approach developed for cylindrical geometry. To this end, Microplane level stress–strain relations for tension, compression and shear for both longitudinal and transverse directions are formulated. An explicit algorithm is formulated and coded into a VUMAT user subroutine for use with ABAQUS. The model is suitable for the analysis and design of large structures made of fiber reinforced polymer composites unlike the competing mesoscale models because of its much lower computational cost. A detailed metodology for the calibration of the model is identified. The model is calibrated against the common test data available in the literature using this metodology and the predictive capacity of the model is demonstrated by comparing the model predictions with test data in longitudinal tension, lateral tension, longitudinal compression, lateral compression, longitudinal shear and rare size effect in bending tests including post-peak fracture available in the literature.

Quantification of the critical displacement initiating collapse of steel moment frames due to an interior column loss

Quantification of the critical collapse displacement is important to establish the collapse-resistant design method for the steel frame under a column-removal scenario. In this paper, the critical collapse displacement of steel frames due to an interior column loss is studied. The Bai-Wierzbicki fracture model of steel is incorporated into ABAQUS via user subroutine, in which the effects of both stress triaxiality and Lode angle on the fracture strain of steel are considered. The effects of the depth, span, axial restraint and rotational restraint of the beam on its critical collapse displacement are studied. The relationships of the critical collapse displacement with the depth, span and boundary conditions of the beam are established. The reliability of the proposed method is validated against the test results. It is found that the ratio of the critical collapse displacement to the depth of the beam can be expressed as a linear function of the span-to-depth ratio of the beam. The rotational restraint of the beam mainly affects the transition stage of the resistance-displacement curve of the beam and it has limited effect on the critical collapse displacement of the beam. The axial restraint mainly affects the catenary stage of the resistance-displacement curve of the beam and when the axial stiffness ratio exceeds 0.01, the effect of the rotational restraint on the critical collapse displacement of the beam can be neglected.

Numerical simulation of post-weld shot peening using combined discrete and finite element methods (DEM-FEM)

The numerical simulation of the post-weld shot peening (SP) process was performed to analyze the influence of various SP parameters on the final induced compressive residual stress (CRS). Firstly, the welding process was simulated based on the Finite Element Method (FEM) using ABAQUS CAE and FORTRAN user subroutine code, from which the initial welding-induced tensile residual stresses (TRS) were obtained. The FE model was validated using the results obtained from the TIG welding of the aluminum 2219 alloy (AA 2219) plate. Then, the post-weld SP process was numerically simulated using the combined discrete and finite element methods (DEM-FEM). The computed CRS from the simulation was compared with the experimentally measured outcome to validate the DEM-FEM model, and good agreement was observed from the compared results. Finally, the post-weld SP model was applied to extensively analyze the influence of the various SP parameters related to the peening media and target material on the final CRS.

Evaluation of uncoupled ductile damage models for fracture prediction in incremental sheet metal forming

The formability limit in a typical incremental sheet forming (ISF) far exceeds the conventional estimate assuming necking failure. The material subjected to incremental forming fails by fracture. Although damage models have been used to correlate the failure in incremental forming, a detailed understanding of the failure mechanism spanning strain paths is not clear yet. In this work, three parts with varied shapes (hybrid five lobe, pyramid and variable wall angle conical frustum (VWACF)) were developed using ISF to cover a range of possible strain paths. The failure is predicted using established uncoupled phenomenological damage models. Damage models were calibrated for AA1050 aluminium sheet by the method of inverse approach using carefully designed tensile tests and FE simulations. Three different damage models were implemented as user subroutine in commercial software code, ABAQUS/Explicit and the results predicted were compared. The linear damage accumulation used to develop fracture locus under monotonic loading could not predict the failure limit in a benchmark single groove test. Therefore a non linear damage accumulation rule (NLDA) is implemented to simulate ISF. The parameters of the NLDA were calibrated from the single groove test. The fracture forming limits during ISF was established using circular grid analysis near the failure zone of the formed part. A good agreement can be found between the experimental observations and numerical predictions for fracture location and part height. It was observed that the overall predictive capability of Hosford Coulomb (HC) damage model is better among three damage models investigated for the given range of loading conditions. However all the three models under-predicted the experimental fracture strain measured in ISF. The possible explanation for the discrepancy is explored.

Anti-collapse behavior of modular steel buildings with corrugated panels

This paper numerically studies the anti-collapse behavior of the modular steel building (MSB) in the scenario of interior module removal and corner module removal. The vertical displacement, lateral displacement and internal force of a six-storey MSB in a sudden module loss are studied. The reasonability by using rigid body assumption in collapse analysis of the MSB is examined. A refined fracture model of the steel is incorporated into ABAQUS via user subroutine, in which the effects of both stress triaxiality and Lode angle on the fracture ductility of the steel are considered. In the pushover analyses, the effects of the horizontal connection, opening of the wall panel and thickness of the wall panel on the collapse resistance and failure mode of the MSB are studied. The dynamic amplification factor (DAF) of the MSB at the collapse limit state is also obtained. It is found that the rigid body assumption may significantly underestimate the displacement and maximum shear force in the horizontal connection of the MSB. Two types of failure modes of the structure are observed, namely, connection failure-induced collapse and module unit failure-induced collapse. Among the studied parameters, the effect of the opening size of the wall panel on the behavior of the structure is the most significant. In the scenario of the interior module removal and corner module removal, the DAFs of the structure at the collapse limit state range from 1.20–1.29 and 1.27–1.50, respectively.

A unified framework of cell population dynamics and mechanical stimulus using a discrete approach in bone remodelling

Multiphysics models have become a key tool in understanding the way different phenomenon are related in bone remodeling and various approaches have been proposed, yet, to the best of the author’s knowledge there is no model able to link a cell population model with a mechanical stimulus model using a discrete approach, which allows for an easy implementation. This article couples two classical models, the cell population model from Komarova and the Nackenhorst model in a 2D domain, where correlations between the mechanical loading and the cell population dynamics can be established, furthermore the effect of different paracrine and autocrine regulators is seen on the overall density of a portion of trabecular bone. A discretization is performed using frame 1D finite elements, representing the trabecular structure. The Nackenhorst model is implemented by using the finite element method to calculate the strain energy as the main mechanical stimulus that determines the bone mass density evolution in time. This density is normalized to be added to the bone mass percentage proposed by the Komarova model, where coupling terms have been added as well that guarantee a stable response. In the simulations, the equations were solved employing the finite element method with a user subroutine implemented in ABAQUS (2017) and by applying a direct formulation. The methodology presented can model the cell dynamics occurring in bone remodelling in accordance with the asynchronous nature of this process, yet allowing to differentiate zones with higher density, the main trabecular groups are obtained for the proximal femur. Finally, the model is tested in pathological cases, such as osteoporosis and osteopetrosis, yielding results similar to the pathology behavior. Furthermore, the discrete modelling technique is shown to be of use in this particular application.

Predictive modeling of damage evolution and ductile fracture in bending process

The present study aims to propose a new predictive damage model considering the stress state and the effect of process parameters. To this end, an intelligent model founded based on both data mining and ductile damage model is developed to predict the fracture of AA6061-T6 during the bending process. The normalized Cockroft-Latham criterion is employed to numerically predict the fracture onset. The criterion is calibrated based on the plane strain tension test. Then, the calibrated criterion is implemented into the finite element (FE) Abaqus/explicit software using an appropriate user subroutine. The stress state analysis in the bending and the plane strain test indicate that the calibrated damage model could provide a close description of damage distribution and fracture onset during the bending process, and also the experimental results prove that the FE analysis could predict fracture onset at an error rate of 2.57%. Also, the combination of machine learning tools and the FE damage model is used to propose a predictive model of damage evolution and ductile fracture initiation during the bending process. As a verification of the proposed model, the damage values in various forming conditions that are out of the learning database are compared between machine-learned and FE simulated damage results. In this regard, it is proved that the proposed model efficiently predict the damage evolution successfully for an arbitrary combination of process parameters including thickness, bending radius, and bending angle. From this investigation, the proposed machine-learned predictive damage model is proved to supply a useful optimal soft computing approach in the bending process. Finally, the forming conditions in which the sheet metal could be successfully deformed into U-shape product (bend up to 90 degree) are determined, and also the minimum achievable bending radius in some cases are predicted by the proposed method.

Inversion of dislocation densities under mixed mode fracture

Mixed-mode crack growth is here investigated through experiments and computations for 34X and P2M steels, 7050 aluminium, and Ti-6Al-4V alloys in a compact tension shear (CTS) specimen. In this study, we use the mechanism-based strain gradient (MSG) plasticity theory to evaluate both crack tip dislocation density behaviour and the coupled effect of the material plastic properties and the intrinsic material length on local stress distributions. The constitutive relations are based on Taylor’s dislocation model, which allows to gain insights into the role of the increased dislocation density associated with large gradients in plastic strain near cracks. The material model is implemented in a commercial finite element (FE) software package using a user subroutine, and the nonlinear stress intensity factors (SIFs) are evaluated as a function of the intrinsic material length. As a result of the FE calculations of dislocation density distributions, the effects of both the fracture mode and the stress–strain state are determined. Strain gradient effects associated with dislocation hardening mechanisms elevate crack tip stresses relative to conventional plasticity. Dislocation densities, stress fields and nonlinear SIF solutions are determined for experimental curvilinear crack paths by taking into account the transition from the initial Mode II crack to the mixed-mode fracture.

Finite-element modelling of alkali–aggregate reaction in a concrete hydraulic structure

Alkali–aggregate reaction (AAR) in concrete has been found to cause serious concerns for the operation and integrity of many mass and reinforced concrete structures, hydraulic structures (dams, powerhouses, etc.) and any other concrete structure that is exposed to moisture. It is known that the kinetics of AAR is strongly driven by temperature and moisture, among other parameters, and the induced strain is assumed to be oriented according to the stress state. Due to complexity of the AAR and its multi-physical nature, the use of chemomechanical modelling is very helpful for making predictions in terms of displacements and concrete damage. Moreover, macro-modelling approaches are frequently preferred for engineering work in real structures. In this context, this paper presents the implementation of a chemomechanical model of AAR for concrete using Abaqus/Explicit modelling software. With this approach, the effects of AAR are introduced by way of the Vuexpan user subroutine jointly with the concrete damage plasticity model of Abaqus. Verification of the proposed model was carried out at the material level. Moreover, a case study of a real hydraulic structure affected by AAR located in North America is presented.