Set Of Material Parameters Research Articles

Meso-scale compaction simulation of multi-layer 2D textile reinforcements: A Kirchhoff-based large-strain non-linear elastic constitutive tow model

This paper presents a non-linear, large strain constitutive approach to modelling the mechanics of fibrous tows. The isotropic and linear St. Venant hyperelastic model was modified to extend its applicability to a transversely isotropic continuum. From this model, a set of large-strain material parameters which resemble the linear-elastic moduli was derived. The non-linear constitutive response of fibrous tows was accounted for through a modelling scheme that incorporates the instantaneous microstructural arrangement of the tows. Tow samples were isolated from the studied textile roll and subjected to a compression study utilising a custom-made tow compression rig, from which a set of material functions that fully defined the constitutive model were generated. The developed model was then implemented as a material subroutine and utilised in a mesoscopic compaction simulation of a multi-layer 2D woven reinforcement. It was found that the constitutive model performed very well in predicting the shape changes and stresses in deforming tows.

Modeling the effect of temperature and degree of crystallinity on the mechanical response of Polyamide 6

In the current work, an extensive experimental study is performed, to investigate the influence of both, the applied thermal conditions over a wide range of temperatures and the manufacturing process induced degree of crystallinity on the mechanical response of semi-crystalline polymers. To this end, large-strain tensile experiments with different loading procedures (i.e. monotonic, cyclic, and relaxation tests) are conducted on Polyamide 6 for different loading rates. The experimental data base provides new insights into the complex dependencies of the effective material properties on the aforementioned factors and serves as the foundation for the development of a continuum mechanical constitutive framework.The phenomenological, isothermal model is developed in a reasonably general, thermodynamically consistent manner, to predict the strain rate, temperature and degree of crystallinity dependent large-deformation response of semi-crystalline polymers. A coupled nonlinear visco-elastic, elasto-plastic theory, incorporating nonlinear isotropic and kinematic hardening, is proposed to capture the complex material behavior (e.g. strain recovery and hysteresis loop after cyclic loading-unloading and nonlinear stress relaxation). A staggered characterization method is proposed, to identify a set of material parameters from the experimental data. Finally, validation studies demonstrate the great capabilities of the novel constitutive framework, to accurately predict the significant influence of the temperature and degree of crystallinity on the material response.

Simulation and validation of residual deformations in additive manufacturing of metal parts

Selective laser melting (SLM) is gaining increasing relevance in industry. Residual deformations and internal stresses caused by the repeated layerwise melting of the metal powder and transient cooling of the solidified layers still presents a significant challenge to the profitability and quality of the process. Excessive distortions or cracking may lead to expensive rejects. In practice, critical additively manufactured parts are either iteratively pre-compensated or redesigned based on production experience. To satisfy the need for improved understanding of this complex manufacturing process, CAE software providers have recently developed solutions to simulate the SLM process. This study focuses on the evaluation of two solutions by ANSYS, i.e. ANSYS Additive Print and ANSYS Additive Suite.ANSYS Additive Print (AAP), a user-oriented software, and ANSYS Additive Suite (AAS), a software requiring advanced experience with Finite Element Methods (FEM), are investigated and validated with regard to residual deformations. For the evaluation of the two programs, calibration and validation geometries were printed by SLM in Ti–6Al–4V and residual deformations have been measured by 3D scanning. The results have been used for the calibration of isotropic and anisotropic strain scaling factors in AAP, and for sensitivity analyses on the effect of basic model parameters in AAS. The actual validation of the programs is performed on the basis of different sample geometries with varying wall thickness and deformation characteristic.While both simulation approaches, AAP and AAS, are capable of predicting the qualitative characteristics of the residual deformations sufficiently well, accurate quantitative results are difficult to obtain. AAP is more accessible and yields accurate results within the calibrated regime. Extrapolation to other geometries introduces uncertainties, however. AAS, on the other hand, features a sounder physical basis and therefore allows for a more robust extrapolation. Numerical efforts and modelling uncertainties as well as requirements for an extensive set of material parameters reduce its practicality, however. More appropriate calibration geometries, continuing extension of a more reliable material database, improved user guidelines and increased numerical efficiency are key in the future establishment of the process simulation approaches in the industrial practice.

A phenomenological expression of strain energy in large elastic deformations of isotropic materials

A new model is constructed to validate a non-linear elastic response of rubber-like materials. The proposed model is phenomenological and is the combination of Rivlin expression truncated in invariant $$\mathop I\nolimits_{1}$$ with a term in $$\mathop I\nolimits_{2}$$ coming from the Hart-Smith model and which has been modified by Gornet et al. This model has five rheological parameters adjusted with a genetic algorithm. The genetic algorithm method is used in binary code and two sets of experimental data (Treloar data and Nunes and Moreira data) from the literature to compare the proposed model. With Treloar rubber data the simple tension suffices for the determination of the parameter values. Thus, the fit of experimental data is shown to provide an accurate response and predictive description in biaxial tension and pure shear with the same sets of material parameters. Unlike, with Moreira data, the proposed model has given direct material parameters in simple shear with good correlation. On the other hand, a comparison is made between the proposed model and the Beda model (2007), Gornet et al. model and hybrid integral approach (HIA) model. The proposed model is the most efficient among three other classical models because the proposed model emerges from very small relative errors with Treloar's experimental data: 0.42 ± 0.08 in simple tension, 0.44 ± 0.07 in biaxial tension and 0.42 ± 0.05 in pure shear. This model also incorporates all the criteria for model validation described in the literature.

System Identification of a Soil Tunnel Based on a Hybrid Artificial Neural Network–Numerical Model Approach

System identification of structures is the primary goal of this study. Numerical simulation using methods such as finite element modelling or finite difference modelling is the practical solution to model the structure based on some accurate parameter values that are essential to identify the behaviour law of the structures. An optimisation method integrated with the numerical model (NM) can solve an inverse problem to provide a calibrated parameter set to improve the modelling. For this purpose, an application of artificial neural networks (ANNs) integrated with an NM with the title “hybrid ANN–NM” approach is proposed in this study so that the parameters and resulted deformations of a developed NM of the structure would enter to a designed ANN for training the network. The developed hybrid ANN–NM method to identify the system of a tunnel excavated in the soil is applied and reported in this paper. A set of variable material parameters with a set of displacement and strain data (in $$x$$ and $$y$$ directions) of critical points of the tunnel which obtained from 2D FDM in FLAC 7.00 software is provided to train the ANN. This study is divided into two methods: (1) fitting model to the displacements and strains to show the deformation behaviour of the tunnel related to the parameter changing, and (2) fitting model to the parameters related to the displacements and strains to estimate the optimum parameters for the NM. For the first method, the parameter dataset as input and the displacements and strains as the ANN’s output in one approach separately and in another approach altogether are considered to train the ANN. In the second method, the displacements and strains as input and the physical parameters of the NM as output are submitted to train the ANN. The MSE convergence of implemented ANN shows the power of ANN to model the behaviour of structure based on the token data. Also, the RMSE of residuals refers to the success of the proposed method for the fitting model. Besides, this method was able to provide the optimum parameters for the numerical model in minimum computing time regarding the implementation of the second method.

Refinement of the Hardening Soil model within the small strain range

The popularity of the elasto-plastic Hardening Soil (HS) model is based on simple parameter identification from standard testing and empirical formulas. The HS model is implemented in many commercial FE codes designed to analyse geotechnical problems. In its basic version, the stress–strain behaviour within the elastic range is subject to the hypoelastic power law, which assures the barotropy of the elastic stiffness. However, a proper modelling within the small strain range, i.e. strain-induced stiffness degradation and correct reproduction of the hysteretic behaviour, was one of the most important drawbacks in the HS formulation. The first small strain stiffness extension to the HS model was proposed by Benz (Small strain stiffness of soils and its numerical consequences, 2007), and the new model was called Hardening Soil Small (HSS). Despite the simple isotropic formulation, its applicability was proved in various numerical simulations in geotechnics. However, the HSS formulation exhibits a serious fault known in the literature as overshooting, i.e. uncontrolled reset of the loading memory after tiny unloading–reloading cycles. The authors' main aim was to retain the set of material parameters for the HSS formulation and to propose a new small strain extension to the HS model without overshooting. The new proposal is based on the Brick model which represents the concept of nested yield surfaces in strain space. The implementation aspects of the new HS-Brick model are described, and its performance is presented in some element tests and selected boundary value problems by comparisons with the HSS formulation.

Identification of inelastic parameters of the AISI 304 stainless steel: a multi-test optimization strategy

ABSTRACT The AISI 304 stainless steel has largely been used in industry owing to its good formability characteristics. Therefore, the use of proper material parameters is essential for a successful numerical simulation of forming operations. Within this framework, this work addresses a parameter identification strategy using strategies based on multiple mechanical tests – a multi-test optimization scheme. The technique is used to determine isotropic hardening parameters of the AISI 304 steel using tensile tests of specimens of different geometries. The proposed method makes possible to obtain a representative set of material parameters taking into account the material response up to macroscopic failure. The study shows that the technique provides a minimum improvement of 30.43% in the global error function when comparing against single test results.

Effect of Affinity Discontinuity on Heterojunction p-i-n Solar Cell Performance

Effect of the band alignment on the performance of heterojunction p-i-n solar cell is investigated in the framework of one-dimensional transport model. An affinity-based approach is introduced in order to identify a set of material parameters and doping levels which would result in lower series resistance R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sub> , higher open-circuit voltage V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">oc</sub> , and higher fill factor FF. The search reveals key ingredients of the nongraded heterojunction optimized design, whose efficiency can be as high as 27%. On one hand, certain band alignment determined by affinity-related quantities has to be observed to ensure the case of staggered gaps, once the dimensional requirements in the absorber and in the charge collectors are met. On the other hand, an affinity-based constraint is shown to control the potential profile which mitigates the adverse effect of barriers and accumulation regions at the junction interfaces. The constraint is shown to result in lower R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sub> and, thus, higher FF. Analytical approach reveals a whole region in the affinity space where the reverse saturation current J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> is at minimum, which also strongly overlaps with the constraints pinning down R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sub> . It is concluded that the performance figures are featured with shallow minima as functionals in roughly same region of the affinity space, where those numbers are relatively stable versus changes in the band alignment. The findings aim to make material screening more feasible and widens significantly the options in the design of new p-i-n cells.

Experimental evaluation of stress concentration ratio of soft soil reinforced with stone column

Ground improvement using stone column reinforcement is an effective treatment technique to increase the stiffness and reduce the total and differential settlement of the soft deposits. Even though stone column reinforcement is a well-established technique, detailed experimental study regarding the load-sharing characteristics and parameters influencing the stress concentration ratio (SCR) between the soil and column is very limited. In the present study through a systematic laboratory investigation, time-dependent and peak SCR between the stone column and surrounding soil has been evaluated for a practical range of stone column, surrounding soil, top sand blanket thickness and column material properties. Stress experienced from the foundation loading to stone column and surrounding soil in a unit cell has been measured using earth pressure cells. Peak SCR has been observed to be in the range of 4–6 for the set of material parameters investigated in the present study. Peak SCR is in the range of 4–5.5 when internal friction angle of the stone column material varies from 38° to 42°. SCR is significantly influenced by the thickness of the top sand blanket. However, an insignificant influence of sand blanket material properties on the SCR has been observed. Additionally, surrounding soil strength significantly influences the SCR of the composite system. The results of the present study will be useful for the estimation of stiffness and settlement of the stone column-reinforced composite ground.

Energy-Based Modelling of Adhesive Wear in the Mixed Lubrication Regime

Adhesive wear in dry contacts is often described using the Archard or Fleischer model. Both provide equations for the determination of a wear volume, taking the load, the sliding path and a set of material parameters into account. While the Fleischer model is based on energetic approaches, the Archard formulation uses an empirical factor—the wear coefficient—describing the intensity of wear. Today, a numerical determination of the wear coefficient is already possible and both approaches can be deduced to a local accumulation of friction energy. The aim of this work is to enhance existing energy-based wear models into the mixed lubrication regime. Therefore, the pressure distribution within the contact area will be determined numerically taking real surface topographies into account. The emerging contact area will be divided into one part of solid and a second part of elastohydrodynamically lubricated (EHL) contacts. Based on the resulting pressure and shear stress distribution, the formation of micro cracks within the loaded volume will be described. Determining a critical number of load cycles for each asperity, a locally resolved wear coefficient will be derived and the local wear depth calculated.

A constitutive model for granular materials subjected to a large stress range

Abstract A simple constitutive model is presented to represent the behavior of granular materials subjected to a large stress range by introducing a revised normal compression line with a limit void ratio (eL) in the double logarithmic scale and a state parameter (ξ) to quantify the state of granular materials. In the proposed model, a drop-shaped yield surface is employed and a unified hardening parameter (H) that is a function of the state parameter (ξ) is developed to govern the hardening process. The model is first established in the triaxial compression stress state and then is extended to the general stress state through the transformed stress (TS) method. Adopting a single set of material parameters, the proposed model is verified against the experimental results of various tests on Cambria sand at different confining pressures (0.25–68.9 MPa), including the data of isotropic compression test and drained/undrained triaxial compression/extension tests.

Pseudo-elastic cavitation model: part I\u2014finite element analyses on thin silicone adhesives in fa\xe7ades

This study investigates the structural behavior of adhesive bonds of glass and metal using thin, structural silicones in heavily constrained applications. This special type of connection may lead to triaxial stress conditions under axial loading, which can lead to dilatation failure due to the abrupt growth of cavities (cavitation effect). Cavitation failure leads to significant stress softening and loss of stiffness; however, it increases connection’s ductility. These material deformations should be considered when designing glass-metal connections. Therefore, a constitutive model is developed to account for cavitation in hyperelastic materials. The volumetric component of the model is equipped with a non-linear Helmholtz free energy function that accounts for isotropic void growth under hydrostatic loading. An energy coupling term is then added that numerically explicates strain energy under isochoric deformation, while also guaranteeing physical material behavior. The energy contribution is calculated internally by analysing the geometric evolution of inherent voids. The extended volumetric–isochoric split enables one to numerically calculate heavily constrained silicone joints under arbitrary deformation modes. Three-dimensional finite element calculations on uniaxial tension, bulge, and pancake tests validate the constitutive model. All experiments could be validated with one set of material parameters through numerical simulations. The numerical calculations were robust and efficient without any underlying mesh dependencies.

The forward and inversion analysis of high rock-fill dam during construction period using the node-based smoothed point interpolation method

PurposeThe meshfree node-based smoothed point interpolation method (NS-PIM) is extended to the forward and inversion analysis of a high gravelly soil core rock-fill dam during construction periods.Design/methodology/approachAs one member of the meshfree methods, the NS-PIM has the advantages of “softer” stiffness and adaptability to large deformations which is quite indispensable for the stability analysis of rock-fill dams. In this work, the present method contains a reconstruction procedure to deal with the existence or nonexistence of the construction layers. After verifying the validity of the NS-PIM method for nonlinear elastic model during construction period, the convergence features of the NS-PIM and FEM methods are further investigated with different mesh schemes. Furthermore, the NS-PIM and FEM methods are applied for the forward analysis of a high gravelly soil core rock-fill dam and the convergence features under complex stress conditions are also studied using the rock-fill dam model. Finally, the NS-PIM method is used to calculate the Duncan–Chang parameters of the deep overburden under the high gravelly soil core rock-fill dam based on the back-propagation neural network method.FindingsThe results show that: the NS-PIM solution for construction analysis still possesses the property of upper bound solution even under complex stress conditions and can provide comparatively more conservative results for safety evaluation. Furthermore, it can be used to evaluate the accuracy of results and mesh quality together with the FEM solution which has the property of lower bound solution; the inversion analysis in this work provides a set of material parameters for the deep overburden under high rock-fill dam during construction period and the calculated results show good agreement with the measured displacement values and it is feasible to apply the NS-PIM to the forward and inversion analysis of high rock-fill dams on deep overburden during construction periods.Research limitations/implicationsIn further study, the feasibility of three-dimensional problems, elastic–plastic problems, contact problems and multipoint inversion can still be probed in the NS-PIM solution for the forward and inversion analysis of high rock-fill dams on deep overburden.Practical implicationsThis paper introduced a method for the forward and inversion analysis of high rock-fill dams during construction period using the NS-PIM solution. The property of upper bound solution ensures that the NS-PIM can provide more conservative results for safety evaluation. The inversion analysis in this work provides a set of material parameters for the deep overburden under high rock-fill dam during construction periods.Originality/valueFirst, the analysis from forward to inversion for high rock-fill dams during construction period using the NS-PIM solution is accomplished in this work. A procedure dealing with the existence or nonexistence of the construction layers is also developed for the construction analysis. Second, it is confirmed in this work that the NS-PIM still possesses the property of upper bound solution even under complex stress conditions (the forward analysis of high rock-fill dams during construction period). Thus, more conservative results can be provided for safety evaluation. Furthermore, it can be used to evaluate the accuracy of results and mesh quality together with the FEM solution which has the property of lower bound solution. Third, the calculated material parameters of the deep overburden in this work can be used for further studies of the high rock-fill dam.

A comparative study of different model families for the constitutive simulation of viscous clays

SummaryThe simulation of the viscous behavior of some clays is of high importance in many geotechnical problems. The literature offers a vast amount of constitutive models able to simulate the rate dependence observed on these materials. Although most of these models are calibrated to very similar experimental observations and share similar definitions of material parameters, some discrepancies of their response have been detected, which are related to their mathematical formulations. In this work, the causes of these discrepancies are carefully studied. To that end, four different model families are analyzed, namely, nonstationary flow surface (NSFS) models, viscoplasticity with overstress function (OVP), viscoplasticity with Norton's power law (NVP), and visco‐hypoplasticity (VHP). For the sake of a fair comparison, single constitutive models using the same set of material parameters, and following other requirements, are developed for each model family. Numerical implementations of the four resulting models are performed. Their response at different tests are carefully analyzed through simulation examples and direct examination of their constitutive equations. The set includes some basic tests at isotropic stress states and others as responses envelopes, undrained creep rupture, and an oedometer test with loading, unloading‐reloading, creep, and relaxation. The article is concluded with some remarks about the observed discrepancies of these model families.

Experiments and numerical simulation of damage and fracture of the X0-specimen under non-proportional loading paths

The paper deals with new non-proportional biaxial experiments and corresponding numerical simulations to analyze the load-history dependence on damage and fracture behavior of ductile metals. The numerical simulations are based on a phenomenological, thermodynamically consistent anisotropic continuum damage model considering the effect of stress triaxiality and Lode parameter on damage behavior. The model as well as the corresponding material parameter identification is discussed and a full set of material parameters is given. The new biaxial experiments take into account non-proportional and corresponding proportional loading paths with special focus on stress states changes from shear to tension dominated cases. The corresponding strain fields of critical regions of the X0-specimen have been analyzed by digital image correlation technique and indicate good accordance with the numerically predicted ones. The evolution of the calculated plastic and damage fields elucidate the relevance of both independent mechanisms. The results are supported by scanning electron microscopy images of the fracture surfaces. This experimental–numerical technique applied to the non-proportional biaxial experiments provides new insights of the stress and load-history dependent damage and fracture processes.

Microstructurally-guided explicit continuum models for isotropic magnetorheological elastomers with iron particles

This work provides a family of explicit phenomenological models both in the F−H and F−B variable space. These models are derived directly from an analytical implicit homogenization model for isotropic magnetorheological elastomers (MREs), which, in turn, is assessed via full-field numerical simulations. The proposed phenomenological models are constructed so that they recover the same purely mechanical, initial and saturation magnetization and initial magnetostriction response of the analytical homogenization model for all sets of material parameters, such as the particle volume fraction and the material properties of the constituents (e.g., the matrix shear modulus, the magnetic susceptibility and magnetization saturation of the particles). The functional form of the proposed phenomenological models is based on simple energy functions with small number of calibration parameters thus allowing for the description of magnetoelastic solids more generally such as anisotropic (with particle-chains) ones, polymers comprising ferrofluid particles or particle clusters. This, in turn, makes them suitable to probe a large set of experimental or numerical results. The models of the present study show that in isotropic MREs, the entire magnetization response is insensitive to the shear modulus of the matrix material even when the latter ranges between 0.003-0.3MPa, while the magnetostriction response is extremely sensitive to the mechanical properties of the matrix material.

Simulation of anisotropic crack growth behavior of nickel base alloys under thermomechanical fatigue

Allvac®718Plus is a nickel base superalloy possessing outstanding abilities to withstand both mechanical and thermal loads, which make this material ideal for turbine disks and blades in aero engines. The thermomechanical fatigue (TMF) crack growth behavior depends on temperature, in-service load cycles and material orientation. In particular, the crack growth at high temperature dwell time shows significant influence of material anisotropy due to platelet-like precipitations, which leads to crack deflection from the maximum principal stress direction. These specific properties must be taken into account properly to ensure a safe design and the desired lifetime of components. The focus of this paper is to develop an appropriate fracture criterion to describe crack deflection for such kind of anisotropic material resistance. This criterion and the associated crack growth laws for thermomechanical fatigue are implemented in the FEM software Procrack for fully automated three-dimensional crack propagation simulation. The numerical simulation of experiments on CT-specimens with various material orientations provided evidence that the software can capture the actual fracture behavior. Moreover, a suitable set of material parameters could be identified. The performance of the presented approach to predict realistic three-dimensional TMF crack propagation is demonstrated by an application to a typical turbine component made from Allvac® 718Plus.

Electric field driven transformations of orientational structure in chiral nematic systems with large flexoelectricity

Transformation of the orientational structure in a plane-parallel cell of a cholesteric liquid crystal, was studied depending on the magnitude of the voltage applied to the boundary planes of the cell. The equilibrium configuration of the liquid crystal director field was found minimizing the free energy both analitically and numerically. We took into account the orientational elastic energy in the volume and at the cell boundaries, the energy of an inhomogeneous electric field, and the flexoelectric effect. Systems with large flexoelectric coefficient were studied in detail. It was found, that when small voltages are applied to the cell boundaries, a continuous Fredericks transition occurs first, and then, when certain characteristic voltage is achieved, a significant jump-like change in the orientational structure takes place, and this structure gradually changes with the further voltage increase. It is shown that various scenarios of transformation of the orientational structure can be implemented depending on the actual values of the set of material parameters of the system studied. The discovered ability to control the structure of a liquid crystal in a cell can find use in various technical devices.

Estimation of Stress Distribution in Timber Beams under Steady Changes of Moisture Content

The paper relates to the effect of steady-state change of moisture content in timber elements. Several constitutive models of the material used in the literature have been presented. For the purpose of the simulation, a three-dimensional FEM model was prepared. Results were presented based on four models of moisture distribution and three sets of material parameters. Based on the literature and simulation results, the necessity of changing the practical design approach was discussed.

Bayesian inference of non-linear multiscale model parameters accelerated by a Deep Neural Network

We develop a Bayesian Inference (BI) of the parameters of a non-linear multiscale model and of its material constitutive laws using experimental composite coupon tests as observation data. In particular we consider non-aligned Short Fibers Reinforced Polymer (SFRP) as a composite material system and Mean-Field Homogenization (MFH) as a multiscale model. Although MFH is computationally efficient, when considering non-aligned inclusions, the evaluation cost of a non-linear response for a given set of model and material parameters remains too prohibitive to be coupled with the sampling process required by the BI. Therefore, a Neural-Network (NNW) is first trained using the MFH model, and is then used as a surrogate model during the BI process, making the identification process affordable.