Measurement and Calibration of the Parameters of Hypoplasticity Model for an Iraqi Soil

2020 ◽  
Vol 857 ◽  
pp. 243-252
Author(s):  
Aysar Hassan Subair ◽  
Ala Nasir Aljorany
Keyword(s):  
Constitutive Model ◽  
Void Ratio ◽  
Initial Stresses ◽  
Model Parameters ◽  
State Variables ◽  
Soil Behavior ◽  
Dense Sand ◽  
Hypoplastic Model ◽  
Material Parameters ◽  
Wide Range

There are many constitutive models that have been used to model the mechanical behavior of soils. Some of these models are either unable to represent important features such as the strain softening of dense sand or required many parameters that can be hard to obtain by standard laboratory tests. Because of that, a more reliable constitutive model, which is capable to capture the main features of the soil behavior with easily obtained parameters, is required. The Hypoplasticity model is considered as a promising constitutive model in this respect. It is considered as a particular class of rate non-linear constitutive model at which the stress increment is expressed in a tensorial equation as a function of strain increment, actual stress, and void ratio. The hypoplastic model required only eight material parameters (critical friction angle critical, maximum and minimum void ratio respectively), granular stiffness hs and the model constants n, α, β). The appealing feature of the hypoplastic model is that the material parameters are separated from the state variables (void ratio and the initial stresses). This feature enables the model to simulate the soil behavior under a wide range of stresses and densities with the same set of material parameters. In this research, a brief description of the Hypoplasticity model is presented. Detailed discussions regarding the measurement and calibration of the model parameters of an Iraqi soil are then exposed. It is concluded that only Consolidated Drained (CD) triaxial test, oedometer test, and the well-known limit density tests are needed to get all the parameters of the hypoplasticity model.

scholarly journals A Micromechanics Based Constitutive Model for Brittle Failure at High Strain Rates

2012 ◽  
Vol 79 (3) ◽  
Author(s):  
Harsha S. Bhat ◽  
Ares J. Rosakis ◽  
Charles G. Sammis
Keyword(s):  
Constitutive Model ◽  
Damage Mechanics ◽  
Time Derivative ◽  
Rate Sensitivity ◽  
Model Parameters ◽  
Strain Rates ◽  
Failure Strength ◽  
Loading Rates ◽  
Brittle Solids ◽  
Wide Range

The micromechanical damage mechanics formulated by Ashby and Sammis, 1990, “The Damage Mechanics of Brittle Solids in Compression,” Pure Appl. Geophys., 133(3), pp. 489–521, and generalized by Deshpande and Evans 2008, “Inelastic Deformation and Energy Dissipation in Ceramics: A Mechanism-Based Constitutive Model,” J. Mech. Phys. Solids, 56(10), pp. 3077–3100. has been extended to allow for a more generalized stress state and to incorporate an experimentally motivated new crack growth (damage evolution) law that is valid over a wide range of loading rates. This law is sensitive to both the crack tip stress field and its time derivative. Incorporating this feature produces additional strain-rate sensitivity in the constitutive response. The model is also experimentally verified by predicting the failure strength of Dionysus-Pentelicon marble over strain rates ranging from ∼10− 6to 103s− 1. Model parameters determined from quasi-static experiments were used to predict the failure strength at higher loading rates. Agreement with experimental results was excellent.

scholarly journals Formulation of a Basic Constitutive Model for Fine - Grained Soils Using the Hypoplastic Framework

2021 ◽  
Vol 0 (0) ◽  
Author(s):  
Filip Gago ◽  
Alessandro Valletta ◽  
Juraj Mužík
Keyword(s):  
Constitutive Models ◽  
Constitutive Modelling ◽  
Tensor Function ◽  
Elastoplastic Model ◽  
Soil Response ◽  
Hypoplastic Model ◽  
Material Parameters ◽  
Fine Grained ◽  
Wide Range ◽  
Geotechnical Problems

Abstract A hypoplastic approach to constitutive modelling was developed by Kolymbas 1996 considering a non-linear tensor function in the form of strain and stress rate. However, the implicit formulation of the hypoplastic model with indirect material parameters severely limits its applicability to real-world geotechnical problems. In many cases, the numerical analysis of geotechnical problems relies on simple elastoplastic constitutive models that cannot model a wide range of soil response aspects. One promising paradigm of constitutive modelling in geotechnics is hypoplasticity, but many of the hypoplastic models belong to advanced models. In the article, we present the simple hypoplastic model as an alternative to the widely used Mohr-Coulomb elastoplastic model.

A critical-state constitutive model for liquefiable sand

2005 ◽  
Vol 42 (3) ◽  
pp. 830-855 ◽  
Author(s):  
SM Reza Imam ◽  
Norbert R Morgenstern ◽  
Peter K Robertson ◽  
David H Chan
Keyword(s):  
Constitutive Model ◽  
Critical State ◽  
Stress Ratio ◽  
Large Strain ◽  
Triaxial Compression ◽  
Loose Sand ◽  
Inherent Anisotropy ◽  
Dense Sand ◽  
Peak Point ◽  
Wide Range

This paper presents a critical-state constitutive model for sands over a wide range of void ratios and consolidation pressures in a triaxial plane. A single set of parameters, including a unique critical-state line reached at large strain, is also used in the model, and differences in behavior in triaxial compression and extension are modeled by accounting for anisotropy at small and medium ranges of strain. The model uses a capped yield surface (YS), which is characterized by its size and shape. Following evidence in past literature, the stress ratio at the peak point of the capped YS of loose sands is approximated by the stress ratio measured at the peak point of their undrained effective stress path. Yielding parameters obtained using this stress ratio are also applied in modeling dense sand behavior and drained loading. These parameters account for the effects of inherent anisotropy, void ratio, and confining pressure on yielding stresses and are readily determined from laboratory tests, but further research is required on their determination from field data. The model accounts for stress-induced and inherent anisotropies, using different parameters, which develop and evolve independently. Emphasis is placed on proper modeling of aspects of loose sand behavior that affect their susceptibility to flow liquefaction.Key words: constitutive modeling, liquefaction, loose sand, critical state, dilatancy, hardening.

scholarly journals Offshore anchor piles under mooring forces: numerical modeling

2013 ◽  
Vol 50 (2) ◽  
pp. 189-199 ◽  
Author(s):  
Mohamed I. Ramadan ◽  
Stephen D. Butt ◽  
R. Popescu
Keyword(s):  
Parametric Study ◽  
Three Dimensional ◽  
Element Model ◽  
Model Parameters ◽  
Test Results ◽  
Dense Sand ◽  
Centrifuge Tests ◽  
Wide Range ◽  
Inclination Angles ◽  
Mooring Forces

A parametric study was carried out to study the behavior of offshore anchor piles under mooring forces in dense sand using a three dimensional (3-D) finite element model (FEM). The Mohr–Coulomb plastic model has been used to model the soil, and has been calibrated based on the centrifuge tests discussed in a Ph.D. thesis (published by Ramadan in 2011). The selection of model parameters and comparison of calibrated results with the centrifuge test results are discussed. In the parametric study, different pile lengths and diameters were considered to have different pile–soil rigidities. The pile was loaded at different load inclination angles to examine a wide range of loading conditions. From the current parametric study, design methods and design recommendations are given to help in improving the design of offshore anchor piles under monotonic mooring forces.

Bending Capacity of Tubes With Lüders Bands: Influence of Geometric and Material Parameters

2012 ◽  
Author(s):  
Julian F. Hallai ◽  
Stelios Kyriakides
Keyword(s):  
Finite Elements ◽  
Constitutive Model ◽  
Parametric Study ◽  
Limit States ◽  
Material Response ◽  
Material Parameters ◽  
Unstable Behavior ◽  
Wide Range ◽  
Bending Capacity ◽  
Lüders Bands

Previous studies involving a combination of experiment and analysis have shown that when bending a pipe with material that exhibits Lüders bands to strain levels of 2–3% the structure develops instabilities that can lead to premature collapse. It has been demonstrated that the associated events and limit states can be consistently simulated using 3D finite elements and an elastic-plastic constitutive model with an up-down-up material response. This paper presents the results of a parametric study of the problem in which a wide range of geometric and material parameters are considered. The parametric study quantifies the boundary between stable and unstable behavior and shows that the bounding values of Lüders strain depend on the D/t of the tube and the level of the Lüders stress.

Constitutive model for rate-independent behavior of saturated frozen soils

2016 ◽  
Vol 53 (10) ◽  
pp. 1646-1657 ◽  
Author(s):  
S.A. Ghoreishian Amiri ◽  
G. Grimstad ◽  
M. Kadivar ◽  
S. Nordal
Keyword(s):  
Constitutive Model ◽  
Mechanical Behavior ◽  
Solid Phase ◽  
Fluid Pressure ◽  
Frozen Soil ◽  
Reverse Direction ◽  
State Variables ◽  
Soil Behavior ◽  
Frozen Soils ◽  
Strain Behavior

The mechanical behavior of frozen soils is strongly affected by the amount of ice. The amount of ice depends on the temperature and the applied mechanical stresses. The influence of ice content and temperature on the mechanical behavior and the coupling effects on the reverse direction can be mentioned as the main difference between frozen and unfrozen soils. In the light of this difference, an elastoplastic constitutive model for describing the stress–strain behavior of saturated frozen soils is proposed. By dividing the total stress into fluid pressure and solid phase stress, in addition to consideration of the cryogenic suction, the model is formulated within the framework of two-stress state variables. The proposed model is able to represent many of the fundamental features of the behavior of frozen soils, such as ice segregation phenomenon and strength weakening due to pressure melting. In the unfrozen state the model becomes a conventional critical state model. Typical predictions of the model for simulating the characteristic trends of the frozen soil behavior is described qualitatively. Model predictions are also compared with the available test results and reasonable agreement is achieved.

scholarly journals Identifiability of Tissue Material Parameters from Uniaxial Tests using Multi-start Optimization

2020 ◽  
Author(s):  
Babak N. Safa ◽  
Michael H. Santare ◽  
C. Ross Ethier ◽  
Dawn M. Elliott
Keyword(s):  
Experimental Data ◽  
Constitutive Model ◽  
Constitutive Models ◽  
Data Fitting ◽  
Tissue Mechanics ◽  
Lateral Strain ◽  
Model Parameters ◽  
Fiber Reinforced ◽  
Material Parameters ◽  
Tissue Material

AbstractDetermining tissue biomechanical material properties from mechanical test data is frequently required in a variety of applications, e.g. tissue engineering. However, the validity of the resulting constitutive model parameters is the subject of debate in the field. Common methods to perform fitting, such as nonlinear least-squares, are known to be subject to several limitations, most notably the uniqueness of the fitting results. Parameter optimization in tissue mechanics often comes down to the “identifiability” or “uniqueness” of constitutive model parameters; however, despite advances in formulating complex constitutive relations and many classic and creative curve-fitting approaches, there is no accessible framework to study the identifiability of tissue material parameters. Our objective was to assess the identifiability of material parameters for established constitutive models of fiber-reinforced soft tissues, biomaterials, and tissue-engineered constructs. To do so, we generated synthetic experimental data by simulating uniaxial tension and compression tests, commonly used in biomechanics. We considered tendon and sclera as example tissues, using constitutive models that describe these fiber-reinforced tissues. We demonstrated that not all of the model parameters of these constitutive models were identifiable from uniaxial mechanical tests, despite achieving virtually identical fits to the stress-stretch response. We further show that when the lateral strain was considered as an additional fitting criterion, more parameters are identifiable, but some remain unidentified. This work provides a practical approach for addressing parameter identifiability in tissue mechanics.Statement of SignificanceData fitting is a powerful technique commonly used to extract tissue material parameters from experimental data, and which thus has applications in tissue biomechanics and engineering. However, the problem of “uniqueness” or “identifiability” of the fit parameters is a significant issue, limiting the fit results’ validity. Here we provide a novel method to evaluate data fitting and assess the uniqueness of results in the tissue mechanics constitutive models. Our results indicate that the uniaxial stress-stretch experimental data are not adequate to identify all the tissue material parameters. This study is of potential interest to a wide range of readers because of its application for the characterization of other engineering materials, while addressing the problem of uniqueness of the fitted results.

scholarly journals Temperature-Dependent Fatigue Characteristics of P91 Steel

2020 ◽  
Vol 14 (2) ◽  
pp. 69-78
Author(s):  
Władysław Egner ◽  
Piotr Sulich ◽  
Stanisław Mroziński ◽  
Halina Egner
Keyword(s):  
Constitutive Model ◽  
Internal State ◽  
Low Cycle Fatigue ◽  
Constitutive Modelling ◽  
Thermomechanical Coupling ◽  
Irreversible Processes ◽  
Experimental Investigations ◽  
Model Parameters ◽  
State Variables ◽  
P91 Steel

AbstractIn this paper, the experimental investigations, constitutive description and numerical modelling of low-cycle fatigue behaviour of P91 steel in non-isothermal conditions are presented. First, experimental tests are performed to recognise different aspects of material behaviour. Then, an appropriate constitutive model is developed within the framework of thermodynamics of irreversible processes with internal state variables. The model describes two phases of cyclic softening, related to plastic mechanisms. An important goal of the presented research is to include thermomechanical coupling in the constitutive modelling. Next, the model parameters are identified based on the available experimental data. Some parametric studies are presented. Finally, numerical simulations are performed, which indicate the significant influence of thermomechanical coupling on the response of the constitutive model in thermomechanical fatigue conditions.

scholarly journals On the optimisation efficiency for the inverse identification of constitutive model parameters

2021 ◽  
Author(s):  
Miguel Guimarães Oliveira ◽  
João Miguel Peixoto Martins ◽  
Bernardete Coelho ◽  
Sandrine Thuillier ◽  
António Andrade-Campos
Keyword(s):  
Constitutive Model ◽  
Mechanical Tests ◽  
Model Parameters ◽  
Inverse Identification ◽  
Full Field ◽  
Material Parameters ◽  
Gradient Based ◽  
Identification Techniques ◽  
Optimisation Algorithms ◽  
Optimisation Technique

The development of full-field measurement techniques paved the way for the design of new mechanical tests. However, because these mechanical tests provide heterogeneous strain fields, no closed-form solution exists between the measured deformation fields and the constitutive parameters. Therefore, inverse identification techniques should be used to calibrate constitutive models, such as the widely known finite element model updating (FEMU) and the virtual fields method (VFM). Although these inverse identification techniques follow distinct approaches to explore full-field measurements, they all require using an optimisation technique to find the optimum set of material parameters. Nonetheless, the choice of a suitable optimisation technique lacks attention and proper research. Most studies tend to use a least-squares gradient-based optimisation technique, such as the Levenberg-Marquardt algorithm. This work analyses optimisation algorithms, gradient-based and -free algorithms, for the inverse identification of constitutive model parameters. To avoid needless implementation and take advantage of highly developed programming languages, the optimisation algorithms available in optimisation libraries are used. A FEMU based approach is considered in the calibration of a thermoelastoviscoplastic model. The material parameters governing strain hardening, temperature and strain rate are identified. Results are discussed in terms of efficiency and the robustness of the optimisation processes.

Constitutive Equations for the Thermomechanical Response of Rene´ 80: Part 1—Development From Isothermal Data

1993 ◽  
Vol 115 (4) ◽  
pp. 351-357 ◽  
Author(s):  
V. S. Bhattachar ◽  
D. C. Stouffer
Keyword(s):  
Constitutive Model ◽  
Arrhenius Equation ◽  
Flow Equation ◽  
Dislocation Climb ◽  
Deformation Mechanisms ◽  
Nickel Base Superalloy ◽  
State Variables ◽  
Material Parameters ◽  
Nickel Base ◽  
Base Superalloy

This work describes the development of a unified nonisothermal constitutive model to predict the thermomechanical fatigue (TMF) response of a Nickel base superalloy, Rene´ 80. Nonisothermal deformation mechanisms are modeled using state variables. The flow equation of the Ramaswamy-Stouffer model was rewritten in the form of an Arrhenius equation with explicit temperature dependence. The isothermal predictions were correlated with the test data at four test temperatures between 538°C and 982°C. Material parameters were verified using nonisothermal tensile calculations. This verification showed that modeling the transition between planar slip and dislocation climb accurately is crucial for obtaining reliable TMF predictions. The revised constitutive model could successfully predict Rene´ 80 response from several TMF tests between 760°C and 982°C.

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