Masoud K. Darabi

Comparing finite element and constitutive modelling techniques for predicting rutting of asphalt pavements

This paper focuses on a comprehensive evaluation of the effects of different finite element (FE) modelling techniques and material constitutive models on predicting rutting in asphalt pavements under repeated loading conditions. Different simplified 2D and more realistic 3D loading techniques are simulated and compared for predicting asphalt rutting. This study also evaluates and compares the rutting performance predictions using different material constitutive behaviours such as viscoelastic–viscoplastic, elasto-viscoplastic and coupled viscoelastic, viscoplastic and viscodamage behaviours. The simulations show that the assumption of the equivalency between a pulse loading and an equivalent loading, which are commonly used as simplified loading assumptions for predicting rutting, is reasonable for viscoelastic–viscoplastic and elasto-viscoplastic constitutive behaviours. However, these loading assumptions and material constitutive models overestimate rutting as damage grows. Results show that the 2D plane strain FE simulations significantly overestimate rutting as compared with the rutting performance predictions from more realistic 3D FE simulations.

Application of a large deformation nonlinear-viscoelastic viscoplastic viscodamage constitutive model to polymers and their composites

Based on the effective stress concept in continuum damage mechanics and the large deformation theory, a viscodamage model, coupled with Schapery-type nonlinear-viscoelasticity and Perzyna-type viscoplasticity constitutive models, is used in order to simulate and predict the inelastic and time-dependent damage behavior of polymeric materials and their composites. The thermo-viscodamage model is presented as a function of temperature, total effective strain, damage history, and a damage-driving force expressed in terms of the deviatoric stress invariants in the undamaged configuration. This expression for the damage force allows for the distinction between the influence of compression and extension loading conditions on damage nucleation and growth. Also, the ability of the constitutive model for predicting the tertiary creep, which shows the nonlinear behavior of polymers during damage growth and nucleation, is presented. The numerical algorithm for integrating the coupled constitutive model is implemented in the finite element software Abaqus via the user-material subroutine UMAT. The model capability in predicting the nonlinear-viscoelastic, viscoplastic, and damage behavior of polymers is demonstrated through comparison of model predictions with experimental measurements in different loading conditions including creep tests and constant strain rate tests over a range of stress levels, strain rates, and temperatures. Also, a general thermodynamic framework for deriving a coupled viscoelastic–viscoplastic–viscodamage constitutive model is presented.

Continuum Coupled Moisture–Mechanical Damage Model for Asphalt Concrete

Despite the detrimental effects of moisture damage in asphalt pavements, few macroscale models are capable of modeling this important phenomenon. Existing models have limitations in accounting for the irreversibility and time dependency of moisture-induced damage. This study presents a moisture damage model based on continuum damage mechanics. Adhesive and cohesive moisture damage phenomena are modeled independently; this procedure allows for the introduction of fundamental mechanical properties for each process and for modeling the transition between adhesive and cohesive damage. Two- and three-dimensional simulations are performed, and the results of the simulations are presented to demonstrate the applicability and utility of these micromechanical computational models. It is shown that the proposed moisture damage model can simulate the effect of moisture damage on the mechanical response of asphalt concrete subjected to different loading conditions. The model also provides useful insight into the effect of mixture design and material properties on resistance to moisture damage.

Cyclic Hardening-Relaxation Viscoplasticity Model for Asphalt Concrete Materials

AbstractA cyclic hardening-relaxation model is proposed that significantly enhances the prediction of the viscoplastic (VP) strain of asphalt concrete under cyclic compressive-loading conditions at high temperatures. The hardening-relaxation mechanism is physically tied to the changes in the material’s microstructure during the rest period. A memory surface that memorizes the viscoplastic deformation history is defined in the viscoplastic strain space as the general initiation and evolution criteria for the hardening-relaxation mechanism. The proposed model is coupled to the classical Perzyna-type viscoplastic model and Schapery’s nonlinear viscoelastic model, and the associated numerical algorithms are implemented in the finite element software ABAQUS through the user-defined material subroutine UMAT. Model predictions show that the proposed model predicts well both the axial and radial viscoplastic responses of asphalt concrete subjected to the cyclic creep tests at various loading times, unloading time...

Three-dimensional microstructural modelling of coupled moisture–mechanical response of asphalt concrete

Three-dimensional (3D) microstructural representation of asphalt concrete subjected to moisture diffusion and mechanical loading is simulated and analysed. The continuum moisture–mechanical damage mechanics framework and the moisture damage constitutive relationship developed by the authors are used in this study to couple the detrimental effects of the mechanical loading and moisture diffusion on the complex response of asphalt concrete. A 3D finite element (FE) microstructural representation of a typical asphalt concrete is used for these simulations. The 3D microstructure is reconstructed from slices of two-dimensional X-ray computed tomography images that consist of the matrix and the aggregates. Results show that the generated 3D FE microstructure along with the coupled moisture–mechanical constitutive relationship can be effectively used to simulate the overall thermo-hygro-mechanical response of asphalt concrete. The analyses provide insight into the impact of the microstructure on the overall response of asphalt concrete.

A straightforward numerical technique for finite element implementation of non-local gradient-dependent continuum damage mechanics theories

This paper presents a direct algorithm to implement non-local gradient-enhanced damage mechanics theories in the existing finite element codes with minor modifications and without the need to formulate a higher-order element. This method extends the algorithm of Abu Al-Rub and Voyiadjis (2005) to gradient-dependent damage theories and to three-dimensional (3D) problems. The presented algorithm is implemented in the well-known finite element code Abaqus via the user material subroutine UMAT. The potential of the proposed numerical algorithm for non-local gradient-enhanced damage theories in eliminating mesh-dependent simulations is validated by conducting various numerical tests of the localised damage.

Modelling moisture-mechanical damage in asphalt mixtures using random microstructures and a continuum damage formulation

This paper presents a computational modelling approach to evaluate moisture damage in hot mix asphalt (HMA) materials. The approach combines a methodology to randomly generate HMA microstructures and a finite element (FE) formulation that uses a coupled mechanical-moisture continuum damage constitutive relationship. Probable two-dimensional HMA microstructures were randomly generated and treated as independent replicates. These replicates were used as the model geometry in FEs and they were computationally subjected to a one-year moisture-conditioning regime. During this time, a mechanical load was periodically applied. The mean and dispersion values related to the evolution of moisture and mechanical damage were identified, and the effects of varying the air void content and the moisture diffusion coefficient of the asphalt matrix were assessed. The results demonstrate that the modelling approach is a viable tool to probabilistically quantify the impact of physical and volumetric properties of HMA mixtures on their susceptibility to undergo moisture-mechanical damage.

Comparing rutting of airfield pavements to simulations using Pavement Analysis Using Nonlinear Damage Approach (PANDA)

This study presents the rutting performance results of full-scale pavement test sections subjected to F-15E and C-17 aircraft wheels at two different temperatures. Pavement structures for the tests were constructed under shelter in the U.S. Army Engineer Research and Development Centers (ERDC) pavement test facility. The full-scale test results are used to validate viscoelastic, viscoplastic and hardening-relaxation constitutive relationships implemented in the Pavement Analysis Using Nonlinear Damage Approach (PANDA) model. PANDA is a mechanistic-based model which incorporates nonlinear viscoelastic, viscoplastic, hardening-relaxation, viscodamage, moisture-induced damage and ageing constitutive relationships. Results of dynamic modulus and different repeated creep-recovery laboratory tests are analysed to extract the parameters associated with viscoelastic, viscoplastic and hardening-relaxation constitutive relationships implemented in PANDA. Once calibrated, PANDA is used to predict the rutting performance observed in full-scale pavement test sections. The simulation results illustrate that PANDA is capable of predicting the rutting of airfield pavements subjected to heavy aircraft wheel loads at intermediate and high temperatures. It is shown that PANDA successfully predicts the effect of shear flow and upheaval at the edges of the wheel. The data from simulation suggested that PANDA, once calibrated, can provide insight into the critical locations of tensile and compressive stresses within the pavement structure. PANDA simulations not only provide a tool for evaluating existing structures, but also can be used in designing more sustainable pavement structures and materials.

Constitutive Modeling of the Coupled Moisture-Mechanical Response of Particulate Composite Materials with Application to Asphalt Concrete

AbstractA novel continuum damage mechanics-based framework is proposed to model the detrimental effect of moisture on the response of particulate composite materials. This framework extends the well-known Kachanov’s effective (undamaged) configuration and the concept of effective stress space to moisture-susceptible materials by introducing wet-undamaged and wet-damaged natural configurations. Physically based moisture-induced damage internal variables are introduced within the proposed framework to consider the moisture aggravation effect and to couple moisture-induced damage with mechanical responses. Constitutive relationships are proposed for the time-dependent evolution of adhesive and cohesive moisture-induced damage to explicate the fundamental processes associated with the moisture-induced damage phenomena. The material properties associated with the moisture-induced damage constitutive relationship are determined based on the available experimental data from asphalt concrete materials. Comparison...

Damage healing in asphalt pavements: theory, mechanisms, measurement, and modeling

A background of evidence of the importance and impact of damage healing in asphalt pavements from laboratory and field studies is presented. Mechanisms of healing that result in a measurable recovery of engineering properties are presented, including descriptions of the impact of crack wetting, reversal of the fracture process, and the influence of molecular morphology. Methods are described to measure intrinsic healing in asphalt binders using a dynamic shear rheometer (DSR). Healing measurements on binders are compared to the proposed mechanisms of healing in asphalt binders, including measurements of surface free energy, which are shown to be consistent with the proposed mechanisms. Later, a continuum model and a concomitant test methodology are used to measure healing characteristics in asphalt mixtures that are independent of the testing conditions, sensitive to the duration of the rest period during which healing occurs, and sensitive to the level of damage that precedes the rest period. Thermodynamic considerations of damage and microdamage healing processes are discussed in considerable detail, concluding with a general thermodynamic framework for derivation of damage and microdamage-healing constitutive relationships, with a focus on accurate estimation of stored and dissipated energies. Finally, incorporation of microdamage healing in constitutive relationships for pavement performance modeling is discussed, and future directions are identified.