The Application of Micromechanical Modeling to Study the Non-Linear Behavior of Flexible Pavement under Truck Loading in Pakistan

This report presents the work done to create a 3D model of a pavement structure, which incorporates in a most accurately way non damaging effects of asphalt layer considering the different layers that compose the pavement structure. A series of approaches have been considered from linear viscoelastic modelling using Maxwell model via UMAT subroutine to integrated viscoelastic response using Prony series available in the built in ABAQUS material library. During this work also other effects have been considered including the viscoelastic behaviour caused by creep, which is included via a separate CREEP subroutine. In order to predict also the non-linear effect of viscous elastic response the well-known Schapery's nonlinear viscoelastic constitutive model was considered. Implementing 3D Schapery’s model can be done by using again user material subroutine UMAT. It was found that the use of a nonlinear viscoelastic model substantially transformed the pavement response. A series of python scripts have been developed to create a multi-step analysis to simulate a cyclic loading application. Keywords: Viscoelasticity, Abaqus. Asphalt, Damage, Simulation, Modelling, Pavement design

Effect of Degradation and Osteoarthritis on the Viscoelastic Properties of Human Knee Articular Cartilage: An Experimental Study and Constitutive Modeling

Articular cartilage, as a hydrated soft tissue which covers diarthrodial joints, has a pivotal role in the musculoskeletal system. Osteoarthritis is the most common degenerative disease that affects most individuals over the age of 55. This disease affects the elasticity, lubrication mechanism, damping function, and energy absorption capability of articular cartilage. In order to investigate the effect of osteoarthritis on the performance of articular cartilage, the mechanical behavior of human knee articular cartilage was experimentally investigated. Progressive cyclic deformation was applied beyond the physiological range to facilitate degradation of the tissue. The relaxation response of the damaged tissue was modeled by means of a fractional-order nonlinear viscoelastic model in the framework of finite deformations. It is shown that the proposed fractional model well reproduces the tissue’s mechanical behavior using a low number of parameters. Alteration of the model parameters was also investigated throughout the progression of tissue damage. This helps predict the mechanical behavior of the osteoarthritic tissue based on the level of previous damage. It is concluded that, with progression of osteoarthritis, the articular cartilage loses its viscoelastic properties such as damping and energy absorption capacity. This is also accompanied by a loss of stiffness which deteriorates more rapidly than viscosity does throughout the evolution of tissue damage. These results are thought to be significant in better understanding the degradation of articular cartilage and the progression of OA, as well as in the design of artificial articular cartilages.

Dynamics Modeling of Corneal Deformation under Air Puff with Linear and Nonlinear Viscoelastic Model

Background: The aim of the study is to model the corneal dynamic deformation under an air puff excitation. The deformation response of the cornea was modeled by using linear and nonlinear viscoelastic models. The corneal deformation responses generated from the linear and nonlinear viscoelastic model were correlated with the clinical results, which were obtained from Corneal Visualization Scheimpflug Tonometer (Corvis ST) to evaluate the comparable biomechanical parameters of the cornea. Methods: A prompt deformation occurs when the external force applied to the cornea. Then a continuous deformation follows. A simple mass, spring and dashpot system were used to model human eyeball. Results: In linear viscoelastic model, the corneal elastic stiffness commanded behavior of the corneal deformation and its maximum, when the viscous component affected for its lateral shifting and marginally alter the magnitude.Whereas, in the nonlinear viscoelastic model, the corneal material nonlinearity commanded the behavior and maximum of the corneal deformation, while the viscous component marginally contributed for its lateral shifting and demonstrated the minimum affect on the magnitude and form. A multi-objective genetic algorithm-based optimization procedure was used to identify the material properties in the nonlinear viscoelastic model for 29 eyes of 20 normal people. Conclusion: The corneal deformation response model with nonlinear viscoelastic model showed to have a better fit with the corneal dynamic deformation behavior under an air pulse excitation. The biomechanical properties of the cornea in vivo can be evaluated by using and analysing dynamic deformation of the cornea under an air puff excitation model.