Saravanan Umakanthan

Realistic traffic conditions involve random load amplitude, frequency (speed) and rest periods between load applications. Out of these three factors, one factor which can be controlled and which can throw light on the viscoelastic response of the asphalt concrete mixtures and to some extent on the “healing” nature of these materials is rest period between loadings. An experimental investigation was designed to study the behavior of asphalt concrete mixture during the rest period of a creep and recovery test by providing a prolonged rest period between each set of 100 test cycles. The test was carried out in the unconfined and confined condition, at temperatures of 20, 40 and 55°C. During rest periods, the confinement pressure was maintained in the material. It was observed that during the rest period, the material attained a beneficial internal structural state that required a lesser time to reach a ‘stable’ state when loaded afterwards. It was noted that confinement pressure was necessary for assisting the healing of the material. To model this response of the material, a non-linear upper convected Burgers’ model was used. The model parameters could explain the material behavior and the conditions under which any beneficial internal structural changes happen in the material.

Major challenge in heart valve tissue engineering for paediatric patients is the development of an autologous valve with regenerative capacity. Hybrid tissue engineering approach is recently gaining popularity to design scaffolds with desired biological and mechanical properties that can remodel post implantation. In this study, we fabricated aligned nanofibrous Bio-Hybrid scaffold made of decellularized bovine pericardium: polycaprolactone-chitosan with optimized polymer thickness to yield the desired biological and mechanical properties. CD44+, αSMA+, Vimentin+ and CD105− human valve interstitial cells were isolated and seeded on these Bio-Hybrid scaffolds. Subsequent biological evaluation revealed interstitial cell proliferation with dense extra cellular matrix deposition that indicated the viability for growth and proliferation of seeded cells on the scaffolds. Uniaxial mechanical tests along axial direction showed that the Bio-Hybrid scaffolds has at least 20 times the strength of the native valves and its stiffness is nearly 3 times more than that of native valves. Biaxial and uniaxial mechanical studies on valve interstitial cells cultured Bio-Hybrid scaffolds revealed that the response along the axial and circumferential direction was different, similar to native valves. Overall, our findings suggest that Bio-Hybrid scaffold is a promising material for future development of regenerative heart valve constructs in children.

In this paper, a methodology to control the surface error on a Doppler antenna using the concept of a variable geometry truss structure is proposed. A genetic algorithm is used to find the optimal location and number of actuators with the objective to minimize the construction and runtime costs. The optimization also takes into account the limitations in actuation. A piezoceramic-based actuator is used to demonstrate the effectiveness of the methodology. A simple illustration of 2D trusses, which could form a part of a Doppler antenna structure, is used to show the efficacy of the method. An analysis of the effectiveness of such a design is presented. The influence of the variables in the problem is examined and observations made. This study concludes that the smart antenna concept is a viable, feasible and effective option in design.

The test method of ASTM C512 (ASTM. 2015. Standard test method for creep of concrete in compression. ASTM C512/C512M. West Conshohocken, PA: ASTM) dictates the use of a spring-loaded creep frame to perform a creep test on concrete. The main thesis of the study is that tests performed using these spring-loaded frames is not a creep test in the sense that the force acting on the specimen is not held constant while the specimen undergoes time-dependent strain. Analysis of this frame is performed using a linear viscoelastic model to represent concrete and isotropic Hooke's law to represent the steel rods and springs. The internal force and displacement in concrete and steel rods at any given instant of time is found using equilibrium equations and compatibility conditions. It is observed that the force transmitted to the concrete does not remain constant throughout the test duration but is a function of the spring and rod stiffness and the viscoelastic properties of the concrete. Hence, a drop in the magnitude of the force transmitted to the concrete specimen occurs in experiments when a spring-loaded creep frame is used. Experimental validation is also carried out by comparing the response of a spring-loaded creep frame with theoretical results. In this work, optimal spring and rod stiffness values to minimize the drop in the force transmitted to the concrete specimen are established. Thus, this study could be used to determine the linear viscoelastic properties of concrete in a creep frame.

We develop a methodology for solving truss problems involving viscoelastic materials where, of all the member forces that satisfy the nodal force equilibrium equation and nodal displacements that satisfy the displacement boundary conditions, those member forces and nodal displacements that satisfy the constitutive relation are sought. Since a rate type viscoelastic constitutive relation involves the rate of the stress or strain, this study explores the use of member forces, nodal displacements, and support reactions or their rates as independent variables. Assuming small deformations, the nodal force equilibrium and the displacement boundary condition results in a linear equality constraint between the independent variables. Then we find the unknown independent variables such that the root mean squared error in the constitutive relation of the members of the truss is minimized subject to the satisfaction of the linear constraint at selected times. The objective function is evaluated at selected times or integrated over subintervals of time. We explore six possible solution methods and benchmark them for their accuracy and efficiency. We study statically determinate and indeterminate truss whose members are modeled using rate and integral type viscoelastic constitutive relations for creep and oscillatory loading. For the standard linear solid model, we find that the proposed methods are more accurate than ABAQUS and, at times, require lesser computational wall time. We also demonstrate the applicability of the proposed methodology to fractional order and nonlinear viscoelastic constitutive relations.

In this article, the appropriateness of modelling asphalt using linear viscoelastic constitutive relations is explored. Towards this, a rate-type nonlinear viscoelastic model recently proposed by Santoshreddy et al. (2011) is used and it is shown that the stress relaxation and creep response of asphalt are influenced by the type of experiment, initial rate of displacement (or loading) and the stress and strain measure used. Furthermore, it is shown that the creep and stress relaxation functions are not dependent functions as required by the linear viscoelastic theory, even in the range of values of engineering strain and stress in which the linearity property holds approximately. These observations lead one to conclude that linear viscoelastic models maybe inappropriate to model asphalt. © 2012 Copyright Taylor and Francis Group, LLC.

This article reports a multi-field numerical formulation for solving plane problems involving viscoelastic materials. Stress fields satisfying equilibrium equations are constructed using Airy's potentials which are expressed as a linear combination of C2 basis functions. The strain field is derived from a continuous displacement field obtained from a linear combination of C0 basis functions. An appropriate linear combination of these stress and displacement basis functions is determined such that the resulting stress and strain fields satisfy the constitutive relation subjected to the satisfaction of the constraints arising from the boundary conditions. Since a viscoelastic constitutive relation involves stress, strain, and their rates, stress and displacement degrees of freedom or their rates can be considered as optimization variables for minimizing the error in satisfying the constitutive relation. Two Algorithms are proposed based on this choice of optimization variable. Accuracy and efficiency of the proposed algorithms are studied through five different boundary value problems involving four forms of the viscoelastic constitutive relations and for two loading histories. Using the developed rectangular element, viscoelastic beam bending problem is solved for the different constitutive relations studied.

We show that even when the constitutive relations for the elastic response of compressible bodies satisfy the empirical, Baker-Ericksen and ordered forces inequalities and ellipticity restrictions, they could result in physically unrealistic response. We pick a constitutive relation that satisfies the above inequalities and is also globally elliptic but its response to both uniaxial extension and inflation at constant length is physically unrealistic, based on the available experimental observations. However, the Coleman-Noll, pressure-compression, tension-extension and invertibility of force-stretch conditions and that proposed in Carroll and McCarthy (1995) are violated by this constitutive relation. © 2011 Elsevier Ltd. All rights reserved.

Asphalt for highway and runway construction is processed by either air blowing or blending with different petroleum streams. In the blending process, petroleum pitch, a by-product of solvent deasphalting of the vacuum residue is mixed with heavy extract to produce asphalt of the desired specifications. The rheological response of blended asphalt hence depends to a large extent on the constitutive property of petroleum pitch. In an aim to develop robust models for blended asphalt, modeling the mechanical behavior of petroleum pitch hence becomes necessary. In this work reported here, petroleum pitch from crude sources such as Basrah Light, Arab Mix and Arab Light are subjected to steady shear for 99 min at temperatures ranging from 70 to 120 °C for different shear rates. Each of these material exhibited different stress overshoot and decay during steady shear depending on the temperature and shear rate. A viscoelastic fluid model of the rate type is selected to model the response of the material. Using the recent thermodynamic framework based on Gibbs potential proposed by Rajagopal and Srinivasa [27], restrictions on the proposed model are obtained. The rotational flow problem is solved and the material parameters are estimated. The model predictions are corroborated with the experimental observations and they are found to be reasonably good. © 2010 Elsevier Ltd. All rights reserved.

Here mixture theory is used to capture the changes in cement concrete exposed to sodium sulfate till cracks develop. Toward this, the mixture is assumed to be made of eleven constituents of which the sodium sulfate and water move relative to themselves and the remaining nine solid constituents. The nine solid constituents constrained to move together are the eight relevant chemical constituents in concrete that react with sodium sulfate and all the other remaining chemical constituents of concrete that do not react with sulfates. Constitutive assumptions needed to be made within this mixture theory framework are the same as those reported by Gouder and Saravanan (Acta Mech 227(11):3123–3146, 2016). Within this framework of mixture theory, the radial ingress and reaction of sodium sulfate solution with the concrete cylinder sealed at top and bottom, exposed to a constant concentration of sodium sulfate at its outer surface, are formulated. The resulting nonlinear governing differential equations are converted into a system of nonlinear algebraic equations using a forward finite difference scheme in space and a backward difference in time. The nonlinear algebraic equations are solved simultaneously using constrained minimization technique till the water reaches the center of the cylinder. The results obtained for ingress without chemical reactions agree with those predicted by Fick’s equation. The axial expansion of the cylinder and the increase in the value of Young’s modulus of the part of concrete which reacted with sulfates agree qualitatively with the experiments.