Microstructural modeling of asphalt concrete using a coupled moisture–mechanical constitutive relationship

The coupled effect of moisture diffusion and mechanical loading on the microstructure of asphalt concrete is studied. The traditional Continuum Damage Mechanics (CDM) framework is modified to model detrimental effects of moisture and mechanical loading. Adhesive/cohesive moisture-induced damage constitutive relationships are proposed to describe the time-dependent degradation of material properties due to moisture. X-ray two-dimensional (2D) computed tomography-imaging technique is used to construct finite element (FE) microstructural representation of a typical dense-graded asphalt concrete. After being calibrated against pull-off experiments, the proposed moisture-induced damage constitutive relationship, which is coupled to thermo-viscoelastic–viscoplastic–viscodamage mechanisms, is used to simulate the microstructure of asphalt concrete. Simulation results demonstrate that the generated 2D FE microstructural representation along with the coupled moisture–mechanical constitutive relationship can be effectively used to model the overall thermo-hygro-mechanical response of asphalt concrete.     

Two-phase damage modeling of concrete affected by alkali–silica reaction under variable temperature and humidity conditions

In the present work the concrete affected by alkali–silica reaction (ASR) is represented as a two-phase material made of a solid skeleton and a wet expanding gel, which exerts a pressure capable of severely damaging the concrete surrounding the reactive sites. Both the effects of temperature and humidity conditions on the kinetic of the chemical reaction and on the final value of the consequent expansion are included in the proposed model. The mechanical degradation induced by the ASR is described by a phenomenological isotropic damage model. The constitutive model, implemented in a finite element code, is used for the analyses of structures made of reactive concrete in the presence of temperature and moisture gradients. Firstly the temperature and humidity fields are obtained through uncoupled heat and moisture transport analyses and then the chemo-mechanical analysis is performed starting from the values of temperature and humidity preliminary calculated.     

Comments on “Stability analysis of Caputo fractional-order nonlinear systems revisited”

Nonlinear Dynamics - This note points out that the proof for a widely and mostly used (uniform) asymptotic stability theorem for Caputo fractional-order systems, presented by the article...     

A note on the “nonlinear control of electrical flexible-joint robots”

Robust tracking control of electrically flexible-joint robots is addressed in this paper. Two important practical situations are considered. The fact that robot actuators have limited voltage and that current measurement is subjected to noise. Let us notice that a few solutions for the voltage-bounded robust tracking control have been proposed. In this paper, we contribute to this subject by presenting a new form of voltage-based control strategy. It proves that the closed loop system is BIBO stable, while actuator/link position errors are uniformly–ultimately bounded stable in agreement with Lyapunov’s direct method in any finite region of the state space. As a second contribution of this paper, we present a robust adaptive control scheme without the need for computation of regressor matrix and current measurement, with the same result on the closed loop system stability. This novelty gives a simple robust tracking control scheme for both structured and unstructured uncertainties based on the function approximation technique. The analytical studies as well as experimental results produced using MATLAB/Simulink external mode control on a flexible-joint electrically driven robot demonstrate high performance of the proposed control scheme.

     

Addendum to “Concentration and Lack of Observability of Waves in Highly Heterogeneous Media”

In [1] we introduced a class of 1?d wave equations with rapidly oscillating Hölder continuous coefficients for which the classical boundary observability property fails. We also established that these examples could be used to contradict Strichartz-type inequalities for the wave equation with low regularity coefficients. The object of this addendum is to further analyze this issue. As we will see, the argument in [1] only provides sharp counter-examples to the Strichartz estimates when the coefficient ρ belongs to L ^∞. We carefully analyze these counter-examples for Hölder continuous coefficients. We also give a new application of our construction which shows that some eigenfunction estimates for elliptic operators due to Sogge can fail when coefficients are not smooth enough.     

Coincidence between linear and global nonlinear stability of non-constant throughflows via the Rionero “Auxiliary System Method”

A system modeling fluid motions in horizontal porous layers, uniformly heated and salted from below, is analyzed in the case of variable thermal and solutal diffusivities. The boundedness and uniqueness of solutions are shown. A class of non-constant throughflows is found and their stability is analyzed via a new approach. Conditions of global nonlinear stability, in closed form, are obtained.     

Shear-stress measurements “in situ” of soils subjected to vibratory loads

A miniaturized shear cell is described. Results of measurements “in situ” with this shear cell are compared with those of pressure cells. Vibratory (sinusoidal) loads were generated by a mechanical oscillator on the surface of the soil and transmitted to both types of cells buried underneath. A soil of predominantly homogenous and non-cohesive characteristics was selected so that as a first approximation the elastic theory for biaxial-stress distribution could be applied in the analysis. The standard deviations of the corresponding shear and normal stresses did not exceed 5.3 percent.     

Rebuttal of “Existence of attractor and control of a 3D differential system” by Z. Wang

A paper, ??Existence of attractor and control of a 3D differential system,?? was published in the journal Nonlinear Dynamics. The author tries to prove the existence of horseshoe chaos by means of the Shilnikov criterion. To do that, he uses the undetermined coefficient method to analytically demonstrate the existence of heteroclinic orbits in a Lorenz-like system. Unfortunately, his proof is not correct for two reasons. Firstly, he considers an odd function to represent the heteroclinic orbit whereas such an orbit joining two saddle-focus equilibria can never have that symmetry. Secondly, he looks for a structurally unstable Shilnikov heteroclinic orbit by means of uniformly convergent series expansions: This would imply that the dynamical object found is structurally stable.     

Developable Surfaces as Generators of the “Isobaric Solutions” to the Euler Equations

The simplest solutions to the Euler equations (1.1) for which the pressure vanishes identically are those representing the motion of lines parallel to a fixed direction moving in the same direction (each line with an independent, given, constant velocity). Are there many other solutions to this problem? If yes, is there a simple characterization of all the initial data (volume occupied by the fluid at time t = 0 and initial velocity that gives rise to the general solutions? In this paper we show that the answer to both questions is positive. We prove, in particular, that there is a natural correspondence between solutions in R² of this problem and (Cartesian pieces of) developable surfaces in R³. See Theorem 3.     

Photoelastic Analysis of Edge Residual Stresses in Glass by Automated “Test Fringes” Methods

Since the glass is a birefringent material, the analysis of residual stress in glass is usually carried out by means of photoelastic methods. This paper considers the automation of the “test fringes” method which is based on the use of a Babinet compensator or of a beam subjected to bending. In particular, two automated methods are proposed: the first one is based on the use of the centre fringe method in monochromatic light and the second one is based on the use of RGB photoelasticity in white light. The proposed methods have been applied to the analysis of membranal residual stresses in some tempered glasses, showing that they can effectively replace manual methods of photoelastic analysis of residual stresses in glass.