On the definitions of effective stress and deformation gradient for use in MD: Hill’s macro-homogeneity and the virial theorem

The continuum notions of effective deformation gradient and effective stress for homogenization problems with large deformations are reviewed. The “local” problem to be homogenized can include inertia effects to allow for a link between continuum homogenization and the estimation of average properties for particle ensembles via molecular dynamics. The focus of this paper is on the role played by boundary conditions in: defining a meaningful space average of deformation, defining a meaningful space average of stress, and establishing a connection between the idea of effective stress from micro-mechanics and that based on the virial theorem.     

Eshelby’s inclusion problem in the gradient theory of elasticity: Applications to composite materials

We extend Eshelby’s integral representations for elastic inclusion problems to the case of gradient theory of elasticity. Gradient elastic effects associated with the existence of an interphase layer, within a simple and robust gradient model whose properties are described by the harmonic equation, are discussed. The decomposition of the corresponding solution into “classical” and “gradient” components is established. It is shown that the aforementioned Eshelby-type integral formulas for gradient elasticity can be expressed in the same form as in the standard theory of elasticity, but only for the “classical” part of the solution. The implementation of Eshelby’s approach in determining the effective properties of composites by the three-phase method requires the derivation of a complete solution for the gradient model. An example of application of the so-obtained generalized gradient method for determining the effective properties of composites with size effects due to cohesion and surface forces is given.     

A dynamic theory for polarizable and magnetizable magneto-electro thermoviscoelastic,electrically and thermally conductive anisotropic solids having magnetic symmetry

A unified dynamic theory for polarizable and magnetizable magneto-electro thermoviscoelastic. electrically and thermally conducting anisotropic solids possessing magnetic symmetry is developed for time-dependent electromagnetic fields. Considering the Chu formulation of the Maxwell equations, the balance equations of nonrelativistic continuum mechanics and the boundary conditions together with the constitutive equations for anisotropic materials are given. It is shown that the entropy, polarization. magnetization and the nondissipative part of stress are derivable from a potential while the dissipative part of stress, electric current and heat flux vectors satisfy a residual inequality. The theory developed for a general anisotropy and interaction is applied to special cases.     

Finite element method and experimental investigation on the residual stress fields and fatigue performance of cold expansion hole

Cold expansion is an efficient way to improve the fatigue life of an open hole. The residual stress fields of cold expansion holes are vital for key components designing, manufacturing and fatigue properties assessment. In this paper, three finite element models have been established to study the residual stress fields of cold expansion hole, experiments were carried out to measure the residual stress of cold expansion hole and verify simulation results. Three groups of specimens with different cold expansion levels are examined by fatigue test. The fracture surfaces of specimens are observed by scanning electron microscope. The finite element method (FEM) results show, with interference values develop, the maximum values of circumferential residual compressive/tensile stresses increase in “infinite” and “finite” domain, and a higher positive stress values are obtained at the boundary of “finite” domain. The effects of the friction between the mandrel and the hole’s surface and two cold expansion techniques on the distribution of residual stress is local, which only affects the radial residual stress around the maximum value and the circumferential residual stress near the hole’s edge. Crack always initiates near entrance face and the crack propagation speed along transverse direction is faster than that along axial direction.     

Bi-velocity hydrodynamics: Single-component fluids

Acceptance of the Navier-Stokes-Fourier (NSF) equations as the fundamental equations of single-component continuum fluid mechanics for liquids and gases is noted to be inseparably linked to Euler’s implicit, but unproved, hypothesis that but a single-velocity field is required to characterize the four physically different, context-specific, velocities appearing in the mass, momentum, and energy equations. To test Euler’s hypothesis, velocity is added to the usual list of quantities requiring constitutive formulation - namely the heat flux q and viscous stress T - in order to effect closure of the mass, momentum, and energy equations. Establishment of this enlarged set of constitutive relations is effected by using conventional linear irreversible thermodynamics (LIT) principles governing the behavior of simple fluid continua, importantly including satisfaction of Onsager reciprocity as a fundamental continuum requirement. The resulting analysis shows that, in general, two velocities rather than one are required and, concomitantly, that additional driving forces must be added to each of the standard constitutive equations for the Fourier’s-law heat flux q = −k∇T and the Newton’s-law viscous stress (wherein the “mass velocity” v_m is the context-specific velocity appearing in the continuity equation ∂ρ/∂t + ∇ · (ρv_m) = 0). For the particular case of dilute gaseous continua explicit expressions are established for the phenomenological coefficients appearing in these additional constitutive contributions. Determination of these coefficients is effected using data derived from the Chapman-Enskog-Burnett constitutive expressions for q and T, the latter obtained by solving the Boltzmann equation at small Knudsen numbers, including so-called rarefied-gas contributions. These coefficients are found to be nonzero, confirming the conclusion, inter alia, that two velocities are constitutively required to quantify hydrodynamic behavior for gases and, by inference, for liquids too. Collectively, these velocity, heat flux, and stress constitutive findings collectively negate the current belief that the NSF equations fully describe the physics of viscous fluid continua. Rather, they do so only in limiting cases where the additional constitutive terms than we have found necessary for completeness are asymptotically small.     

Management of climatic heat stress risk in construction: A review of practices,methodologies, and future research

Climatic heat stress leads to accidents on construction sites brought about by a range of human factors emanating from heat induced illness, and fatigue leading to impaired capability, physical and mental. It is an occupational characteristic of construction work in many climates and the authors take the approach of re-engineering the whole safety management system rather than focusing on incremental improvement, which is current management practice in the construction industry. From a scientific viewpoint, climatic heat stress is determined by six key factors: (1) air temperature, (2) humidity, (3) radiant heat, and (4) wind speed indicating the environment, (5) metabolic heat generated by physical activities, and (6) “clothing effect” that moderates the heat exchange between the body and the environment. By making use of existing heat stress indices and heat stress management processes, heat stress risk on construction sites can be managed in three ways: (1) control of environmental heat stress exposure through use of an action-triggering threshold system, (2) control of continuous work time (CWT, referred by maximum allowable exposure duration) with mandatory work-rest regimens, and (3) enabling self-paced working through empowerment of employees. Existing heat stress practices and methodologies are critically reviewed and the authors propose a three-level methodology for an action-triggering, localized, simplified threshold system to facilitate effective decisions by frontline supervisors. The authors point out the need for “regional based” heat stress management practices that reflect unique climatic conditions, working practices and acclimatization propensity by local workers indifferent geographic regions. The authors set out the case for regional, rather than international, standards that account for this uniqueness and which are derived from site-based rather than laboratory-based research.     

Bleustein-Gulyaev SAW and its associated quasi-particle

Following a previous work that dealt with the case of Rayleigh surface waves in pure elasticity, here it is shown that a quasi-particle with Newtonian point-like mechanics (equation of inertial motion, expression of the kinetic energy) can be associated with the celebrated Bleustein-Gulyaev surface waves of linear piezoelectricity. This association is based on the integration over a vertical band of the sagittal plane of the canonical balance laws that accompany, via Noether’s theorem, the basic field equations. It accounts for the boundary conditions at the limiting surface, the periodicity of the solution in propagation space, and the vanishing of all fields at infinity in the substrate or in the outside vacuum. The proof benefits from the fact that the average (over one wavelength of propagation) of the Lagrangian density at the limiting surface is proportional to the satisfied “dispersion relation”, and hence is zero. The expression found for the “mass” of the said quasi-particle is informative in that it contains information about the frequency, the amplitude of the signal, and the electromechanical coupling.     

Illustration of the WPS benefit through BATMAN test series: Tests on large specimens under WPS loading configurations

To study the effects of warm pre-stressing on the toughness of reactor pressure vessel steel, the “Commissariat à l’Energie Atomique”, in collaboration with “Electricité De France” and AREVA-NP, has made a study combining modeling and a series of experiments on large specimens submitted to a thermal shock or isothermal cooling. The tests were made on 18MND5 ferritic steel bars, containing a short or large fatigue pre-crack.The effect of “warm pre-stressing” was confirmed, in the two cases of a fast thermal shock creating a gradient across the thickness of the bar and for gradual uniform cooling. In both cases, no propagation was observed during the thermal transient. Fracture occurred under low temperature conditions, at the end of the test when the tensile load was increased. The failure loads then recorded were substantially higher than during pre-stressing.To illustrate the benefit of the WPS effect, numerical interpretations were performed using either global approach or local approach criteria. WPS effect and capability of models to predict it were then clearly shown.     

A.A. Ilyushin’s works: an appraisal from Paris

An appraisal of some of the scientific contributions of A.A. Ilyushin on account of recent developments in thermo-mechanics and continuum physics is presented. An attempt is made to place these original contributions in a more international landscape.     

Two visual systems in monitoring of dynamic traffic: Effects of visual disruption

Studies from neurophysiology and neuropsychology provide support for two separate object- and location-based visual systems, ventral and dorsal. In the driving context, a study was conducted using a change detection paradigm to explore drivers’ ability to monitor the dynamic traffic flow, and the effects of visual disruption on these two visual systems. While driving, a discrete change, such as vehicle location, color, or identity, was occasionally made in one of the vehicles on the road ahead of the driver. Experiment results show that without visual disruption, all changes were detected very well; yet, these equally perceivable changes were disrupted differently by a brief blank display (150 ms): the detection of location changes was especially reduced. The disruption effects were also bigger for the parked vehicle compared to the moving ones. The findings support the different roles for two visual systems in monitoring the dynamic traffic: the “where”, dorsal system, tracks vehicle spatiotemporal information on perceptual level, encoding information in a coarse and transient manner; whereas the “what”, ventral system, monitors vehicles’ featural information, encoding information more accurately and robustly. Both systems work together contributing to the driver's situation awareness of traffic. Benefits and limitations of using the driving simulation are also discussed.     

Investigations on the high temperature behaviour of welded martensitic joints

As in Germany in the year 2015 the next generation of fossil fired power plant with steam parameters up to 700 °C will be erected, intensive R&D work focused on materials capable of operating in that high temperature regime is ongoing. Modern nickel-based alloys offer the possibility to be used for components for the highest temperatures and pressures in such power stations. Nevertheless, martensitic heat resistant 9-12% chromium steels will be widely used for the majority of the components subjected to “lower” temperatures up to 650 °C maximum, as they are much cheaper than nickel-base alloys. By welding these martensitic components, the heat affected zone (HAZ) has to be considered as a location of premature failure due to the change in the material’s microstructure i.e. size and number of precipitates, dislocation density, etc. Thus fully loaded weldments are of specific interest with regard to their possible optimization as well as their inspection during service. To be able to develop optimization strategies to increase the lifetime of welded martensitic components, the better knowledge of the HAZ’s dimension and extent in addition with the understanding of the development of the time dependent complex stress and strain states in this area are mandatory. In this paper, the results of an national funded project [1] focused on the optimization of creep loaded welds in pipes made of 9-11% Cr-steels will be reported. The project was aimed on the evaluation of the influence of the mismatch (in terms of different creep deformation of weld and base metal respectively) on the formation of the local stress situation in the weldment. In the frame of this work, extensive numerical and experimental investigations on component like specimens and lab specimens but also on measuring the temperature fields during the welding process lead to a comprehensive picture of the HAZ. Data of creep tests of thermally simulated HAZ material at various peak temperatures ranging from 780 °C to 1300 °C, of the weld metals but also of the base material formed the basis for a realistic simulation of the weldment’s behaviour. Finally the component tests under long term creep loading situations have been used for the validation of the numerical simulation.     

Comments and authors’ reply on “Linear stress-strain relations in nonlinear elasticity” by A. Chiskis and R. Parnes, (Acta Mech. 146, 109–113, 2001)

Summary. Recently this journal published a work stating that the idea of geometrical nonlinearity within Hookes law is no more than a widely accepted illusion since the linear stress-strain laws hold only for very nontrivial measures representing the corresponding strain tensor which depend on material parameters. Since the linear stress-strain relations with nonlinear strains are, indeed, widely used in research and design the arguments of the authors of this work should be considered. Below it is shown where a flaw in these arguments is and why Hookes law with nonlinear strains is correct.     

Application of the “K-ε” model to open channel flows in a magnetic field

In magnetohydrodynamic (MHD) flows turbulence reduction occurs due to the Joule dissipation. It results in heat transfer degradation. In open channel flows, heat transfer degradation is also caused by the turbulence redistribution near the free surface. Both effects can be significant in fusion applications with low-conductivity fluids such as molten salts. In the present study, the “K-ε” model equations for turbulent flows and the free surface boundary condition are adjusted with taking into account MHD effects. Different orientations of the magnetic field, perpendicular and parallel to the main flow, have been considered. The model coefficients have been tuned by a computer optimization using available experimental data for the friction factor. The effect of free surface heat transfer degradation due to the turbulence redistribution has been implemented through the variation of the turbulent Prandtl number. As an example, the model is used for the analysis of a turbulent MHD flow down an inclined chute with the heat flux applied to the free surface.     

Short sample training behavior of Nb-Ti fibers at 4.2 K

Experimental results are presented for the stress required to cause quenching during successive runs when bare fibers of Nb-Ti are carrying subcritical currents with no cross field. The data fall into two distinct regimes attributed to regions of magnetic flux stability and instability. Microplastic deformation is believed to supply the energy to initiate the flux jump process in the magnetic instability regime, and is the only source of heat available for triggering a quench when the fiber is magnetically stable. In both cases, quenching is observed at stresses well below the mechanically observed elastic limit. Simple techniques for one-step training and detraining are also described.     

Ultra-strength materials

Recent experiments on nanostructured materials, such as nanoparticles, nanowires, nanotubes, nanopillars, thin films, and nanocrystals have revealed a host of “ultra-strength” phenomena, defined by stresses in a material component generally rising up to a significant fraction of its ideal strength - the highest achievable stress of a defect-free crystal at zero temperature. While conventional materials deform or fracture at sample-wide stresses far below the ideal strength, rapid development of nanotechnology has brought about a need to understand ultra-strength phenomena, as nanoscale materials apparently have a larger dynamic range of sustainable stress (“strength”) than conventional materials. Ultra-strength phenomena not only have to do with the shape stability and deformation kinetics of a component, but also the tuning of its physical and chemical properties by stress. Reaching ultra-strength enables “elastic strain engineering”, where by controlling the elastic strain field one achieves desired electronic, magnetic, optical, phononic, catalytic, etc. properties in the component, imparting a new meaning to Feynman’s statement “there’s plenty of room at the bottom”. This article presents an overview of the principal deformation mechanisms of ultra-strength materials. The fundamental defect processes that initiate and sustain plastic flow and fracture are described, and the mechanics and physics of both displacive and diffusive mechanisms are reviewed. The effects of temperature, strain rate and sample size are discussed. Important unresolved issues are identified.     

Pressure drops in the Graetz-Nusslt problem extended on a gap flow of power law fluids

Summary In this paper a new approximative method of calculating the pressure drops in laminar forced convection heat transfer to power law fluids with temperature dependent rheological properties is proposed. These fluids exhibiting a non-Newtonian rheological behaviour are assumed to flow through a flat gap formed by two wide, parallel plane plates. The method to be proposed here makes it possible to calculate the values of local Fanning's friction factor if only one value of the so called film temperature has been previously measured. The main advantage of the procedure presented is that it holds for various thermal wall conditions (constant and variable wall temperature as well as wall heat flux). In addition, the effects of viscous dissipation on pressure drops have been taken into account. The results of numerical computations are shown in form of graphs and some comparisons are presented tabularly. The present work is intended to be a supplement to the previous paper by the same authors [1].With 3 Figures     

Strain-gauge compliance measurements near the crack tip for crack closure evaluation: Applicability and accuracy

The “plasticity-induced crack closure” phenomenon is the leading mechanism which controls the main effects on fatigue crack growth (e.g. stress ratio and load interaction effects) in metallic materials. Experimental tests, consisting mainly in global and/or local compliance measurements of the considered specimen, are usually carried out to quantify the physical phenomenon, but some aspects concerning the elaboration of acquired local compliance signals are not yet clear. From the analytical point of view, the so-called “Strip-Yield” model has proven to be the most versatile and powerful tool for estimating crack closure levels, but its application to structural steels is not yet straightforward due to the delicate calibration process.The present work tries to add some new ideas on the elaboration of local compliance experimental data, obtained from a M(T) specimen, simulating the measurements by means of an optimised Strip-Yield model implementation enriched by a novel module based on the Westergaard’s elastic complex potentials. The application of the method to the calibration of the Strip-Yield model has been already successfully faced elsewhere, so here the analytical results gave the possibility, together with dedicated FEM analyses, to investigate some of the different parameters and to state some conclusions about the reliability and applicability of local compliance measurements.     

A Strip-Yield algorithm for the analysis of closure evaluation near the crack tip

“Plasticity-induced crack closure” phenomenon is the leading mechanism of different effects (R-ratio, overload retardation, … ) acting on crack growth rate in many metallic materials. Experimental tests are carried out to quantify the physical phenomenon, while Strip-Yield analytical models have been developed for predicting life of components. In the present work, an additional module to be applied to a Strip-Yield model is proposed in order to derive the strains near the crack tip. Particularly, the module is based on the Westergaard’s elastic complex potential. The presented algorithm allowed us to obtain the correlation between “local compliance” experimental results and the corresponding Strip-Yield analyses. This method can be taken as a semi-analytical procedure for calibrating the constraint factor, i.e., the most delicate parameter for Strip-Yield models.