A statistical micromechanical model of multiple cracking for ultra high toughness cementitious composites

A new statistical micromechanical model of multiple cracking is proposed in which a general expression of the fiber bridging stress laws in the crack plane is established. In this model, the random distribution properties of fibers are considered. And the Weibull function is adopted to represent the flaw size distribution. The relationships of stress versus strain and crack width versus strain are proposed. The formulas of the crack width, crack space, strain capacity and fracture energy density at the end of multiple cracking processes are also deduced. The validity of the proposed model was demonstrated by experimental results.     

Dynamic behavior of an aggregate material at simultaneous high pressure and strain rate: SHPB triaxial tests

Low velocity impacts on energetic materials induce plastic deformations and sliding friction which can lead to ignition. If some ignition criteria have been proposed, the remaining difficulty is to characterize the mechanical behavior of the material when submitted to the corresponding solicitations (high pressure and high strain rate). Thus, a technique based on the Split Hopkinson Pressure Bars system is proposed to carry out a triaxial compression test. A cylindrical specimen is placed into a confining ring and is compressed by the system of bars. The ring prevents the radial extension of the specimen and creates a lateral confining pressure. The material and dimensions chosen for the ring maintain a constant radial pressure during the test. Some tests were carried out on an inert aggregate material and proved the validity of this experimental device. The experimental data processing shows the influence of both the pressure and the strain rate. The shear stresses, which contribute to thermal dissipation and then to the ignition threshold, increase according to the pressure.     

The Hermite radial basis function control volume method for multi-zones problems; A non-overlapping domain decomposition algorithm

In this paper a non-overlapping non-iterative multi-domain formulation for the control volume Hermite radial basis functions (CV-HRBF) method is proposed, where the local Hermitian RBF meshless collocation method is used to satisfy a physical matching condition at the sub-domain boundaries. In addition, the robustness of the Hermite interpolation is exploited even further to apply multiple flux continuities for those cases where more than two sub-domains converge in the same point. The algorithm is first validated in one-dimensional advection diffusion problems for which an analytical solution is known. Its accuracy is compared with a classic CV approach and a local radial basis function collocation method (LRBFCM). More general applications in two and three-dimensional domains are then considered. A heat transfer problem in strongly heterogeneous materials, and a groundwater flow problem in presence of geological layers characterised by different hydraulic conductivity, are taken as engineering applications to test the capabilities of the CV-HRBF method to handle multi-zone problems. Finally, the transport of a single species is simulated in a one-dimensional channel consisting of two adjacent zones that feature different Peclet numbers.     

A monolithic energy conserving method to couple heterogeneous time integrators with incompatible time steps in structural dynamics

A new hybrid multi-time method for multi-time scales structural dynamics simulations is described. A monolithic method in a Schur dual domain decomposition framework is proposed and allows to consider heterogeneous time integrators with their own time discretization and possible large ratio between the time steps for each subdomain. In the proposed method, zero numerical dissipation is ensured at the interface. This implies that the global stability of the coupling method is governed by the stability of each time integrator without influence of the interface. For that purpose, velocity continuity is ensured in a weak sense at the interfaces, and time integrators (Newmark, HHT, Simo, Krenk, Verlet) are introduced in a unified framework (incremental velocity formulation). Furthermore, dynamics governing equations are introduced from a weak formulation in time. In other words, equilibrium equation is no more ensured in a strong sense at a given time step, but rather on average on a time interval. Some numerical examples illustrate the efficiency and the robustness of the proposed method, for ratio of time scales close to 1000 without any numerical dissipation at the interfaces.     

Performance-based Aeolian risk assessment and reduction for tall buildings

The design of tall buildings subject to wind actions can be developed in the framework of Performance-Based Wind Engineering (PBWE). The structural performances are described by a set of measurable attributes, the decision variables, which are functionally related to proper measures of the structural damage, in turn dependent on proper engineering demand parameters characterizing the structural response, and measures of the intensity of the wind field. In this paper, PBWE is applied to the assessment of the comfort requirement and the structural reliability for a 74 storey building. Probabilistic calculations of the structural response are carried out in frequency and time domains, and the parameters of the wind velocity field are calibrated on the basis of the time-histories of the global floor forces derived by experimental tests on a rigid 1:500 scale model of the building. The occupant comfort is related to the motion perception under moderate winds, and quantified by the probability of exceeding threshold values of the across-wind acceleration at the top of the building. The structural reliability is related to the lateral deformation capacity under strong winds, and quantified by the probability of exceeding threshold values of the maximum inter-storey drift ratio. The results of numerical analyses suggest the use of a tuned mass damper to enhance the building performances.     

Variability response functions for stochastic systems under dynamic excitations

The concept of variability response functions (VRFs) is extended in this work to linear stochastic systems under dynamic excitations. An integral form for the variance of the dynamic response of stochastic systems is considered, involving a Dynamic VRF (DVRF) and the spectral density function of the stochastic field modeling the uncertain system properties. As in the case of linear stochastic systems under static loads, the independence of the DVRF to the spectral density and the marginal probability density function of the stochastic field modeling the uncertain parameters is assumed. This assumption is here validated with brute-force Monte Carlo simulations. The uncertain system property considered is the inverse of the elastic modulus (flexibility). The same integral expression can be used to calculate the mean response of a dynamic system using a Dynamic Mean Response Function (DMRF) which is a function similar to the DVRF. These integral forms can be used to efficiently compute the mean and variance of the transient system response together with time dependent spectral-distribution-free upper bounds. They also provide an insight into the mechanisms controlling the dynamic mean and variability system response.     

Structural behaviour of ferrocement–brick composite floor slab panel

This study introduces a semi-fabricated system for the construction of floor slab. The slab panel consists of two layers joined together using truss type shear connectors. The first layer is a precast ferrocement layer which acts initially as a formwork, while the second layer consists of bricks and mortar. Continuous truss shear connectors are used to connect the two layers. The paper experimentally investigates the structural response of ferrocement–brick composite panel under flexural load. Four full scale specimens were cast and tested under two-line loads. The study highlights the effect of shear connectors and brick layout on the overall structural response of the slab. The results in terms of load–deflection, crack pattern, strain distribution and failure loads indicate that the response of the composite slab to the flexural loading is satisfactory and can be used as a floor slab in construction sector.     

Thermal state of electronic assemblies applied to smart building equipped with QFN64 device subjected to natural convection

The performance and reliability of electronic components and assemblies strongly depend on their thermal state. The knowledge of the temperature distribution throughout the assembly is therefore an essential element to ensure their correct operation. This is the main objective of this work that examines the case of a conventional assembly equipped with a quad flat non-lead QFN64 subjected to free convection. This active electronic package is welded on a PCB which may be inclined by an angle varying between 0° and 90° (horizontal and vertical positions respectively) and generates during its operation a high power ranging from 0.1 to 1 W. Thermoregulation of the assembly is ensured by natural convection, given its many well known advantages in this engineering field. Accurate relationships are proposed to determine the temperature on different areas of the device and the PCB. They are determined by means of a 3D numerical approach based on the finite volume method confirmed by measurements on an actual installation. These relationships allow reliability improvement of these electronic assemblies that are widely used in many engineering fields such as computing industry, mobile telephony, home automation, automotive, embarked electronics and the smart building applications considered in this work. The present survey complements a recent study which quantifies the natural convective heat transfer on the considered electronic assembly equipped with the QFN64 device, for the same power range and angle of inclination.     

Phase transition and related electronic and optical properties of crystalline phenanthrene: An ab initio investigation

This investigation discusses a structural phase transition of organic crystalline phenanthrene and the resulting changes of its electronic and optical properties investigated by ab initio calculations based on density functional theory (DFT). The structure of phase I has been optimized then its electronic and optical properties have been calculated. Our computational results on phase I (at ambient pressure) get along well with the available experimental data.Calculating the electronic and optical properties of phase II are proceeded in the same way and the results, particulary Raman spectra, reveal a crystallographic phase transition indicated by abrupt changes in lattice constants which are accompanied by rearrangement of the molecules. This results in modifications of the electronic structure and optical response. For both phases the band dispersion of the valence and conduction bands are anisotropic, whereas the band splitting is strongly noticeable in phase II. By calculating the imaginary part of the dielectric function of phase II, we have found the appearance of new peaks at the lowest z-polarized absorption and about 30 eV in all absorption components. Excitonic effects in the optical properties of phases I and II have been investigated by solving the Bethe–Salpeter equation (BSE) on the basis of the FPLAPW method. Phase II shows four main excitonic structures in the energy range below band gap, whereas phase I shows two. The excitonic structures in the optical spectra of phase II show a red shift in comparison to phase I. The calculated binding energies of spin-singlet excitons in phase II are larger than the ones in phase I.     

Hyperspectral image compression based on lapped transform and Tucker decomposition

In this paper, we present a hyperspectral image compression system based on the lapped transform and Tucker decomposition (LT-TD). In the proposed method, each band of a hyperspectral image is first decorrelated by a lapped transform. The transformed coefficients of different frequencies are rearranged into three-dimensional (3D) wavelet sub-band structures. The 3D sub-bands are viewed as third-order tensors. Then they are decomposed by Tucker decomposition into a core tensor and three factor matrices. The core tensor preserves most of the energy of the original tensor, and it is encoded using a bit-plane coding algorithm into bit-streams. Comparison experiments have been performed and provided, as well as an analysis regarding the contributing factors for the compression performance, such as the rank of the core tensor and quantization of the factor matrices.