Nicolas Angellier

Acoustic emission technique for fracture analysis in wood materials

Understanding the failure mechanisms of construction materials, as well as their damage evolution, constitute two key factors to improving structural design tools. Depending on the failure modes to be highlighted and studied, several test methods and analysis tools have been developed. One such development, the acoustic emission technique (AET), is an experimental tool appropriate for characterizing material behavior by means of monitoring the fracture process. Despite the widespread uses of acoustic emission techniques to characterize and monitor the damage evolution of composite materials, only a few research studies have focused on using AET to characterize the mechanical behavior of wood materials. In the present work, the failure process in Douglas fir under monotonic loading is studied by comparing three experimental methods: force-displacement curve analysis, acoustic emission measurements, and digital image acquisition. First of all, results show good correlation and complementarities among the methods employed. Second, analyzing acoustic emission signals by considering the event number and the cumulative events yields interesting information on crack initiation and growth without the material. Moreover, an additional analysis of acoustic emission data (involving the determination of source locations and the study of amplitude distributions) opens the possibility to characterize the fracture process zone which is a key damage mechanism in wood materials.

Crack Analysis of Wood Under Climate Variations

The knowledge of crack driving forces such as energy release rate and stress intensity factors is very important in the assessment of the reliability of timber structures. This work deals with static and creep fracture tests in opening mode crack growth. They are performed in climatic chamber, which allows reproducing the real environment effects on the cracking of Double Cantilever Beam specimens machined in Douglas and White Fir species. The evolutions of the crack length are posted versus time. The aim of this work is to compare the obtained experimental results with numerical tools given by a finite element model. The numerical model is based on a new analytical formulation of the A-integral, generalized to viscoelastic orthotropic material, which allows taking into account the effect of thermal and hydric loads in the cracking process.

Experimental and numerical aspects in diffusion process characterization in tropical species

The development of timber structures in tropical regions using local species requires characterization of mechanical properties in respect to moisture content. In this context, the estimation of moisture content distribution relies on knowledge of diffusion properties in terms of sorption hysteresis and diffusion kinetics. This paper deals with experimental protocols and numerical approaches used in the 3D orthotropic diffusion characterization for Moabi and Ozigo species.

Behavior of a Double-Layer Tensegrity Grid under Static Loading: Identification of Self-Stress Level

AbstractThe determination of the state of internal stress is important to define the rigidity of a tensegrity structure and its stability. Several methods can be used; some are based on direct measurements of the forces in the elements, but are not easily transferable to a real structure. The authors opt for indirect measurement techniques, which seem more appropriate for implementation on-site. One can consider the vibratory analysis of the elements, the vibratory analysis of the whole structure, or the analysis of the structure’s behavior under static loading. Here, the node displacement fields of a tensegrity structure in different states of self-stress under several strategies of static loadings is studied by comparing the measurement obtained by a tachometer with simulations. The aim of this work is to show the feasibility of a displacement field to identify the state of self-stress by this analysis. It is shown that under certain conditions, plans can be made to replace the direct measurement of the...

Acoustic techniques for fatigue cracking mechanisms characterization in hot mix asphalt (HMA)

This article deals with the investigation of the damage process in Asphalt Concrete (AC) using Acoustic Emission technique (AE). The AE response, particularly how it can change as a function of applied stresses, is known to be promising for micro-cracking detection. AE events are correlated with mechanical damage analysis, and allow determining fatigue cracking mechanisms. Nevertheless, the pertinence of AE approaches is closely related to the comprehension of wave propagation phenomenon within the tested material. In fact, wave propagation is affected by attenuation and velocity variation due to the viscoelastic behavior of the AC. For understanding this phenomenon, ultrasonic techniques can be used for determining both the attenuation curves and variation of the propagation velocity.

Numerical and experimental approaches to characterize the mass transfer process in wood elements

The scope of this paper is an experimental characterization of diffusion parameters for wood material. Based on a nonlinear mass transfer algorithm, the present study focuses on the need to capture experimental moisture profiles in the sample, along with its evolution in weighting during both the desorption and adsorption phases, especially when the moisture content of the samples is far from the equilibrium state inducing a great gradient between heart and exchange surfaces of specimen. These moisture profiles are derived by means of a gammadensimetry laboratory method based on the water adsorption of gamma rays. Determination of the diffusion parameters is obtained through optimizing a simulation by means of implementing the mass transfer kinetics into a finite difference method. Both the diffusion coefficient and convective exchange coefficient are deduced by considering a Nelder–Mead simplex inversion method. This work highlights the efficiency of the approach dedicated to uncoupling nonlinear diffusion in the cross sections from boundary conditions in terms of convective exchanges and equilibrium moisture. Scale effects and boundary conditions are also investigated herein.

Approaches to the Moisture Content Monitoring Intimber Elements: Development of a Resistive Method

The scope of this work is based on the use of resistive method to quantify the water content in timber elements. For an in-depth mapping, we adapted a multiplexed technique derived from geophysics based on a maximum crossing of current lines to obtain topography of measures sweeping the space boundary conditions being defined by the sample. The calibration of these measures is completed by a gammadensimetry laboratory method which allows access to water profiles along a preferred direction. Nevertheless, the resistive method is penalized by logarithmic laws linking moisture and resistivity. So, we develop a hybrid method for coupling the data obtained to diffusion models: it will provide complementary information where resistivity and gammadensimetry are no longer effective. The developed experimental protocol allows employing the selected method and optimizing the diffusion properties (diffusion coefficient and convective exchange coefficient) injected into a characterization algorithm (Nelder-Mead’s simplex inversion method) based on a finite difference method for the Fick’s diffusion law determination integrating orthotropic and non- linear properties. Overlap between electric field and density measurements and the numerical simulation tool are possible.

Etude de la déformée d'une grille de tenségrité pour l'identification de son niveau d'autocontrainte

ABSTRACT We study the possibility to use the measurement of the displacement fields of the nodes of a tensegrity structure under static loading to obtain a new method for the identification of its self-stress state. We try to determinate the correlation between the precision of this identification and the precision of the measure. With a tacheometer we obtain a precision of identification as good as the standard method using efforts measurements.

Corrélation entre modes propres et identification de l'état de contrainte d'une structure de tenségrité

ABSTRACT The structures of tensegrity are structures in equilibrium by an initial stress state. This stress state is the composition of elementary selfstress states which form a base. In order to identify the stress state of the structure, a nondestructive method based on the vibratory analysis has been used. The structure is subjected to a sinewave excitation for a given frequency. It appears that certain frequencies allow better an identification than others. This study tries to establish a relation between eigen mode, selfstress state and effectiveness of the identification. The article is based on a plane double layer tensegrity grid having six elementary selfstress states. The numerical simulations show the utility of the study for the identification of this structure.