Rostand Moutou Pitti

Characterization of a cracked specimen with full-field measurements: direct determination of the crack tip and energy release rate calculation

The crack characterization in a pre-cracked aluminum specimen is investigated in this study using the grid method. The images of this grid are analyzed to provide the crack tip location as well as the displacement and strain fields on the surface of the specimen during a tensile test. Experimental data are used to calculate the energy release rate with the compliance method A fracture analysis is also performed using the invariant M

Generalization of Integral Parameters to Fatigue Loading in Room Temperature

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Experimental Characterization of Semi-Rigidity of Standardized Lattice Beam Using the Grid Method

θ integral in which both real and virtual displacement fields are introduced. This integral is implemented in the finite element software Cast3M. Both approaches give similar results in 2D case.

An Incremental Constituve Law for Damaging Viscoelastic Materials

The reliability analysis applied to viscoelastic and orthotropic materials, in the case of mixed mode configuration, is studied in this work. The M integral, separating mixed mode during creep crack initiation in viscoelastic field, is used in the analytical approach. The main development, based on conservative law, and a combination of real and virtual displacement fields, is proposed. In order to provide mixed mode configuration, a Compact Tension Shear (CTS) specimen is used in the numerical process. Simultaneously the fracture and the viscoelastic procedures are coupled with reliability analysis in order to take account for model and parameter uncertainties. In this case, the random parameters related to model factors, elastic constants are defined in the reliability analysis of time dependent fracture materials subjected to complex loading. As results, the reliability levels are computed and discussed according to various mixed-mode loading scenarios

Reliability-Based Design of Structures Under Seismic Loading: Application to Timber Structures

In this paper a numerical approach coupling independent path integrals, such as M-integral, to compute the crack driving forces namely the stress intensity factors, and empirical models, for instance Paris-Erdogan’s law, to assess the cumulative fatigue damage (i.e. crack size) during the crack growth process, is proposed. The M-integral derived from Nother’s theorem combines the real and virtual mechanical deformation and stress fields. A finite element routine is developed in order to compute the energy release rate according to the stress intensity factors. Results are given for a simple standard Al7075-T6 tensile test specimen. Finally, numerical estimates are compared to experimental data for various crack length in order to prove the efficiency and the accuracy of the proposed model.

An incremental constitutive law for ageing viscoelastic materials: a three-dimensional approach

The mechanical behavior, of Earlywood (EW) and Latewood (LW) of Abies Alba Mills (white fir of Massif Central) during drying under mechanical loading, is investigated. The grid method is used to measure the strain field and distinguish the heterogeneities at rings scale. Images are recorded by a Sensicam CCD camera. A 50-kN Zwick testing machine is used to apply the load. The tested specimen is first conditioned to a moisture content of 57.42 %. It is then tested and regularly removed in order to measure the moisture content. A tracking system ensuring constant positioning of the sample with respect to the camera has also been developed. Before loading, images have been taken to highlight the impact of moisture conditioning. Images are also grabbed during the loading steps and between them after totally unloading. All the strain maps are determined through a reference image (zero strain) taken before water conditioning of the specimen. During the experiments, strain gradients clearly appear between EW and LW, highlighting the variability of the mechanical properties at the rings scale. The effect of moisture content changes in the mechanical behavior of wood is also shown.

A proposed mixed-mode fracture specimen for wood under creep loadings

Timber structures often exhibit shear and tension perpendicular to grain. This phenomenon induces brittle failure if it is not controlled. This is particularly the case in joining zones, and even more when the beam elements are thin. These thin elements can be found for example in lattice beams. Standardized lattice timber beams appear as an efficient solution for economical, ecological and mechanical aspects. This study focuses on the mechanical behavior of notched beams. Experiments are carried out with a classic loading device and LVDT measurements as well as with the grid method which provides full-field displacement and strain measurements. Tests are conducted for various orientations of annual rings of the wood. The evolutions of strains in the zone affected by shear and tension stresses are analyzed.

Incremental constitutive formulation for time dependent materials: creep integral approach

An incremental formulation suitable for modelling of materials with damaging viscoelastic behaviours is proposed in this work. A constitutive law based on linear viscoelasticity coupled with strain dependent damage is developed. The viscoelastic model is represented by a generalized Maxwell’s chain. It is governed by a set of internal stress variables attached to the branches of the Maxwell’s chain. The damage evolution is governed by values gained by a pseudo strain. The coupled law is turned into an incremental form suitable for the numerical analysis of damaging time dependent structures. Taking advantage of the incremental form, the coupled damaging viscoelastic law is implemented as a step-by-step procedure. The calculation procedure consists of a damaging elastic step followed by a number of damaging viscoelastic steps. The damage variable is adjusted at each step, according to the value gained by the pseudo strain. Exemplary calculations are worked out for two cases of uniaxial and biaxial variable or cyclic loadings. The results show the efficiency of the incremental model. It is worth noticing that the time increment used for the calculations is not necessarily small. As a consequence, precise analysis of damaging time dependent structures can be performed for low calculation cost.