Julien Réthoré

An extended and integrated digital image correlation technique applied to the analysis of fractured samples The equilibrium gap method as a mechanical filter

To reduce the measurement uncertainty, a measurement technique is proposed for estimating full displacement fields by complementing digital image correlation with an additional penalization on the distance between the estimated displacement field and its projection onto the space of elastic solutions. The extended finite element method is used for inserting discontinuities independently of the underlying mesh. An application to the brittle fracture of a silicon carbide specimen is used to illustrate the application. To complete the analysis, the crack tip location and the stress intensity factors are estimated. This allows for a characterization of the measurement and identification procedure in terms of uncertainty.

Digital volume correlation analyses of synchrotron tomographic images

Three-dimensional (3D) images of nodular graphite cast-iron samples obtained with synchrotron tomography are analysed by resorting to digital volume correlation. This technique uses the material microstructure to measure displacement fields within a sample submitted to mechanical loading. Compared to classical two-dimensional (2D) digital image correlation, the reconstructed volumes used by DVC tend to increase the uncertainty in the correlation calculations, yet elastic strains of the order of 10−3 can be measured. Displacement fields around the front of a fatigue crack have also been measured by an extended version of DVC, and are used to extract stress intensity factors (in modes I, II, and III) along the crack front. The values are in good agreement with finite element calculations when the experimentally measured displacements prescribed on the sample boundaries are considered as input in the numerical simulations.

Experimental investigation of localized phenomena using digital image correlation

This paper is dedicated to the use of enriched discretization schemes in the context of digital image correlation. The aim is to capture and evaluate strong or weak discontinuities of a displacement field directly from digital images. An analysis of different enrichment performances is provided. Two examples of strain localization illustrate the discussion.

Extraction of stress intensity factors for 3D small fatigue cracks using digital volume correlation and X-ray tomography

This paper describes a methodology used to compute Stress Intensity Factor values along the curved front of a fatigue crack inside a nodular cast iron. An artificial defect is introduced at the surface of a small sample. The initiation and growth of a fatigue crack from this defect during constant amplitude cycling is monitored in situ by laboratory x-ray tomography. The method for processing the 3D images in order to compute SIF values is described in detail. The results obtained show variations of the stress intensity factor values along the crack front.

Recent progress in digital image correlation: From measurement to mechanical identification

Displacement field measurements based on digital image correlation are discussed for 2D pictures in terms of principles and possible exploitations. The presentation will focus on the study of cracked media.

A Precis of Two-Scale Approaches for Fracture in Porous Media

A derivation is given of two-scale models that are able to describe deformation and flow in a fluid-saturated and progressively fracturing porous medium. From the micromechanics of the flow in the cavity, identities are derived that couple the local momentum and the mass balances to the governing equations for a fluid-saturated porous medium, which are assumed to hold on the macroscopic scale. By exploiting the partition-of-unity property of the finite element shape functions, the position and direction of the fracture is independent from the underlying discretisation. The finite element equations are derived for this two-scale approach and integrated over time. The resulting discrete equations are nonlinear due to the cohesive crack model and the nonlinearity of the coupling terms. A consistent linearisation is given for use within a Newton—Raphson iterative procedure. Finally, examples are given to show the versatility and the efficiency of the approach.

CARPIUC benchmark overview: crack advance, reorientation, propagation and initiation under complex loadings

A series of experiments has been performed to build a benchmark for the numerical modeling of mixed mode crack propagation in concrete. Two tests are selected so that the propagation is almost always stable although the material is quasi-fragile. Moreover the variation of mode mixity enables the study of the reorientation of the crack, as well as more complex phenomena such as branching or link-up. To control stability and mode mixity, the loading is imposed with a 6-axis testing machine while the crack propagation is assessed by full-field measurement. In this paper, a description of the experimental setup is provided as well as numerical simulation results illustrating the sensitivity to boundary conditions and geometry. Links to data needed to perform numerical simulations of these benchmark experiments are given, together with references to publications giving full descriptions of the protocol and experiment results.

3D Analysis of a Fatigue Crack in Cast Iron Using Digital Volume Correlation of X-ray Tomographic Images

Three-dimensional (3D) images of a thumbnail corner fatigue crack are obtained in a sample of nodular graphite cast iron using laboratory X-ray computed tomography. The crack is initiated in situ from an artificial defect created by laser machining, its development is followed in situ and Digital Volume Correlation (DVC) analysis of the 3D images gives access to the 3D displacement field at the tip of the crack (mainly mode I opening).

Fatigue mechanisms of brazed Al-Mn alloys used in heat exchangers

The ratio of aluminium alloys used in the automotive industry tends to increase as a consequence of the enforcement of tougher environmental regulation (minimization of vehicles weight). For example, thanks to their good thermal, corrosion and mechanical properties, aluminium alloys have steadily replaced copper alloys and brass for manufacturing heat exchangers in cars or trucks. Such components have been constantly optimized in terms of exchange surface area and, nowadays, this has led to Al components in heat exchangers with a typical thickness of the order of 0.2 to 1.5 mm. With such small thicknesses, the load levels experienced by heat exchangers components has drastically increased leading to an important research effort in order to improve the resistance to damage development during service life.

A multiscale molecular dynamics / extended finite element method for dynamic fracture

A multiscale method is presented which couples a molecular dynamics approach for describing fracture at the crack tip with an extended finite element method for discretizing the remainder of the domain. After recalling the basic equations of molecular dynamics and continuum mechanics the discretization is discussed for the continuum subdomain where the partition-of-unity property of finite element shape functions is used, since in this fashion the crack in the wake of its tip is naturally modelled as a traction-free discontinuity. Next, the zonal coupling method between the atomistic and continuum models is described, including an assessment of the energy transfer between both domains for a one-dimensional problem. It is discussed how the stress has been computed in the atomistic subdomain, and a two-dimensional computation is presented of dynamic fracture using the coupled model. The result shows multiple branching, which is reminiscent of recent results from simulations on dynamic fracture using cohesive-zone models.