Bertrand Wattrisse

Analysis of strain localization during tensile tests by digital image correlation

This paper presents an imaging technique developed to study the strain localization phenomena that occur during the tension of thin, flat steel samples. The data are processed using digital speckle image correlation to derive the two in-plane components of the displacement vectors. The authors observe that the calculation of the intercorrelation function reveals a systematic error and propose a numerical method to limit its influence. Plastic incompressibility and thin-sheet assumptions are used to derive the third displacement component and, hence, the various strain and strain rate components. Numerous checks are presented at each step in processing the data to determine the final accuracy of the strain measurements. It is estimated that this accuracy is quite sufficient to track the inception and the development of localization. Examples of possible application are presented for mild steels whose strain localization mechanisms appear to be precocious and gradual.

Kinematic manifestations of localisation phenomena in steels by digital image correlation

Abstract The aim of this paper is to show that recent advances made in the field of speckle image processing give valuable information useful in understanding and modelling of localisation phenomena. The potentialities of the proposed imaging method are illustrated by examples extracted from tensile tests performed on steel specimens. Having introduced the underlying motivations of this experimental work, this paper briefly focuses on the image processing technique and its reliability. Then, it describes the characteristics of the strain field within and outside of a propagating Luders band. The properties of strain states associated with diffuse and localised necking are also investigated. The catalyst role of possible geometrical defects is pointed out. Finally, a method is proposed to construct, despite localisation, a local stress–strain correspondence.

Local energy analysis of high-cycle fatigue using digital image correlation and infrared thermography

This paper presents the first results provided by an experimental set-up developed to estimate locally the terms of the energy balance associated with the high-cycle fatigue (HCF) of DP 600 steel. The experimental approach involves two quantitative imaging techniques: digital image correlation and infrared thermography. First, a variational method is used to derive stress fields from the displacement fields. Patterns of deformation energy per cycle can then be determined on the basis of stress and strain data. Second, a local form of the heat equation is used to derive separately the thermoelastic and dissipative sources accompanying HCF. Energy balances show that around 50 per cent of the deformation energy associated with the mechanical hysteresis loop is dissipated while the rest corresponds to stored energy variations.

Calorimetric analysis of polymer behaviour using a pixel calibration of an IRFPA camera

A pixel calibration of an IRFPA camera was developed to detect very small temperature variations induced by quasi-static loading of materials. The thermographic data were then used to estimate heat sources accompanying the deformation of PMMA and PC polymers during cyclic tests. The calorimetric balance analysis led us to define several possible ways of introducing the thermoelastic coupling effects in viscothermoelastic models.

Thermomechanical effects accompanying the localized necking of semi-crystalline polymers

This paper presents an application of quantitative infrared thermography to the analysis of the mechanical behavior of materials. We particularly focus on the thermomechanical behavior of a semi-crystalline polymer below its glass transition temperature. During a quasi-static tensile test, the temperature distribution at the surface of a thin flat sample is recorded by an infrared camera. Using a local expression of the heat balance, the distribution of heat sources is derived from thermal measurements. These calorimetric data are then correlated with strain and stress fields. Indeed, a second optical device gives displacement fields using correlation methods on speckle images. Strain and strain-rate are derived from displacement data by numerical differentiation. The load-displacement curve shows three stages: first the load increases with the stretching, then a significant softening of the sample occurs until it flows at constant load. Both calorimetric and kinematic measurements indicate that the sample softening is associated with a progressive localization of heat sources and strain-rates, while the loading plateau corresponds with a regular expansion of the neck. A local construction of stress-strain diagram is then proposed. Several stress-strain curves are finally analyzed taking into account the loading stage characteristics and the expansion mode of the necking region.

An inverse method applied to the determination of deformation energy distributions in the presence of pre-hardening stresses

This paper presents an experimental procedure to estimate the deformation energy distribution within plane samples submitted to mechanical loading. This procedure combined a digital image correlation (DIC) technique giving in-plane displacement fields with an identification method that separately provided fields of material properties and stress distributions developed during the loading. The method was first applied to simulated data to characterize the capabilities of the image processing. Finite element computations were first performed on a complex structure using a standard linear kinematical hardening model to generate multistage loadings leading to heterogeneous displacements and distributions of deformation energy. Loads and displacements were then used as inputs to check the robustness of the image processing by comparing the identified deformation energy fields with the computed ones. The procedure was then applied to experimental data. Tests were conducted under conditions similar to the numerical tests. The identification of a linear kinematical hardening model gave deformation energy patterns showing a good agreement with the simulated results, even in the presence of residual stresses induced by a pre-hardening.

Calorimetric analysis of coarse-grained polycrystalline aluminum by IRT and DIC

A novel method is presented in this paper which aims at achieving grain scale energy balances at finite strain in mechanically-loaded polycrystalline metallic specimens. For this purpose, two complementary imaging techniques were used to investigate material thermomechanical behaviour: Digital Image Correlation and InfraRed Thermography to separately reach the kinematic and the thermal responses of the material. A calorimetric analysis can be conducted by combining these techniques. The aim of this paper is to present and to validate a novel IR data processing method that can be used to perform local thermal field measurements. The procedure was validated on numerical data associated with the response of aluminum polycrystalline aggregates.

Energy balance properties of steels subjected to high cycle fatigue

This paper presents an experimental protocol developed to locally estimate different energy balance terms associated with the high cycle fatigue (HCF) of steels. Deformation and dissipated energy are respectively derived from displacement and temperature fields obtained using digital image correlation (DIC) and quantitative infrared thermography (QIRT) techniques. The combined processing of visible and infrared images reveals the precocious, gradual and heterogeneous development of fatigue localization zones. It also highlights the plastic character of dissipative heat sources (i.e. proportional to the loading frequencies), and the progress of fatigue dissipation, observing the drift of the mean dissipation per cycle for a given loading. The substantial of internal energy variations during HCF loading are finally underlined. The paper ends with a discussion on the consequences of such energy balance properties in terms of HCF modeling.

Multiscale thermomechanical approaches to SMA behaviour

This paper presents, in a synthetic way, several works performed on shape memory alloys (SMAs). Three scales of description are used according to whether one seeks to numerically predict the possible microstructural configurations of phases in equilibrium or the behavior of a mono- and polycrystal of SMA during a phase transition.

Terahertz thermometry system to measure temperature in the thickness of a solid polymer

Abstract This paper deals with thermal attenuation of a transmitted THz signal (165 GHz) across a POM polymer sample subjected to a thermal gradient in the thickness direction. The paper describes the experimental setup, including a THz sensor, a system to impose thermal loading and an infrared camera used to measure temperature variations at the surface. The thermal dependence of the transmitted THz signal through the sample was studied along a spatial profile. A simple polynomial model, validated through finite element analysis and thermal imaging measurement, was used to estimate temperature variations in the thickness direction. The correspondence between the 2D transmitted THz signal and the 3D temperature distribution allowed us to estimate the thermal sensitivity of the absorption coefficient in the THz range. This study showed that the THz sensor was sensitive enough to measure THz signal variations due to small temperature variations. The mean temperature may be determined along the thickness direction once the thermodependence of the transmission and reflection coefficients are known.