Jean-Michel Muracciole

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.

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.

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.

Thermomechanical Analysis of the Yield Behaviour of a Semicristalline Polymer

The cold drawing of semi crystalline polymer has been observed in the early 1930s by Carothers and Hill [1]. Ever since, various attempts were made to explain this phenomenon in terms of crystallinity modification. This behaviour, usually related to a rearrangement of the crystallites, depends on the nature of the stretched materials. In some materials, during necking, part of chains in crystalline blocks are unfolded and transfer into amorphous phase with more or less orientation (see Peterlin and Olf [2], Gent et al. [3]). In other polymers, Strauch and Schara [4] exhibit realignment of crystallite. Other authors, Waddon and J., Karttunen [5], emphasize solid-solid phase transition. Nevertheless, everyone agree that a local rise of temperature occurs in the neck shoulders.

Speckle and Infrared Images Processing Applied to the Analysis of the Thermomechanical Behaviour of a Semicrystalline Polymer

The mechanical behaviour of thermoplastics with glassy amorphous regions is characterised by a strong dependence in temperature and strain-rate. It seems thus relevant to take a thermomechanical point of view on to account. As these materials may also exhibit both tough dissipation and strong strain localisation, we have decided to use two complementary local techniques to study their response.