ABSTRACTmThis 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.KEY WORDS--Digital image correlation, strain rate measurement, necking, material behavior, steel Over the past two decades, the simulation tools used in mechanics of materials have become increasingly powerful. Progress made in the field of scientific computation is giving rise to a new branch of mechanics called computational mechanics of solids.1 It allows the use of more realistic, and also more complex, models of behavior.Of course, the consistency of the simulation results depends very much on the validity of the constitutive equations implemented in the computational codes. These phenomenological equations are, most often, identified on the basis of experiments carried out on particular structures for which stress and strain patterns are supposed to be known. For instance, we assume that strain and stress fields are homogeneous in the gage part of a sample during a simple tensile test. This homogeneity hypothesis, often implicitly assumed, is necessary to estimate the stress and the strain from load and displacement measurements. Naturally, as soon as a strain localization occurs--local necking, LiJders bands, transformation bands and so on--the mechanical fields are B.
This paper presents an infrared image processing procedure that was developed to study calorific effects accompanying material fatigue. This method enables us to separately estimate patterns of thermoelastic and dissipative sources. Heat sources were estimated on the basis of partial derivative operators present in a local form of the heat equation by using a set of approximation functions that locally fits the temperature field and takes the spectral properties of the sought sources into account. Numerical examples were used to check the validity of the method and to highlight its capabilities along with its limits. The paper concludes with examples of thermal image processing extracted from fatigue tests performed on a dual-phase steel. The coupling sources were compared to the theoretical predictions induced by a basic thermoelastic model, while the heterogeneous character of the fatigue development was highlighted in terms of dissipation sources.
The paper aims to highlight the advantages of using data supplied by digital image correlation (DIC) and infrared thermography (IRT) to study the thermomechanical behaviour of materials. It describes an experimental procedure for the determination of mechanical energy and heat sources involved locally during a heterogeneous tensile test. This procedure involves two complementary imaging techniques: DIC provides in‐plane displacement fields, while IRT enables the temperature distribution at the specimen surface to be monitored. Numerous different application examples are successively proposed to underline the promising potential of this experimental approach. Kinematical assessments can reveal the extent of homogeneity of the deformation state for a given gauge length. They can also help to determine the relevance of the variables and/or material parameters introduced in the behavioural description at the length scale imposed by the spatial resolution of optical systems (typically 0.1 mm). Moreover, infrared and kinematical data can be used to derive heat source fields induced by the specimen loading and then to generate information on the dissipative or coupled nature of the deformation mechanisms.
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