Quantitative Strain Analysis of the Large Deformation at the Scale of Microstructure: Comparison between Digital Image Correlation and Microgrid Techniques

Abstract

A comparative study has been carried out to assess the accuracy of the Digital Image Correlation (DIC) technique for the quantification of large strains in the microstructure of a Interstitial Free (IF) steel used in automotive applications. A microgrid technique has been used in this study in order to validate independently the strain measurements obtained with DIC. Microgrids with a pitch of 5 microns were printed on the etched microstructure of the IF steel to measure the local in-plane strain distribution during a tensile test carried out in a Scanning Electron Microscope (SEM). The progressive deformation of the microstructure with microgrids has been recorded throughout the test as a sequence of micrographs and subsequently processed using DIC to quantify the distribution of local strain values. Strain maps obtained with the two techniques have been compared in order to assess the accuracy of the DIC measurements obtained using the natural patterns of the revealed microstructure in the SEM micrographs. Although results obtained with the two techniques are qualitatively similar, therefore demonstrating the reliability of DIC applied to microstructures, even after large deformations in excess of 0.7. However, an average error of about 16% was found in the strain values calculated using DIC.

Failure mechanisms in DP600 steel: Initiation, evolution and fracture

“…9b shows that a maximum local strain value of about 80% is measured within the islands close to the phase boundaries. However it is worth noting that an average error of about 16% may exist in the measured strain values in the used DIC technique and SEM micrographs as reported elsewhere [20].…”

“…This could be due to the different microstructural morphology in the tested materials. Additionally, the selected DIC parameters used for strain calculation play an important role in the calculated strain values [20] and the selected grid size for strain measurement in [11] might be considerably larger than the one used in the present study that would result in a large variation in the measured strain values.…”

“…The technique that was developed to measure strain distributions within the deformed microstructure as well as the local strain values at the onset of damage for the identified mechanisms is explained elsewhere [9,20]. The distribution of the strain component along the loading direction that is in the vertical direction of the image is shown in Fig.…”

“…The measured local strain values are dependent not only on the carbon content, the volume fraction of the martensite phase, the microstructural morphology of the material, but also the selected parameters to calculate strain values using the DIC algorithms [20]. It has been reported that the carbon content of austenite increases by reducing the grain size [25].…”

Tribological behaviour and local mechanical properties of magnesium-alumina composites

“…Verification of the quality through various tests is essential outside monitored dimensional characteristics [1][2][3][4]. One of the basic tests is the tensile test, by which we can characterize the plastic properties of materials.…”