Visualization of yielding phenomena on carbon steel under tensile loading by speckle pattern interferometry

A previously observed optical interferometric band pattern interpreted as representing localized strain is compared with a mechanical band structure known as the Luders' line. The interferometric band pattern is observed in fringe systems formed with the subtraction method of electronic speckle-pattern interferometry applied to in-plane displacement of plastically deforming metal specimens. The Luders' line is known to be representing localized strain. Experiments have been carried out to compare the locations of the optical band pattern and the Luders' line under the same stress condition. The experimental results indicate clear coincidence of the optical band pattern and the Luders' line, confirming the previous interpretation that the optical band pattern represents localized strain. It has been observed that the optical band pattern begins to appear at 11% lower stress than the Luders' line, indicating that strain localization occurs prior to the appearance of the first Luders' line.