Repeated plastic instability accompanying serrated yielding in stress–strain curves and localization of deformation is observed during plastic deformation of many metallic alloys when tensile specimens are deformed under certain experimental conditions of temperature, strain rate, and pre-deformation. This phenomenon is referred to as the Portevin- Le Chatelier (PLC) effect. TMW alloy, a newly developed Ni–Co base superalloy for aircraft engine application, also exhibit PLC effect during tensile test at temperatures ranging from 300 ℃ to 600 ℃, which are also the temperature range for engine working. In this paper, a 3D digital image correlation (3D DIC) measurement system was established to observe the localization of deformation (PLC band) in a tensile test performed on TMW alloy specimen at temperature of 400 ℃. The 3D DIC system, with displacement measurement accuracy up to 0.01 pixels and strain measurement accuracy up to 100 με, has a high performance in displacement field calculation with more than 10000 points every second on a 3.1G Hz CPU computer. The test result shows that, the PLC bands are inclined at an angle of about 60° to the tensile axis. Unlike tensile test performed on aluminums alloy, the widths of PLC bands of TMW alloy specimen, ranging from 4 mm to 4.5 mm, are much greater than the specimen thickness (0.25 mm).
Digital image correlation (DIC) method is an effective way for full-field strain measurement. Optical flow estimation
methods combined with a global searching strategy for displacement field measurement are introduced in this paper.
Compared with the conventional DIC method, this strategy can lessen possible mismatching between the reference
image and warped image. By minimization the energy function of displacement field, displacement continuity and
displacement gradients continuity among calculation points are achieved. For detecting large displacements, a coarse-tofine
strategy is also employed. More importantly, the architecture parallelization of optical flow estimation and searching
strategy can decrease the running time of this method for time-critical conditions.
This proposed method is universally applicable to the images with shadows, rotation, and large deformation. Several
pairs of simulated digital speckle images were used to evaluate the performance of this novel DIC method, and the
experimental results clearly demonstrate its robustness and effectiveness.