Strain field investigation of limestone specimen under uniaxial compression loads using particle image velocimetry

The particle image velocimetry (PIV) method was used to investigate the full-field displacements and strains of the limestone specimen under external loads from the video images captured during the laboratory tests. The original colorful video images and experimental data were obtained from the uniaxial compression test of a limestone. To eliminate perspective errors and lens distortion, the camera was placed normal to the rock specimen exposure. After converted into a readable format of frame images, these videos were transformed into the responding grayscale images, and the frame images were then extracted. The full-field displacement field was obtained by using the PIV technique, and interpolated in the sub-pixel locations. The displacement was measured in the plane of the image and inferred from two consecutive images. The local displacement vectors were calculated for small sub-windows of the images by means of cross-correlation. The video images were interrogated in a multi-pass way, starting off with 64×64 images, ending with 16×16 images after 6 iterations, and using 75% overlap of the sub-windows. In order to remove spurious vectors, the displacements were filtered using four filters: signal-to-noise ratio filter, peak height filter, global filter and local filter. The cubic interpolation was utilized if the displacements without a number were encountered. The full-field strain was then obtained using the local least square method from the discrete displacements. The strain change with time at different locations was also investigated. It is found that the normal strains are dependant on the locations and the crack distributions. Between 1.0 and 5.0 s prior to the specimen failure, normal strains increase rapidly at many locations, while a stable status appears at some locations. When the specimen is in a failure status, a large rotation occurs and it increases in the inverse direction. The strain concentration bands do not completely develop into the large cracks, and meso-cracks are not visible in some bands. The techniques presented here may improve the traditional measurement of the strain field, and may provide a lot of valuable information in investigating the deformation/failure mechanism of rock materials.