3D Crack Propagation in Cement Materials with Aggregates using digital volume correlation
Topic(s) : Special Sessions
Co-authors :
Riki NAGAO (JAPAN), Wataru SATO , Kosuke TAKAHASHI (JAPAN), Takashi NAKAMURAAbstract :
In this study, in-situ X-ray Computed Tomography (XCT) tests of cement specimens under compressive loading are carried out to visualize the difference in crack propagation inside cement with and without aggregates. Cylindrical cement specimens of 1.0 × 2.0 mm2 with and without aggregates were fabricated. Glass spheres of uniform size were used as aggregates to eliminate the influence of size and shape. The test was performed at BL20XU of SPring-8, which is a synchrotron radiation facility located in Hyogo, Japan. The small compression testing machine, driven by a piezoelectric actuator mounted on top, was developed for this study (Fig. 1). This testing machine can be fixed on the XYZ-θ stage on the beamline for the in-situ observation [1]. The periphery of the specimen in the testing machine is a circular tube of PMMA, through which X-rays penetrate the specimen.
Observations were first carried out with no load, and then with 20 MPa load. Thereafter, the compressive load applied to the specimens was increased by 5 MPa at each stress level. Specimens with and without glass spheres were fractured at compressive loads above 50 MPa. To characterize the crack propagation as the deformation increases, the segmentation techniques of aggregates and the digital volume correlation (DVC) method were applied to XCT images. Firstly, the entropy filter was applied to the glass spheres to extract areas with uniform luminance values compared to the surrounding cement matrix areas. Next, the morphological transformation was applied to smooth the surface of the glass spheres, fill in the glass spheres, and remove the noise. Finally, the active contour method was used to determine the edge of the glass spheres. Three-dimensional images of the cracks were then visualized using the DVC method, which maps the relative deformation between successive XCT images. Regions with a maximum principal strain of above 2000 μm were considered to be cracks in this study and were overlaid on segmented glass spheres.
As a result, we succeeded in visualizing the cracks in cement specimen and the positional relationship between cracks and glass spheres in the cement specimen with glass spheres (Fig. 2). The longer stable crack growth of the cement specimen with glass spheres suggests that the glass spheres are effective in suppressing unstable crack growth. The crack initiation was observed only from the bottom surface, but no crack was originated from the interface between the cement matrix and the glass spheres although it has been thought to be the weak regions.
Observations were first carried out with no load, and then with 20 MPa load. Thereafter, the compressive load applied to the specimens was increased by 5 MPa at each stress level. Specimens with and without glass spheres were fractured at compressive loads above 50 MPa. To characterize the crack propagation as the deformation increases, the segmentation techniques of aggregates and the digital volume correlation (DVC) method were applied to XCT images. Firstly, the entropy filter was applied to the glass spheres to extract areas with uniform luminance values compared to the surrounding cement matrix areas. Next, the morphological transformation was applied to smooth the surface of the glass spheres, fill in the glass spheres, and remove the noise. Finally, the active contour method was used to determine the edge of the glass spheres. Three-dimensional images of the cracks were then visualized using the DVC method, which maps the relative deformation between successive XCT images. Regions with a maximum principal strain of above 2000 μm were considered to be cracks in this study and were overlaid on segmented glass spheres.
As a result, we succeeded in visualizing the cracks in cement specimen and the positional relationship between cracks and glass spheres in the cement specimen with glass spheres (Fig. 2). The longer stable crack growth of the cement specimen with glass spheres suggests that the glass spheres are effective in suppressing unstable crack growth. The crack initiation was observed only from the bottom surface, but no crack was originated from the interface between the cement matrix and the glass spheres although it has been thought to be the weak regions.