Microscopic investigation of the breakage characteristics of calcareous grains under one-dimensional compression

Abstract

Particle breakage is an important subject in the compression of coarse-grained soils at high stress levels. The overall mechanical response would change along with the fragmentation process. Breakage, which may happen to individual particle, needs to be examined at grain scale to reveal its failure mechanism, to estimate its influence on soil structure, and then to be used to predict the subsequent changes in stress-strain behavior. In order to bridge the microscopic characteristics and the macro responses, this research offers a way to incorporate X-ray micro focused computed tomography (μCT), a non-destructive analyzing tool, into the analysis of breakage under one-dimensional compression.

This research first resolved the problem of high-quality image processing of μCT scanned images. A systematic analysis method was introduced, including image processing and quantitative data analysis. Image processing is critical to the quality of subsequent microscopic measurements. An innovative watershed method was proposed and validated to resolve image segmentation in highly complex media, e.g. highly irregular crushed particles. The proposed method has been proved to be applicable to specimens in various grading conditions. Methodology to obtain 3D measurements was then demonstrated with specific algorithms. The algorithms for grain and contact related analyses had been verified from QICPIC and DEM respectively. Parameters from an image-based analysis had undergone careful inspection and their accuracy was discussed. For example, the computed contact normal directions was found to be particularly sensitive to the image resolution and therefore a code modification was applied based on contact’s size and spatial configurations.

The one-dimensional compression test under X-ray μCT scan was conducted on 3 different graded carbonate sand, namely uniformly graded, well graded and gap graded samples. It was found that they behaved differently during breakage. A microscopic observation made from an image-based analysis provided information for their deviated grain morphology and contact evolution. Study on the packing and fabric configurations further explained for their different fragmentation behaviors.

The ultimate stable state in one-dimensional compression was explored by conducting sample reconstitution and repetitive loading. Results from sieving and QICPIC analysis indicated that the stable state exhibits fractal characteristics. By comparing test results on different settings, it was found the ultimate stable state is stress dependent. At relatively lower stress, particle sizes tend to affect the location of the ultimate grading and shape. A larger fractal dimension D would be resulted from a uniformly graded sample comprised of larger particles. At higher pressures, a unique PSD and grain morphology tend to be arrived by diversely graded samples, whose initial PSD lies below D=2.5. An elevated peak normal stress would promote the fractal dimension and corresponding shape characteristics in ultimate stable state, however at a much neglected rate at about 36 MPa. The maximum compression limit in 1D condition was highly possible to occur at around D=2.6.