Getting into the mechanical behaviour of sand with X-ray Tomography

Getting into the mechanical behaviour of sand with X-ray Tomography

Speaker: Dr Edward Ando, Laboratoire 3SR (Grenoble, France)
28th February 2019

In this seminar, the speaker with an engineering background introduced how the observation of sand with X-ray tomography can be very useful in identifying the mechanical behaviour of sand. This will help our understanding of sand better and can be applied in geological interpretation. 

Firstly, I think it will be helpful to explain what is -ray tomography. This x-ray Computed Tomography is basically based on similar technology of the one that is widely used in medical profession. Instead of taking just one 2D X-ray image or radiograph, the images are taken as the object is rotated from a very slightly different angle. From then, the series of 2D radiographs can then be constructed into 3D radiograph, mathematically.

A research comparing the mechanics of transitional soil with sand with plastic fines and sand with non-plastic fines shows that there is no significant difference in shearing at any point of stress and of the initial volume of the soil stiffness. The only significant difference is in the volume remaining after isotropic compression and monotonic compression shearing. (Shipton and Coop, 2015). During the talk, Ando did highlight that the dense sand behaviour is of that the volume increases when compression occur resulting in the shear band. But this is not a homogeneous respond hence the need to observe the mechanical behaviour at finer scale using the X-ray tomography method. The ‘critical state’ density is stronger at microscale.

To make a CT scan, a narrow beam of X rays sweeps across an area of the body, moving through a slight angle after each X-ray pulse. Using the resulting images, a computer produces a three-dimensional X-ray image of the body site. © Encyclopædia Britannica, Inc.

In this investigation, Ando started with back projection with the total of 1120 projections. These massive data comprise of different direction, at different scale in term of both distance and grain size. In addition to that, the strain calculation is recorded by observing the rate of change of the displacement, shortening of the sand, and the degree of grain displacement. The project observes the behaviour progressively on different scale.

The finding of his experiment is that the progressive concentration of rotation forming isolated shear band. The system is trying to loosen itself in order to create a runaway mechanism. 

Besides this, Ando and a few others, have developed a Python software, SPAM, the Software for the Practical Analysis of Materials, which is a numerical model of predicting behaviour of the sands on grain scale such as changes in shape and frictions. This can be used to handle and correlate 3D fields for application of material science, built on top of Numpy and Scipy, including some optimised C/C++ functions for speed. More info on SPAM here.

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