Rock Mechanics and Rock Engineering | 2019
An Improved Digital Volumetric Speckle Photography Technique with X-ray Microtomography and its Applications to Investigating Strain Localization in Red Sandstone
Abstract
Strain localization is a common phenomenon in geomaterials. To investigate the onset and development of strain localization is important for understanding the failure mechanism of geomaterials, which raises a formidable task to the experimental mechanics field. As full-field measurement, many optical measurement techniques have been applied to 2D or 3D surface displacement measurement (Zhang et al. 2013; Munoz et al. 2016; Cheng et al. 2017; Xing et al. 2017). However, surface observations are limited in their capability to resolve the geometric complexities and heterogeneity in geomaterials. In fact, when strains are localized, stress and strain variables cannot be derived from boundary measurements of loads and displacements (Viggiani et al. 2004). It is urgent to develop 3D interior stress/strain measurement technique. By combining 3D printing technology and frozen stress photoelastic technique, Ju et al. (2014, 2017) have produced 3D-printed rock-like samples with photopolymer to visualize and quantify the internal stress distribution via specimen slicing. However, due to the fact that the mechanical property of photopolymer is different from that of rock, its potential application to rock mechanics is rather limited. In recent years, the nuclear magnetic resonance (NMR) technique and the X-ray computed tomography (CT), both are capable of acquiring 3D volumetric images, have been used to investigate the internal structures, deformation localization, and failure of rock materials (Baker et al. 2012; Cnudde and Boone 2013; Liu et al. 2017; Mathews et al. 2017; Ju et al. 2018). As a direct 3D extension of a 2D Digital Image Correlation (DIC) method, Digital Volume Correlation (DVC) method emerges as a novel and effective tool for measuring the internal deformation of solid objects (Buljac et al. 2018). By combining in situ CT scanning and DVC, some studies have been carried out on a number of geomaterials, such as rocks (Lenoir et al. 2007; Charalampidou et al. 2011, 2014; Renard et al. 2018; McBeck et al. 2018) and granular materials (Hall et al. 2010; Adam et al. 2013), in which strain localizations were detected. In the DVC method, surrounding an interrogated point, a cubic subset (subvolume) of voxels is selected in the reference volumetric image, its corresponding position is registered in the deformed volumetric image, from which its 3D displacement vector is retrieved. The registration process can be carried out either in the spatial domain or frequency domain. In the frequency domain, Fast Fourier Transform (FFT) is used to increase the calculation efficiency. Derived from the 2D digital speckle photography (DSP) technique developed by Chen et al. (1993) and Chiang (2009), using a Fourier transform approach following the Fourier optics analysis used in the classical speckle photography and speckle interferometry techniques, we have proposed a 3D whole-field displacement/strain measurement technique called digital volumetric speckle photography (DVSP) with the help of X-ray computed tomography (Chiang and Mao 2015). This technique has been successfully * Lingtao Mao [email protected]