This study presents the results of a series of investigations on the effects of testing parameters (including the rate of loading, machine stiffness, and contact conditions) on material's Young's modulus and Poisson's ratio. A 100 mm Aluminium cube (grade 7075-T6) was selected as the reference sample and tested both with a 10 MN MTS compression machine and a true triaxial testing system. Deformation and strain of the Aluminium reference cube were measured with the help of strain gauges and a system of linear variable differential transformer (LVDT) transducers placed at the centre and along the sample's length, respectively. Results indicate that Young's modulus measured from LVDT's recordings (placed between the two platens of the loading frame) is always smaller than that obtained from strain gauges (which are mounted directly on the sample) by up to 30%. Overall, the measured elastic modulus was observed to increase with the stiffness increase of contact plates and the loading frame. At high loading rates, a more pronounced reduction in the modulus of elasticity was recorded particularly during loading scenarios compared with the unloading cases. In contrast, no obvious changes in the Poisson's ratio were confirmed by changing the testing conditions above.

1. Introduction

The estimation of Young's modulus and Poisson's ratio plays an important role in mechanical characterization procedures to define strength and deformability of engineering and natural materials including rock and compositions containing rock aggregates (Serati, 2014; Serati and Williams, 2015). In the design of underground structures, however, these elastic properties for rock masses are often not well-known or easily measured. Therefore, the available standards and computer codes often utilize the modulus of the intact rock as input parameters to compute the rock mass modulus for deformation analyses. There have been many studies on how to reliably estimate intact rock elastic parameters using laboratory testing techniques including uniaxial compression (Dehghan S et al, 2010; Diyuan Li et al, 2011) triaxial compression (Yang SQ et al. 2011; Sheng-Qi Y et al. 2012; Zhidong Wang et al. 2019) and true triaxial testing procedures (Xia-Ting et al. 2015; Xibing Li et al. 2018). According to standard recommendations (ISRM; ASTM D 3148-02; ASTM D7012-14; BS 1610; DIN 5130), elastic modulus can be obtained by calculating the ratio of axial stress changes to the corresponding axial strains produced by the stress change in a target material. Poisson's ratio is then the slope of the axial stress-strain curve to the slope of the diametric stress-strain curve at each stress level. While electrical resistance strain gauges (with a minimum length of at least ten-grain diameters in magnitude) are commonplace to determine the axial and lateral strains, external linear variable displacement transformers (LVDTs) can instead be used provided they have a minimum deformation reading accuracy of 0.002 mm. Compressometers, fibre optics, laser and optical devices with strain sensitivity of the order of 5 × 10−6, and more recently digital image correlation (DIC) cameras and Moire interferometry techniques for full-field deformation measurement can also be adopted. Several previous papers have been already devoted to the study of material's elastic properties dependency to testing parameters including loading path (Yang SQ et al., 2011), contact friction (Bowden, 1950; Xia-Ting et al. 2015; Xia-Ting et al. 2017), and types of the contact platen (Onyekachi N. et al, 2018). This study aims to: (i) compare the results of localized on-specimen strain measurement using strain gauges with LVDT averaged recordings along the sample length, and (ii) understand the effect of loading rates, loading and unloading cycles, and machine and contact plate stiffness on elastic modulus and Poisson's ratio for a reference isotropic and homogeneous sample with known properties.

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