Recognition of the Stress-Strain Curve Based on the Local Deformation Measurement of Soil Specimens in the Triaxial Test

Abstract

A digital image technique was developed and used to measure the deformation distribution over the entire surface of soil specimens in triaxial tests. The measured deformation process shows that the specimen apparently exhibits three states during the test, i.e., pre-failure, in-failure, and post-failure, in correspondence to the different features of its deformation. The deformation feature in each state is then analyzed by the displacement and strain contours. Additionally, the stress level, S, is calculated to determine whether the soil is in failure at a point (representative element volume (REV)) on the surface of the specimen. Next, the failure zone, namely the shear band, was considered to be enveloped by the curve defined by S = 1 on the stress-level contour map. The stress level is calculated based on the strain, Young's modulus and Poisson's ratio. According to analysis of the deformation feature and the failure process, we recognized the following properties: (i) the deformation of the specimen was approximately uniform in the pre-failure state; (ii) failure occurred from a point and developed gradually until the shear band cleaves the specimen; (iii) in the post-failure state, deformation was exclusively due to the blocks of the specimen sliding along the shear band; (iv) the deformation feature in the shear band was quite different from that outside the shear band. In conclusion, the stress-strain curve of the specimen revealed a structural response, not an elementary response, especially in the in-failure and post-failure states, in which the deformation features of different corner points were different, and the observed deformation for the entire specimen may be the combination of local deformations. Therefore, it is not appropriate to build the constitutive model for soil according to the stress-strain curves of the entire deformation process and to take the specimen as a uniform element in the entire process.

A plane strain testing apparatus characterized by flexible loading and noncontact deformation measurement and its application to the study of shear band failure of sand

“…It means that our usual method for measuring the stress–strain relationship of the soil sample with a plane strain test is only a macroscopic approximation, which may not be appropriate to be used for determination the constitutive law of the soil. Recently, Shao et al 33 gave the similar results for the triaxial test.…”

“…To confirm the reliability of measurement results, the measurement precision is estimated by comparing the strain from the digital image system with that from the strain gauges. 33 It is given that the difference between the ''true values'' and the ''test values'' of the strain is approximately 0.0001, which means that the measurement precision of the plane strain test can be satisfied.…”

“…All the corner points are taken as the nodes of four-node element, and the strains are calculated thereafter. 33 Mapping element and four-node mother element are shown in Figure 5(a) and (b), respectively.…”

“…32 Recently, the State Key Laboratory of Structural Analysis for Industrial Equipment in Dalian University of Technology has been devoted to the development of the digital image technique to surface deformation measurement in the triaxial tests. 33 Now, the digital image technique is extended to develop a new type of plane strain apparatus. Compared with the LVDT measurement, there are a number of advantages of the new plane strain apparatus: large deformation and noncontact measurement, and the ability to obtain the strains for every point on the surface of the specimen, which has important significance in the analysis of the development of strain localization and formation of shear bands.…”

Modern Displacement Measuring Systems Used in Geotechnical Laboratories: Advantages and Disadvantages

“…Currently, researchers are using photogrammetry in a triaxial tests [ 110 , 111 , 112 ] and in torsional shear tests on a hollow cylindrical specimen [ 11 ]. The cylindrical shape of the test cell and the sample in the triaxial apparatus require correcting the algorithms for data analysis and taking countermeasures (especially in the case of shear torsion).…”