ASTM International

A Hollow Cylinder Torsional Simple Shear Apparatus Capable of a Wide Range of Shear Strain Measurement

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Courtesy of ASTM International

The lack of high-quality data at small strains under monotonic loads has led to the assumption of linear elasticity in small strain analysis and the use of low values of stiffness for analysis of geotechnical structures under working loads. In addition, since most simple shear apparatuses cannot measure lateral stresses, various assumptions have to be made in order to define the failure strength in terms of principal stresses. In this paper a hollow cylinder torsional simple shear system that is capable of measuring shear strains accurately from a strain level as small as 10-6 to failure in addition to measuring all stress components is described. This system is also capable of consolidating specimens along both normal and overconsolidated stress paths and during shearing is able to apply extremely small load reversals with little or no backlash effect. Using the special features of this system, the behavior of kaolin specimens under various stress histories from very small strains to failure was studied. The results show that irrespective of the consolidation stress history, the major part of the principal stress rotation occurs at the early stages of undrained simple shear, and the final direction near the peak strength is always a few degrees below 45°. The small strain behavior indicates that there exists a very small elastic zone of the order of 2 × 10-5 shear strain, and that the behavior beyond this region is highly nonlinear. The trend in the variation of the peak strength with OCR obtained in this system is similar to that obtained in the conventional simple shear apparatus as reported in the literature.

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