Over the years, special cells and special transducers have been designed and built to conduct static and dynamic tests on hollow cylinders. The testing and data acquisition systems are fully automated and are properly interfaced to allow microcomputer controlled testing. The Soil Dynamics laboratory at CWRU is equipped with a computer-controlled freeze-thaw room for sample conditioning and the following advanced devices to study the dynamic characteristics of soils.
Apparatus for Static and Slow Cyclic (Torsional) Loading - Stress Controlled.
At present, the only reliable way to study the response of anisotropic soils to principal stresses which are inclined on the axis of symmetry is to incline those stresses and not the specimens. To obtain meaningful results, one must have the capability of controlling the inclination of the principal stresses during each test. The torsional shear devices available at CWRU are capable of meeting this requirement with the aid of a pneumatic analog computer by applying suitable combinations of axial and normal stresses. For a given test, the inclination of the principal planes is preset and maintained during the whole test, and the computer will automatically apply the necessary combination of axial force and torque to satisfy the requirement described above, in spite of the changing cross section during testing. The computer can also control the one dimensional consolidation of specimens in the triaxial cell, prior to their shearing.
An electro-pneumatic system connected to the constant stress torsional shear device allows the application of cyclic stresses at any inclination of the principal stresses on the axis of symmetry of the hollow cylinders according to the following combinations: axial stresses can be maintained constant and torsional stresses varied, or vice versa; or both changed simultaneously. This device has been modified to allow the application of random torsional stress with the aid of a digital to analog converter residing in a microcomputer.
Apparatus for Static and Cyclic Loading - Strain Controlled.
While the apparatus described above applies stresses, automatic units based on the same principles have been used to design an electro-pneumatic strain controlled system also capable of maintaining a fixed inclination of the principal stresses on the axis of symmetry of the material. This capability allows testing of dense samples beyond the peak and under combined stresses. Strain softening which is impossible to study in stress controlled devices can thus be examined for anisotropic materials.
Hydraulic Torsional Shear Device.
This device is similar to the previous two machines except that it contains a hydraulic torsional drive that allows the application of torsional loads with a frequency of up to 1Hz, as well as random loads with the aid of a D\A converter. Tests with either shear stress \ strain \ torque or angular displacement control can be conducted with this device. It is particularly suited for shear modulus and damping characterization as well as for liquefaction testing.
Resonant Column Devices
Two resonant column devices are available for low strain shear modulus and damping characterization of soils:
Both devices are fully automated
This fully automated device allows true triaxial testing of soils. It has been used to study the shear band propagation in soils.
Special Triaxial Cell
A triaxial cell specially built to allow for the application of axial and torsional stresses to hollow cylinders was developed at CWRU. The friction in the bushings is automatically compensated for by the pneumatic analog computer in static tests. However, this cannot be done in dynamic tests where the direction of application of the stresses is constantly reversed. Also, if it is desired to synchronize axial and torsional stresses, friction must be nearly eliminated. This problem has been solved with a special bushing involving a combination of high precision longitudinal and rotational ball bearings and a piston with a variable cross section. The cell is complemented with the presence of interior and exterior axial and torsional transducers for data collection.
Cyclic Triaxial Device
A pneumatic cyclic triaxial device has been assembled and used in several funded projects.