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Soil mechanics

Bender elements

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General description
General description
Bender elements allow to measure the maximum shear modulus (Gmax) of a soil sample and from this data to evaluate the stiffness of a soil. Gmax is generally associated with shear strain levels of about 0.001% and is a key parameter in small strain dynamic analysis, such as those to predict soil behaviour  or soil structure interaction during earthquakes, explosion or machine and traffic vibrations.
Theory on shear wave propagation in an elastic body tells us that the value of the shear modulus Gmax of the soil from measurement of shear wave velocity Vs is given by:             
Gmax = r · (Vs)2
were r  is the mass density of the soil sample.
The system consists of a piezoceramic transmitter, which is energized to produce the shear waves through the soil sample, and the receiver that receive the electrical signal. The travel time of the shear wave from the transmitter to the receiver is determined via a specific software that allows the user to quickly and easily calculate the shear wave velocity. The bender elements have to be powered by the 28-WF4190 Power and measuring system. See accessories.
The bender elements have to be used with the Triaxial cells for advanced applications.
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Ordering info

Bender elements for compression test only

28-WF4057/B 
Bender element, 50 mm top cap, base pedestal with benders, for 70 mm dia. cell

28-WF4077/B 
Bender element, 70 mm top cap, base pedestal with benders, for 70 mm dia. cell

28-WF4077/B1 
Bender element, 70 mm top cap, base pedestal with benders, for 100 mm dia. cell

28-WF4107/B 
Bender element, 100 mm top cap, base pedestal with benders, for 100 mm dia. cell

28-WF4157/B 
Bender element, 150 mm top cap, base pedestal with benders, for 150 mm dia. cell

Bender elements Vacuum type, also for extension test

28-WF4058/B
Bender element, 50 mm vacuum top cap, base pedestal with benders for 70 mm dia. cell

28-WF4078/B
Bender element, 70 mm vacuum top cap, base pedestal with benders for 70 mm dia. cell

28-WF4078/B1
Bender element, 70 mm vacuum top cap, base pedestal with benders for 100 mm dia. cell

28-WF4108/B
Bender element, 100 mm vacuum top cap, base pedestal with benders for 100 mm dia. cell

28-WF4158/B
Bender element, 150 mm vacuum top cap, base pedestal with benders for 150 mm dia. cell
 

Accessories

28-WF4190 
Power and measuring system.

Description
The bender elements have to be powered by a waveform signal generator. The output signal is converted into digital and transmitted to PC via an interface and processed with the virtual oscilloscope software. The complete system includes waveform generator, analog-to-PC interface, virtual oscilloscope software and connecting cables. PC not included.

Additional Information

The piezoceramic bender element is an electro-mechanical transducer, which is capable of converting mechanical energy (movement) either to or from electrical energy.  The single bender element consists of two thin piezoceramic plates, which are rigidly bonded together with conducting surfaces between them and on the outsides. The polarization of the ceramic material in each plate and the electrical connections are such that when a driving voltage is applied to the element, one plates elongates and the other shortens. The net result is a bending displacement, which is greater in magnitude than the length change in either of the two layers. On the other end, when the bender element is forced to bend, one layer will go into tension and the other into compression: this will result in an electrical signal, which can be measured. In the soil application the bender elements are encapsulated and mounted into inserts, which are fixed into the pedestal and top cap of a triaxial cell. They protrude edge-first into the soil specimen as cantilevered. When excited the bender element bends from side to side  pushing the soil in a direction perpendicular to the length of the element and thus having a large coupling factor with the soil. This produces a shear wave, which propagates parallel to the length of the element into the soil sample. On the other end of the soil sample another bender element is forced to bend and produces an electrical signal that can be measured.

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