Shear stress transducer

Figure 8.2.6 also illustrates a sliding plate shear rheometer design. If a wall shear stress transducer is not used, then the total force on the fixed plate is usually measured with a strain gage load cell. The linear actuators used have time constants in the... 

Wall shear stress transducer mounted in a sliding plate rheometer. Shear between the sample and the wall causes die cantilever to deflect slighdy, which motion is detected by the capacitance probe. Adapted from Interlaken (1992). 

The drive system of a rotational rheometer is likely to be optimized in one of two ways depending upon its preferred mode of operation. The most common form of rheometer is a controlled-rate (controlled-speed) device. This configuration is also used in most viscometers and has been around for decades. A shear rate is applied to a rotor by the motor controlling the viscometer s speed. The rotor is normally a flat plate or cylindrical cup. The stator is thus a cone or plate for the first two geometries or a cylindrical bob for the third (Figure HI. 1.1). The stator is linked to the rotor via the sample, which acts to couple the input signal like an automobile transmission. Thus, the torque on the stator when measured by a transducer is used to derive the shear stress in the sample. 

In thermomechanical analysis (TMA) the deformation of the sample under stress is monitored against time or temperature while the temperature increases or decreases proportionally to time. Changes are detected by mechanical, optical, or electrical transducers. The stress may be a compression, penetration, tension, flexure, or torsion. Generally the instruments are also able to measure the sample dimensions, a technique called thermodilatometry. The stress F/A) expressed in N/m or Pa may be a normal tensile stress cr, a tangential shearing stress x, or a pressure change Ap the force applied is F and A is the area. 

To calculate shear stress, shear rate and melt viscosity, the melt pressure and piston velocity must be monitored. The former is measured using a 0 to 20,000 psi pressure transducer mounted in the rheometer barrel just above the capillary. A linear potentiometer is used to monitor piston position during the test. Utilizing the piston displacement data and the HP real time clock, the piston velocity is calculated. 

FIG. 21-32 iShear rotary, full annulus split cell, illustrating normal load weight application, rotational base, and shear stress/torque measurement. Vertical displacement of lid is monitored by displacement transducer. (Courtesy E G Associates, Inc.)... 

There are several reasons for the scatter of the data (a) nonhnear contributions of the transducers and measurement equipment are superimposed on the measurement effect (b) the samples bend during a test, producing both tensile and shear stress in the bonded interface and (c) there are expected to be statistical variations of the properties of the epoxy bonds themselves, due to different microstructures and curing. Furthermore, depending on the dynamic behavior of the interfaces, the deviation of the phases of the transmitted waves from their values in the nonhysteretic case must be taken into account in the evaluation of the interaction forces, especially if the ultrasonic excitation is high enough to obtain ultrasonic stresses comparable with the bond strength. 

Two commercial viscometers were used at higher shear stresses a Rheometrics Mechanical Spectrometer with sensitive transducer (data kindly provided by P. J. Whitcomb of General Mills Chemicals Co.), and a Contraves instrument (data kindly provided by W. Gale and B, Boseck of Exxon Production Research Co.). Viscosity standards from Cannon Instrument Co. were used to check the Contraves instrument. 

The viscoelastic behavior of biomaterials is typically measured using DMA. In rheological terms, viscoelastic is the concomitance of viscous (fluid-like) and elastic (solid-like) elements. The proportion of viscous and elastic properties is depending on the used material as well as on the measuring conditions such as the temperature. In DMA measurements, a sinusoidal shear load is applied to the sample while measuring the shear stress (cr ) with a stress transducer. The strain induced... 

The first main feature of the System 4 RMS is its ability to characterize a wider range of the above systems than any other current rheometer. Several drives and stress transducers are required the wide range of modes, rates and materials to be handled would mean that no single drive/transducer combination would be sufficiently flexible and accurate. Instead, the System 4 RMS uses a turret with four different drive or transducer units one unit is used for steady shear tests, one for oscillatory shear, one for tension-compression and bending and one unit is used for low-viscosity-high-shear-rate tests on fluids. The second main feature of the new machine is its completely automatic microprocessor control coupled with data acquisition and reduction. Without doubt, this new rheometer will result in rheo-metrical tests being carried out more reliably, accurately and quickly than previously. 

The cone and plate viscometer can be used for oscillatory shear measurements as well. In this case, the sample is deformed by an oscillatory driver which may be mechanical or electromagnetic. The amplitude of the sinusoidal deformation is measured by a strain transducer. The force deforming the sample is measured by the small deformation of a relatively rigid spring or tension bar to which is attached a stress transducer. On account of the energy dissipated by the viscoelastic polymer system, a phase difference develops between the stress and the strain. The complex viscosity behavior is determined from the amplitudes of stress and strain and the phase angle between them. The results are usually interpreted in terms of the material functions, p, G, G" and others [33-40]. 

This rheometer is similar in all respects to that discussed in section 3.2.1 except for the fact that it has a slit orifice cross-section rather than a circular one. The major credit for the development of the concept and use of this rheometer goes to Han [69,71,72] though others [73] have also used it for polymer melt studies. The instrument makes use of a series of flush mounted transducers located along the flow channel wall which measure the pressure gradients along the flow direction. These are then converted into wall shear stress values [69] as follows ... 

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