Stress measurement special instruments

Although Ey and are analogous to fj. and v, respectively, in that all these quantities are coefficients relating shear stress and velocity gradient, there is a basic difference between the two kinds of quantities. The viscosities n and v are true properties of the fluid and are the macroscopic result of averaging motions and momenta of myriads of molecules. The eddy viscosity and the eddy diffusivity are not just properties of the fluid but depend on the fluid velocity and the geometry of the system. They are functions of all factors that influence the detailed patterns of turbulence and the deviating velocities, and they are especially sensitive to location in the turbulent field and the local values of the scale and intensity of the turbulence. Viscosities can be measured on isolated samples of fluid and presented in tables or charts of physical properties, as in Appendixes 8 and 9. Eddy viscosities and diffusivities are determined (with difficulty, and only by means of special instruments) by experiments on the flow itself. 

The failure load of a fracture toughness specimen depends on the rate of load application. If a cracked compact tension specimen of 5 mm thick polycarbonate is loaded in a tensile testing machine, there is time for a neck to develop from the crack tip, so plane stress fracture occurs at a crack velocity of 5ms . However, if the load is applied in 1ms by impact loading, plane strain fracture occurs at a low Xjc value, and the crack velocity exceeds 200 ms . Special instrumentation, having a quartz crystal force gauge that responds in 0.1ms, and a grid of resistance lines on the surface to monitor the crack velocity, are needed to measure the Kic value. 

It will be clear that in the field of block copolymers, fallow aretis are present to a high degree. The difficulty of the measurement of viscoelastic properties is the sensitiven of the gels to (shear) deformations linear viscoelasticity is, in general, only measurable at very small deformations, so that very small stresses have to be measured. Special sensitive instruments are needed for these studies. 

Analytical methods used to determine the final stability of the broken or unbroken emulsion have varied. It has been shown that measurements of water content yielded unreliable results [J5S, J63], Viscosity measurements show correlation to emulsion stability and provide a more reliable measure of emulsion stability but special instrumentation is required [J69, 170], Simple viscometers without controlled shear or stress rates yield unreliable results. 

There are two basic designs of drag flow rheometers controlled strain with stress measurement and controlled stress with strain measurement. Below we Hrst discuss strain control and torque measurement (Section 8.2.2) followed by instrument alignment problems (Section 8.2.3) and normal stress measurement (Section 8.2.4). Then we treat special design issues for stress control. Both designs use the same type of environmental control system, as discussed in Section 8.2.6. 

The effect of temperature on the mechanical properties of a liquid can be investigated using a special type of dynamic mechanical analyser called an oscillatory rheometer. In this instrument the sample is contained as a thin film between two parallel plates. One of the plates is fixed while the other rotates back and forth so as to subject the liquid to a shearing motion. It is possible to calculate the shear modulus from the amplitude of the rotation and the resistance of the sample to deformation. Because the test is performed in oscillation, it is possible to separate the shear modulus (G) into storage (G ) and loss modulus (G") by measuring the phase lag between the applied strain and measured stress. Other geometries such as concentric cylinders or cone and plate are often used depending on the viscosity of the sample. 

Hardness values, indentation moduli, strain hardening exponents and viscoelastic properties can be measured with the instrumented indentation test, also the fracture toughness of very brittle polymers as well as the influence of residual stresses. If needed and a suitable device provided measurements can be done with high spatial resolution and with very small indentation depths. A special application of the testing devices is the characterization of the elastic behaviour of miniaturized components or the realization of micro compression tests, i.e. using the machines like a small universal testing machine. 

Recent papers on the relationship between optical rotatory dispersion and circular dichroism of optically active systems have stressed the superiority of the latter in interpreting the data. Unfortunately, circular dichroism measurements have been either difficult or expensive to obtain. Recently available instruments are costly and the reliability of the data these instruments afford certainly has not reached its maximum. Universal polarimeters have been described, but they require either major changes in existing insttuments or special construction of new instruments. This paper describes a method of modifying the RUDOLPH Spectropolarimeter, Model 200 AS, to measure the ellipticity of elliptically polarized light and thus indirectly measure circular dichroism. This modification requires no major changes and virtually no expense. 

Thermomechanical analysis (TMA) measures the deformation of a material contacted hy a mechanical prohe, as a function of a controlled temperature program, or time at constant temperature. TMA experiments are generally conducted imder static loading with a variety of probe configurations in expansion, compression, penetration, tension, or flexime. In addition, various attachments are available to allow the instrument to operate in special modes, such as stress relaxation, creep, tensile loading of films and fibers, flexural loading, parallel-plate rheometry, and volume dilatometry. The type of probe used determines the mode of operation of the instrument, the manner in which stress is apphed to the sample, and the amount of that stress. 

Calibration is a special form of preventive maintenance whose objective is to keep measurement and control instruments within specified limits. A standard must be used to calibrate the equipment. Standards are derived from parameters established by the National Bureau of Standards (NBS). Secondary standards that have been manufactured to close tolerances and set against the primary standard are available through many test and calibration laboratories and often in industrial and university tool rooms and research labs. Ohmmeters are examples of equipment that should be calibrated at least once a year and before further use if subjected to sudden shock or stress. 

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