Shear measuring device

Lap shear testing Shear can be applied in a number of ways cyclic, intermittent, static (or constant), or increasing. A simple overlap shear test is described in ASTM-D-1002. This can be illustrated again using two strong microscope slides. Here, the microscope slides are adhered in parallel to one another except offset. After the appropriate set-up time, the top and bottom of the slide combination are attached to the shear tensile-measuring device and the experiment is carried out. 

Very much less useful are the various kinds of simplified measuring devices found in many industrial plants and even in their technical support laboratories. Such devices may not be capable of determining shear stresses for known shear rates or may not be capable of operation at shear rates that are appropriate to the process under consideration. Instruments that are capable of absolute viscosity measurements provide the most useful information. 

Blood is obtained from the test subjects by venipuncture and mixed with K-EDTA (1 mg/ml) or heparin (5 IU/ml heparin sodium) to prevent clotting. Erythrocyte aggregation is determined in whole blood of 40 % haematocrit. A sample of 40 xl blood is transferred to the measuring device. The red cells are dispersed at a shear rate of 600/s. After 20 s, flow is switched to stasis and the extent of erythrocyte aggregation is determined photometrically. 

In a rotational viscometer the solution is filled in the annulus between two concentric cylinders of which either the external (Couette-Hatschek type) or the internal (Searle type) cylinder rotates and the other, which is connected to a torsion-measuring device, is kept in position. Let R, and Ro be the radii of the inner and outer cylinders, h the height of the cylinder which is immersed in the solution or its equivalent height if end effects are present, a> the angular velocity of the rotating cylinder, and T the torque (or moment of force) required to keep the velocity constant against the viscous resistance of the solution. It can be shown that the shearing stress (see, for example, Reiner, 1960) ... 

In summary, various shear and tensile strength measuring devices have been developed, and many are commercially available. Though they are more complex and often more difficult to use than the other methods, they yield data on a more scientific basis, which allows a mechanistic approach to flow problems. 

Whorlow (15) and others (16,17) described very useful experimental techniques. Very often, measurements are made with a suspension sample placed in the annulus between two concentric cylinders. The shear stress is calculated from the measured torque required to maintain a given rotational velocity of one cylinder with respect to the other. Knowing the geometry, the effective shear rate can be calculated from the rotational velocity. Less useful are the various kinds of simplified measuring devices found in many industrial plants and even in their technical support laboratories. Such devices may not be capable of determining shear stresses for known shear rates or may not be capable of operation at shear rates that are appropriate to the process under consideration. Instruments that are capable of absolute viscosity measurements provide much more useful information. 

These mixing apparatuses were built to ensure pulse-free flow to obtain a uniform cross-linker concentration throughout the gel. In addition, the mixing devices were connected directly to the viscosity measuring device, so that the gel is continuously sheared from the time of cross-linker injection. Finally, it is recommended that metal content of gels be analytically verified to ensure that the proper amount of cross-linker has been delivered. We have analyzed for titanium with X-ray fluorescence and atomic adsorption spectroscopy. 

The importance of shear stress measurement is even more crucial for small-scale devices, i.e., MEMS applications, due to their higher surface to volume ratio. There have been many efforts in literature for successful shear stress measurements. The success of these efforts primarily relies on the complexity of the flow, the nature of solid boundaries, and limitations of the measurement techniques. The other drawback of shear stress measurements is its smaller magnitude, i.e., the estimated value of shear stress of a typical car moving at 100 km/h is about 1 Pa. Hence, highly sensitive shear stress measuring devices are required for successful measurements of surface shear stress, and it is essential to have proper understanding of the various noise sources that can effect shear stress measurements. 

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