1,3-Strain control

Fig. 4. Fatigue data for high temperature alloys (a) As vs cycles to failure for various alloys tested under strain control. (°) = testing at RT in air ...

The main concern regarding the utilization of Monascus pigments relates to the production of the citrinin mycotoxin in Monascus cultures. Several methods for controlling the mycotoxin production were proposed, including selection of non-toxinogenic strains, control of citrinin biosynthesis, and modifications of culture conditions. Despite their wide and traditional food applications in Asian countries, Monascus pigments have not been approved for use in the United States or European Union. 

Mechanical rheometry requires a measurement of both stress and strain (or strain rate) and is thus usually performed in a simple rotating geometry configuration. Typical examples are the cone-and-plate and cylindrical Couette devices [1,14]. In stress-controlled rheometric measurements one applies a known stress and measures the deformational response of the material. In strain-controlled rheometry one applies a deformation flow and measures the stress. Stress-controlled rheometry requires the use of specialized torque transducers in conjunction with low friction air-bearing drive in which the control of torque and the measurement of strain is integrated. By contrast, strain-controlled rheometry is generally performed with a motor drive to rotate one surface of the cell and a separate torque transducer to measure the resultant torque on the other surface. 

This relationship is the simplest of those describing the connection between stress and strain controlled measurements. Of course it also follows that complex terms must be similarly related ... 

The model system used by Mabille et al. [149, 150] was a set of monodisperse dilute (2.5 wt% of dispersed oil) emulsions of identical composition, whose mean size ranged from 4 p.m to 11 p.m. A sudden shear of 500 s was applied by means of a strain-controlled rheometer for durations ranging from 1 to 1500 s. All the resulting emulsions were also monodisperse. At such low oil droplet fraction, the emulsion viscosity was mainly determined by that of the continuous phase (it was checked that the droplet size had no effect on the emulsion viscosity). The viscosity ratio p = t]a/t]c = 0.4 and the interfacial tension yi t = 6 mN/m remained constant. 

The structure of the cyclic ketone is of utmost importance. Reduction of cyclic ketone by complex hydrides is started by a nucleophilic attack at the carbonyl function by a complex hydride anion. The approach of the nucleophile takes place from the less crowded side of the molecule (steric approach or steric strain control) leading usually to the less stable alcohol. In ketones with no steric hindrance (no substituents flanking the carbonyl group or bound in position 3 of the ring) usually the more stable (equatorial) hydroxyl is generated (product development or product stability control) [850, 851, 852, 555]. The contribution of the latter effect to the stereochemical outcome of... 

The kind of test that is appropriate in the two cases is different in the first case, load should be increased until failure while in the second, some sort of deformation (or strain) control is appropriate. (A practical case of the latter is the Marshall stability test for evaluating the performance of asphalt concrete). In reality some combination of stress and strain capacity is required. 

The strain-control test has the advantage that information on the strain capacity is obtained, as well as the maximum stress that can be sustained, the latter value being similar to that obtained in a conventional test with constant loading rate. In the following we shall discuss behaviour in compression in tests with a constant strain rate. 

Steric approach control, 67 Steric strains, 71 Steric strain control, 69 Steroid hydrogenation, 111 5/3-Stigmast-22-en-3-one, 130 Stigmasterol, 266 Sulfur dichloride, 459 Sulfur tetrafluoride, 459, 472 Sulfur tetrafluoride fluorination, 471 Surface catalysts, 157... 

The terms steric strain control and product stability control are preferred by H. C. Brown and H. R. Deck, J. Amer. Chem. Soc., 87, 5620 (1965). 

The high diastereoselectivity found on base-promoted cyclization of ethyl 7-bromo-2-methylheptenoates (bearing 3-, 4- or 6-methyl substituents) has been attributed to folding strain control of incipient cyclohexane formation.40 The selectivity is only moderate for the 5-methyl substrate but increases with the bulk of the substituent Ph < Me Et < z -Pr < r-Bu. 

Strasser P, Koh S, Anniyev T, et al. Lattice-strain control of the activity in dealloyed core -shell fuel cell catalysts. Nat Chem. 2010 2 454-60. 

As mentioned above, for a creep stress exponent, n<3, the crack tip stress field for a continuously growing crack is the applied A -field. For this case, based on the model of Purushothaman and Tien48 the crack growth rate is, for strain-controlled failure,... 

Snyder, J. P. Monocyclic enediyne collapse to 1,4-diyl biradicals a pathway under strain control, J. Am. Chem. Soc. 1990,112, 5367-5369. 

Fig. 1 Torsion vs. axial analyzers The PerkinElmer Diamond DMA (A) is an axial analyzer while the ATS Rheo (B) is a torsional instrument. Both are controlled stress but can act as strain controlled because of the feedback programmed in. Photos taken by the author of Equipment at the University of North Texas (C) shows the TBA. (Courtesy of Dr. John Enns of Polymer Network Characterizations, Inc.) (View this art in color at www.dekker.com.)...

From R D to quality control, rheology measurements for each phase of the product development life cycle involve raw materials, premixes, solutions, dispersions, emulsions, and full formulations. Well-equipped laboratories with stress- and strain-controlled oscillatory/steady shear rheometers and viscometers can generally satisfy most characterization needs. When necessary, customized systems are designed to simulate specific user or process conditions. Rheology measurements are also coupled with optic, thermal, dielectric, and other analytical methods to further probe the internal microstucture of surfactant systems. New commercial and research developments are briefly discussed in the following sections. 

Dynamic mechanical strain-controlled measurements for both concentrated fabric softeners are shown in Figure 4.26. There are significant differences between the two products as regards the magnitude of the complex viscosity and complex modulus components and their strain dependence. Product B exhibits a higher viscosity and markedly longer linear region. The zero shear viscosity of product B is approximately 95 mPa s whereas that of product A is approximately half of this value at 50 mPa s. 

I. Acid Base Balance. Trauma from whatever cause may strain control of the acid-base balance by increasing production of acids, especially where there is tissue anoxia, by diminishing power of lungs and kidneys to eliminate them, and by lowering of buffering capacity through anemia and hypoproteinemia [for review, see Walker (W2)]. 

Strain-controlled applications. In these applications, a definite strain is applied. The failure mode for the part may be fracture from exceeding the maximum elongation for the material, permanent deformation from exceeding the yield point, or fracture over time from strain close to the maximum elongation and subsequent creep rupture. 

---