A 12 Strain

To measure electrodeformation there is a technique that builds a bimorph actuator and calculates the deformation ratio Ifom the curvature [101]. As this method exaggerates even a 1% strain, it is effective for qualitative... 

Figure 17.9 Creep stress inducing a 1% strain as a function of time for aiioy 800 grade 1 [31].

Figure C2.1.17. Stress-strain curve measured from plane-strain compression of bisphenol-A polycarbonate at 25 ° C. The sample was loaded to a maximum strain and then rapidly unloaded. After unloading, most of the defonnation remains.

This result is the shear equivalent to Eq. (3.42) for tensile deformation. Note the modulus is a constant independent of strain for shear, while this is only true for a = 1 in the case of tension as shown by Eq. (3.43). 

Of the models Hsted in Table 1, the Newtonian is the simplest. It fits water, solvents, and many polymer solutions over a wide strain rate range. The plastic or Bingham body model predicts constant plastic viscosity above a yield stress. This model works for a number of dispersions, including some pigment pastes. Yield stress, Tq, and plastic (Bingham) viscosity, = (t — Tq )/7, may be determined from the intercept and the slope beyond the intercept, respectively, of a shear stress vs shear rate plot. 

Instant Active Dry Yeast. Instant ADY (lADY or HADY) production is similar to ADY production but requires a different strain of yeast. After pressing, the yeast is extmded into noodles 0.2—0.5 mm in diameter and 1—2 cm long and deposited on a metal screen or perforated plate in a fluid-bed air dryer. Drying time is shorter than with ADY, about 1—2 hours in practice, with a final moisture level of 4—6%. Instant active dry yeast does not require separate rehydration. It is always packaged in a protective atmosphere or under vacuum. On an equivalent soHds basis, the activity of lADY is greater than that of regular ADY, but stiU less than that of compressed yeast. 

To combat more tolerant, ie, resistant coccidia, particularly E. acerulina and E. maydma a 1 1 combination of narasin and nicarbazin, eg, 0.45 kg premix containing 36 g of each compound, has been marketed as Maxiban by Elanco. These two compounds work synergisticaHy against less susceptible field strains. 

Quinacrine (49) is an acridine that was used extensively from the mid-1920s to the end of World War 11. It acts much like chloroquine and is reasonably effective. Because it causes the skin to turn yellow and, in high doses, causes yellow vision, the dmg is no longer in use as an antimalarial. Pyronaridine (77), a 1-azaacridine developed in China, appears to be effective against mefloquine-resistant, but not entirely against chloroquine-resistant, strains of P falciparum. 

Fig. 4. Paitial hysteresis loop for a feiioelastic material. After the appHed stress is removed a permanent strain - 0.0064 remains (see eq. 1).

Alginate impression materials must have a compressive strength of at least 0.34 MPa (49 psi) 8 min after the start of the mix at least 3.5 min of this time interval should be in storage at 37 1°C. They should have a strain in compression of 4—20% between stresses of 9.8—98 kPa (1.42—14.2 psi) per specification method and they should not have a permanent deformation exceeding 3% after a 12% strain is appHed for 30 s. 

Prior to anv machineiy alignment procedure, it is imperative to check for machine pipe strain. This is accomplished by the placement of dial indicators on the shaft and then loosening the hold-down bolts. Movements of greater than 1 mil are considered indication of a pipe strain condition. 

Ultimate tensile strength the maximum stress value as obtained on a stress-strain curve (Figure 30.1). 

Proof stress (F/Aq at a permanent strain of 0.1%) (0.2% proof stress is often quoted instead. Proof stress is useful for characterising yield of a material that yields gradually, and does not show a distinct yield point.)... 

The stress which produces a permanent strain equal to a specified percentage of the specimen length. A common proof stress is one corresponding to 0.1% permanent strain. 

One example of this occurs with stress relaxation. If a polymer is deformed to a fixed strain at constant temperature the force required to maintain that strain will decay with time owing to viscous slippage of the molecules. One measure of this rate of decay or stress relaxation is the relaxation time 0, i.e. the time taken for the material to relax to 1/e of its stress on initial application of strain. 

As indicated above, the stress-strain presentation of the data in isochronous curves is a format which is very familiar to engineers. Hence in design situations it is quite common to use these curves and obtain a secant modulus (see Section 1.4.1, Fig. 1.6) at an appropriate strain. Strictly speaking this will be different to the creep modulus or the relaxation modulus referred to above since the secant modulus relates to a situation where both stress and strain are changing. In practice the values are quite similar and as will be shown in the following sections, the values will coincide at equivalent values of strain and time. That is, a 2% secant modulus taken from a 1 year isochronous curve will be the same as a 1 year relaxation modulus taken from a 2% isometric curve. 

As before, a similar result could have been achieved by taking a section across the creep curves at 1.5% strain, plotting an isometric graph (or a 1.5% modulus/time graph) and obtaining a value for modulus at 1 year (see Fig. 2.8)... 

J7 In a tensile test on a plastic, the material is subjected to a constant strain rate of 10 s. If this material may have its behaviour modelled by a Maxwell element with the elastic component f = 20 GN/m and the viscous element t) = 1000 GNs/m, then derive an expression for the stress in the material at any instant. Plot the stress-strain curve which would be predicted by this equation for strains up to 0.1% and calculate the initial tangent modulus and 0.1% secant modulus from this graph. 

In a small polypropylene component a tensile stress of 5.6 MN/m is applied for 1000 seconds and removed for 500 seconds. Estimate how many of these stress cycles could be permitted before the component reached a limiting strain of 1%. What is the equivalent modulus of the material at his number of cycles The creep curves in Fig. 2.5 may be used. 

Then if a straight line is drawn from the point 0.523, 1000 to 0.691, 10,000 on Fig. 2.4 then this may be extrapolated to 1% strain which occurs at approximately t = 9 x 10 seconds. This is the total creep time (ignoring recovery) and so the number of cycles for this time is... 

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