The Effects of Strain Rate

The effect of straining rate on maximum radical production in drawn fibres is not great, as Fig. 24 shows although we have already noted that under ste y load the number of radicals increases with time for periods of up to twenty minutes. In the plastic deformation of polycarbonate al ), radical (xrncentration increased only two-fold over two decades of strain rate Much la r effects have been observed in plastically deformed, cross-linked glasses, as Fig. 22 drows. Here, for cis-polybutadiene at 500% pre-strain, the radical concentration at fracture has a peak value of 7 X 10 spins/g at intermediate strain rates, and decreases to 5 x 10 at 

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Darabiha, N., Candel, S., and Marble, R, The effect of strain rate on a premixed laminar flame. Combust. Flame, 64, 203, 1986. 

A simple bubble machine is devised and successfully applied in characterising lightly crosslinked PE resins for foam expansion. The biaxial stress-strain relationship is deduced from the air injection rate and pressure. The effects of strain rate, temperatnre and crosslinker level on the stress-strain behavionr are investigated. Uniaxial extension experiments are also performed and compared with biaxial extension data. 5 refs. 

The effect of strain rate, sy at - 50 °C and 50 °C is shown in Fig. 16. Qualitatively, increasing the strain rate is analogous to decreasing temperature (the temperature and strain rate dependencies of the stress-strain curve shapes are summarised in Fig. 17). However, it is worth noting that an equivalence temperature-strain rate does not apply over the whole stress-strain curve. 

The effects of strain rate and temperature are correlated, and can be modeled (Kinloch and Young, 1983, Kinloch, 1985). For different temperatures and strain rates, GIc and the time to failure, tf, were measured. Using the time-temperature superposition principle, shift factors (aT) applicable to the time to failure tf, were determine. Shift factors plotted against (T — Tg) are independent of the type of test used (Fig. 12.14). The construction of a typical master curve GIc versus tf/aT is shown in Fig. 12.15 (Hunston et al., 1984). The value of GIc may be predicted for any strain rate/temperature combination. This model can also be applied to rubber-modified epoxies (See chapter 13). 

Hence, a complete model for viscosity of a reacting polymer must contain the effects of strain rate, 7, temperature, T, and degree of cure, c, such as... 

The theory did not model accurately the rise in pressure with increasing extrusion ratio, due to the omissicHi of the effects of strain rate and pressure on the flow stress. 

Cook et al. also recently reported studies on the yielding of model epoxy thermosets. They focused first on the effects of strain rate and composition [169] and then on the effects of physical aging [170]. Their results are consistent with the trends summarized above. Networks cured with aromatic amines had higher Gcy than those cured with aliphatic amines. The value... 

Fig 14. The effect of strain rate on dislocation microstructures of copper single crystals. The pulse duration of these simulations is 1.3 nanoseconds. The dislocation microstructures of each strain rate are shown in slices within that RVE. (a) =2xloVs, (b) =7xloVs, (c) =5xl0Vs... 

Schematic representation of the effect of strain rate on SCC and hydrogen-induced cracking. Redrawn from Ref 118...

G.F. Carrier, F.E. Fendell, and F.E. Marble. The effect of strain rate on diffusion flames. SIAM J. Appl. Math., 28 463-500, 1975. 

Figure 12 shows the effect of strain rate on the stress-strain curves in the moderate environment. Reducing the strain rate gave the adhesive more time to stress relax, thus lowering the yield strength. At lO /s, lO /s, and lO Vs strain rates, the average yield strengths are 45.7, 35.4, and 30.0 MPa, respectively, in the moderate environment and 29.8, 25-2, and 23.5 MPa, respectively, in the humid/hot environment. 

The effect of strain-rate sensitivity on stability in extensional plastic flow... 

Now consider the effects of strain rate. Ejqieriments show that Kic increases with increasing crack speed, as illustrated in Figure 5.18, but the resulting shift in fracture stress with increasing strain rate is usualfy smaller than the increase in yield stress. Consequent, a deoreases. As a result, impact and other forms of rapid loading tend to cause brittle failures. 

In general, the scale ratio M (the ratio of the model dimension to the prototype dimension) should be not less than 1 4. For a model with a scale ratio of 1 4 or larger, the effect of strain rate dependence on the material s mechanical properties will be negligibly small. The effect of strain rate dependence for typical materials... 

The hemispherical dome (HD) test provides more reproducibility of results than the Olsen and Erichsen cup tests. For low-carbon steels, the dome height, which is determined at the point of maximum load, increases linearly with the n value. For other materials such as brass, aluminum alloys, and zinc, optimum correlation has been found between the dome height and the total elongation, which includes the effects of strain rate hardening and limiting strains. 

The development of the WPCs for load-bearing structural applications necessitates the characterization of their strain rate-dependent mechanical properties. In this regard, the effect of strain rate on flexural properties of WPG was addressed by Tamrakar and Lopez [49]. The strain at failure was not significantly influenced by the strain rate variation. A prediction model for the effects on strain rate on the modulus of elasticity (MOE) of WPG material was demonstrated based on the viscoelastic standard solid model. Yu et al. [50] analyzed the variability of the dynamic young s modulus of WPG, which was measured by different non-destructive test (NDT) methods. They also estimated the correlativity between the dynamic Young s modulus and the static MOE of WPG. 

Note that the impact BDT in AI2O3, as indicated above, is 900 °C, whereas for sapphires it was 1000 °C. The reason for this may be that, in polycrystalline AI2O3, grain-boundary sliding contributed to the onset of plastic flow, which is absent in single crystal AI2O3. The effect of strain rate on BDT is observed also in superplastic deformation, discussed below in Sect. 2.2c. 

Not unlike the case of superplastic ceramics, ductility and strength relations are influenced by strain rate. The conditions of the experiment must be above the DBT to observe plastic flow, which is different for various ceramics. An illustration of the effect of strain rate and temperature on the strain (ductility) at some stress level can be seen in monolithic Si-C-N. Silicon-nitride-based ceramics are quite promising candidates for mechanical applications at elevated temperatures. Specimens were prepared by hot isostatic pressure (henceforth HIP) of pyrolyzed powder compact at 1500 °C and 950 MPa, without any sintering additives. These compression tests were conducted at temperatures from 1400 to 1700 °C in a nitrogen atmosphere with a servo-hydraulic-type testing machine at constant crosshead speed in an induction heating furnace. In Fig. 2.5, stress-strain curves... 

In the case of high strain rates, SHPB experiments were conducted to determine the effects of strain rate, water content, moisture absorption, and low temperature. In each SHPB experiment, five or more specimens were tested to ensure repeatability under the same test conditions. 

J. Fembock, R. Stickler, A. Vinckier The effect of strain rate and heating rate on the tensile behavior of W and W—Th02 between room temperature and 1400 °C, Proc. 11th Plansee Seminar, Vol.1 (Plansee... 

FIG. 7—The effect of strain rate on low cycle CF In the C-Mn sleel/hlgh-temperature water system. The dissolved oxygen content of the pure water environment was varied, as Indh caled by the weight-paits-per-mlllion values in the legend [3S]. 

Reproduced with permission from Vincent. Plastics, 26, 141 (1961).) (b) Diagram illustrating the effect of strain rate on the brittle-ductile transition (—) low strain rate (—) high strain rate... 

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