Strain rate effect

I.O. Bashkin, E.G. Ponyatovsky, O.N. Senkov, and V.Yu. Malyshev, The strain-rate effect on the hydrogen-induced workability improvement of titanium alloy VT20 at temperatures 500-800°C, Phys. Met. Metall., 69 167 (1990). 

The mechanical response of polypropylene foam was studied over a wide range of strain rates and the linear and non-linear viscoelastic behaviour was analysed. The material was tested in creep and dynamic mechanical experiments and a correlation between strain rate effects and viscoelastic properties of the foam was obtained using viscoelasticity theory and separating strain and time effects. A scheme for the prediction of the stress-strain curve at any strain rate was developed in which a strain rate-dependent scaling factor was introduced. An energy absorption diagram was constructed. 14 refs. 

Describe temperature, molecular weight, and strain-rate effects on the mechanical properties of polymers. 

A review by Bird and Wiest [6] gives a more complete list of existing viscoelastic models. The upper convective model and the White-Metzner model are very similar with the exception that the White-Metzner model incorporates the strain rate effects of the relaxation time and the viscosity. Both models provide a first order approximation to flows, in which shear rate dependence and memory effects are important. However, both models predict zero second normal stress coefficients. The Giesekus model is molecular-based, non-linear in nature and describes thepower law region for viscosity andboth normal stress coefficients. The Phan-Thien Tanner models are based on network theory and give non-linear stresses. Both the Giesekus and Phan-Thien Tanner models have been successfully used to model complex flows. 

The form of EBU expression is mainly based on dimensional arguments. The ratio k/ is the turbulent time scale. If the turbulence intensity is high, so is the fuel consumption. For the prediction of secondary species, such as CO, HC1, and soot, more advanced models using flamelets [37] have been used. The flamelets (and state relations) can be determined either experimentally [39] or computationally, using detailed models for combustion chemistry [40] that incorporate strain rate effects. 

Malvern, L.E., Propagation of Longitudinal Waves of Plastic Deformation In a Bar of Material Exhibiting a Strain-rate Effect, J. App. Mech., Vol. l8, pp. 203-8. 

The observation that an increase in temperature or a decrease in rate both result in the same fracture response points toward a viscoelastic influence on thermoset fracture behavior, especially crack initiation. This characteristic behavior of epoxies has been explained qualitatively by consideration of the temperature and strain rate effects on the plasticity of the material at the crack tip . In effect, test conditions which promote the formation of a so-called crack tip plastic zone, or blunt the crack by a ductile process, promote unstable crack propagation. This aspect of unstable fracture is subsequently discussed in more detail. 

These two examples illustrate two particular situations where strain rate effects are similar to temperature effects, the spurious hysteretic heating having been isolated. In most cases, strain rate effects (strain hardening, increase of the mechanical properties with speed) are mixed with heating effects due to mechanical energy absorption (softening, decrease of the material properties with temperature). 

Fig. 15. The craze stress versus loading frequency in the case of a cyclic loading, in experimental conditions where hysteretic heating occurs in PMMA at the crack-tip. Note the increase of the craze stress due to strain rate effects at low frequencies, and the decrease of craze stress due to hysteretic heating above 100 Hz. From Refs. and courtesy of Society of Plastics Engineers Edn.

Fig. 35. Normalized life-time showing pure frequency effects. Open stars 50 °C test, shaded stars 23 °C test. At 23 °C, strain rate effects occur (life-time increases at high frequencies), whereas at 50 C and high frequency, hysteric heating occurs, giving a drop of the fibrils life-time. From Ref. courtesy of Society of Plastics Engineers Edn.

Substrate composition and phases Surface hardness and roughness at interface Grain size/microstructure Anisotropy in structure and properties Areal variation and batch-to-batch variation Elastic/plastic mechanical properties Fracture mechanics Flaw population and distribution Strain rate effects... 

The second term on the right-hand side of Eq. (8.5) describes the increase in true yield stress with true strain rate. If the initial yield stress is measured in a tensile test, the low strain means there is no contribution from orientation hardening (see the next section), and there is insignificant heating. Consequently, the strain rate effect can be isolated. The initial yield stress was foimd, for HOPE at 20 °C, to vary with the true strain rate according to... 

Plastics products are most likely to fail in a brittle manner under impact conditions, both due to strain rate effects and because large forces can be generated by low energy impacts on stiff structures. A variety of impact tests are used. Usually a weight falls from a height of the order of Im to hit the test specimen with a velocity of about 5ms . This simulates the strain rates that occur when a product is dropped about a metre, but not the higher strain rates in vehicle collisions or ballistic impacts. The uses and limitations of three types of impact tests will be discussed. 

In the case of high loading rates an increase in material stiffness and strength compared to the static behavior may occur. This is addressed as strain rate effect. This effect is important in dynamic hnite element calculations. Actually, in taking account of this effect the material may be designed more lightly. A strain rate effect has been demonstrated for aramid paper honeycombs. ... 

S. Heimbs, S. Schmeer, P. Middendorf, and M. Maier. Strain rate effects in phenolic composites and phenolic-impregnated honeycomb structures. Compos. Sci. Tech., 67(13) 2827-2837, October 2007. 

Se is the strain rate effect (varies between 1 and 2 as a function of velocity)... 

Combined strain amplitude and strain rate effects on shear modulus. (After Isenhower, W.M., and Stokoe n, K.H., Strain-rate dependent shear modulus of San Francisco Bay mud. International Conference on Recent Advances in Geotechnical Earthquake Engineering and Soil Dynamics, University of Missouri, Rolla, MO, pp. 597-602,1981.)... 

Dynamic properties of soils are dependent on a number of factors, such as OCR, effective stress, void ratio, and saturation (Athanasopoulos and Richart, 1983a, 1983b Wu et al., 1984). The measurement of these dynamic properties is often influenced by strain rate effects (Isenhower and Stokoe, 1981). The strain rate effect of the measured variation of shear modulus with shear strain amplitude is shown in Figure 9.31. Laboratory... 

Cheng, S. K., and Chen, C. Y. 2004. Mechanical properties and strain-rate effect of EVA/PMMA in situ p>olymerization blends. European Polymer Journal 40(6) 1239-1248. 

Kar Karger-Kocsis, J., Friedrich, K. Temperature and strain-rate effects oti the fracture toughness of poly (ether ether ketone) and its short glass-fibre reinforced composite. Polymer 27 (1986)... 

Strain rate effect on toughening of nano-sized PEP-PEO block copolymer modified epoxy. Acta Mater. 57 (2009) 2691-2701. 

Swan, G., Cook, J., Bruce, S., Meehan, R. (1989). Strain Rate Effects in Kimmeridge Bay Shale. International Journal of Rock Mechanics and Mining Sciences Geomechanics Abstracts, 26,135-149.doi 10.1016/0148-9062(89)90002-8... 

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