Strain softening effect

The strain softening effect can be characterised by its amplitude, SSA, defined as ... 

Tt has been shown that some block copolymers and their blends with - corresponding homopolymers exhibit the so-called strain softening effect (1,2,3, 4, 5). When the plastic specimen is stretched beyond the yield point, it becomes rubbery and exhibits high-elasticity with large recoverable deformation attributable to the break-up of their original rigid... 

Fig. 8.18 Compression stress-strain curves of nearly glassy PET (crystalline content 9%) for seven temperatures reaching Tg, showing strong yield phenomena and strain-softening effects that decrease with increasing temperature, having a relatively temperature-independent entropic strain-hardening contribution (from Zaroulis and Boyce (1997) courtesy of Elsevier).

A number of experimental data have shown that a great variety of filled rubber vulcanizates exhibit the dynamic strain softening effect schematically illustrated in Figure 5.42, at given frequency and temperature. The limiting... 

AS.5.2 Modeling the Dynamic Strain Softening Effect Elastic modulus ... 

It is instructive to describe elastic-plastic responses in terms of idealized behaviors. Generally, elastic-deformation models describe the solid as either linearly or nonlinearly elastic. The plastic deformation material models describe rate-independent behaviors in terms of either ideal plasticity, strainhardening plasticity, strain-softening plasticity, or as stress-history dependent, e.g. the Bauschinger effect [64J01, 91S01]. Rate-dependent descriptions are more physically realistic and are the basis for viscoplastic models. The degree of flexibility afforded elastic-plastic model development has typically led to descriptions of materials response that contain more adjustable parameters than can be independently verified. 

However, when considering temperature or strain dependencies of strain softening, the absolute stress difference (ay - apf) does not allow one to discriminate between a change of the absolute values of ay and crpf and an intrinsic effect on the strain softening behaviour. 

It is interesting to notice that similar effects on yielding characteristics have been reported [54] when increasing pressure to 80 kbar, as shown in Fig. 69. Here too, the decrease of strain softening with increasing pressure is opposite to behaviour observed for PMMA (Sect. 3.1.1.3). 

Fig. 47b), which impacts the slope of the stress-strain cycles. This softening effect results from the drop of the strain amplification factor Xmax with increasing pre-strain, which has been determined by an extrapolation of the adapted values, shown in the insert of Fig. 47a, with the power law approximation Eq. (53). 

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). 

A difficulty in comparing stress-softening data of various authors is the different degree of emphasis given by them to the extent of recovery of the sample before determination of the softening effect. When the sample is swelled in the vapor of a good solvent, elastic recovery should be very nearly complete and, indeed, there is very little permanent set, even under conditions of severe pre-strain. Under these conditions amorphous gum rubbers show no detectable softening, but black rubbers do. 

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