Strain softening phenomena

At small strains, polymers (both amorphous and crystalline) show essentially linear elastic behavior. The strain observed in this phase arises from bond angle deformation and bond stretching it is recoverable on removing the applied stress. The slope of this initial portion of the stress-strain curve is the elastic modulus. With further increase in strain, strain-induced softening occurs, resulting in a reduction of the instantaneous modulus (i.e., slope decreases). Strain-softening phenomenon is attributed to uncoiling... 

Second, the strain-softening phenomenon of a styrene-butadiene-styrene tri-block copolymer and its blend with polystyrene both having alternating lamellar microdomains of the two components i.e., "strain-induced plastic-to-rubber transition" is investigat-d in terms of change of the alternating lamellar microdomains to fragmented sty-... 

If the compressive force exceeds a critical value, /c, a mechanical instability occurs. This results in a large bending. The behavior of bending above the critical strain, kc, requires less force (f[Pg.157]

It is well known that at extremely low shear rates the slope of the r/y curve (Figure 3.26) is constant and that there exists some very low but finite threshold shear rate beyond which deviation from linearity commences. The slope of the initial linear portion of the curve is known as the limiting viscosity, the zero shear viscosity, or the Newtonian viscosity. Beyond this low shear rate region (initial Newtonian regime) the material is shear-softened (i.e., becomes pseudoplastic), a phenomenon which has its counterpart in the solid state where it is known as strain-softening. 

The work-softening phenomenon is due then to a reduction in dislocation density with increasing strain. This was shown clearly by Donlon et al. [142], who measured... 

The Mullins effect [79] is a strain induced softening phenomenon, which is associated mainly with a significant reduction in the stress at a given level of strain during the unloading path as compared with the stress on initial loading in stress-strain cyclic tests [80] (Fig. 18). 

Another softening phenomenon which manifests the dependence of the stress upon the entire history of deformation is the so-called Payne effect. Like the Mullins effect, this is a softening phenomena but it concerns the behavior of carbon blackfilled rubber subjected to oscillatory displacement. Strain dependence of the storage and loss moduli (Payne effect) at 70 °C and 10 Hz for a rubber compotmd with different concentration of carbon black filler [7] (Fig. 26). Indeed, the dynamic part of the stress response presents a rather strong nonlinear amplitude dependence, which is actually the Payne effect [8, 16, 43]. 

Stage II The polymer chain segments start to move and orientation starts to occur. The characteristic of this stage is strain softening, i.e., the stress decreases with increase in strain. (The stain softening phenomenon disappears if true stress and true strain are used in the stress-strain curve. However, the... 

Natural rubber exhibits unique physical and chemical properties. Rubbers stress-strain behavior exhibits the Mullins effect and the Payne effect. It strain crystallizes. Under repeated tensile strain, many filler reinforced rubbers exhibit a reduction in stress after the initial extension, and this is the so-called Mullins Effect which is technically understood as stress decay or relaxation. The phenomenon is named after the British rubber scientist Leonard Mullins, working at MBL Group in Leyland, and can be applied for many purposes as an instantaneous and irreversible softening of the stress-strain curve that occurs whenever the load increases beyond... 

Stress softening is a phenomenon related to filler reinforcement. When a material is extended to a certain strain, returned to zero strain and stretched again, the second stress-strain curve lies below the first one. There are several reasons for this phenomenon, including incomplete elastic recovery, conversion of the hard phase... 

When a strip of a filler-reinforced rubber is extended, returned to the unstressed state and then re-extended, the second stress-strain curve is found to lie below the original one, at least up to the elongation of the first extension. This phenomenon, known as stress-softening, has been the subject of much study as well as controversy. It is frequently referred to as the Mullins Effect, although it was well known even before the extensive work of Mullins and collaborators. The subject was thoroughly reviewed by Mullins (181) in 1969 and no attempt will be made here to cover it in detail. Instead, only a brief summary will be given, along with some relevant observations not emphasized in the Mullins review. 

However, not all properties are improved by filler. One notable feature of the mechanical behaviour of filled elastomers is the phenomenon of stress-softening. This manifests itself as a loss of stiffness when the composite material is stretched and then unloaded. In a regime of repeated loading and unloading, it is found that part of the second stress—strain curve falls below the original curve (see Figure 7.13). This is the direct opposite of what happens to metals, and the underlying reasons for it are not yet fully understood. 

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