Deformation crazing and

Locally Heterogeneous Deformation Crazing and Slow Crack Growth. .. 22... 

PMMA, deformed, crazes, and appearance of damage due to electron beam irradiation [2, 3] ... 

The aim of this contribution has been to link the basic macroscopic phenomena associated with polymers with the unique features of their structure, the most obvious being the presence of long, flexible molecular chains. The important role of the conformational entropy of flexible chains, not only for rubber elasticity but for polymer dynamics in general, has been demonstrated. Moreover, the concept of an entanglement network, which underpins much of the theory of polymer dynamics in the melt, has also been shown to have important repercussions for the high strain behavior of solid polymers, namely plastic deformation, crazing, and fracture. 

As an indication of the changes in deformation modes that can be produced in ionomers by increase of ion content, consider poly(styrene-co-sodium methacrylate). In ionomers of low ion content, the only observed deformation mode in strained thin films cast from tetra hydrofuran (THF), a nonpolar solvent, is localized crazing. But for ion contents near to or above the critical value of about 6 mol%, both crazing and shear deformation bands have been observed. This is demonstrated in the transmission electron microscope (TEM) scan of Fig. 3 for an ionomer of 8.2 mol% ion content. Somewhat similar deformation patterns have also been observed in a Na-SPS ionomer having an ion content of 7.5 mol%. Clearly, in both of these ionomers, the presence of a... 

As one example, in thin films of Na or K salts of PS-based ionomers cast from a nonpolar solvent, THF, shear deformation is only present when the ion content is near to or above the critical ion content of about 6 mol% and the TEM scan of Fig. 3, for a sample of 8.2 mol% demonstrates this but, for a THF-cast sample of a divalent Ca-salt of an SPS ionomer, having only an ion content of 4.1 mol%, both shear deformation zones and crazes are developed upon tensile straining in contrast to only crazing for the monovalent K-salt. This is evident from the TEM scans of Fig. 5. For the Ca-salt, one sees both an unfibrillated shear deformation zone, and, within this zone, a typical fibrillated craze. The Ca-salt also develops a much more extended rubbery plateau region than Na or K salts in storage modulus versus temperature curves and this is another indication that a stronger and more stable ionic network is present when divalent ions replace monovalent ones. Still another indication that the presence of divalent counterions can enhance mechanical properties comes from... 

The formation of voids in the rubbery phase in HIPS influences its mechanical properties. The formation of voids is believed to facilitate the energy dissipating deformation processes, i.e., crazing and shearing. Crazing and shearing are facilitated under conditions in that the rubber particles can easily cavitate. 

However, at lower constant loads the rate of crystal plastic deformation decreases and (at 80 °C) disentanglement becomes competitive leading to the development of isolated planar craze-like defects extending perpendicular to the tensile axis (Fig. 15). The ensuing concentration of stress will further localize most of the sample deformation in such creep crazes and lead to a macroscopic ductile-brittle transition—in this material observed at 20 MPa (Fig. 14 [67]). 

Depending on the polymer chemical structure and MW and on the deformation conditions (temperature and strain rate), two types of deformation heterogeneities are observed crazes and shear deformation zones. 

At intermediate temperature (between 10 and 80 °C for all the copolymers), two types of deformation are observed to coexist, i.e. crazes and SDZs. At temperatures just above the transition temperature, T12, from CSCs to mixed deformation, the crazes are of a high-aspect ratio, and the SDZs are generally restricted to the craze-bulk interfaces (Fig. 55a), al-... 

Above 70-80 °C, the deformation remains mixed, but crazing is dominant and corresponds to CDCs, as revealed by the behaviour of the strain to craze and the strain rate dependence this temperature of occurrence of CDCs is denoted T223. Furthermore, the SDZs become less widespread and tend to be accompanied at the tips of the main crazes by regions of multiple crazing (Fig. 56)... 

It illustrates, in the case of a bulk fracture, the way crazes and shear deformation zones interact... 

In terms of tonnage, polyolefins are by far the most important polymeric materials for structural applications, and there is consequently enormous interest in optimising their fracture properties. A rational approach to this requires detailed understanding of the relationships between macroscopic fracture and molecular parameters such as the molar mass, M, and external variables such as temperature, T, and test speed, v. Considerable effort is therefore also devoted to characterising the irreversible processes (crazing and shear deformation) that accompany crack initiation and propagation in these polymers, some examples of which will given. 

Fig. 24 TEM micrograph showing crazing and rubber particle break-down close to the fracture surface in an iPP/EPR CT specimen deformed at about 7ms [19, 26]...

Crazes usually form under tensile stress when a critical strain is surpassed they do not occur under compressive stress applying hydrostatic pressure during tensile deformation can even inhibit their development. Crazes always nucleate preferentially at points of triaxial stress concentration. It is the dilatational strain which initiates crazes and cracks. 

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