Shear craze

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. 

Shearing Shearing + crazing A Multiple eraring x Single craze... 

In simple tension and tension-compression fatigue, HIPS deforms by craze nucleation and growth while ABS deforms primarily by shear. Crazes develop in ABS prior to fracture but at a later stage than does shear deformation. 

Harris and Ward have observed conventional crazes nearly normal to the tensile stress axis (tensiile crazes), as well as what appear to be crazes along the shear direction (shear crazes), in uniaxially drawn, crystalline PETP sheets. Tensile crazes were formed parallel to the initial draw direction. The shear crazes which were the first of their kind to be reported, were seen in specimens under all directions of orientation. They formed always at the same angle as that of the shear bands which appeared subsequently upon yielding. An explanation for this shear-craze -phenomenon was offered by Brady and Yeh based on their own studies of crazes and shear bands in amorphous and crystalline, isotactic PS films. First they produced a set of crazes and shear bands by stretching a film in one direction. When this film was then redrawn in a second direction, the authors observed that the stress component, which was... 

Since the balance between these shear/craze propensities is dependent on many external variables, such as temperature, frequency and stress level, we can expect... 

In general, the development of crazes is associated with dilatational stresses (Kambour, 1973). In one case, crystalline poly(ethylene terephthalate), so-called shear crazes have been reported to lie along shear bands induced by yielding. While such crazes are not yet understood, it is reasonable to assume that a dilatational stress component must somehow be involved. If such crazes exist in other systems, however, the argument in this section should not be affected. 

The authors of the Ref [19] studied the plastic deformation mechanisms for polymers within the temperatures wide range. They showed that for PC and polyphenyleneoxide (PPO) the transition from shear to crazing was observed at testing temperature approach to the glass transition temperature of those polymers. The indicated transition was observed at temperatures 373 393 K for PC (compare with the data of Fig. 9.1) and -413 K for PPO [19]. The authors of Ref [20] considered the transition shear-crazing as nonequilibrium phase transition and obtained its universal criterion within the frameworks of deformable solid body synergetics [21, 22]. 

Hence, the adduced above S5niergetic analysis of nonequilibrium phase transition shear-crazing with the experimental and theoretical data for glassy linear and cross-linked amorphous pol5miers at different temperatures allows to establish, that ... 

Bashorov, M. T, Kozlov, G. V, Ovcharenko, E. N., Mikitaev, A. K. (2008). The Nanostructures in Polymers Synergetics if the Nonequilibrium Phase Transition Shear-Crazing . Nano-i Microsistemnaya Tekhnika, 11, 5-7. 

When crazing limits the ductility in tension, large plastic strains may still be possible in compression shear banding (Fig. 23.12). Within each band a finite shear has taken place. As the number of bands increases, the total overall strain accumulates. 

Internal stresses occur because when the melt is sheared as it enters the mould cavity the molecules tend to be distorted from the favoured coiled state. If such molecules are allowed to freeze before they can re-coil ( relax ) then they will set up a stress in the mass of the polymer as they attempt to regain the coiled form. Stressed mouldings will be more brittle than unstressed mouldings and are liable to crack and craze, particularly in media such as white spirit. They also show a characteristic pattern when viewed through crossed Polaroids. It is because compression mouldings exhibit less frozen-in stresses that they are preferred for comparative testing. 

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... 

Thermal treatment and the nature of the casting solvent can also affect the deformation modes achieved in strained films of ionomers. For example, in films cast from polar dimethylformamide (DMF), the solvent interacts with ion-rich clusters and essentially destroys them, as is evident form absence of a second, higher temperature loss peak in such samples. As a result, even in a cast DMF sample of Na-SPS ionomer of high ion content (8.5 mol%), the only deformation mode observed in tensile straining is crazing. However, when these films are given an additional heat treatment (41 h at 210°C), shear... 

Studies of PMMA-based ionomers also demonstrate the influence of thermal treatment on deformation modes (16). For Na salts of PMMA-based ionomers of 6 and 12 mol% that were cast from DMF, only crazes were observed on straining. However, after an additional heat treatment (48 h at 160°C), which also removes any DMF solvent that is present, shear deformation zones are induced. Hence, the ionic cluster phase, which was destroyed by the polar solvent, has been restored by the heat treatment. 

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 combined effects of a divalent Ca counterion and thermal treatment can be seen from studies of PMMA-based ionomers [16]. In thin films of Ca-salts of this ionomer cast from methylene chloride, and having an ion content of only 0.8 mol%, the only observed deformation was a series of long, localized crazes, similar to those seen in the PMMA homopolymer. When the ionomer samples were subject to an additional heat treatment (8 h at 100°C), the induced crazes were shorter in length and shear deformation zones were present. This behavior implies that the heat treatment enhanced the formation of ionic aggregates and increased the entanglement strand density. The deformation pattern attained is rather similar to that of Na salts having an ion content of about 6 mol% hence, substitution of divalent Ca for monovalent Na permits comparable deformation modes, including some shear, to be obtained at much lower ion contents. 

Friedrich, K. Crazes and Shear Bands in Semi-Crystalline Thermoplastics. Vol. 52/53, pp. 225-274. 

Takemori, M. T. Competition Between Crazing and Shear Flow During Fatigue. Vol. 91/92, pp. 263-300. 

---