Craze structure

When the stress that can be bom at the interface between two glassy polymers increases to the point that a craze can form then the toughness increases considerably as energy is now dissipated in forming and extending the craze structure. The most used model that describes the micro-mechanics of crazing failure was proposed by Brown [8] in a fairly simple and approximate form. This model has since been improved and extended by a number of authors. As the original form of the model is simple and physically intuitive it will be described first and then the improvements will be discussed. 

FEG-SEM of crack-tip deformation in a [3 nucleated CT specimen and Fig. 23 shows the corresponding TEM images. These confirm deformation to be less localised than in a spherulites at comparable test speeds, with diffuse regions of cavitation and lamellar shear coexisting with relatively well-defined crazes. The craze structures themselves are nevertheless similar to those in a-iPP. 

Fig. 4 Description of a craze (a) with a Dugdale zone, and the local analysis (b) as a long strip representing the anisotropic craze structure made of main fibrils oriented along direction 2, connected with lateral cross-tie fibrils along direction 1...

Fig. 5 a Schematic description of the craze structure, b Idealization of the craze process according to Kramer and Berger [32] for the craze thickening after initiation, c Representation of crazes by discrete cohesive surfaces... 

Much attention has been focused on the microstructure of crazes in PC 102,105 -112) in order to understand basic craze mechanisms such as craze initiation, growth and break down. Crazes I in PC, which are frequently produced in the presence of crazing agents, consist of approximately 50% voids and 50% fibrils, with fibril diameters generally in the range of 20-50 nm. Since the plastic deformation of virtually undeformed matrix material into the fibrillar craze structure occurs at approximately constant volume, the extension ratio of craze I fibrils, Xf , is given by... 

The structural analysis of intrinsic crazes in PC has been carried out by SAXS. The fibrillar microstructure of these crazes gives rise to pronounced scattering effects which enable a detailed analysis of the craze structure in terms of both the voliune fraction of craze fibrils and of the fibril diameter. This analysis showed that the microstructure of intrinsic and extrinsic crazes is largely different. There exists some evidence that the distinct microstructure primarily reflects the different stress-strain states of the matrix at craze initiation. Further investigations are necessary to answer... 

It was shown recently by Brown that in PS the craze structures are similiar in form for thin and thick films and that, however, the fibrils are about three times larger in thin films. 

For ionomer samples with low ion. content (less than 5 mol %), only crazes are formed. Figure 24 shows a typical TEM picture of a craze in a deformed thin film of an ionomer with low ion content. This can be compared with the craze structure of starting PS (Fig. 12b). Also, in Fig. 25 two views of the craze microstructure in PS (Fig. 25a and b) are compared with corresponding views (Fig. 25c and d) of the craze structure of the ionomer containing 4.8 mol % ion content. These micrographs show typical structural features of crazes of glassy polymers a) a midrib of lower fibril... 

For ionomer samples with high ion content (more than 6 mol %), it is noted that fewer crazes are formed, and the direction of some of these is not perpendicular to the stress axis (Fig. 26a). Also, bifurcation of crazes is observed (Fig. 26b). In addition to these anomalies in craze structure, some shear character, such as localized shear deformation, also appears (Fig. 27a). For comparison, a TEM micrograph taken for a polycarbonate (PC) sample under the same experimental conditions is shown in Fig. 27b. PC is known to deform by shear deformation at room temperature In Fig. 27b, it is seen that shear bands are formed at approx. 45° to the stress direction. Similar structural features, although in smaller degree, are seen for ionomer samples with high ion content (Fig. 27a). Also, in these samples interactions between crazes and shear bands are noted (for example. Fig. 26a). Interaction effects have also been observed in other glassy polymers... 

In sulphonated PS ionomers of low ion content, crazing appears to be the dominant deformation mode in tension and the resulting craze structure is similar to that observed in PS. For higher ion contents (above 5 mol %) fewer crazes appear and some degree of localized shear deformation is present. 

Craze structural parameter S(x) Craze surface stress distribution... 

Fig. 13. Craze stiffness above and below the critical temperature in PMMA. Note the transition near T., indicating a change in the craze structure. From Ref. courtesy of Chapman and Hall, Ltd.

Optical interferometry k basically inadequate for invratigating the craze structure directly. Nevertheless, that structure may be investigate through its mechanical properties, for example, the stiffness. Early work on crazes used that approach to evaluate the craze fibrils structure... 

In the experiment described below, small changes in the craze structure due to low scale molecular motion between craze fibrik will be detected by means of optical interferometry measuring stiffness of the craze material. The experimental procedure is a follows ... 

Whereas in Sect. 2 the use of optical interferometry to study qualitatively the morphology of the running crack-tip craze has been shown, this section shows several quantitative craze material models adapted to the experimental results obtained from optical interferometry in the case of a running crack-tip. As mentioned in Sect. 1, the lack of information about the inner craze structure confines the choice to models not sensitive to details in the craze structure. The proposed mechanisms are the following in the case of a steady-state propagating crack-craze system, with breakage in the craze midrib, the fibril breaks at the oldest part. The drawing... 

Then, using Eqs. 17 and 18, the numerical value of the craze structural parameter S is experimentally known ... 

Fig. 46. Craze structural paraincier S. as defined by Eq. 17 and as determined by means of Eq. 19. This parameter includes craze fibril volume fraction and the tensile modulus of the bulk. The dashed i ojc encloses the values of for all the crazes in air, whereas the dots correspond to the crazes...

Fig. 47. Craze structural parameter versus craze material volume fraction v, from Eq. 20. The numerical values of = 0.048 0.007 leads to = 0.3 0.08, and indicates that the craze material volume fraction is not effected by the toluene vapors. From Ref. by permission of the publishers, Butterworth and Co. Ltd.

The lack of information about inner craze structure confines the interpretation to models not sensitive to details in craze structure. 

Fig. 4a d. Transmission electron micrographs of craze structure formed by the PB phase cavitation ntechanism in several dibtocks a) SBS h) SB6 e) SB8 d) SBIO... 

Fig. 48. High magnification views of craze structure of PE samples of higher and lower molecular weights...

Previous studies have shown that the formation and failure of the craze structure ahead of the crack tip is the precursor to fracture in polyethylene (PE). A knowledge of the craze development and its structure should lead to an understanding of the crack growth behaviour. However, to date there have been very few studies of the craze behaviour from its initiation and growth to eventual breakdown. 

It is well documented [2-4] that the precursor to fracture in PE is the failure of the craze structure ahead of the crack tip during SCO, The formation of the craze and the mechanism that leads to craze breakdown have been described frequently. The craze nucleation is characterised by the formation of a highly localised zone ahead of the crack tip which consists of multiple voids. Their growth and subsequent coalescence leads to the formation of a fibrous structure. Depending on the stability of the craze structure, the craze may widen by drawing material from the craze-bulk interface into the craze fibrils and eventually rupture at the midribs, or fail at the craze-bulk interface with little or no signs of material fibrillation [5],... 

Figures 5(a) and 5(b) show the damage evolution and the corresponding measured TS curve of PESO at 50 mm/min. In Figure 5(a), the damage evolution is similar and is characterLsed by the abrupt rupture of the craze structure formed across the ligament area for all LBA ratios. Figure 5(b) shows that the separation distance increases with increasing LBA ratio, but the increase is much smaller than at 0.05 mm/min.

It is important to note that the model developed in this section is based on the simpler set of processes occurring in the spherical-morphology diblock. The process in the rod-morphology diblock requires considerable idealization, as comparison of the craze-structure micrographs of Fig. 11.20(a) and Fig. 11.21 should make clear immediately. 

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