Crack-local deformation zone

It is known [5], that at elastoplastic behavior a system crack-local deformation zone deviates from thermodynamical equilibrium and for its analysis a principles, correct for close to equilibrium systems, for example, Griffith theory, are inapplicable. Besides, prefailure zone structure is differed from elastically deformed material structure (Fig. 5.3) that complicates additionally process analysis. As it was noted above, for polymers this effect is displayed as the formation of local deformation zones near crack tip, containing microvoids and oriented material (crazes) or oriented material only (ZD) [20]. Therefore, for fracture analysis in such cases fracture fractal theory is applied, using fractal analysis and general principles of synergetics [28]. 

The characteristic mechanical property of the amorphous polymers is high strength and a brittle up to ductile deformation behavior. The reason for this behavior is the formation of localized deformation zones under load, such as crazes, deformation bands, or shear bands [12]. The typical type of deformation seen in the amorphous brittle, glassy polymers is the craze. Crazes are often visible with the naked eye in reflected light see Fig. 1.4. The word craze recalls a macroscopic crack-like appearance craze comes from an old English word. 

The studies carried out earlier have shown that polymer film samples strength to a considerable extent is defined by growth parameters of stable crack in local deformation zone (ZD) at a notch tip [1-3], As it has been shown in Refs. [4, 5], the fiactal concept can be used successfully for the similar processes analysis. This concept is used particularly successfully for the relationships between fracture processes on different levels and subjecting fracture material microstructure derivation [5]. This problem is of the interest in one more respect. As it has been shown earlier, both amorphous polymers structure [7] and Griffith crack [4] are fractals. Therefore, the possibility to establish these objects fractal characteristics intercommunication appears. The authors of Refs. [8, 9] consider stable cracks in polyarylatesul-fone (PASF) film samples treatment as fractals and obtain intercommunication of this polymer structure characteristics with samples with sharp notch fracture parameters. 

In order to supplement micro-mechanical investigations and advance knowledge of the fracture process, micro-mechanical measurements in the deformation zone are required to determine local stresses and strains. In TPs, craze zones can develop that are important microscopic features around a crack tip governing strength behavior. For certain plastics fracture is preceded by the formation of a craze zone that is a wedge shaped region spanned by oriented micro-fibrils. Methods of craze zone measurements include optical emission spectroscopy, diffraction... 

It is understood that the (local) Gibbs energy depends on the local stress, and thus aH(NH) and (A/h) reflect the self- and coherency stresses in the Me-H system. In addition, if coherency is lost due to plastic deformation or cracking, the Me atoms in the deformation zone may well become mobile and Me then is well defined near the interface. This could explain the fact that aK(N (P)) (= aH(Ajj(a))) corresponds, in essence, to the value of the a/p equilibrium calculated using independent thermodynamic data. 

The structure of the diffuse weld interface resembles a box of width X, with fractal edges containing a gradient of interdiffused chains as shown by Wool and Long. When the local stress at a crack tip exceeds the yield stress, the deformation zone forms and the oriented craze fibrils consist of mixtures of fully entangled matrix chains and partially interpenetrated minor chains. 

It appears that the toughening effect near a crack tip occurs as follows after initial deformation, cavitation occurs within or aroimd the soft phase in a zone surrounding the crack tip. This zone is known as the process zone. The newly created surface around these cavities in the process zone allows the latter to grow plastically, thus absorbing some energy. Localized shear deformations, often referred to as shear bands, may also grow from these cavities. If the volume fraction... 

The behaviour to fracture of a rigid polymer matrix can be positive influenced by the introduction of some finely divided particles, which increase material s toughness. As Figure 3.468 shows, these particles can determinate large zones of local deformation, in the proximity of the centres of stress/tension high concentration, as the cracks tips are, causing an important absorption of energy [1255, 1265], Fig. 3.468... 

In Ref [4] it has been shown that increase from I up to 2 by its very nature represents itself the transition from ideally sharp notch D = 1) to cavity = 2), that is, the crack with blunted tip. Thus, the physical significance of fractal model corresponds completely to the offered in Ref. [2] treatment, namely, to the mechanism of notch blunting by local plastic deformation zone. 

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