Craze-bulk interface

Fig. 5 TEM micrographs of thin films a craze in MGIM76 at 0 °C (From [51]), b an active zone at the craze-bulk interface in polystyrene (From [21])...

The craze thickening, associated with the craze growth, implies an increase of fibril length. This is achieved by pulling out polymer chains from the craze-bulk interface, according to a behaviour analogous to plastic flow within the active layer (5-10 nm thick), as shown in Fig. 5b [21]. 

Concerning the proportionality constant, it involves quantities difficult to determine experimentally, such as the thickness and rate of advance of the craze-bulk interface, the coefficients ef and ne in the constitutive equation of non-Newtonian flow ... 

Under conditions where chain mobility is high enough, typically at high temperature and low strain rate, the loss of entanglement in the active layer at the craze-bulk interface can occur by chain disentanglement, resulting in chain disentanglement craze (CDC). 

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-... 

Fig. 13a, b Fibrillar deformation in a third generation HDPE subjected to fatigue testing in air. a Overview of the fibrillar zone and b detail of the craze-bulk interface... 

Fig. 20 a Side-view of the crack-tip damage zone in a CT specimen of iPP with Mw of 455 kg mol-1 deformed at about 3 m s 1. b Oblique view of the damage zone showing the curved deformation front, c TEM micrograph of the collapsed fibrillar structure of the crack-tip craze, d Detail of structure at the craze-bulk interface [19]... 

These observations appear to be in contradiction with a creep mechanism for craze fibrillation, and the currently accepted description refers to the drawing-in mechanism due to Kramer [31,32], Kramer argued that fibrillation takes place within a thin layer (about 50 nm) at the craze/bulk interface, in which the polymer deforms into highly stretched fibrils similar to the mechanism of drawing of polymer fibers, as illustrated in Fig. 2. Craze thick-... 

Fig. 2 Description of the craze thickening process according to Kramer [31] as drawing in new polymer chains from the craze/bulk interface into the fibrils. The fibrils have a diameter D and spacing of D0...

As discussed in Sect. 3.2, once a mature fibril is created, further thickening occurs by a viscoplastic drawing mechanism which involves intense plastic deformation at the craze/bulk interface [32], Instead of using a non-Newtonian formulation as in [32] or a formulation based on Eyring s model [45], but on the basis of a preliminary study of the process [36], the craze thickening is described with a similar expression as the viscoplastic strain rate for the bulk in Eq. 3 as [20]... 

During this transformation from primitive to mature fibril, the force distribution acting on the craze/bulk interface remains constant so that a = F/Sq... 

The crack frequently initiates from the breakdown of a craze that formed at an internal defect, as a void or impurity particle. Then, as shown by various investigators, as crack speed increases, the crack jumps rapidly from one craze bulk interface to another and from one craze to another. This can lead to a so-called mackerel type pattern on the fracture surface or to a craze island type structure see also Chapter 1 and 3. As crack length increases and local stress rises, numerous secondary fractures, as shown in Fig. 2 b, are generated ahead of the crack front. 

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],... 

While theoretically the full-field continuum solution, Eq. (13), is an approximation for the stress in the last fibril, simulations that take into account the discrete nature of the craze and the detailed displacements of the craze/bulk interface due to fibril drawing near the crack tip [51, 54] show that it, and thus Eq. (19), are very good approximations. Note however that Eq. (19) is meaning-... 

Crazes thicken by drawing in new material, across the craze-bulk interface that stretches to a characteristic extension ratio. The plastic deformation occurs under plane strain conditions (Appendix C), in the plane containing the tensile stress direction and the craze advance direction. The bulk polymer, above and below the craze, remains elastic, but it does not constrain the craze opening, because the void creation means that the craze Poisson s ratio is zero. 

The stress through most of the craze has a constant value acrazeJ this crazing stress is essentially a material constant for a given type of polymer. This stress is carried primarily by the main craze fibrils which run perpendicular to the craze/bulk interface. However, there are in addition to the main craze fibrils cross-tie fibrils, which connect the main craze fibrils laterally. These permit some transfer of stress in the lateral direction (see figure 7.7), with the result that there is a stress concentration at the crack tip. It is this stress concentration that causes the breakdown of the last load-bearing fibril and the growth of the crack. At the crudest level we can model the craze as an elastic continuum and write the stress as a function of the distance x from the end of the crack as... 

In Fig. 24.1 we show a part of a craze. The parameter/) is the (mean) craze fibril diameter while Do is the craze fibril spacing. Both D and Do increase somewhat with increasing He. Berger [42] traced the craze fibril breakdowns to the formation of small pear-shaped voids at the craze/bulk interface. The results in [42] confirm the microscopic model of Kramer and Berger [38] which we see in Fig. 24.1. 

Extensive studies of crazes and their behavior imder loads have been conducted by Kramer and co-workers (30-40). We know from this work that there are two unique regions within the craze (1) the craze/bulk interface, a thin (10-25 nm) strain-softened polymer layer in which the fibrillation (and thus craze widening) takes place and (2) the craze midrib, a somewhat thicker (50-100 nm wide) layer in the craze center, which forms immediately behind the advancing craze. The relative position of the midrib does not change as the craze widens. By contrast, as the phase boundaries advance, continuously new locally strain-softened regions are generated, while strain-hardened craze fibrils are left behind. 

Craze thickening, ie opening of the craze-bulk interfaces and extension of the fibrils in the Y direction (Fig. 16). 

Micro plastic zones occur even in the brittle fracture of polymers in front of the crack tip. Crazes are localized bands of plastically deformed polymer material, which always appear perpendicular to the stretching direction. They are constituted hy polymer fibrils of about 5 -15 nm diameter, which are stretched in the loading direction and separated by elongated voids with diameters up to about 50 nm. The craze-bulk interface is relatively sharp and only about 10 nm thick. Crazing is connected with volume increase of the material. In Part II, Figs. 1.4 and 1.5 and those figures that follow show typical examples of crazes in PS. Crazes in other polymers can also possess a coarser internal structure. 

The fibrillated crazes grow through the continuous stretching of new material at the craze boundaries surface drawing or pull-out mechanism). The situation at a craze/bulk interface is illustrated in Fig. 1.7. Stretching of the fibrils occurs up to an elongation that depends on the parameters f and d of the entanglement network. The transition zone (active zone g) forms the craze interphase with a characteristic thickness g. 

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