Crazing tips study

Clearly, if a rubber modified blend contains particles larger than the film thickness (about 1 /xm), the film cannot be representative of the bulk. For the study of craze tips and craze growth, a double tilting stage was used to obtain stereo pairs of the craze tip [396]. 

Brittle crack or craze propagation can be studied by TEM, where the crack or craze was stabilized by filling the notch tip with epoxy resin see Fig. 1.54. A small sample was cut from the material in front of the notch tip and was chemically treated for fixation and staining. Using an ultramicrotome, ultrathin sections are produced from the stained region, containing the crazed zone and the area ahead of the craze (such an area near a craze tip in HOPE is shown in Fig. 2.47 in Part 11). 

In their photoelastic studies of PMMA and PC crack tips Narisawa et al. [127] noted high stress concentrations just before the initiation of a craze and very modest stresses 60 min later. No noticeable new stress concentration appeared at the craze tip. 

A convenient technique for studying the crack tip craze propagation in amorphous polymers deals with optical interferometry. It has been applied to the examination of PMMA behaviour at various temperatures and crack speeds under conditions of stable propagation [44,45]. 

The criteria (Eqs. 11 and 12) are similar and are derived from studies on materials that are elastic at initiation of crazing, while more ductile materials like polycarbonate show a more pronounced sensitivity to the hydrostatic tension. This has been found experimentally by Ishikawa and coworkers [1, 27] for notched specimens of polycarbonate. Crazing appears ahead of the notch root, at the intersection of well-developed shear bands. From a slip fine field analysis, the tip of the plastic zone corresponds to the location of the maximum hydrostatic stress. This has been confirmed by Lai and Van der Giessen [8] with a more realistic material constitutive law. Therefore, Ishikawa and coworkers [1,27] suggested the use of a criterion for initiation based on a critical hydrostatic stress. Such a stress state condition can be expressed by Eq. 11 with erg = 0 and I r = B°/A°. Thus, the criterion (Eq. 11) can be considered general enough to describe craze initiation in many glassy polymers. For the case of polycarbonate, a similar criterion is proposed in [28] as... 

Following the studies on craze initiation, several efforts have focused on the description of glassy polymer fracture, and especially on the characteristics of a craze developed at a crack tip. Kambour [16] has shown that the length and thickness of a craze developed at the tip of a preexisting crack can be measured by interferometry and quantitative predictions have been reported in [29,30,38],... 

Failure Morphologies. Ductile failure of notched polycarbonate specimens has long been recognized to occur with shear yielding from the notch tip (6). This occurs for the block polymers for all rates of test. Hull and Owen (5) recently reported from micrographic studies of impact fracture surfaces that the brittle failure of polycarbonate involves the formation and breakdown of a craze at the notch tip. The ductile-... 

The growth behavior of surface crazes has been intensively studied in various polymers (e.g. 6-80) jjjg fracture mechanics approach provides a basis for an analytical description of the growth in front of a crack tip (e.g. In this Section... 

Studies of craze microstructure and the surrounding displacements of crazes have established that the only parts around a craze that undergo plastic deformation are concentrated into a process zone at the tip of the craze, and into a fringing layer all around the entire craze body. In the process zone craze matter is generated by one of the two processes discussed above, and fibrils are necked down to the final extension ratio. In the fringing layer, additions are made to craze fibrils by drawing polymer out of half space. Outside the idetifiable parts of a craze, the solid polymer remains entirely elastic while inside the craze body the fully drawn fibers carry the required craze tractions purely elastically in their orientation hardened state at the... 

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... 

The most promising development of the method for the future will probably be the study of transparent multiphase polymers and environmental crazing, which is a difficult situation where interferometry is one of the rare applicable methods. It should be noted that the field of applicability is already one of the largest of all techniques used to study microscopial aspects of crack tips during propagation. 

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. 

For 6 nm < a7 < 12 nm, Qc increases sharply with interfacial width, suggesting a transition from failure by simple fracture or pullout, to failure after formation of a craze at the crack tip. This craze is clearly observable in the PS-PMMA studies. Once the transition is passed, the increase in Qc is due to the increase in the maximum stress that can be sustained by the fibrils since all polymers in this study have very similar crazing stresses. 

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