Crazing craze criteria

Parameters in the craze initiation criterion of Sternstein et al. [24] Parameters in the craze initiation criterion of Oxborough and Bowden [26] Critical craze thickness Craze length... 

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... 

The cohesive surface considered in the foregoing is based on observations made under quasistatic conditions. In particular, the incubation time for craze initiation is neglected and a critical stress state for craze nucle-ation is used (Eq. 11). For dynamic loading, a time-dependent craze initiation criterion is to be included in the kinetics, since the characteristic timescale associated with the loading can be comparable to that involved in the craze nucleation process. If the time for craze initiation is accounted for, another timescale is involved in the competition between crazing and shear yielding that determines whether or not crazing takes place. Therefore, a switch... 

Double-grooved specimens were used to study the failure of PC, PC/PE, PET, ABS, and HIPS during transitions from plane stress to plane strain. The yield behavior of PC is consistent with a von Mises-type yield criterion plane strain reduces its elongation. The yield behavior of PC/PE is consistent with a Tresca-type yield criterion plane strain appears to be relieved by voiding around the PE particles. PET undergoes a ductile-to-brittle transition its behavior is consistent with a von Mises-type yield locus intersected by a craze locus. The yield behavior of ABS and HIPS is not significantly affected by the plane-stress-to-plane-strain transition. Plane strain alone does not necessarily cause brittleness. 

The yield criteria of polymers have been reviewed by Ward (7) and more recently by Raghava et al. (8). Except for the craze yield criteria of Sternstein and Ongchin (9) and Bowden and Oxborough (10), most of the yield data can be described by a pressure-modified, von Mises-yield criterion. The corresponding yield surface is everywhere convex. A typical yield locus on the [Pg.103]

Bowden, P. B., Oxborough, R. J., A Critical Strain Criterion for Craze... 

The mechanism for craze nucleation and growth describai here is essentially possible in semicrystalline polymers since the criterion is only related with a stress field due to plastic constraint. Therefore, the size and geometry of a local plastic zone at the notch root is responsible for the formation of crazes (sometimes named internal crazes by the authors). 

Some criteria have been proposed for craze initiation. The earliest criterion states that crazing occurs when the uniaxial tensile stress reaches a critical value (27). Since the crazing stress depends on the strain rate and... 

Sternstein and Ongchin (28) considered that if cavitation occurs in crazes the criterion for crazing initiation should include the dilative stress component. They proposed the criterion to fit the experimental data for surface craze initiation in PMMA when the polymer is subjected to biaxial tension. The segmental mobility of the polymer will increase due to dilative stresses, thus provoking cavitation and the orientation of molecular segments along the maximum stress direction. 

A craze is supposed to initiate by a stress and temperature controlled mechanism and the initiation criterion is motivated by Stemstein s [10] classical work and is taken as... 

To obtain an upper limit for Q0 Oprii is set equal to the failure stress Ojlhrij = Z-efffl, of the continuum fibril structure which gives a failure criterion for the critical crack opening displacement <5 = Sc at the crack tip. Here Z(, is the number of effective connector strands per nominal unit craze area can be less than Z due to the scission of chains during formation of the craze fibrils. 

The experimental results described above can be explained within the basic fracture mechanism map after detailed consideration of the processes necessary to generate a craze at the interface. The criterion lfb > ocmze is a necessary condition for the formation of stable craze fibrils. However, it is not sufficient for the formation of a craze at an interface. Craze initiation is believed to occur by a meniscus instability process that happens within a yield zone (an active zone) at a... 

The onset of crazing follows a completely different stress criterion, as reported by Sternstein (25). Crazing is favored by high tensile stress and a positive mean normal stress. Brittle fracture follows still another stress criterion. The mode of response to a particular type of stress depends upon which critical threshold is first crossed. 

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