Crazes initiation

Id. Thus, it is expected that in the modified PRP-EVA blend, probably due to interface modification by reactive processing, a transesterification between the pendant MAH group in MAH-PP, and acetate groups in the EVA elastomer, as predicted in reaction Scheme 1, the dispersed rubber particles become more efficient in craze initiation. 

The relationship between craze initiation and propagation, and crack initiation and propagation, is not well understood (31). Crazing clearly precedes formation of unstable cracking. For example, in tensile tests on Lustran A-21 S/AN, crazing occurred at 1.8-2.0% elongation, whereas... 

In addition to the rapid propagation of mature crazes discussed above, there is a period of extremely rapid propagation at craze initiation, as discussed by Knight (22). 

Based on our observation, a membrane degradation and failure mechanism under the RH cycling, a pure mechanical effect is theorized as the following sequence electrode-microcracking- - crazing initiation at the electrode/electrolyte interface - crack growth under stress cycling- -fast fracture/instability. 

Presented in this paper are the results of an investigation concerning the link between structure and properties of rubber-toughened plastics. An attempt has been made to assess the importance of the spatial distribution of rubber particles in terms of their effectiveness in controlling craze initiation and growth. Also studied in particular were the effects of rubber particle size on the mechanical properties of HIPS materials. A... 

This study (34) implies that a right dispersion of rubber particles may permit optimum stress field overlap that affords lower craze-initiation stresses and therefore can rapidly dissipate the strain energy in the HIPS. A more homogeneous spatial distribution of rubber particles allow for a uniform development of crazes. Prevention of the strain localization phenomenon to avoid the detrimental situation, where crazes prefer to develop in certain areas and quickly lead to a catastrophic crack, could result in a larger total volume of crazed material. Further, Donald and Kramer (22) discovered no crazes nucleating from an isolated rubber particle with diameter smaller than 1 urn because of an insufficient size of stress-enhanced zone. Since Sample-A has a small average particle size it should contain a large number of small rubber particles. Two small rubber... 

A good dispersion of rubber particles appears to favor the nucleation and growth of a large number of thick crazes uniformly distributed in the polystyrene matrix. This is believed to be an efficient source of energy absorption for the material under mechanical loading. The concepts of stress field overlap and critical volume of stress concentration zone for craze initiation were introduced to explain the observed mechanical behavior of HIPS. 

Many studies have been devoted to the influence of several factors on FCP and the total lifetime to failure however, little information is available in the literature concerning the effect of material and test variables on craze initiation. 

Arnold JC (1995) Craze initiation during the enviromental stress cracking of polymers. J Mater Sci 30(3) 655-660... 

Kawagoe M, Kitagawa M (1987) Craze initiation in poly(methyl methacrylate) exposed to n-alkanes. J Mater Sci 22(8) 3000-3004... 

Cho K, Lee MS, Park CE (1998) The effect of Freon vapour on fracture behaviour of styrene-acrylonitrile copolymer - I. Craze initiation behaviour. Polymer 39(6—7) 1357—1361... 

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... 

Because crazing is a precursor to failure, pioneering studies on crazing have focused on the conditions for craze initiation. Later on, estimation of the toughness motivated examination of the craze thickening and the conditions for craze breakdown. 

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],... 

Since oy is the major principal stress, we have an = oy > crm = (oy + a2)/2 and the side condition that the normal stress has to exceed the hydrostatic stress for craze initiation is satisfied. Equation 21 defines a critical normal stress which appears to be hydrostatic stress dependent. As long as an < oTr(am), crazing does not occur and when an reaches o/jr(crm) crazing initiates. Once initiated, the craze thickens and the condition (Eq. 21) is no longer relevant. 

Prior to craze initiation, an < cr r (crm) in Eq. 21 and the stiffness has to be infinitely large to ensure that no separation occurs across the cohesive surface. We propose to use... 

After a short transition following craze initiation, the craze thickens and results in an opening An of the cohesive surface at approximately constant normal stress. 

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