Craze development

In the higher temperature range, where a single craze develops at the crack tip, it has been shown [46] that a breaking time r of the craze fibrils under a stress, ac> can be defined from the craze length S and the crack-craze velocity vc by ... 

Before analysing these results, it is interesting to point out that optical and electron microscopies of the sample region ahead of the crack reveal that a craze develops at the crack tip, whatever the type of fracture regime, i.e. stable or unstable [53]. 

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],... 

The diffraction patterns reported here reveal quantitative features. As indicated earlier, exact quantitative data are being collected to continue this research. However some preliminary observations and interpretations on the relations between chemical groups and their influence in crazing development are given for possible future reference. 

Figure 1 illustrates the stressed polystyrene film without crazing. A thin layer of glycerin has been applied to this area with little or no change in the diffraction pattern as shown on the right. The same laser diffraction pattern of a circular halo was also obtained when distilled water was used. However if a big blot of distilled water were introduced and allowed to dry, minute but perfect parallel crazes developed, and a sharp vertical diffracted light was observed (see Figure 2). The possible absorption of atmospheric contaminants by the water or water molecules that entered the polystyrene molecular system and eased the nucleation...

Several compounds having methylene and hydroxyl groups such as ethanol, polyethylene glycol, propanol, isopropyl alcohol, and propylene glycol have been used. All greatly affected crazing development. Note... 

Glycerides of the commercial quality commonly used in the food industry apparently have different effects on crazing development than... 

Mechanically stressed polystyrene without the application of any chemical compound can also produce crazing. Figure 23 shows crazing and its diffraction pattern attributed to bending the polystyrene sheet surface crazing appears on only one side. This contrasts the development of crazing in depth by simple tensile stress (see Figure 24) where crazes developed on both sides of, and sometimes in between, the surfaces. 

Generally all crazes developed perpendicular to the direction of the maximum applied principal stress, which in this case is the vertical direction. The vertical laser diffraction pattern indicates that all horizontal crazes are essentially parallel. According to the theory of light diffraction... 

In simple tension and tension-compression fatigue, HIPS deforms by craze nucleation and growth while ABS deforms primarily by shear. Crazes develop in ABS prior to fracture but at a later stage than does shear deformation. 

Fig. 8. Tensile strength, aj, craze initiation stress, and stress at start of rapid craze development, vs average contour length between molecular entanglement points, for PSAN samples of varying AN content...

In another study by Brown et al. craze fibril diameters were determined by SAXS measurements during fatigue cycling of samples of a commercial PS (M = 3 X 10 ) in 3-point bending at the low rate of 1 cycle in 12 s. Displacements were varied from zero to maximum in a saw-tooth manner and the maximum strain was sufficiently high so that crazes developed on the first cycle. SAXS measurements of... 

However, at higher ion content (above 6 mol %), craze development is partially suppressed and localized shear deformation appears. The observations reported here seem to be consistent with observed fatigue behavior of the same ionomers Both behaviors change significantly at around a critical ion content (ca. 5 mol%). 

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. 

Thus crazing has been directly identified in typical rubber-modified plastics in a plane perpendicular to the direction of the applied stress and initiated at the rubber-matrix interface. These findings are quite consistent with macroscopic model studies performed by Matsuo et al. (1972), who studied the behavior of polystyrene-containing rubber balls (as an analog of ABS morphology). It was found that equatorial crazes developed in tension, as expected, and also that stress field interaction occurred when the balls were close together, resulting in a heavier craze density between the balls. This shows that a principal role of rubber particles is to induce many... 

---