The structure of crazes

If the craze layer extends with complete lateral constraint, the strain in the craze is related to the change in its density. From a relationship between density and refractive index, an equation between strain in the craze and its refractive index can be derived. Although it is usual to start with the Lorenz-Lorentz equation, this may not be the correct relationship for a material having the structure of the craze (9). For the present purposes a linear relationship is assumed. The error introduced is at most 10% and only a few percent for the stretched craze with a high void content. 

The rate of steady state craze advance may be estimated usinp a procedure devised by Fields and Ashby A schematic view of the finger structure of the craze tip is shown in Fig. 3 a. The deforming polymer again is represented by a fluid between two rigid plates spaced a distance h apart (normal to the plane of the diagram). The radius of curvature at the tip of a void finger is h/2 in the plane normal to the craze... 

The contour of the loaded and unloaded craze was investigated by interference optics (Doll Kdnczdl, 1990 Kdnczdl et al, 1990). Additionally the structure of the crazes was investigated by high-voltage TEM. Sections of about 1 pm were microtomed for TEM observations. Structural inspection of the craze zone was done by TEM. The fibrillar structure of a craze zone in SAN is shown in Fig. 3.14. The microstructure of this craze created in a CT sample is similar to the structure investigated in situ in strained semithin sections (Michler, 1990). 

The fibrillar structure of the crazes is t5 ical of PS. Another glassy polymer is styrene-acrylonitrile (SAN) copolymer with a PS content of usually about 74%. In this material the dominant deformation structures are homogeneously deformed zones, but in many cases they coexist with the fibrillated crazes (2,29). Depending on the loading conditions (stress state, loading velocity, temperature), the deformation character can be shifted from one to another. Another typical glassy polymer is PMMA with a t5 ical appearance of homogeneously deformed crazes at room temperature (29). 

A craze in a block copolymer with PBMA cylinders is shown in Figure 19b with an internal cellular structure of the craze. The PBMA cylinders are cavitated in the craze and, therefore, appear bright. This is followed by a large plastic deformation of the PS parts (appearing dark) up to fibrils. 

The structure of crazes in bulk specimens was studied by Kambour [15], who used the critical angle for total reflection at the craze/polymer interface to determine the reliactive index of the craze, and showed that the craze was roughly 50 per cent polymer and 50 per cent void. Another investigation involved transmission electron microscopy of polystyrene crazes impregnated with an iodine-sulphur eutectic to maintain the craze in its extended state [33, 34]. The structure of the craze was clearly revealed as fibrils separated by the voids that are responsible for the overall low density. 

Since the molecular crazing criteria require a substantial amount of detailed information about the molecular structure of the solid polymer and no clear correlation to the macroscopic phenomena observed experimentally exists, phenomenological criteria analogous to those for shear yielding were proposed. The... 

Fig. 11 Craze in commercial polystyrene showing the characteristic steps nucleation through void formation in a pre-craze zone, growth of the fibrillar structure of the widening craze by drawing-in of new matrix material in the process zone, and final breakdown of the fibrillar matter transforming a craze into a crack (the crack front is more advanced in the center of the specimen, shielded by a curtain of unbroken fibrils marked by the arrow). The fibril thickness depends—of course—on the molecular variables, the strain rate-stress-temperature regime of the crazing sample and on its treatment (preparation, annealing) and geometry (solid, thin film) for PS typical values of between 2.5 and 30 nm are found [1,60,61]...

Cavitation is often a precursor to craze formation [20], an example of which is shown in Fig. 5 for bulk HDPE deformed at room temperature. It may be inferred from the micrograph that interlamellar cavitation occurs ahead of the craze tip, followed by simultaneous breakdown of the interlamellar material and separation and stretching of fibrils emanating from the dominant lamellae visible in the undeformed regions. The result is an interconnected network of cavities and craze fibrils with diameters of the order of 10 nm. This is at odds with the notion that craze fibrils in semicrystalline polymers deformed above Tg are coarser than in glassy polymers [20, 28], as well as with models for craze formation in which lamellar fragmentation constitutes an intermediate step [20, 29] but, as will be seen, it is difficult to generalise and a variety of mechanisms and structures is possible. 

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]... 

Fig. 5 a Schematic description of the craze structure, b Idealization of the craze process according to Kramer and Berger [32] for the craze thickening after initiation, c Representation of crazes by discrete cohesive surfaces... 

As the craze microstructure is intrinsically discrete rather than continuous, the connection between the variables in the cohesive surface model and molecular characteristics, such as molecular weight, entanglement density or, in more general terms, molecular mobility, is expected to emerge from discrete analyses like the spring network model in [52,53] or from molecular dynamics as in [49,50]. Such a connection is currently under development between the critical craze thickness and the characteristics of the fibril structure, and similar developments are expected for the description of the craze kinetics on the basis of molecular dynamics calculations. 

A more recent hypothesis is that the craze tip breaks up into a series of void fingers by the Taylor meniscus instability - . Such instabilities are commonly observed when two flat plates with a layer of liquid between them are forced apart or when adhesive tape is peeled from a solid substrate jjjg hypothesis in the case of a craze is that a wedge-shaped zone of plastically deformed and strain softened polymer is formed ahead of the craze tip (Fig. 3 a) this deformed polymer constitutes the fluid layer into which the craze tip meniscus propagates whereas the undeformed polymer outside the zone serves as the rigid plates which constrain the fluid. As the finger-like craze tip structure propagates, fibrils... 

Craze nucleation appears controlled by the nucleation of voids in localized regions undergoing large unstable plastic deformation. The sensitivity of this process to the nature of the flaw structure of the surface makes a detailed comparison of data between different experimental groups or between experiment and theory very difficult. 

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