Crazing overview

At room temperature, PP is a tough material with expanded plastic deformations in form of shear yielding. These processes are connected with drastic rearrangements on a lamellar and spherulitic level. However, a brittle fracture can be initiated in materials with large sphemlites and a reduced interspherulitic bonding 181- Lower temperatures, high strain rates, and high stress concentrations at crack tips favour the formation of crazes (overview in [9]). 

Fig. 13a, b Fibrillar deformation in a third generation HDPE subjected to fatigue testing in air. a Overview of the fibrillar zone and b detail of the craze-bulk interface... 

A third method which recently provided considerable insight into the role of crazes in deformation and fracture of amorphous polymers is the optical interference measurement of crazes (preceding a crack). Since the pioneer work of Kambour, this method has been widely used to determine characteristic craze dimensions and critical displacements. W. Doll gives an overview on recent results and on their interpretation in terms of fracture mechanics parameters (stress intensity factor, plastic zone sizes, fracture surface morphology, fracture energy). 

The second half of this volume is reserved to a discussion of specific craze problems encountered in practical application of polymer materials. J. A. Sauer and C. C. Chen analyze the fatigue behavior (mostly of rubber modified polymers). They show quantitatively the important effects of test variables and sample morphology on fatigue response. K. Friedrich gives an overview on the shear and craze phenomena in semicrystalline polymers. 

The following is a brief overview of the subject matter which introduces the reader to the main features of craze morphology and current theories on their initiation, growth, and failure. The reader is pointed in particular to excellent reviews on many aspects of crazing by Rramer (116), Kramer and Berger (117), Kambour (118), and Donald (119,120), and much of the subsequent discussion follows the development in those reviews. 

The in situ deformation of amorphous polymers by shear deformation and craze growth has been observed in optical microscope studies by Donald and Kramer [381]. Grids with thin films of various polymers and polymer blends were prepared on copper grids which were strained in air on a strain frame held in an optical microscope. The films were precracked in an electron microscope by a method more fully described by Lauterwasser and Kramer [382]. Many crazing studies are evaluated by in situ methods, and optical microscopy plays a major role in providing an overview of the deformation structure. Crazing studies will be more fully explored in the next section. 

Overview of initiation zone of crazes at a surface notch in a deformed thin film in low magnification in TEM ... 

PMMA, tensile bar after loading in overview with crazes ... 

Figure 3.25 In situ deformation of a rubber-toughened polymer (HIPS - high impact polystyrene) (a) overview of the area under load and (b) area in front of a crack tip (in micrograph (a) at the bottom) with rubber particles (gray) in a matrix (black) with crazes (bright) 2-pm-thick deformed section, deformation direction see arrow, in lOOOkV HEM.

Such an overview of the field naturally connects to a definition of framework materials. Porous materials are not new—activated carbon, zeolites, and many resin systems are among the diverse examples long in wide practical uses. The recent craze in porous framework research is partly driven by the appeal of rational design associated with the modular assembly process using distinct, prefabricated building blocks. Although the ability to correlate molecular and crystal structures remains far from perfection, the selection of shape and function in the molecular building blocks does lend considerable... 

In the previous sections on crazing primarily mechanical and molecular parameters have been discussed while the chemical environment was not considered as a variable. At this point an overview over the physico-chemical response of a crazable material to an active chemical environment will be given. (The chemical response to an active physical environment such as photodegradation or ozonolysis has been treated or referenced in 8 III). 

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