Crazing, definition

Such a phenomenological definition of the critical craze thickness Acr hides much of the underlying physics. Further insight is expected to reveal how this parameter changes with loading rate as well as its temperature dependence, which could then be incorporated in the present framework. 

If the strain derived is entirely due to craze matter production, there will be no significant change in the cross-sectional area A of the part during such (dilational) flow. Then, by definition, the imposed tensile strain rate e must be... 

Argon and Salama have demonstrated that this form of the expression accoimts for the measured craze growth kinetics in homogeneous glassy polymers very well, giving for values in the range of 1.5-2.0, which is quite consistent with the definition of this quantity in Eq. (51) and the known extension ratio of 4 in craze matter and back stresses that correspond to this extension ratio. 

In the case of cyclic loading, the definition of the life-time of craze fibrils remains the same. But in that case, another quantity may be defined the number of cycles No during which the fibrils may carry a certain load level before breakage. 

In PEI the DCG process, as in any polymer, is active. The epsilon CTPZ, however, was not observed. No plane strain shear bands have yet been observed. Some form of localized crack tip shear process can be activated, however, as evidenced by the inversion transition that occured at higher stresses (at the higher temperatures). The fracture surface did not show fracture to occur on a slanted 45 degree plane. The fracture plane was still normal to the leading direction. The fracture surface, however, was not smooth, as seen with craze fracture, but has a definite roughened texture which is associated with active localized shearing. This texture is often described as honeycomb or tufted. 

The above definition thus includes the classical fibrillar crazes (formed on surfaces, in thin films or in the bulk at lower or higher temperatures in preoriented or isotropic material, in the presence or absence of a solvant or diluant) as well as those crazes which contain so many voids and cross-ties within the heavily deformed craze matter that it becomes difficult to identify individual fibrils. The Editor is extremely grateful to all authors of this volume and to Dr. G. H. Michler for frequent discussions on the above subject and for their constructive comments which led to the above Jointly proposed definition. 

This work is a contribution to the definition of an experimental protocol which aims in identifying the parameters involved in a description of crazing within a cohesive surface methodology. The results obtained for PMMA are valuable for the calibration to perform in connection to the numerical work of Estevez et al. [2]. The method of preparation needs to be improved for more ductile material in order to characterize the failure by crazing only. 

It is often difficult to predict the color of the gum arabic solutions on the basis of the color of the dry tears or powder. The size and condition of the lumps and powder affect judgment considerably. The smaller the size and the more frosted, the lighter will their color appear. A dark gum when finely powdered loses its color. When the gum is finely powdered or its surface is crazed, it presents so many minute facets at all angles that practically all the light is reflected and scattered before it has traversed more than the outermost layers of the substance. Proper comparison of color should be made in solutions of a definite concentration. According to Hamy (7), the rotatory power of solutions of gum from Accu ia verek is negative, that from other species of acacia is positive. 

Table 14.4, which lists some requirements for CDs, allows the elimination of some contending plastics. Polystyrene has too low a resistance to crazing and stress cracking, and the disc birefringence would be too high because the relatively elastic melt has a high stress-optical coefficient (defined in Eq. 9.9). Values for melts differ from values for glassy polymers given in Table 11.5. PVC has too low a heat distortion temperature, and its lack of thermal stability makes the injection moulding of high definition surfaces difficult. Silicate glass cannot be moulded with sufficient surface detail, and is brittle.

The most extensive and definitive experiments on craze microstructure in homopolymers were carried out by Kramer and co-workers (Kramer 1983 Kramer and Berger 1990). In these experiments crazes were formed in thin, electron-transparent cast films of polymer deposited on copper TEM grids and firmly bonded to them by solvent vapors. When such films were subsequently stretched slightly, crazes were formed in the stretched films, permitting in-situ study of their microstructure and its mechanical response in the elastically stretched state, under tension, eliminating any possibility of relaxation in craze matter that might have occurred in unloaded fibrils. 

To achieve craze plasticity with substantial potential to alleviate brittleness, well under the stress levels given above, a definite strategy needs to be followed, which is not possible through surface-initiated growth of crazes. 

What are the definitions of the following terms impact resistance tensile strength failure envelope fatigue crack propagation craze ... 

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