Craze initiation mechanism

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... 

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. 

Bucknall and Smith (17) concluded that crazing is the dominant mechanism to toughen high impact polystyrene and related polymers. One important function of the rubber particles is to serve as craze initiators and stabilizers in the glassy matrix. However Newman and Strella (18) concluded from optical microscope studies that cold drawing is responsible for toughness in ABS. 

Much attention has been focused on the microstructure of crazes in PC 102,105 -112) in order to understand basic craze mechanisms such as craze initiation, growth and break down. Crazes I in PC, which are frequently produced in the presence of crazing agents, consist of approximately 50% voids and 50% fibrils, with fibril diameters generally in the range of 20-50 nm. Since the plastic deformation of virtually undeformed matrix material into the fibrillar craze structure occurs at approximately constant volume, the extension ratio of craze I fibrils, Xf , is given by... 

Valuable information on the intrinsic craze phenomenon has been obtained from studies on pre-oriented material The effect of pre-orientation on craze initiation is central to the discussion of the molecular craze mechanism in Section 4.2. To facilitate the presentation of the experimental results, the following nomenclature is introduced. Quantities which refer to the pre-orientation and to the crazing experiment are labelled with indices one and two, respectively. If no index is used, the quantity refers to the result of both experiments. 

In this Section the kinetics of craze growth at crack tips in air will be considered in some detail. We shall not be concerned with the initiation phase and any micro mechanism (e.g. leading to craze initiation. 

Ascertaining the causes of the increased effectiveness of the composite particles in craze initiation requires analysis of the mechanical properties of the particles. A preliminary study of this type is presented below. 

Since then the research activity may even have increased. Central themes were the molecular mechanisms in craze initiation, the influence of molecular weight and presence of entanglements, the nature of plastic instabilities, the role of crazes as precursors to cracks, and last but certainly not least, the formation of crazes in semicrystalline and multiphase polymers. Although considerable progress has been made in the above mentioned fields, some important questions are still open today. 

The experimental results described above can be explained within the basic fracture mechanism map after detailed consideration of the processes necessary to generate a craze at the interface. The criterion lfb > ocmze is a necessary condition for the formation of stable craze fibrils. However, it is not sufficient for the formation of a craze at an interface. Craze initiation is believed to occur by a meniscus instability process that happens within a yield zone (an active zone) at a... 

Figure 8. Three-stage mechanism of multiple crazing (a) stage 1 stress concentration and craze initiation at rubber particles (b) stage 2 superposition of stress fields (small interparticle distance, high rubber volume content) and formation of broad craze bands and (c) stage 3 limitation of crack length and crack stopping at rubber particles.

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