Crazing process

Processes that occur at a size scale larger than the individual chain have been studied using microscopy, mainly transmission electron microscopy (TEM), but optical microscopy has been useful to examine craze shapes. The knowledge of the crazing process obtained by TEM has been ably summarised by Kramer and will not be repeated here [2,3]. At an interface between two polymers a craze often forms within one of the materials, typically the one with lower crazing stress. 

We present the major results established in the description of crazing and the recent developments in this field. Crazing has been investigated within continuum or discrete approaches (e.g., spring networks or molecular dynamics calculations to model the craze fibrils), which have provided phenomenological or physically based descriptions. Both are included in the presentation of the crazing process, since they will provide the basis for the recent cohesive surface model used to represent crazing in a finite element analysis [20-22],... 

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... 

The three stages of the crazing process discussed in the foregoing subsections are combined by the traction-opening law... 

We illustrate in Fig. 6 the full traction response to a constant widening rate An derived from Eq. 23. The three regimes of the craze process can be readily distinguished ... 

Basu and Van der Giessen [9] extended the above isothermal formulation to account for thermal effects associated with the heat dissipated by plastic dissipation of the bulk and from the craze process. The plastic energy dissipation rate per unit volume is specified in Eq. 4, so that the energy balance inside the material can be written as... 

Finally, our interest will be limited here exclusively to the phenomenon of crazing in heterogeneous polymers. Thus, apart from the considerations of improving toughness by manipulation of the processes that govern the craze flow stress and, thus, rendering the extrinsic flaws inoperable that result in craze fracture, we will not consider the mechanics of fracture of crazable polymers. A brief survey of this subject related to the crazing process can be found elsewhere... 

The role of various polybutadienc molecular and morphological parameters can be better understood in the light of a mechanistic view of the crazing process. The mechanism of craze growth suggests the importance of various rubber domain... 

The crazing process does not induce large-scale energy dissipation in pure Polypropylene so the cavitation / dewetting process is the main toughening mechanism in CaC03 filled Polypropylene. The basic micro-mechanisms depend on the material properties and on the loading conditions. 

Motivated by the Kramer and Berger [3] description of the crazing process, Tijssens et al. [9] proposed a viscoplastic crazing model within the fiamework of a cohesive zone methodology. The traction-separation law proposed in [9] comprises three parts corresponding to initiation, thickening and breakdown of the craze. 

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