Deformation, fibrillar

The formation of fibrillar structures during the crystallization of deformed solutions and melts under various conditions of mechanical treatment was observed by many authors22,24,25 who studied the crystallization in stirred or flowing solutions. In all cases... 

Stage III Plastic deformation of the fibrillar structure after neck formation is complete. 

Properties of composites obtained by template poly condensation of urea and formaldehyde in the presence of poly(acrylic acid) were described by Papisov et al. Products of template polycondensation obtained for 1 1 ratio of template to monomers are typical glasses, but elastic deformation up to 50% at 90°C is quite remarkable. This behavior is quite different from composites polyacrylic acid-urea-formaldehyde polymer obtained by conventional methods. Introduction of polyacrylic acid to the reacting system of urea-formaldehyde, even in a very small quantity (2-5%) leads to fibrilization of the product structure. Materials obtained have a high compressive strength (30-100 kg/cm ). Further polycondensation of the excess of urea and formaldehyde results in fibrillar structure composites. Structure and properties of such composites can be widely varied by changes in initial composition and reaction conditions. 

Before discussing specific aspects of micro deformation and fracture in bulk polyolefins, some basic notions of microdeformation and the micromechanics of fracture mediated by generation and breakdown of cavitated or fibrillar deformation zones or crazes are introduced. SCG in PE and rate-depen-dent fracture in iPP are then considered in more detail. 

The existence of a wedge-shaped cavitated or fibrillar deformation zone or craze, ahead of the crack-tip in mode I crack opening, has led to widespread use of models based on a planar cohesive zone in the crack plane [39, 40, 41, 42]. The applicability of such models to time-dependent failure in PE is the focus of considerable attention at present [43, 44, 45, 46, 47]. However, given the parallels with glassy polymers, a recent static model for craze breakdown developed for these latter, but which may to some extent be generalised to polyolefins [19, 48, 49], will first be introduced. This helps establish important links between microscopic quantities and macroscopic fracture, to be referred to later. 

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... 

Fig. 18 Fibrillar deformation in bulk a iPP deformed at room temperature, embedded in PMMA and post-stained with Ru04 vapour...

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]... 

We have presented a multi-scale method to simulate a fibrillar structure such as a collagen tissue equivalent. The method is able to predict macroscopic behavior based on microscopic properties, and it also demonstrates the microscopic restructuring that can occur during deformation. Although the method is computationally demanding, the potential for parallelization is high, and three-dimensional problems should not be out of reach. 

A further difference between both a and / -modifications is the extent of strain-hardening they exhibit. Convergent sets of data highlight the more prominent strain-hardening of the /3-phase in the post-yield range compared to its non-nucleated counterpart [109,140,172,179]. This fact results most probably from (i) the easier plastic deformation of the /1-crystals and of (ii) the transformation into the fibrillar structure at earlier stages than for the a-phase. 

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... 

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