Craze micro structure

Brown, H. R. and Kramer, E. J. (1981) Craze micro structure from small angle X-ray scattering (SAXS), /. Macromol. Sci. - Phys., B19, 487-522. 

In Eq. (10) that gives the toughness in dilatational plasticity the factors C and A are dependent on craze microstructure and will not vary significantly. The stress and temperature dependence of the craze velocity while quite determinate in the interface convolution process of craze matter production will also be quite sensitive to micro-structural detail of phase distribution in block copolymers. The appUed stress = Y ... 

Microstructure of crazes in thin films by transmission electron microscope. In thin PS films e (craze initiation) =1% Fibrils of 25-50 nm are a common feature of the microstructure of crazes in cast and bulky films. Major fibrils diameter = 20-30 nm. Minor fibrils connecting major fibrils have a diameter >10 nm and they tend to orient normally to major fibrils. At large total deformation at low e, there is a gradual transition from coarse to fine micro structure of crazes. Beahan, Bevis Hull (1151 PS... 

Farrar, N. R., and E. J. Kramer, Micro structure and Mechanics of Crazes in Oriented... 

When the stress that can be bom at the interface between two glassy polymers increases to the point that a craze can form then the toughness increases considerably as energy is now dissipated in forming and extending the craze structure. The most used model that describes the micro-mechanics of crazing failure was proposed by Brown [8] in a fairly simple and approximate form. This model has since been improved and extended by a number of authors. As the original form of the model is simple and physically intuitive it will be described first and then the improvements will be discussed. 

For these reasons, PMMA and its maleimide and glutarimide copolymers represent very suitable materials for investigating the effect of the chemical structure and of the solid state molecular motions on the plastic deformation, the occurrence of the various micro-mechanisms of deformation (chain scission crazes, shear deformation zones, chain disentanglement crazes), as well as the fracture behaviour. 

The EPDM particles were formd either to initiate crazes or to terminate them, depending on the interfacial bonding, the particle size, the concentration, and the interparticle distances. The variation of notched Izod impact strength of PPBC blends with different EPDM concentrations is shown in Fig. 10.16. The fracture surface of a blend with 10 wt% EPDM, as examined under SEM, is shown in Fig. 10.20. The hemispherical embeddings and hoUows (representing the removed EPDM particles) are clearly visible on the fracture surface. The ribbonlike structures visible on the fractured surfaces are probably the micro-shear bands in the blends. 

The stress-strain curves simulate a homogeneous deformation process of the polymer. However, on the microscale above the linear part of the stress-strain curve (see Fig. 1.15, curves (b), (c), (d)), localized heterogeneous deformation mechanisms occur. Depending on the polymer chemical structure and entanglement molecular weight Mg and on the deformation conditions (temperature and strain rate), several types of heterogeneous deformation are observed micro plastic zones, crazes, deformation zones, and shear bands. Their main features are sketched in Fig. 1.18. 

Micro plastic zones occur even in the brittle fracture of polymers in front of the crack tip. Crazes are localized bands of plastically deformed polymer material, which always appear perpendicular to the stretching direction. They are constituted hy polymer fibrils of about 5 -15 nm diameter, which are stretched in the loading direction and separated by elongated voids with diameters up to about 50 nm. The craze-bulk interface is relatively sharp and only about 10 nm thick. Crazing is connected with volume increase of the material. In Part II, Figs. 1.4 and 1.5 and those figures that follow show typical examples of crazes in PS. Crazes in other polymers can also possess a coarser internal structure. 

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