Brittle system

Under compression or shear most polymers show qualitatively similar behaviour. However, under the application of tensile stress, two different defonnation processes after the yield point are known. Ductile polymers elongate in an irreversible process similar to flow, while brittle systems whiten due the fonnation of microvoids. These voids rapidly grow and lead to sample failure [50, 51]- The reason for these conspicuously different defonnation mechanisms are thought to be related to the local dynamics of the polymer chains and to the entanglement network density. 

Acid anhydride hardeners, in general, result in brittle systems after curing with a relatively high level of gas evolution on irradiation. 

At the same time it was assumed that interaction of the amino groups of the silane should act as additional adhesion promotor to the polyimide surface. The use of the y-aminopropyl methyl triethoxy silane together with diepoxides (Araldite GY 266) did not improve the brittleness remarkably. The use of AMDES, however, should lead to less brittle systems due to the only two-dimensional crosslinking ability of the diethoxy silane. The basic structure formation features are given in Fig. 10. 

The mechanical alloying of mixtures of ductile and brittle components results in the brittle particles being trapped at the layered welded interfaces of the ductile component (Benjamin 1970, Maurice and Courtney 1994). Continued milling results in fi-acture of the brittle particles and the development of a uniform distribution of particles of the brittle phase within the matrix of the ductile phase. This is in contrast to the distribution in conventional powder systems where the dispersoid particles are confined to the prior particle boundaries. Mechanically alloyed CDS alloys are an important practical example of a ductile/brittle system. If the volume fraction of the brittle phase is of the order of 0.5, the characteristic layered structure does not form. Rather, the microstructure consists of a uniformly distributed nanocrystalline mixture of the two phases (Schaffer and McCormick 1990a). 

Ductility of one of the constituent powders is not a requirement for mechanical alloying to occur. A number of brittle/brittle systems have been shown to form solid solutions of intermetallic compounds during milling (Davis et al. 1988, Davis and Koch 1987). In contrast to the layered morphology exhibited by ductile systems, brittle/brittle systems develop a granular morphology. While the alloying mechanism is not well understood with brittle systems it is evident that material transfer between the components plays an important role (Davis et al. 1988). 

As discussed in Section 2.0 (Exploration), the earth s crust is part of a dynamic system and movements within the crust are accommodated partly by rock deformation. Like any other material, rocks may react to stress with an elastic, ductile or brittle response, as described in the stress-strain diagram in Figure 5.5. 

The quality of bonding is related direcdy to the size and distribution of solidified melt pockets along the interface, especially for dissimilar metal systems that form intermetaUic compounds. The pockets of solidified melt are brittle and contain localized defects which do not affect the composite properties. Explosion-bonding parameters for dissimilar metal systems normally are chosen to minimize the pockets of melt associated with the interface. 

The dynamic mechanical properties of VDC—VC copolymers have been studied in detail. The incorporation of VC units in the polymer results in a drop in dynamic modulus because of the reduction in crystallinity. However, the glass-transition temperature is raised therefore, the softening effect observed at room temperature is accompanied by increased brittleness at lower temperatures. These copolymers are normally plasticized in order to avoid this. Small amounts of plasticizer (2—10 wt %) depress T significantly without loss of strength at room temperature. At higher levels of VC, the T of the copolymer is above room temperature and the modulus rises again. A minimum in modulus or maximum in softness is usually observed in copolymers in which T is above room temperature. A thermomechanical analysis of VDC—AN (acrylonitrile) and VDC—MMA (methyl methacrylate) copolymer systems shows a minimum in softening point at 79.4 and 68.1 mol % VDC, respectively (86). 

In cases where the copolymers have substantially lower glass-transition temperatures, the modulus decreases with increasing comonomer content. This results from a drop in crystallinity and in glass-transition temperature. The loss in modulus in these systems is therefore accompanied by an improvement in low temperature performance. However, at low acrylate levels (< 10 wt %), T increases with comonomer content. The brittle points in this range may therefore be higher than that of PVDC. 

---