Molecular basis fracture

Washington, D.C., 20th-24th Aug.2000, p.461-2 MOLECULAR BASIS OF FRACTURE STUDIES... 

Table 11.7 Molecular basis of fracture at block copolymer and homopolymer interfaces...

Ultimately, fracture results from the breaking of atomic bonds. For a brittle solid, the balancing of the energy release rate G and the dissipative processes associated with the creation of new free surface is played out explicitly on an atom by atom basis if one carries out a molecular dynamics simulation of the relevant atomic-level processes. A number of calculations illustrate the level to which such calculations can be pushed using parallel versions of molecular dynamics codes. An especially beautiful sequence of snapshots from the deformation history of a solid undergoing fracture is shown in fig. 12.33. The key point illustrated by... 

The ability of certain tight shales to sieve petroleums on a molecular size basis is considerable. Numerous examples of such fractionative behaviour were described by Karlsen et al. (1997, 1988), i.e. more effective loss of hydrocarbons with small kinetic diameters like methane and ethane (4.4 A) compared with propane (with a length of 6.5 A and a width of 4.9A), not to mention cyclohexane with a diameter of 6.0 A (Breck 1974). Fractionative depletion trends, from a Permian Ravnefjeld Formation shale, towards local fractures and the surface were evident for all molecules apart from methane, which even in this shale showed the ability to move effectively through the whole rock profile, equalizing concentrations, irrespective of local fractures (Karlsen 1987 Karlsen et al. 1988). 

Recently there has been considerable controversy concerning the possibility of scission in the partially stretched state 15, 16). One suggestion, on the basis of Ryskin s yo-yo model IT), is that scission could occur in transient flows. A theoretical treatment has predicted a different molecular weight dependence and a much higher absolute value for critical fracture strain rate and it has predicted central scission less precisely. We have... 

It has long been recognized that oriented polymers (i.e. fibres) are much less strong than would be predicted on the basis of elementary assumptions that fracture involves simultaneously breaking the bonds in the molecular chains across... 

Hence, the offered techniques of limiting mechanical characteristics calculation allow stress and strain of fracture (strength and deformability) estimation on the basis of two independent parameters only and S. Let us remind, that value r is determined by the ratio of two molecular characteristics [48] ... 

It has long been recognised that oriented polymers (i.e. fibres) are much less strong than would be predicted on the basis of elementary assumptions that fracture involves simultaneously breaking the bonds in the molecular chains across the section perpendicular to the applied stress. Calculations of this nature were originally undertaken by Mark [86] and rather more recently by Vincent [87] on polyethylene. It was found that in both cases the measured tensile strength was at least an order of magnitude less than that calculated. 

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