Stress cluster formation

Fe or Ni ion irradiation with energies of a few MeV is mostly used for studies of radiation damage of RPV steels. Previous studies have provided information on the effects of Cu, Mn and other elements, carbides, dose rate and tensile stress on hardness, matrix damage evolution and solute cluster formation in model alloys and commercial steels (e.g. Fujii and Fukuya, 2005 Murakami et al., 2009). These data provide clear evidence of the effects of various metallurgical parameters on hardening and microstructural evolution in Fe-based alloys and RPV steels, although these data cannot be directly or quantitatively correlated to data from neutron-irradiated materials. 

It is shown that solute atoms differing in size from those of the solvent (carbon, in fact) can relieve hydrostatic stresses in a crystal and will thus migrate to the regions where they can relieve the most stress. As a result they will cluster round dislocations forming atmospheres similar to the ionic atmospheres of the Debye- Huckel theory ofeleeti oly tes. The conditions of formation and properties of these atmospheres are examined and the theory is applied to problems of precipitation, creep and the yield point."... 

It is important to note that we assume the random fracture approximation (RPA) is applicable. This assumption has certain implications, the most important of which is that it bypasses the real evolutionary details of the highly complex process of the lattice bond stress distribution a) creating bond rupture events, which influence other bond rupture events, redistribution of 0(microvoid formation, propagation, coalescence, etc., and finally, macroscopic failure. We have made real lattice fracture calculations by computer simulations but typically, the lattice size is not large enough to be within percolation criteria before the calculations become excessive. However, the fractal nature of the distributed damage clusters is always evident and the RPA, while providing an easy solution to an extremely complex process, remains physically realistic. 

Statistical analysis of the microbial communities associated with the root tips of iron stressed and nonstressed plants revealed the formation of distinct communities in response to plant iron nutritional status (Fig. 5B and C). Communities associated with the older root parts clustered similarly, whereas sites of lateral root emergence and nongrowing root tips differentiated to a lesser degree than the communities associated with the rapidly growing primary root tips (data not shown). 

Regardless of these short-ranged cohesive forces, the formation and stability of particle clusters in a fluidized bed appears to be a multistep process [27], Some shear (as in two particles grazing each other) may be needed to promote collisional cooling, but less than that perhaps in the dense emnlsion of a fluidized bed. Perhaps the lower particle concentration in a babble provides the environment where clnster stability is promoted for the smaller particles. Collisional stresses in the emnlsion may be too high and the cohesive forces may be too low to have long-lasting particle clusters. Indeed, the only evidence of particle clnsters in fluidized beds offered here is that the clusters are located near the bubbles. 

Fig. 21 Proposed mechanisms of lipoplex formation (a) vesicle titration (DNA initially in excess) -DNA coats the vesicle surfaces as the latter are added to the DNA solution - with increase of the vesicle concentration, clusters of DNA-coated vesicles form and consequently rupture (b) DNA titration (lipid initially in excess) - DNA encounters with bare membranes result in vesicle associations - vesicle-DNA-vesicle adhesion generates stresses, which lead to vesicle rupture, followed by continued aggregation and growth of the complex upon further addition of DNA. (reproduced with permission from [67] copyright (2000) Biophysical Society)...

Lourie, O., Wagner, H.D., Evidence of stress transfer and formation of fracture clusters in carbon nanotube-based composites, Comp. Sci. and Techn., 59, 1999, 975-977. 

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