585 research outputs found
Provisions for the enrichment of the curriculum of small Massachusetts high schools
Thesis (Ed.M.)--Boston University, 1938. This item was digitized by the Internet Archive
Similarity between craze morphology and shear-band morphology in polystyrene
The formation of shear bands and crazes in thin films as well as in bulk samples of polystyrene were examined in the electron microscope using a variety of replication techniques. The morphologies of shear bands and crazes are quite similar both depending initially upon the relative shear displacement of 400 to 1000 Ă
domains. As deformation continues and orientation increases, fibrils varying from 50 to 700 Ă
are formed within the deformation zone, lateral constraint of the normal Poisson contraction causing voids to form in the crazes but not in the shear bands. Shear-band width was found not to be a unique function of either temperature or strain-rate and both craze and shear-band morphologies were found not to be strong functions of molecular weight. Regardless of molecular weight, fibrils formed within the deformation zone were always on the order of a few hundred Angstroms in diameter. However, for thin films of molecular weight less than 20 000 insufficient numbers of tie molecules between fundamental structural units or domains made it difficult for these fibres to span the craze width.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/44785/1/10853_2004_Article_BF00632758.pd
Continuum field description of crack propagation
We develop continuum field model for crack propagation in brittle amorphous
solids. The model is represented by equations for elastic displacements
combined with the order parameter equation which accounts for the dynamics of
defects. This model captures all important phenomenology of crack propagation:
crack initiation, propagation, dynamic fracture instability, sound emission,
crack branching and fragmentation.Comment: 4 pages, 5 figures, submitted to Phys. Rev. Lett. Additional
information can be obtained from http://gershwin.msd.anl.gov/theor
Preparative fractionation of a random copolymer (SAN) with respect to either chain length or chemical composition
The possibilities to fractionate copolymers with respect to their chemical
composition on a preparative scale by means of the establishment of
liquid/liquid phase equilibria were studied for random copolymers of styrene
and acrylonitrile (san). Experiments with solutions of san in toluene have
shown that fractionation does in this quasi-binary system, where demixing
results from marginal solvent quality, take place with respect to the chain
length of the polymer only. On the other hand, if phase separation is induced
by a second, chemically different polymer one can find conditions under which
fractionation with respect to composition becomes dominant. This opportunity is
documented for the quasi-ternary system dmac/san/polystyrene, where the solvent
dimethyl acetamide is completely miscible with both polymers. The theoretical
reasons for the different fractionation mechanisms are discussed
Fracture initiation and crack propagation of acrylonitrile-butadiene-styrene (ABS) in organic solvents
The effects of organic liquid environments on the fracture behaviour of acrylonitrile-butadiene-styrene (ABS) have been investigated. Fracture initiation experiments showed that K i 2 , ( K i being the stress intensity factor at crack/craze initiation), could be meaningfully correlated with the solvent solubility parameter ( ÎŽ s ) of the different liquid environments and had a minimum value at ÎŽ s = ÎŽ p , where ÎŽ p was the solubility parameter of ABS. For the range of organic liquids used, hydrogen bonding did not have any significant effects on the correlations. It was demonstrated that the K i 2 â ÎŽ s correlations could also be usefully extended to other materials such as plain and glass-filled polystyrenes. At a common crack speed ( Ă„ ), the fracture toughness ( R ) values in âcrazingâ liquids (i.e. alcohols) were greater than those in âcrackingâ solvents (i.e. acetone, benzene, toluene, etc.) which usually caused a âdissolutionâ effect on the plastic. From crack propagation experiments, and using fracture mechanics analyses, definite R ( Ă„ ) and K c ( Ă„ ) relationships for ABS immersed in toluene, carbon tetrachloride and methanol were determined. These experimental results showed that crack propagation was relaxation controlled and agreed well with a recent theoretical analysis due to Williams and Marshall for environmental crack and craze growth in polymers. Finally, SEM pictures were presented to show the remarkable differences in the fracture morphologies of ABS in both âcrazingâ and âcrackingâ liquid environments.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/44670/1/10853_2004_Article_BF00551442.pd
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