ISSN:
0021-8995
Keywords:
Chemistry
;
Polymer and Materials Science
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
,
Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
,
Physics
Notes:
The relationship between biaxial stress-rupture behavior and polymer morphology has been investigated for a series of compression-molded disks of linear polyethylene. Rupture data were obtained over a range of temperatures on polymers of several melt indices that had been solidified at two rates. Two failure mechanisms were observed: one at high stress levels, which was ductile; the other at low stress levels, which was brittle. The stress level at which the mechanism changed from ductile to brittle decreased as the measurement temperature increased, as the melt index of the polymer increased, and as the rate or solidification decreased. It was shown that initial pressurization of the disks causes the formation of microscopic surface fissures along spherulite boundaries as well as within individual spherulites. The extensive growth of these fissures at high stress levels sufficiently raises the stress on the sound portions of the polymer to cause large-scale macroscopic drawing of the spherulites. At low stress levels the initially formed fissures grow by a localized drawing process at their ends. A macroscopic crack forms by a chance coalescence of a number of individually growing fissures. This process is more rapid in disks of polymer that form large spherulites when cooled slowly from the melt. The spherulite size in disks of polymer of low melt index is much smaller under the same conditions of cooling and is less sensitive to the cooling rate than in disks of polymer of high melt index. Annealing of rapidly cooled disks whose microstructure contains relatively small spherulites reduces the stress at which the failure mechanism changes without appreciably altering the spherulite size. Oxidation of the polymer also reduces the transition stress.
Additional Material:
10 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/app.1964.070080435
Permalink
