Failure modes stress relaxation

Several cautions are, however, in order. Polymers are notorious for their time dependent behavior. Slow but persistent relaxation processes can result in glass transition type behavior (under stress) at temperatures well below the commonly quoted dilatometric or DTA glass transition temperature. Under such a condition the polymer is ductile, not brittle. Thus, the question of a brittle-ductile transition arises, a subject which this writer has discussed on occasion. It is then necessary to compare the propensity of a sample to fail by brittle crack propagation versus its tendency to fail (in service) by excessive creep. The use of linear elastic fracture mechanics addresses the first failure mode and not the second. If the brittle-ductile transition is kinetic in origin then at some stress a time always exists at which large strains will develop, provided that brittle failure does not intervene. 

For example, the most common failure modes for springs are fracture due to fatigue and excessive load stress relaxation. The reliability of a spring will therefore depend on the material, design characteristics and the operating environment. NSWC-94/L07 models attempt to predict spring reliability based on these input characteristics. 

The work discussed here has related not only to structure relationships but also to means of protection of the macromolec-ular material and the protective functions of these materials. There are many modes of failure, by chemical reaction, failure by fracture, environmental stress cracking and creep. Further there are complicating interactions arising from chemical reaction during relaxation of polymer networks, and in multiphase polymer systems and cos osites, failure at interfaces by adhesive failxire or stress-stress dilation. 

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