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Fracture in polymers
Polymer fracture is the study of the fracture surface of an already failed material to determine the method of crack formation and extension in polymers both fiber reinforced and otherwise.〔John Scheirs, “john wiley and sons”, 30-oct-2000 “(and Failure Analysis of Polymers: A Practical Approach )”〕 Failure in polymer components can occur at relatively low stress levels, far below the tensile strength because of four major reasons: long term stress or creep rupture, cyclic stresses or fatigue, the presence of structural flaws and stress-cracking agents.
Formations of submicroscopic cracks in polymers under load have been studied by x ray scattering techniques and the main regularities of crack formation under different loading conditions have been analyzed. The low strength of polymers compared to theoretically predicted values are mainly due to the many microscopic imperfections found in the material. These defects namely dislocations, crystalline boundaries, amorphous interlayers and block structure can all lead to the non-uniform distribution of mechanical stress.
==Long term stress or Creep Failure==
Taking into account the viscoelastic path at small strain based on thermally activated rate processes. When strain attains higher values, high enough to lead to failure, its slope versus time exhibits an abrubt change. At this specific time the creep function appears a minimum.〔G. Spathis, E. Kontou, “Creep failure time prediction of polymers and polymer composites”〕 In most cases DMTA (Dynamic mechanical thermal analysis) can be used to determine the viscoelastic behavior of samples as a function of time. A classic case is when the rubber hose ruptures due to creep after many years of service. DMTA can be used for o-rings and gaskets to measure the creep rates.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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