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Creep in metals

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Title: Creep in metals
Author(s): Saarinen, Juho
Contributor: University of Helsinki, Faculty of Science, Department of Physics
Discipline: Physics
Language: English
Acceptance year: 2013
Creep is time dependent plastic malformation of solids, that happen in static stress and temperature when threshold values are met. Creep occurs at high temperature, meaning temperature more than 30% of material's absolute melting temperature (this limit is a little lower with plastics, and higher in ceramics). The malformations it causes can lead to rupture, which usually happen in a short time compared to the duration of the whole process. The creep effect itself is known from already the 19th century, and for metals it's quite clear that diffusion is always present in creep (Coble and Nabarro-Herring creep), and that dislocations can increase the rate of creep strain. Effects of creep can be seen e.g. in power plants and engines, where turbine blades, turbines, pipes and vessels are all the time at high temperature and stress. Also creep relaxation is often 'loosening' bolts which needs to be retightened. In regular office creep can be seen in paper clips, especially in plastic ones, which relax and lose grip fast because of the low melting point of plastics. Creep, because it usually needs long time to be visible, has been part of accidents, too, e.g. in '9/11'. Creep appears in 3 stages (primary (transient), secondary (steady-state), tertiary), and depending on the application, either secondary or tertiary is the most important one. The secondary creep is important for displacement-, buckling- and relaxation-limited situations, and tertiary for the rupture-limited ones.

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