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In the second case, the most significant factor, which is determined by the temperature, is the mobility of the structural defects such as grain boundaries, point vacancies, line and screw dislocations, stacking faults and twins in both crystalline and non-crystalline solids. The movement or displacement of such mobile defects is thermally activated, and thus limited by the rate of atomic diffusion. <ref name="Dav">Davidge, R.W., '''Mechanical Behavior of Ceramics''', Cambridge Solid State Science Series, Eds. Clarke, D.R., et al. (1979)</ref><ref name="Zar">Zarzycki, J., '''Glasses and the Vitreous State''', Cambridge Solid State Science Series, Eds. Clarke, D.R., et al.(1991)</ref>

The concept of a [[rigid body]] can be applied if the deformation is negligible.

 

 

Depending on the type of material, size and geometry of the object, and the forces applied, various types of deformation may result. The image to the right shows the engineering stress vs. strain diagram for a typical ductile material such as steel. Different deformation modes may occur under different conditions, as can be depicted using a [[deformation mechanism map]].

 

 

 

Linear elastic deformation is governed by [[Hooke's law]], which states:

Note that not all elastic materials undergo linear elastic deformation; some, such as concrete, gray cast iron, and many polymers, respond in a nonlinear fashion. For these materials Hooke's law is inapplicable.<ref>Fundamentals of Materials Science and Engineering, William D. Callister, John Wiley and Sons, 2nd International edition (September 3, 2004), ISBN 0-471-66081-7, ISBN 978-0-471-66081-1, p.184</ref>

 

This type of deformation is irreversible. However, an object in the plastic deformation range will first have undergone elastic deformation, which is reversible, so the object will return part way to its original shape. Soft [[thermoplastics]] have a rather large plastic deformation range as do ductile metals such as [[copper]], [[silver]], and [[gold]]. [[Steel]] does, too, but not [[cast iron]]. Hard thermosetting plastics, rubber, crystals, and ceramics have minimal plastic deformation ranges. One material with a large plastic deformation range is wet [[chewing gum]], which can be stretched dozens of times its original length.

 

Under tensile stress, plastic deformation is characterized by a [[strain hardening]] region and a [[necking (engineering)|necking]] region and finally, fracture (also called rupture). During strain hardening the material becomes stronger through the movement of [[dislocation|atomic dislocations]]. The necking phase is indicated by a reduction in cross-sectional area of the specimen. Necking begins after the ultimate strength is reached. During necking, the material can no longer withstand the maximum stress and the strain in the specimen rapidly increases. Plastic deformation ends with the fracture of the material.

 

Another deformation mechanism is [[metal fatigue]], which occurs primarily in [[ductile]] metals. It was originally thought that a material deformed only within the elastic range returned completely to its original state once the forces were removed. However, faults are introduced at the molecular level with each deformation. After many deformations, cracks will begin to appear, followed soon after by a fracture, with no apparent plastic deformation in between. Depending on the material, shape, and how close to the elastic limit it is deformed, failure may require thousands, millions, billions, or trillions of deformations.

 

Metal fatigue has been a major cause of aircraft failure, especially before the process was well understood (see, for example, the [[De Havilland Comet#Accidents and incidents|De Havilland Comet accidents]]). There are two ways to determine when a part is in danger of metal fatigue; either predict when failure will occur due to the material/force/shape/iteration combination, and replace the vulnerable materials before this occurs, or perform inspections to detect the microscopic cracks and perform replacement once they occur. Selection of materials not likely to suffer from metal fatigue during the life of the product is the best solution, but not always possible. Avoiding shapes with sharp corners limits metal fatigue by reducing stress concentrations, but does not eliminate it.

 

 

Usually, compressive stress applied to bars, [[column]]s, etc. leads to shortening.

 

In long, slender structural elements — such as columns or [[truss]] bars — an increase of compressive force ''F'' leads to [[structural failure]] due to [[buckling]] at lower stress than the compressive strength.

 

{{See also|Concrete fracture analysis|Fracture mechanics}}

This type of deformation is also irreversible. A break occurs after the material has reached the end of the elastic, and then plastic, deformation ranges. At this point forces accumulate until they are sufficient to cause a fracture. All materials will eventually fracture, if sufficient forces are applied.

 

A popular misconception is that all materials that bend are "weak" and those that don't are "strong." In reality, many materials that undergo large elastic and plastic deformations, such as steel, are able to absorb stresses that would cause brittle materials, such as glass, with minimal plastic deformation ranges, to break.<ref>{{Cite book|title=Structural glass|url=https://books.google.com/books?id=7t9wgJEUWHYC&pg=PA33|page=33|author=Peter Rice, Hugh Dutton|publisher=Taylor & Francis|isbn=0-419-19940-3|year=1995}}</ref>

 

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*[[Artificial cranial deformation]]

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{{Reflist}}

 

*[http://virtualexplorer.com.au/special/meansvolume/contribs/jessell/glossary/defmech.html Table of deformation mechanisms and processes]