Strain

Johann Maier, winter semester 2015/16

Introduction and Basics

Strain is the response of a system to an applied stress. When a material is loaded with a force, it produces a stress, which then causes a material to deform. Engineering strain is defined as the amount of deformation in the direction of the applied force divided by the initial length of the material. This results in a unitless number [%] although it is often left in the unsimplified form, such as [milli]meters per [milli]meter. For example, the strain in a bar that is being stretched in tension is the amount of elongation or change in length divided by its original length. ^[1]

$//<![CDATA[ \begin{array}{l}Strain, ε=Elongation/(Original Length)=ΔL/L_0\end{array} //]]>$

If the stress is small, the material may only strain a small amount and the material will return to its original size after the stress is released. This is called elastic deformation, because like elastic it returns to its unstressed state. Elastic deformation only occurs in a material when stresses are lower than a critical stress called the yield strength. If a material is loaded beyond it elastic limit, the material will remain in a deformed condition after the load is removed. This is called plastic deformation. ^[1]

Measurement

The most common technique in strain measurement are strain gauges.

Strain gauge

A strain gauge (or strain gage) is the most common device used to measure strain on an object. Invented by Edward E. Simmons and Arthur C. Ruge in 1938, the most common type of strain gauge consists of an insulating flexible backing which supports a metallic foil pattern. The gauge is attached to the object by a suitable adhesive […]. As the object is deformed, the foil is deformed, causing its electrical resistance to change. This resistance change, usually measured using a Wheatstone bridge, is related to the strain by the quantity known as the gauge factor. ^[2]

Literature

1. https://www.nde-ed.org/EducationResources/CommunityCollege/Materials/Mechanical/StressStrain.htm
2. https://en.wikipedia.org/w/index.php?title=Strain_gauge&oldid=690172261

January 27th, 2016