An extensometer (also called an Instron or tensometer) exerts a force on an object while recording deflection (deformation) using a strain gage. Strain gages indicate deflection by measuring electrical resistance through a small metal beam attached to your sample. As the length of the beam increases, electrical resistance increases. This setup allows measurement of a sample's stiffness, extensibility and strength at breaking.
The steeper the stress/strain relationship (the higher Young's Modulus, E), the more stiff the material. Breaking strain and stress are the amounts of strain and stress observed at the breaking point. The more ductile the material, the more strain at breakage; the stronger the material, the higher the breaking stress.
Ductile materials will deform more than stiff materials before breaking; however, ductile materials are not usually as strong a stiff materials. Compare, for example, a brittle object, say beach glass, and a ductile object, perhaps a rubber chicken. Glass may be strong (high breaking stress) but it is not ductile (low strain (i.e., little deformation) at breaking). A rubber chicken extends quite a bit before it breaks (long live rubber chickens) but can not withstand as much stress.
This discussion only covers linearly elastic materials for which stress and strain are directly proportional. Most materials are only linearly elastic for low stresses and strains. Strain hardening, necking and plastic (unrecoverable) deformation are just some of the other interesting processes which complicate analyses of material properties. Since failure must occur through the process of crack propagation, you might want to consider how irregularities in a material may promote or slow crack growth. (See Stressed Out in 3-D.)
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copyright Elizabeth Nelson, Judith Connor 1999, 2000 Non-profit educational uses permitted.