Does Sample Strain Decrease At Failure?

[person123]

TL;DR

Say a sample undergoes uni-axial tension. It undergoes first linear and then plastic deformation and finally fails. Does the deformation of the sample decrease at failure?

My guess is that the deformation immediately before would be the sum of elastic and plastic deformation, and the deformation after would be just the plastic deformation, and it therefore would decrease. Is this correct?

 

[jrmichler]

A typical uniaxial tension test is done at constant velocity, so the rate of strain is constant. Immediately after fracture, the stress in the sample goes to zero, so the elastic strain goes to zero while the plastic strain stays where it was at fracture.

Note that strain is a more specific term than deformation. It is more correct in this case.

 

[person123]

Note that strain is a more specific term than deformation. It is more correct in this case.

Could you explain that a bit more? What would the inaccuracies be about discussing it in terms of deformation instead of strain?

 

[Astronuc]

Could you explain that a bit more? What would the inaccuracies be about discussing it in terms of deformation instead of strain?

See - http://www.engineeringarchives.com/les_mom_truestresstruestrainengstressengstrain.html

Deformation (displacement) is dimensional, while strain is a dimensionless ratio. At some point, one has to apply a dimension to strain.

During a test under load, a material will deform (strain), which is includes elastic and plastic (permanent) deformation. When a specimen breaks, the material will 'snap back', i.e., giving back the elastic deformation (strain) and leaving the plastic or permanent deformation.

Note that most materials are used well below yield strength, so as to prevent any possibility of exceeding yield during an overload condition (service transient). As stress in a material approaches yield, especially where service temperature becomes greater than about 0.35 homologous temperature, creep becomes an issue, and a designer must consider creep, which is a slow permanent deformation of a material. Think of high temperature components such as gas-fired turbine blades.

There is some convention regarding creep and flow of a material, but I'm not sure how universal it is. Creep usually refers to slow permanent deformation under load where the stress is below yield. Flow refers to a slow, but some what faster permanent deformation when the stress is between yield and ultimate tensile.

Some tensile testing may be conducted at a faster rate up to some level below yield, or proportional limit, then at a slower rate for the remainder of the test. Also, some programs have looked at ranges of strain rate, since strain rate does affect the recorded value of yield and ultimate tensile strength, especially at high temperature.