Does Sample Strain Decrease At Failure?

In summary, during a uniaxial tension test, the deformation is a combination of elastic and plastic deformation. After fracture, the stress goes to zero, but the plastic strain remains. Strain is a more accurate term than deformation in this context. Creep and flow refer to slow permanent deformation under different levels of stress. Strain rate can also affect the recorded values of yield and ultimate tensile strength.
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person123
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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?
 
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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.
 
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jrmichler said:
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?
 
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person123 said:
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.
 
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1. What is sample strain and how is it measured?

Sample strain is a measure of the deformation or change in shape of a material when subjected to stress. It is typically measured using strain gauges or extensometers, which can detect even small changes in length or shape of a material.

2. Does sample strain always decrease at failure?

No, sample strain does not always decrease at failure. In some materials, such as ductile metals, the strain may continue to increase until the material eventually breaks. In other materials, such as brittle ceramics, the strain may decrease rapidly before failure.

3. What factors can affect the decrease of sample strain at failure?

The decrease of sample strain at failure can be affected by various factors, such as the type of material, the amount of stress applied, the rate of loading, and the presence of any defects or imperfections in the material.

4. How does sample strain at failure relate to the strength of a material?

The sample strain at failure can provide valuable information about the strength of a material. In general, materials with higher strain at failure tend to be more ductile and have higher toughness, while materials with lower strain at failure tend to be more brittle and have lower toughness.

5. Can sample strain at failure be used to predict the lifespan of a material?

While sample strain at failure can provide important information about the behavior of a material, it cannot be used to accurately predict the lifespan of a material. This is because the lifespan of a material is affected by many other factors, such as environmental conditions and the type of loading it experiences.

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