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Let's assume that in a metal sample without any dislocation, there is a void. Then we conduct a uniaxial tensile test.

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  • $\begingroup$ Probably lower yield and tensile strengths? $\endgroup$
    – Algo
    Oct 18, 2020 at 12:46
  • $\begingroup$ Big void or little void? For big voids , specification will list the maximum size and quantity of permitted voids ( that will not prevent a part from functioning). $\endgroup$ Jul 28, 2021 at 20:55

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My opinion:

When there is void in cross sectional area of the testing sample, cross sectional area of the material is smaller. The strength of the material itself doesn't change, but the ultimate force and rupture force will be reduce due to the void. This causes by reduction of the cross sectional area of testing specimen.

If you consider the void and the metal as a composite material, then the material should have lower yield point and lower rupture point.

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What happens in dislocations depends very much on the structure of the material. For example in BCC materials (e.g. Iron) what you get when you have a dislocation and you apply stress is the following:

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This is mainly responsible for the presense of a yield point in the stress curve strain.

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In a tensile test, it’s assumed that the sample has a circular cross-sectional area. The force applied to the sample is normalized by this area to obtain a stress. As previously noted, a void in the center of this sample will decrease the actual cross-sectional area, causing the sample to fail with a smaller applied force. If this force is normalized by the nominal cross-sectional area, it will appear that the sample had a lower yield strength than a fully dense sample.

If you assume dislocations are active, the material will begin work-hardening once plastic deformation begins. The stress-strain curve will slope upwards after the yield point, as increasing strain generates additional dislocations and strengthens the material. If dislocations are assumed to be inactive, the material is “perfectly plastic”, and the stress-strain curve will be a horizontal line starting at the yield point.

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  • $\begingroup$ Tubulars and flat stock ( to about one inch thickness ) have rectangular test sections. $\endgroup$ Mar 31, 2021 at 0:25
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In theory, the stress-strain curve should be identical for bars with and without void (hole). However, some minor variant is expected because of the different stress patterns at the critical crosssection that inevitably affect the measurement.

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Note: The typical tensile test specimen is as shown below. Pay attention to the statement below the figure.

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