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Mechanical properties and fracture

For materials such as aluminum alloys that show no distinct yield point, which correctly describes how the 0.2% offset yield strength is determined?

  1. ARead the stress on the stress–strain curve directly at the point where the strain is 0.2%
  2. BTake the stress where a line parallel to the elastic line, offset by a strain of 0.2%, intersects the stress–strain curve
  3. CTake the stress equal to 0.2% of the tensile strength obtained from the maximum load
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Answer: B. Take the stress where a line parallel to the elastic line, offset by a strain of 0.2%, intersects the stress–strain curve

The 0.2% offset yield strength is defined as the stress that leaves a permanent (plastic) strain of 0.2% when the load is removed. On the stress–strain curve, shift the elastic line by 0.002 along the strain axis and read the stress where it meets the curve. Reading the stress at a strain of 0.2% directly differs from the definition because that strain includes elastic strain, and it gives a lower value. The maximum load corresponds to the tensile strength, a different quantity from the offset yield strength. For materials such as mild steel that show distinct upper and lower yield points, the yield point value is used.

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