Testing Talk: Tensile strengths

Typical points of interest when tensile testing a material include ultimate tensile strength or peak stress; modulus of elasticity, offset yield strength, which represents a point just beyond the onset of permanent deformation; and elongation at break.

The modulus of elasticity is calculated as the slope in the linear elastic region of the stress-strain curve. The linear elastic region is prior to the proportional limit (see point A – Figure 1). Beyond the proportional limit, the material undergoes permanent plastic deformation.

The offset yield strength is reported as a stress (psi, MPa, etc) and is defined as the point where a line drawn parallel to the modulus line intersects the stress-strain curve (see point B and line XB – Figure 1). Offset distance, 0X, in Figure 1 is the product of sample gauge length and percent offset. Offset distance, 0X, for example would be 0.004 in for a 2 in. gauge length sample at 0.2% offset (0X = 2 in. x 0.002 = 0.004 in.).

Successful reporting of the offset yield value is therefore dependent on how well the testing program has determined the modulus of elasticity.

Tensile tests sometimes produce stress-strain curves that do not follow the "classic" shape shown in Figure 1. As a result, the calculated slope of the modulus line may be incorrect, leading to an invalid offset yield strength.

Reasons for invalid offset yield strength values:

1. Non-linearities and or discontinuities in the linear elastic region of the stress-strain curve. These anomalies may be the result of misapplication of the extensometer to the sample and/or extensometer slippage during the test.
2. The extensometer collecting strain data was removed from the test specimen before sufficient strain data could be collected to satisfy the specified offset yield.
3. Specimen slipping in grips.

See images below for an example of discontinuity causing a bad offset yield value and a fix for this problem:

Figure 2: Extensometer slipping during the initial loading causing a distinct change in slope

Things you can do with this type of curve to get valid offset yield strength (see Figure 3): Raise your data logging threshold so that the discontinuity in the initial part of the stress-strain curve is not reported. Select points for modulus calculation in the linear part of the curve.

Figure 3: Logging threshold was raised to eliminate discontinuity

For more information, call 1-800-667-3220, visit ADMET or email info@ADMET.com.

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