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 gage length and percent offset. Offset distance, 0X, for example would be 0.004 in for a 2 in gage 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.
Figure 1 - ENGINEERING STRESS-STRAIN CURVE
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:
- 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.
- The extensometer collecting strain data was removed from the test specimen before sufficient strain data could be collected to satisfy the specified offset yield.
- 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.