determining tensile test offset yield strengths using extensometer
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7/28/2019 Determining Tensile Test Offset Yield Strengths Using Extensometer
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Determining Tensile Test Offset Yield Strengths using Extensometer
Posted by Marc Venet on Fri, Jan 15, 2010 @ 12:16 PM
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 theonset 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 regionis prior to the proportional limit (see point A - Figure 1). Beyond the proportional limit, the material undergoes permanent plasticdeformation.
The offset yield strength is reported as a stress (psi, MPa, etc) and is defined as the point where a line drawn parallel to themodulus line intersects the stress-strain curve (see point B and line XB - Figure 1). Offset distance, 0X, in Figure 1 is the productof sample gage length and percent offset. Offset distance, 0X, for example would be 0.004 in for a 2 in gage length sample at0.2% 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 modulusof 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, thecalculated 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 theresult of misapplication of the extensometer to the sample and/or extensometer slippage during the test.
1.
The extensometer collecting strain data was removed from the test specimen before sufficient strain data could be collectedto satisfy the specified offset yield.
2.
Specimen slipping in grips.3.
See images below for an example of discontinuity causing a bad offset yield value and a fix for this problem:
mining Tensile Test Offset Yield Strengths using Extensometer http://info.admet.com/blog/bid/25956/Determining-Tensile-Test-O
5/23/2013
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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.1.Select points for modulus calculation in the linear part of the curve.2.
Figure 3 - Logging threshold was raised to eliminate discontinuity.
All ADMET testing systems are fully customizable. If you have any questions regarding an application or system, please call800-667-3220, [email protected], or fill anAsk an Engineerform.
Tags: Troubleshooting, Testing Tips
mining Tensile Test Offset Yield Strengths using Extensometer http://info.admet.com/blog/bid/25956/Determining-Tensile-Test-O
5/23/2013
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7/28/2019 Determining Tensile Test Offset Yield Strengths Using Extensometer
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mining Tensile Test Offset Yield Strengths using Extensometer http://info.admet.com/blog/bid/25956/Determining-Tensile-Test-O
5/23/2013