Are you testing for upper yield strength, but not seeing a “dip” in your stress/strain curve? This is often the result of using improper test control parameters. During yielding, the strain rate needs to be as constant as possible. This is best achieved by using crosshead position or strain control*.
For example, if you run a test in stress control at the onset of yielding, the testing machine will accelerate to maintain the desired stress rate. Incorrectly running in load control causes unwanted acceleration. This prevents the stress from dropping relative to the increase in strain. As a result, the upper yield strength calculation will fail because it can’t find the dip in the stress-strain curve (a zero or negative slope).
To correct this situation, set up the test to use stress control during the first half of the elastic portion. Prior to the onset of yielding, switch to either position or strain control. We have software packages that are designed to allow for control transition.
*Additional help for determining the allowable test rates during yielding can be found in the following Testing Solutions: ASTM E8, ISO 6892 or EN10002.
For example, if you run a test in stress control at the onset of yielding, the testing machine will accelerate to maintain the desired stress rate. Incorrectly running in load control causes unwanted acceleration. This prevents the stress from dropping relative to the increase in strain. As a result, the upper yield strength calculation will fail because it can’t find the dip in the stress-strain curve (a zero or negative slope).
To correct this situation, set up the test to use stress control during the first half of the elastic portion. Prior to the onset of yielding, switch to either position or strain control. We have software packages that are designed to allow for control transition.
*Additional help for determining the allowable test rates during yielding can be found in the following Testing Solutions: ASTM E8, ISO 6892 or EN10002.
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