Hello, For measuring the softening kinetics I am planning to do the stress relaxation tests for cylindrical sample on a dilatometer. After the first hit, from my understanding the sample is held at constant strain between the tool and force is measured. How is the softening kinetics realised for a material by this method of force measurement? Thank you.
Dear Akhil,
Under the condition of constant total length (or, equivalently, total strain), the plastic length change of the sample is equal and opposite to any elastic length change, which reduces the stress in the sample. The plastic deformation rate can, therefore, be found as
$\dot\epsilon_p = - \dot\sigma / E_{eff}$
with $E_{eff}$ being the combined stiffness of the sample and the dilatometer. Machine contributions to the stiffness are usually not negligible and should be quantified beforehand by using, for instance, an elastic (ceramic) sample.
Once the relation between plastic deformation rate and stress, i.e. $\dot\epsilon_p(\sigma)$ is established, you can start to analyze this relation and draw your conclusions.
Please clarify your question,, what material? polymer? what stress mode.. tensile?
Im a bit confused.. a dilatometer is usually a device that measures mechanical size difference, the force is there just to keep the sample in contact with the sensor. Not saying that what you mention must be wrong :-)
Generally temperature dependent kinetics may be approached with Arrhenius equation - not sure if you are able to do different temperatures, and if you are interested in activation energy of your process, but the Ar.Eq. is a good starting point.
Thanks a lot @Vit Jan for responding to my question :-) Actually I am working on low carbon steel and doing a compressive load hit using the dilatometer and then force is measured under constant strain of the sample. So according to my understanding, the sample is stressed condition after the compressive hit and the force measurement corresponds to stress relaxation? But I have seen small fluctuations in the load vs time graph. I coudn't reach a conclusion for that effect.
Dear Akhil,
Under the condition of constant total length (or, equivalently, total strain), the plastic length change of the sample is equal and opposite to any elastic length change, which reduces the stress in the sample. The plastic deformation rate can, therefore, be found as
$\dot\epsilon_p = - \dot\sigma / E_{eff}$
with $E_{eff}$ being the combined stiffness of the sample and the dilatometer. Machine contributions to the stiffness are usually not negligible and should be quantified beforehand by using, for instance, an elastic (ceramic) sample.
Once the relation between plastic deformation rate and stress, i.e. $\dot\epsilon_p(\sigma)$ is established, you can start to analyze this relation and draw your conclusions.
Dear Akhil,
Are you using Baehr 805A/D dilatometer ? If your institute has also the optical diameter strain option, you can use the diametrical strain to get rid of the machine compliance. But in that case, you should not have so much barelling in the sample.
Kind Regards,
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