Nice and easy to understand lecture series of creep deformation!! I have a query , at 21:58 you mentioned steady state creep rate is reduced or delayed in case of constant load tests, but the reason you proposed is because of reduction in cross sectional area. But reduction in cross sectional area and necking including internal void formation is expected to occur in tertiary stage only. Can someone pls clarify?
Dear Mr. Sunando, What I meant is that the secondary creep stage also known as the steady state creep stage is reduced in a constant load test compared to a constant stress test. I am not talking of creep rate. I am only talking of the size of the secondary creep stage. In tensile creep as the material elongates, there is a reduction in cross sectional area for constancy of volume during plastic deformation. Hence if you are doing constant load tests, the uniform reduction in cross-sectional area during elongation will lead to an increase in stress experienced by the sample. Please note that there is a difference between uniform reduction in cross-sectional area and necking. Necking is localized reduction in cross-sectional area. Hence in a constant load test, the strain rates of deformation will continue to increase with increase in sample elongation (creep strain). Hence the chances of attaining a steady state creep rate (constant creep rate) is reduced. Hence secondary creep stage in constant load creep tests is small or reduced. In a constant stress test, one will notice a clear steady state or secondary creep stage. The stress is maintained constant by decreasing the load when the cross-sectional area of the sample reduces. The tertiary creep stage in a constant stress creep test will be observed when necking happens or internal voids start forming. Necking as I mentioned is localized reduction in area. Hope this answers your question
@@0notout Thank you for the explanation . You are absolutely correct. The point I missed is the " uniform reduction in cross sectional area" due to increase in length during secondary creep stage, which is different from " necking".
Nicely Explained Sir
Nice and easy to understand lecture series of creep deformation!! I have a query , at 21:58 you mentioned steady state creep rate is reduced or delayed in case of constant load tests, but the reason you proposed is because of reduction in cross sectional area. But reduction in cross sectional area and necking including internal void formation is expected to occur in tertiary stage only. Can someone pls clarify?
Dear Mr. Sunando, What I meant is that the secondary creep stage also known as the steady state creep stage is reduced in a constant load test compared to a constant stress test. I am not talking of creep rate. I am only talking of the size of the secondary creep stage.
In tensile creep as the material elongates, there is a reduction in cross sectional area for constancy of volume during plastic deformation. Hence if you are doing constant load tests, the uniform reduction in cross-sectional area during elongation will lead to an increase in stress experienced by the sample. Please note that there is a difference between uniform reduction in cross-sectional area and necking. Necking is localized reduction in cross-sectional area.
Hence in a constant load test, the strain rates of deformation will continue to increase with increase in sample elongation (creep strain). Hence the chances of attaining a steady state creep rate (constant creep rate) is reduced. Hence secondary creep stage in constant load creep tests is small or reduced. In a constant stress test, one will notice a clear steady state or secondary creep stage. The stress is maintained constant by decreasing the load when the cross-sectional area of the sample reduces. The tertiary creep stage in a constant stress creep test will be observed when necking happens or internal voids start forming. Necking as I mentioned is localized reduction in area.
Hope this answers your question
@@0notout Thank you for the explanation . You are absolutely correct. The point I missed is the " uniform reduction in cross sectional area" due to increase in length during secondary creep stage, which is different from " necking".