For my experiments, I am pulling the sample prepared as per ASTM E8M standard and then stopping the crosshead at the specified strain. But when I am trying to retrieve the sample back the jaws are too tight to get the sample back even after zeroeing the load on the console! I can get back the sample only after breaking it. Breaking will not serve my puspose as i have to study the microstructural changes happening after giving sample different strains. I am using Lloyd's EZ 50 machine with a B1 class extensometer.
What shold I do to get the sample back without breaking it?
Will reducing the cross sectional area of the sample in the middle third
of the sample be enough reduction to the applied force such that the
jaws do not bind. I would think that large diameter jaws and small diameter
in the area of strain would help solve this problem.
Pre strained samples are nothing special. Just pull it to desired strain and compensate for elastic part (spring back effect). You should be able to take sample from jaws without problem. Please check if machine has some safety feature preventing opening of jaws before sample is broken.
Normally this should not happen. After elastic spring back, the sample should be safely recovered. Try an equipment having jaws clamped through hydraulic pressure rather than screw mechanism. Larger diameter jaws can also help in solving this problem. Depending upon the type of material, the rise in the temperature of the deformed part may be one of the cause if you unmounting it too quickly after the test stoppage.
I would go with David on this. There shouldn't be any problem removing the sample. Read the manual again or get someone with experience on the machine to assist.
The desired strain level in the plastic regime can be well controlled - as said above just pull it to the desired level and relax the load. You will have a plastic strain at zero load. Another elegant way is to use a trapezoidal specimen, in which you can have variable plastic strain across the specimen lenght!
As mentioned, relaxing the load, could release elastic strain and you could take the sample out.
Yes, David describes how the elastic load must be relieved before removing the sample. For the desired pre-strain (plastic strain), you will have to sacrifice a couple of samples to determine the work-hardening behavior of the material. The elastic strain recovered will include the final stress divided by elastic modulus (not just the original yield stress). Once you determine the stress-strain curve, you can deform your specimen to the point where the strain is the total of the final stress/young's modulus plus the desired residual strain.
Please note that for low melting point materials, these points can be strain rate dependent.
In thinking about your problem, look at how the machine is designed.
What is the release mechanism after failure, and how can i modify it
such that the forces can be reversed? Do you need to add springs to
it, or a pressure relief valve, or a method of reversing the pressure from
tension to compression?
Perhaps another manufacturers machine will solve the problem.
Have you tried to go a bit on compression side, just before bending the sample?
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