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True strain rate experiment
Sun, 2008-06-15 22:46 - raju_pro
I am planning to perform few "true-strain-rate" experiment under compression (where the crosshead velocity is not constant sice the height of the specimen keeps decreasing/ area of cross section keeps increasing, hence requiring the velocity to decrese to attian true stress-true strain), is there any option available in Bluehill or Max software of Instron 8874to suit my requirement? As an alternative method, are there any options for me to import my own X-Y data (either time vs position or time vs velocity) to the system so that i can perform this true strain rate testing.
As Currently this is an urgent requirement it would be very thankful if you can reply ASAP!
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Piecewise linear approximation?
Hi Raju_Pro,
For a given constant true strain rate, say eps_dot, you can compute the stretch, lambda, by solving the differential equation
lambda_dot/lambda=eps_dot. If the inital condition (at t=0) is the undeformed configuration, the stretch expression becomes
lambda(t) = exp ( eps_dot * t ). From this expression, --depending on the height of your specimen-- you can compute the position
of the cross-head for a given true strain or time, eps = eps_dot * t. This non-linear function can be approximated by using
some linear lines (for moderate rates 5-10 lines should be enough) connecting the selected points. You might like to use finer
divisions in the begining and coarser towards the end. I hope this will be helpful.
Regards,
Serdar Goktepe