The various messages all circle around the original question, but my comment is that there is no good reason for you to see a variation in modulus. It is a property that, in the elastic range, depends on interatomic spacing and crystal orientation. At the bottom end of the curve, variations in machine setup such as mechanical slack and surface connections play roles. At the higher end of strain the beginnings of yielding/lattice slip have an effect, and this is where higher strain rates will move the measured yield higher in steels.
At much higher strain rates, you can end up with effects on the data due to resonances in the test machine assemblies.
My guess would be that apparent variations in modulus are a result of measurement errors such as imperfect extensometer connection with the test sample, or data sampling rate. The latter stands in for the problems found testing on older machines where the response speed of the electronics and/or the mechanical pen would lose registration with the actual signal.
I also am curious where a yield of 70 MPa came from (forgive me but I have to convert, I get ~10 kpsi)? This is way too low for any steel. I normally expect carbon steel to be =>250 MPa. If your colleague does come up with an explanation for the unexpected, it would be interesting to hear the outcome.
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Paul Tibbals, P.E.
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Original Message:
Sent: 12-04-2020 16:53
From: Jose Flores Herrera
Subject: Strain rate sensitivity
The high-strain rate tests were performed in a ASTM E8 2" gauge standard specimen in a hydraulic universal testing machine with an extensometer. As I mentioned, I was not directly involved with the testing but a colleague reached me to discuss about the results, and what I was explained is that there was a range from 124 GPa to 190 GPa for elastic modulus, but the yield stress was in a range of 70 MPa.
All the specimens were extracted from the same heat treated steel section and usually at standard tensile testing there is a variation up to 35 MPa in yield strength and UTS. However in high strain rate there was variation but not in a straigthforward trend such as a strength increase with increasing strain rate. I aknowledge that first a statistic analysis should be done on the data from the results, because I am speaking from a subjective perspective.
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Jose Flores Herrera
Metallurgist Intern
Fundicion Lerma
San Nicolas de los Garza
5218717957482
mariano@flores-h.com
Original Message:
Sent: 12-04-2020 01:04
From: Kenneth Vecchio
Subject: Strain rate sensitivity
Jose,
Before I reply to the group thread on your question, I'd like to ask a few questions about your comment and question.
You state 'There was variation in strength (both yield and UTS) and elastic modulus, the latter is what puzzles me......'.
Can you explain if these variations are from one sample to another all conducted at high strain rates, or are these interrupted tests of a single sample being tested on a split-Hopkinson tensile bar setup? I do a good amount of high strain rate testing, both compression and tension, and tension testing at high rates has a lot of caveats about the results that need to be considered. If you can provide more specific about your test concerns, I am sure I can provide better advice.
Prof. Kenneth Vecchio
UC San Diego
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Kenneth Vecchio FASM
Professor Kenneth S. Vecchio
University of California San Diego
San Diego CA
Original Message:
Sent: 12-03-2020 09:49
From: Jose Flores Herrera
Subject: Strain rate sensitivity
Hello,
What I have read is that it is due to inelastic deformation such as creep, however for some brittle materials there is also time-dependent cracking. Actually, recently a colleague had an issue to interpret the results from a set of high strain-rate (0.01 1/s - 1000 1/s) tensile tests on quench and tempered low carbon steel. There was variation in strength (both yield and UTS) and elastic modulus, the latter is what puzzles me because I think it should be constant . So if anyone could comment anything on this I would appreciate it.
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Jose Flores Herrera
Metallurgist Intern
Fundicion Lerma
San Nicolas de los Garza
5218717957482
mariano@flores-h.com
Original Message:
Sent: 12-02-2020 23:51
From: Ankit Yadav
Subject: Strain rate sensitivity
Why material shows the strain rate sensitivity??
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Ankit Yadav
Bansur
(774) 269-6124
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