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Strain rate sensitivity

  • 1.  Strain rate sensitivity

    Posted 12-02-2020 23:52
    Why material shows the strain rate sensitivity??

    Ankit Yadav
    (774) 269-6124

  • 2.  RE: Strain rate sensitivity

    Posted 12-03-2020 09:50

    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.

    Jose Flores Herrera
    Metallurgist Intern
    Fundicion Lerma
    San Nicolas de los Garza

  • 3.  RE: Strain rate sensitivity

    Posted 12-04-2020 01:04

    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

    Kenneth Vecchio FASM
    Professor Kenneth S. Vecchio
    University of California San Diego
    San Diego CA

  • 4.  RE: Strain rate sensitivity

    Posted 12-04-2020 16:54
    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.

    Jose Flores Herrera
    Metallurgist Intern
    Fundicion Lerma
    San Nicolas de los Garza

  • 5.  RE: Strain rate sensitivity

    Posted 12-05-2020 23:10
    As a co-op student in the late-middle 60's we had to perform a number of tensile tests using a Riehle hydraulic test machine with a clip-on extensometer. The Chief Metallurgist at the time warned us to make sure we did not run the test machine too fast as the paper recorder could not keep up with the signal from the extensometer. He noted that a part of the E8 spec at the time was to note the strain rate of the test. This could be done in a number of ways. One way was the free running cross-head speed at the valve setting used in the test. We settled on one particular spot on the valve and then documented what the speed was with no sample with a stop watch and dial indicator. He said that was the best way to get consistent results. After we got the elastic portion of the curve we were allowed to speed up the test a bit to save time. You could see a variation on the strip chart curve when the speedup occurred.

    This is not a metallurgical answer but a practical answer of the limitations of the test equipment.

    James Mikoda

  • 6.  RE: Strain rate sensitivity

    Boston Admin
    Posted 12-06-2020 17:11
    ​Limitations of testing instruments has always been a challenge in the space of strain rate sensitivity. Rest assured that modern instruments and, more importantly, software improvements allowed for quite accurate strain rate controlled testing.  You should be able to discern materials differences in regards to strain rate sensitivity much easier now. If you have particular issues/challenges, we have an applications lab at Instron that could assist in demonstrating some of those capabilities. Best of luck! jim

    James Ritchey
    Innovation Director-Gloabl Services
    Norwood MA
    (781) 929-2866

  • 7.  RE: Strain rate sensitivity

    Posted 12-06-2020 19:21
    I am sure great advancements have been made in 50+ years. The point I should have made was the documentation of the test speed with the tests should be considered with the data and could explain the variation he mentioned and could correlate his data with the added variable.

  • 8.  RE: Strain rate sensitivity

    Posted 12-06-2020 21:16
    Edited by Paul Tibbals 12-06-2020 21:40
    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.

    Paul Tibbals, P.E.

  • 9.  RE: Strain rate sensitivity

    Posted 12-07-2020 10:53
    I really appreciate the comments on the topic. 
    Regarding the yield strength I meant that the variation is around 70 MPa, I should have said the yield strength nominally is 120 ksi, but the results are up to 130 ksi.

    Jose Flores Herrera
    Metallurgist Intern
    Fundicion Lerma
    San Nicolas de los Garza

  • 10.  RE: Strain rate sensitivity

    Posted 12-04-2020 09:33
    This discussion is restricted to ambient temperatures, and creep effects are excluded.

    1. Variations in elastic modulus
    • Elastic modulus is surprisingly difficult to measure precisely. The number you get from the slope of a stress-strain curve well below yield often varies by a factor of 2 (i.e. from 1/2 to 2X) the "true" modulus.
    • Tests with strain gauges applied directly to the sample are more accurate, but bending still needs to be considered and factored out of the answer.
    • Anelastic effects can distort the very low stress (<5% of yield) behavior.
    • There appears to be a positive correlation between elastic modulus and yield strength. I haven't yet found a good explanation for that.
    • Sonic resonance or speed of sound measurements can provide a highly accurate (+/- 3%) elastic modulus. The right techniques and equipment can also provide shear modulus.
    • Theoretically, the sonic (dynamic) modulus is slightly greater than the static (relaxed) modulus. In actual practice, this difference is less than the other experimental errors. 
    2. "Normal" strain rate sensitivity
    • Most materials show a slight positive strain rate dependence. That may be due to the need to move atoms, but there may be other factors involved as well.
    3. Inverse strain rate sensitivity
    • Some materials exhibit negative strain rate sensitivity. There are several possible reasons for this:
      • Adiabatic shear bands may form in some materials. This has been documented for titanium alloys in very high strain rate (explosions, etc.) testing.
      • Even at lower strain rates, adiabatic heating during deformation may inhibit strengthening reactions, such as the formation of deformation martensite.

  • 11.  RE: Strain rate sensitivity

    Eastern Virginia Admin
    Posted 12-03-2020 09:58
    The simple answer is that different defects have different mobilities (like speed of sound), so the defect type/motion can be dependent on thermodynamics or kinetics.  More complex defect structures may take more time to form, so they will not be apparent at higher strain rates.  

    A fairly straightforward example of this is stress corrosion cracking.  At high strain rates, the metal deforms plastically with dislocation motion; at very low strain rates corrosion occurs faster than plastic deformation; and at a sweet spot in between, corrosion and deformation combine into SCC.  Another example is creep, where diffusion is more thermodynamically favorable at very low strain rates, and dislocation motion is more kinetically favorable at higher strain rates.

    David Sapiro
    King George VA