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Effect of strain rate and control method on the behavior of three aluminum alloys

Effect of strain rate and control method on the behavior of three aluminum alloys

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Effect of strain rate and control method on the behavior of three aluminum alloys

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  1. Effect of strain rate and control method on the behavior of three aluminum alloys Rich Brazill Bob Sirkoch Alcoa Technical Center 2007 May 21

  2. Field problems - Commissioning of new tensile testing equipment - 7175-T6511 extrusion material 2007 May 21

  3. Field problems 2007 May 21

  4. Field problems Problem likely related to improper PID control parameters. 2007 May 21

  5. Materials • Three sheet alloys: • 2024-T3 • 0.094 in. thick • Medium strength, high elongation • 7075-T6 • 0.016 in. thick • high strength, low elongation • 5182-O • 0.050 in. thick • low strength, very high elongation • exhibits discontinuous straining 2007 May 21

  6. Materials • These aluminum alloys not usually considered strain rate sensitive, although discontinuous straining behavior is known to be influenced by strain rate. 2007 May 21

  7. Properties tabulated • Yield strength • Initial elastic slope • Stress rate during elastic deformation • Strain rate during elastic deformation • Strain rate at yield • Stress rate at yield • Time to reach yield • Machine stiffness at yield • UTS • Elongation at fracture* 2007 May 21

  8. Equipment • Instron 4486 test machine, 60-kip capacity • 10-kip piggybacked load cell • Instron 2630-114, 2-in. 10% extensometer • Instron Series IX software (Bluehill did not have strain control module) • Instron universal joint • Alcoa Templin self-acting wedge grips • Load train diameter ~3/4-1 in. • Total load train length about 30 in. 2007 May 21

  9. Notes • Post-yield rate set to switch at 2% total strain. • Up to that point, strain is read from extensometer. • After that point, machine uses crosshead position to calculate an effective strain. • Elongation at fracture calculated from these data. • Force, strain, and time collected at 15 readings per second. 2007 May 21

  10. Strain rate control with Series IX • From help files, we learned • Strain rate control is implemented as an outer control loop on crosshead speed, not actually direct strain rate control. • For strain rate control, you must tell the software at what stress you want to change to strain rate control. • Strain rate control will commence within 2 seconds of the test achieving the specified stress. • Apparently, up to that point, the system is gathering information about the specimen behavior in order to set up for strain control. 2007 May 21

  11. Strain rate control with Series IX • This makes sense because there are no operator-settable parameters for PID control to account for widely varying system stiffness due to different specimen and grip configurations. • It took us three tries to get the parameters set for the 5182 material. However, it seemed to control strain rate surprisingly well considering the discontinuous yielding behavior exhibited by this alloy. 2007 May 21

  12. Specimen • 0.500 in. tapered-width sheet specimen • Parallel length: 2.25 in. • Overall length: 9 in. 2007 May 21

  13. Three Control Modes Three duplicate specimens tested for each condition on each material 2007 May 21

  14. Initial elastic slope 2024-T3 7075-T6 5182-O 2007 May 21

  15. Strain rate during elastic loading 2024-T3 7075-T6 5182-O 2007 May 21

  16. Stress rate during elastic loading 2024-T3 7075-T6 5182-O 2007 May 21

  17. Yield strength Trend to lower yield strength with strain rate control for all three alloys 2024-T3 7075-T6 5182-O 2007 May 21

  18. Strain rate at yield 2024-T3 7075-T6 5182-O Note SRC not at 0.015 2007 May 21

  19. Stress rate at yield 2024-T3 7075-T6 5182-O Note negative stress rates 2007 May 21

  20. Time to yield 2024-T3 7075-T6 5182-O 2007 May 21

  21. Apparent machine stiffness Km at yield 2024-T3 Machine stiffness at yield depends on material 7075-T6 5182-O 2007 May 21

  22. UTS 2024-T3 7075-T6 5182-O 2007 May 21

  23. Elongation at fracture 2024-T3 7075-T6 5182-O 2007 May 21

  24. 2024-T3 SRC 1 2 3 2007 May 21

  25. 7075-T6 SRC 1 2 3 2007 May 21

  26. 5182-O SRC 1 2 3 2007 May 21

  27. 2024-T3 Std SRC Exp 2007 May 21

  28. 7075-T6 Std SRC Exp 2007 May 21

  29. Note effect of strain rate on discontinuous yielding behavior 5182-O Std SRC Exp 2007 May 21

  30. Conclusions • Strain rate control tests produced yield and ultimate results that were slightly lower on all three alloys than when using our “standard” constant crosshead speed based on stress rate. • Strain rate control changed the character of the discontinuous yielding exhibited by 5182-O material. • Strain rate control on 5182-O material worked better than expected even though it exhibits discontinuous yielding. However strain rate as we measured it went outside the limits in the yield region. • Machines improperly set up can produce undesirable results (like the observed drop in stress in yield region). • Because it doesn’t consider machine stiffness, use of the proposed crosshead rate based on 0.015 /min is too slow (5-7 times slower to get to yield) and should be discarded. • The “stress rate” method in the current standard was intended to account for machine stiffness problem with the previous point. • I believe that the stress rate method is misinterpreted by many users (ie, comments received during last ballot). 2007 May 21