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Uncertainty in Hardness Measurement

Uncertainty in Hardness Measurement. By Tassanai Sanponpat Hardness Laboratory, NIMT. What is hardness measurement ?. The measurement of a material’s resistance to plastic deformation caused by a force applied through a hard body (indenter).

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Uncertainty in Hardness Measurement

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  1. Uncertainty in Hardness Measurement By Tassanai Sanponpat Hardness Laboratory, NIMT

  2. What is hardness measurement ? The measurement of a material’s resistance to plastic deformation caused by a force applied through a hard body (indenter)

  3. For example Definition of Rockwell Hardness scale CForcing an indenter (diamond cone, steel or hard metal ball) into the surface of a test piece in two steps under specified conditions. Measuring the permanent depth h of the indentation under preliminary test force after removal of additional test force (ISO 6508-1)

  4. True Value of Hardness The “true value” of hardness is achieved by • ideal operator • ideal standard hardness calibration machine • ideal indenter • ideal test block to be realized according international definition

  5. -Ideal machine

  6. -Ideal machine

  7. Ideal indenter Ideal test block The uniformity should be zero.

  8. In practically, it is impossible to realize the true value because they cannot be made and calibrated without an error. What should we do?

  9. Determine the initial variable xi • Find the mathematical model y = f(xi) • Evaluate the sensitivity coefficient by using partial differential equation etc. ci • Estimate the uncertainty of initial variable uxi • Convert uxi to the unit of depended variable • Combine the uncertainty Note: This method can not always be applied to testing quantity

  10. Source of Uncertainty • The Influence of the machine. - Testing Force - Testing cycle 2. The Influence of the indenter. 3. The Influence of the indentation measurement. 4. The Influence of the test block.

  11. Tip I -The meaning of error in physical scale ERROR = measured value - true value -The meaning of error in hardness scale (testing) ERROR = true value - expected value = true value - defined value they should be controlled and calibrated

  12. The Influence of the machine Preliminary test force effect

  13. Preliminary test force effect

  14. -Sensitivity coefficient of preliminary test force

  15. Total test force effect

  16. Total test force effect

  17. -Sensitivity coefficient of total test force

  18. Tolerances on applied force

  19. Error due to applied force within tolerance

  20. The influence of testing cycle • Preliminary force Dwell Time (s) • Indenter Velocity (m/s) • Total force Dwell Time (s) • Recovery Dwell Time (s)

  21. -Effect of creep and recovery material

  22. H H h h/ v1 t1 t3 t2 t1/ Longer duration time of preliminary test force

  23. Preliminary force Dwell Time Hardness variation due to preliminary force dwell time

  24. -Sensitivity coefficient of preliminary test force dwell time

  25. Indenter Velocity (mm/s) or Load rise Time (s)

  26. Indenter Velocity (mm/s) or Load rise Time (s)

  27. Indenter Velocity (mm/s) or Load rise Time (s)

  28. Faster indentation velocity h v1 t1 t3 LRT+ t2 h v1 t3 t1 LRT/ + t2

  29. Sensitivity coefficients of indenter velocity

  30. H H h/ h v1 t1 t3 t2 t2/ Longer duration time of total test force

  31. Total Force Dwell Time

  32. Sensitivity coefficients of Total Force Dwell Time

  33. Recovery Dwell Time

  34. The influence of indentation measurement • static Hardness measurement - measured under Force - depth measurement - measured without Force - diagonal measurement - diameter measurement • dynamic hardness measurement - speed or position measurement

  35. depth measurement Standard for depth measurement : displacement sensor

  36. The influence on depth measurement • accuracy of standard device < 0.2 mm in condition under preliminary test force • misalignment • gap between indenter and mounting • gap between standard device and indenter • gap between standard device and anvil • gap between anvil and support • force variation • shocks • Calibration system of standard device • operator • deformation of frame (offset reading)

  37. Deformation of frame (offset reading) Dh

  38. Diagonal & Diameter measurement (used microscope) -Operator Repeatability & systematic error Repeatability & Reproducibility -Magnified objective lens Systematic error -Numerical aperture (N,A) Systematic error -Alignment Systematic error -Automatic reading system Systematic error

  39. -Operator effect

  40. -magnified objective lens Higher magnification Higher indentation result better repeatability

  41. -Magnified objective lens & numerical of aperture (NA)

  42. - Numerical of aperture (NA)

  43. -Alignment

  44. Automatic Reading (image analysis)

  45. Automatic Reading (image analysis)

  46. + Standard block How to get the influence of the indenter ?

  47. Conference paper…influence of the indenter shape in Rockwell hardness test* Cone angle 120 +/-2 , tip radius 200 +/- 20 * By Giulio Barbato, Maurizio Galetto IMGC

  48. Barbato G

  49. How can we do in nowadays ? • get the correction value of Force application. • get the correction value of testing cycle. • get the correction value shape shape indenter.

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