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Assessment of error and out of plane pseudo strain

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João Quinta da Fonseca edited this page Dec 15, 2017 · 4 revisions

Error in any DIC measurement can be measured by taking two images of the same area, calculating the displacement field and analysing the strain associated with this strain increment of zero. The following is an example of such a method. A link to the raw data is given at the end.

Material

  • Ni-based superalloy RR1000, 0.5m mm thick
  • Polished P800, P1200, 6 um diamond, 3 um diamond, 1 um diamond, 15 min conc. OPS
  • Au sputter for 6 minutes, remodel in water vapour at 300 C for 3 hr
  • The size of the particles is investigated at 35 kx magnification. Figure 1 and 2 show the Au particle image analysis in Fiji and a histogram of the speckle size distribution, respectively. The modal size of the speckles is ~90 nm.

Figure 1 Au speckles on RR1000 surfacer, imaged at 35 kx magnification to determine size and spacing. a) shows the BSE image, b) the thresholding and c) the particle outlines

Figure 2 Au speckle size histogram

Microscope conditions for error analysis:

  • Magellan
  • 5 keV (including a 2 keV deceleration), 0.8 nA, WD 3.9 mm
  • BSE CBS detector, 10 us dewll time, no integration/averaging, 8 kx (full screen) magnification

Methods and analysis

  • Image is obtained at the focus condition. The stage is then moved in either the positive or negative Z stage direction to defocus the image and simulate out-of-plane strain conditions - will this create a pseudo strain in our in-plane strain hrdic measurement?
    • NB: In FEI microscopes the WD link causes automatic re-focussing when moving stage Z. Therefore unlink Z to prevent automatic re-focus. BE CAREUL! Operating the microscope when Z is not linked makes crashing into the polepiece much easier!
  • Figure 3: Strain maps are calculated as a function of defocus
  • Figure 4: Average Max shear strain is calculated for each map and plotted as a function of defocus
  • Figure 5: Frequency distribution of Max shear strain values is plotted for all defocus positions
  • Figure 6: Modal (most common) value of Max shear strain is plotted as a function of defocus along with the mean max shear strain as shown in Figure 4
  • Figure 7: The components of the in-plane strain tensor are plotted as a function of defocus

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Conclusions

We can therefore state with confidence that, under these imaging conditions, grains that move out-of-plane by less than ~20 um will not give rise to pseudo strains of an appreciable level (strain ~0.03) in regards to the high local strains of 0.2-0.6 that we observe within slip bands. In RR1000 the mean grain diameter is 40 um, and so a grain would need to deform by 50 % plastic strain out-of-plane to give pseudo strains for the in-plane HRDIC calculation. This is unlikely at current deformation levels of 1-3 % global plastic strain, but such a statement requires validation by atomic-force microscopy, or similar, to determine topographical changes as a function of strain for this alloy.

Raw data

  • All raw data for this error assessment can be found here
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