3.6. RING-CORE METHOD 57
but not on the physical dimensions that the images represent. us, if a hole occupies a given
fraction of an image, the same relative measurement resolution will be achieved independent of
whether the hole is physically 100 mm, 1 mm, or 1 m in diameter. Practical DIC hole-drilling
measurements have been done at both ends of this range, the biggest holes produced by cutting
cores out of large concrete members, and the smallest made using a Focused Ion Beam within a
Scanning Electron Microscope (FIB-SEM). e concrete example demonstrates measurements
at a scale too big for conventional strain gauge use, while the microscopic example demonstrates
measurements at a much smaller scale than possible with traditional strain gauges. Figure 3.11
illustrates a DIC hole-drilling measurement done within a scanning electron microscope. e
hole diameter is approximately 1 m. e surrounding dot pattern is deliberately applied to
provide data for the DIC evaluation. e surface deformations in the x and y directions caused
by hole-drilling can then be determined throughout that area.
Figure 3.11: DIC hole-drilling measurement done using a FIB-SEM (image courtesy of Dr. B.
Winiarski, Manchester University, UK).
3.6 RING-CORE METHOD
e Ring-Core Method, schematically illustrated in Figure 3.2b, is closely analogous to the
Hole-Drilling Method, but with the locations of the hole and the measurement area inter-
changed. Instead of having a hole at the center and measurements around the outside, the mea-
surements are at the center and the “hole” becomes an annular groove around the outside. e
Ring-Core Method is an evolution of the excision method described in 1946 by Meriam et al.
ey measured residual stresses in welded steel plates by attaching strain gauge rosettes and then
entirely cutting out the local stressed material by drilling a series of overlapping holes along a
surrounding circular path. e modern form of the Ring-Core Method using an annular groove
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