7.5. DIGITAL IMAGE CORRELATION 151
tails within the two images are then mathematically correlated, and their relative displacements
determined. e algorithms used for doing this have become quite sophisticated, and with a
well-calibrated optical system, displacements of C= 0.02 pixel can be resolved.
e 3-D technique, schematically illustrated in Figure 7.8b, involves imaging the region
of interest with two cameras and using stereoscopic imaging to determine deformations in three
dimensions. e equipment is more complex than for the 2-D technique, and careful setup
and calibration are required. For hole-drilling measurements the out-of-plane displacements
are small compared with the in-plane displacements, so in this case the extra data available from
3-D measurements provide only modest advantage.
(a) (b)
Y
X
Z
Y
X
Z
Camera
Came
r
a
Camera
Figure 7.8: Typical DIC measurement arrangements, (a) 2-D and (b) 3-D.
e DIC method has both advantages and disadvantages compared with the interferomet-
ric methods. Its major disadvantage is its relatively low sensitivity, about an order of magnitude
less when measuring a typical 2 mm diameter hole. It also requires the painting of a speckle
pattern on the specimen surface. However, once the pattern has been applied, the surface is
quite durable and can be cleaned if it becomes dirty. By comparison, a surface for ESPI mea-
surements cannot be touched at all; even very minor abrasions will damage measurement quality.
In addition, DIC can directly evaluate displacements in two in-plane directions, also the out-
of-plane direction if the 3-D technique is used. e large amount of available data improves the
quality of the resulting residual stress evaluation. By comparison, typical ESPI systems provide
displacement data in only one direction. Multi-axis ESPI systems do exist, but are uncommon.
DIC measurements differ in a very significant conceptual way from interferometric mea-
surements. Interferometric measurements are size dependent, with spatial sensitivity defined
relative to the wavelength of the light source used. In contrast, DIC is size independent because
its sensitivity depends only on the pixel density of the imaged area, not on the physical dimen-
sions represented within the image. A drilled hole could be large or small, but its appearance
within a measured image is the same. us, the DIC analysis procedure is the same. Conse-
quently, DIC is an attractive choice for hole-drilling measurements where the hole size is either