126 6. EXAMPLE PRACTICAL PROCEDURES AND RESULTS
gauge elements to the free edges may result in departure from the conditions required for
the published coefficients.
• Distance to a step feature: e proximity of a step feature in the region of the gauge results in
changes to the stiffness of material around the drilled hole that are usually less severe than
changes caused by an edge feature at the same distance. It is unlikely that the presence
of small steps, such as weld beads (Figure 6.4a), will affect the validity of published Inte-
gral Method coefficients for practical ranges of dimension “ds” that can be achieved using
available rosettes. e two adjacent steps shown in Figure 6.4b may also have an effect on
the gauge outputs, but the presence of blend fillets will reduce the severity of this. For both
of these applications, parts of the rosette backing material can be removed to increase the
flexibility of the rosette for ease of installation. Distances from drilled holes to larger steps
should be greater than the tabulated values to avoid excessive uncertainties.
• Radius of curvature: Figures 6.5a–d show the installation of gauges on curved surfaces.
In Figure 6.5a, an open 1/16” pattern gauge rosette is installed on an external cylindrical
surface; Figure 6.5b shows a 1/8” rosette installed on a spherical surface. e rosette in
Figure 6.5c is installed on a fillet surface while the gauge in Figure 6.5d is installed on a
“saddle” surface with both internal and external curvatures.
Curvature of the gauge installation surface causes two significant concerns:
– the required Integral Method coefficients for a curved surface (cylindrical or spheri-
cal) will progressively deviate from the published values for a flat surface as curvature
increases; and
– drilling a flat-bottomed hole into a curved surface results in ambiguity concerning
the selection of the hole datum depth (from which all subsequent depth increments
are measured). is uncertainty is most significant at shallow drilling increments at
surfaces with a small radius of curvature.
In practice, where the radius of curvature falls below the quoted rc
min
value, the effect
on coefficients and shallow increment strains can be investigated using finite element models
and specimens with “known” stresses from applied loads. Figure 6.5b also shows how slitting
of the rosette backing material between the adjacent gauge elements can be used to provide the
required flexibility of the rosette to wrap the elements around the curved specimen surface. A
less severe gauge installation is shown in Figure 6.5c where much of the backing material of
the encapsulated 1/16”-size rosette has been removed to increase the flexibility of the gauge to
facilitate wrapping around the fillet.
• Gauge spacing: e relaxation effects of hole drilling extend beyond the boundaries of the
rosette. Where it is required to install a number of rosettes on a specimen surface (Fig-
ures 6.6a and 6.6b) potential interference between adjacent holes must be considered. For