122 6. EXAMPLE PRACTICAL PROCEDURES AND RESULTS
the increase in thickness can result in excessive bond thickness when bonding to an irregular or
significantly curved specimen surface. In that case, the flexibility of an “open” rosette makes it a
preferable choice.
6.2 PRACTICAL STRAIN GAUGE ROSETTE
INSTALLATIONS
Practical specimens are often less than fully ideal because of their complex geometry and the
presence of local geometric features. e photographs in Figures 6.2–6.6 illustrate some of the
dimensions defined in Table 6.1 and show ways in which the standard rosette patterns can be
used and possibly adapted to accommodate various practical circumstances. e extreme gauge
position dimensions (minimum or maximum) are listed in Table 6.1 for which the published
Integral Method coefficient values remain valid within a range of ca. ˙4%. ese dimensions
relate to the:
• specimen thickness (ts),
• distance to a free edge (de and de1),
• distance to step features (ds and ds1),
• radius of curvature of the surface (rc).
In non-standard examples of gauge installations, coefficients for combinations of extreme
installation dimensions (for example near-edge and small radius of curvature) may lie further
from the ASTM E837 values.
Dimensions relating to the drilled hole, specimen, and gauge position are established as
follows.
• Nominal hole diameter: Typically, D
o
D 0:4D with a recommended range of diameters of
˙0:04D. e use of a smaller hole diameter is a concern because it leads to lower strain
outputs, in particular at depths close to the surface. is reduced strain response increases
uncertainties in the calculated residual stress values. Conversely, the use of a larger hole
diameter is a concern because it can lead to gauge and bond damage close to the innermost
parts of the gauge elements. Accordingly, for 1/16”-size gauges (D D 5.13 mm), a good
choice for the nominal hole diameter is around 2.0–2.2 mm.
• Drilled hole depth: e maximum depth zh
max
to which holes are drilled is defined by the
size and geometry of the gauge pattern. For rosette Types A and B, ASTM E837 specifies
20 0:02D drilling steps for a “uniform” stress measurement (final depth of 0.4D) and
20 0:01D for an incremental measurement (final depth 0.2D). e smaller depth allows
for measurements closer to the material yield stress. e specified drilling steps for the Type
C rosette for an incremental measurement is 25 0:01D (final depth 0.25D). Drilling
6.2. PRACTICAL STRAIN GAUGE ROSETTE INSTALLATIONS 123
(a) Specimen thickness (moderate) (b) Specimen thickness (thin)
Figure 6.2: Gauge rosette installations on thin specimens (photos courtesy of Stresscraft Ltd.).
(a) Distance to a single free edge (b) Distances to two free edges
Figure 6.3: Gauge rosette installations adjacent to edge features (photos courtesy of Stresscraft
Ltd.).
124 6. EXAMPLE PRACTICAL PROCEDURES AND RESULTS
(a) Distance to a single step feature (b) Distances to two step features
Figure 6.4: Gauge rosette installations adjacent to step features (photos courtesy of Stresscraft
Ltd.).
(a) Surface curvature (external, single) (b) Surface curvature (external, double)
(c) Surface curvature (internal, single) (d) Surface curvature (external and external)
Figure 6.5: Gauge rosette installations on curved surfaces (photos courtesy of Stresscraft Ltd.).
6.2. PRACTICAL STRAIN GAUGE ROSETTE INSTALLATIONS 125
(a) Gauge spacing (single) (b) Gauge spacing (multiple)
Figure 6.6: Multiple strain gauge rosette installations (photos courtesy of Stresscraft Ltd.).
beyond these depths produces no useful further information about interior stresses. e
maximum depth to which residual stresses can be evaluated is defined by the size and
geometry of the gauge pattern. is depth is slightly smaller than zh
max
; because the
computed stress within each hole depth increment is associated with the center of the
increment, the effective maximum stress data depth is reduced below the hole depth by
half the thickness of the final calculation increment.
• Specimen thickness: For measurements on a thin specimen, the drilling process causes signif-
icant localized bending of the specimen. For rosette Types A and B, ASTM E837 proposes
a minimum specimen thickness ts
min
D 1:0D. Rosette Type C includes circumferential
elements for which deeper hole depths cause significantly greater changes in stiffness.
Accordingly, the minimum specimen thickness for Type C rosettes is somewhat greater
(around 1.2D). Figures 6.2a and 6.2b show 1/32” rosettes installed on specimens where
the material thickness is to be considered; the thinner specimen is shown in Figure 6.1b.
• Distance to an edge feature: e proximity of a free edge to the drilled hole causes changes
in stiffness and departures from the conditions used to calculate the published Integral
Method coefficients. is is a particular concern when using rosette Type B (Figure 6.3a),
where all the gauge elements are contained within a single quadrant. In extreme cases,
the drilled hole could be positioned very close to an edge. Where the distance between
the gauge center and specimen edge de
min
is less than 0.8D, the validity of published
Integral Method coefficients may lie outside the range ˙4%. e presence of chamfers at
the edge may further influence the validity of the coefficients. Figure 6.3b shows a Type
A rosette installed between two chamfered holes; in this case the close proximity of the
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
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