SAMPE Journal - May/June 2020 - 26

FEATURE / FIBER OPTIC SENSING

Figure 3. G10 fiberglass coupons instrumented with fiber and foil strain
gage rosettes.

EXPERIMENTATION
Individual Fiber Optic Strain Rosettes
Due to the flexibility and high measurement density along an optical fiber, an infinite number of possible rosette configurations can be created10. Two
common rosette configurations were installed on
30.5 cm x 4.45cm x 0.32 cm G10 fiberglass coupons,
as shown in Figure 3. On each coupon, the rosettes
were formed using a single fiber sensor, demonstrating the multiplexing capabilities of HD-FOS.
The sections of the fiber shown in solid yellow were
bonded to the coupon using M-Bond 200 after
the surface was lightly abraded and cleaned. The
strains measured by the fiber rosettes were compared with a traditional FSG rosette adhered to the
coupon center using AE-15 adhesive and standard
installation procedures.
The straight sections of each rosette were approximately 2 cm in length, except for the hypotenuse of the triangular rosette on Coupon A, which
was slightly longer. The fiber rosettes were each
spaced 2.5 cm from the FSG rosette and located
such that principal strains measured by each rosette would be located on the centerline of the coupon.
Figure 4. Two different styles of rectangular rosette used for testing.

The rosettes on Coupon A and B are both of the
0-45-90° configuration, which is sometimes called
a rectangular rosette5. The main difference between these two variations is the spatial relationship of the bonded segments of the rosette to the
point of interest, indicated by the red dot in Figure
4. Coupon B has the bonded rosette segments A,
B, and C intersecting at the point of interest while
on Coupon A the point of interest is located at the
centroid of the rosette triangle. One notable consideration is that where the optical fiber crosses
over itself, such as at the center of the rosette on
Coupon B, the top fiber is spatially separated from
the surface of the test article resulting in a possible
loss of strain transfer from the test article to the optical fiber.
For both coupons, the measured strain values
along segments A, B, and C are averaged to give a
single strain value in that direction: eA, eB, and eC,
respectively. Using these strain values and Equations 1 and 2 below, the principal strains and the
orientation angle, j, can be calculated6. The FSG
rosette used for comparison was also of the rectangular rosette configuration and therefore the same
equations were used to resolve e1, e2, and j from the
foil strain gage data.

[1]
[2]
Fiber Rosette Testing
In order to characterize each of the fiber rosettes,
a series of 4-point bend tests were performed on
the fiberglass coupons (Figure 5). The 4-point bend
test was selected due to its ability to induce a constant strain across the instrumented portion of the
coupons. The bend fixture used in these tests had
outside contact rollers spaced 26.67 cm apart, with
19.05 cm between the internal contacts which were
centered in the fixture. Measurements were zeroed
after the top crossmember of the fixture was added and prior to any loads being applied. Increasing
loads, ranging from 0 to 9 kg, were applied to the
bend fixture as strain data was recorded from both
the fiber and the foil strain gage rosettes. Three cycles of the loading regime were repeated for each
coupon.
Embedded Distributed Strain Rosette
The individual fiber rosette was then extended
to form a distributed lattice of continuous fiber
rosette capable of measuring principal strains
at many locations across a part. This distributed

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SAMPE Journal - May/June 2020

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