SAMPE Journal - May/June 2020 - 30

FEATURE / FIBER OPTIC SENSING
ing) arrows indicate compression and the red (outward pointing) arrows show tension. While these
arrow pairs are always perpendicular to each other,
their orientation relative to the blade is rotated to
describe the angle j. The length of the arrows describes the relative magnitude of the strain values
they represent so that regions of strain concentration can be identified on the part. A similar calculation was carried out for the pressure side of the
blade but is not shown here.
While this technology offers a truly novel method for measuring distributed principal strains and
could be tremendously impactful in industry, it
does have limitations which should be addressed.
It is possible for highly localized strain fluctuations
to be inaccurately represented through the interpolation process used in the distributed rosette. As
with any measurement of this type, the results will
improve in accuracy as the number of rosettes on a
given part increases.
While most of this work has focused on the
ability to resolve distributed principal strains, the
importance of this innovation in structural health
monitoring applications should not be underestimated. Damage in critical components may, at
times, generate a prominent strain signature in one
direction. In past work, optical fiber sensors have
been embedded in composite panels and successfully used to detect and localize impact events (Figure 15)4,5. However, since optical fiber is principally
sensitive to strain along its length, any fiber configuration besides a rosette used in a structural health
monitoring application risks failing to detect damage which generates a strain field perpendicular
to its location. For this reason, distributed rosettes
afford a promising innovation both for characterizing distributed principal strains and for conducting
structural health monitoring.
CONCLUSIONS
In conclusion, this work shows how a single optical fiber can be configured into an extended rosette to measure distributed principal strains and
their orientation across an entire test article. Two
variants of a 0-45-90° rosette were installed on G10
fiberglass coupons and strained in 4-point bend
in order to compare the fiber configurations with
foil rosettes, the incumbent technology. Principal
strains calculated using data from the optical fiber
rosette were shown to have a consistent linear relationship with principal strains calculated using
foil strain gage data. A slight difference in slope between strains from the fiber and foil gage on each
coupon indicate that gage factors may need to be
adjusted to improve agreement and accuracy. A

SAMPE JOURNAL

Figure 11. Calculated orientation angle, j, indicating the angular direction of
the maximum strain from the lengthwise axis of the coupon.
Figure 12. Color map of strain along distributed rosette located
on the suction side of the composite rotor blade.

Figure 13. Strain measurements taken using distributed rosette decomposed into three separate measurement axes and interpolated along the
blade on the suction side.

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SAMPE Journal - May/June 2020

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