SAMPE Journal - May/June 2020 - 27

strain rosette was embedded within a composite
rotor blade manufactured by a rotor blade manufacturer. In order to lay out multiple identical fiber
rosettes, a repeatable manufacturing process was
developed. Additionally, to accurately capture and
represent data from these multiple rosettes across
a surface with complex curvatures, an extensive
measurement and analysis scheme was developed.
Distributed Rosette Layout
It was determined that a modified version of the
triangular rosettes used on Coupon A (0-45-90°
configuration) would be most conducive to forming a distributed rosette. The pattern chosen is
shown in Figure 6 and is formed using a single 10 m
high-definition fiber optic sensor. In Figure 6, each
of the primary measurement axes are shown in a
different color (green [0°], red [45°], or blue [90°]).
The manufacturing process for laying out and embedding the sensor is shown in Figure 7.
Due to the distributed rosette's complexity and
the importance of each segment being oriented correctly, installing the rosettes manually was
not a viable option. Instead, a template was used
to ensure that the fiber was laid out in the correct
orientation and spacing, as seen in Figure 7a. From
the template, the rosette was transferred to Kapton
tape and then to the top sheet of the prepreg material on the mostly-completed blade. The ends of
each straight segment were touched with a localized heat source, causing the uniaxial carbon fiber
prepreg to become tacky and grip the fiber. This
allowed the Kapton tape to be removed while still
holding the fiber in the desired layout as shown in
Figure 7b. A small hole was installed through the
foam core close to the trailing edge so that a single fiber could be used to instrument both the high
and low pressure sides of the blade. Once the distributed rosette was placed on both sides of the
blade, the final prepreg sheet and metal abrasion
strip was then installed using the manufacturer's
standard process. This resulted in a blade where the
embedded rosette is only faintly visible (Figure 7c).
Distributed Rosette Analysis
The challenge with analyzing the distributed rosette was the presence of multiple distinct segments. Each segment needed to be isolated, and
the slight manufacturing variations in each had
to be captured and taken into account when calculating the resulting strain. A largely automated
software program was developed to achieve this.
The first step was to isolate the segments based on
a photo of the rosette pattern. With minimal user
input, the end result was a display of the principal

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Figure 5. The 4-point bend fixture which was used to load the fiberglass
coupons.

Figure 6. (a) A single 0-45-90° rosette. (b) Distributed 0-45-90° rosette
using a single fiber optic strain sensor.

Figure 7. Manufacturing procedure used to incorporate the distributed
rosette into a composite tail rotor blade. (a). The template used to form
the rosette (bottom) and the rosette installed on the prepreg material. (b)
Rosette tacked to prepreg material by localized heating at corners. (c)
Finished blade with embedded distributed rosette.

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SAMPE Journal - May/June 2020

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