SAMPE Journal - May/June 2020 - 23

INTRODUCTION
In an effort to reduce weight, minimize cost, and
improve performance, many industries are incorporating composites and other advanced materials
into critical applications that have historically used
metals. Even as the advantages of these innovative
materials satisfy the above needs, they come with
their own set of challenges. Due to the complexity
of composite failure mechanisms and the associated difficulty in developing accurate failure prediction models, there is a pressing need to experimentally characterize performance and to monitor
the material's mechanical integrity during its operational life1. In particular, the ability to determine
distributed principal strains would be useful to
design engineers for validating models. There are
many technologies that are actively used to determine material performance and conduct structural
health monitoring (SHM). However, each of these
technologies has limitations that prohibit them
from addressing the needs of certain industries
for a lightweight, embeddable, and high-definition
strain measurement.
The rotorcraft industry is one of the markets in
which traditional methods have failed to address
the evolving needs for complex structural health
monitoring in composite parts. Rotorcraft blades
are increasingly made primarily from composite
materials. One promising avenue to improving
safety and reliability would be the ability to monitor
these critical structures for defects, so that blades
can be repaired or replaced prior to a catastrophic
failure. Methods currently in use to monitor blades

for defect formation and growth include visual/
tactile inspection, digital image correlation (DIC),
and ultrasonic testing. Visual and tactile inspection
is highly operator-dependent, potentially incomprehensive, and prone to missing defects that are
not visible on the part surface. DIC can offer good
distributed surface strain mapping, but it is inherently not embeddable and requires a line of sight
from the camera and a proper and repeatable setup
if used in the field. Additionally, DIC can be sensitive to speckle contamination2. Ultrasonic testing
can be effective in locating sub-surface defects, but
this method of testing is expensive, time-consuming, and its efficacy depends on the structure of the
test article3.
High-Definition Fiber Optic Sensing (HD-FOS)
offers a unique, embeddable solution for determining distributed strain profiles, at a 0.65 mm
measurement density, in parts that would be difficult to characterize through traditional methods4,5.
Optical fibers can be installed during the part manufacturing process and remain embedded for the
lifetime of the component. Using a ground-based
interrogation system, the optical strain-sensing fiber can be periodically scanned to detect changes
in the strain profile across the part. Since the optical fibers themselves are very small, flexible, lightweight, and possess mechanical properties similar
to the carbon fibers in the structure, they do not
negatively affect the mechanical properties of the
specimen being evaluated.
The authors will begin by introducing basic rosette formations using optical fiber and then show

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SAMPE Journal - May/June 2020

Table of Contents for the Digital Edition of SAMPE Journal - May/June 2020