SHOWCASE 2012

AEROSPACETESTINGINTERNATIONAL.COM

SHOWCASE 2013

AEROSPACETESTINGINTERNATIONAL.COM

Fiber sensors z

hile each foil gage requires

at least two copper wires

for a single point of

sensing, a distributed fiber-optic

sensor can provide thousands of

sensing points with a single optical

fiber and connection. Optical fiber

sensors are immune to

electromagnetic interference and they

are efficient and cost effective to

install. And because they are small

and lightweight, they have little effect

on the structure being tested.

With distributed sensing using

optical backscatter reflectometry,

introduced by Luna Technologies in

2004, strain can be measured at any

point along the fiber with spatial

resolution as fine as a few millimeters.

This makes it possible to measure

the strain profile of structures at many

locations rather than at a few points.

This is beneficial for the aerospace

industry, which is increasing the use of

composite materials and their complex

strain profiles. The use of composite

materials is being pursued to achieve

material and structural component-

tailored design and to reduce the

number of structural components,

which leads to a valuable reduction in

aircraft weight, and more efficient and

environmentally friendly aircraft. The

challenge in the application of

composite materials resides in their

complex nature at the material level,

For years the aerospace industry has relied on foil strain gauges

to test materials. But an emerging technology, fiber sensing, is

offering advantages over more traditional methods

BY DAWN K. GIFFORD

Appeal to the sensors

leading to the mentioned complex

strain profiles. High-resolution

distributed fiber sensing is ideal for

measuring the non-uniform strain

profiles in composite structures and

identifying defects within the

composite parts well before fatigue

turns to catastrophe.

Despite these advantages, and

though fiber sensing has been available

in some form for more than 20 years, it

has not yet been commonly adopted in

aerospace. However, due to the

advantages of and advances in the

available technology, distributed fiber

sensing is emerging as a desirable

method for aerospace testing.

TESTING THE OPTIONS

One common concern with an

emerging sensing technology is

validating its performance against

known techniques. The National

Research Council Canada (NRC)

recently performed independent

comparative testing of Luna’s

distributed fiber sensing versus

traditional foil gages on a test rig

designed to simulate loading on a

representation of a solid aircraft wing

spar-web structure. NRC operates

Structural Health Monitoring test

facilities for load monitoring and

damage detection sensor testing and

validation. These testbeds present a

range of structural complexity, from

spar structures to the intricacies of the

outer wing of a fighter jet.

The test, conducted earlier this

year consisted of two aluminum

beams clamped to a central pedestal,

acting as the simulated wing root. The

beams were loaded at the tips in a

simple cantilever configuration.

Though the pressure loading

experienced by an aircraft wing would

be better simulated with a distributed

loading configuration, such a

complicated setup was not required

for this initial test.

An optical fiber to be used as a

continuous sensor was bonded to one

side of the beam, first running along

the center line from root to tip and

then returning in a pattern crossing

over the center line at a 45° angle in

four locations, running from tip to

root. Foil strain gage rosettes were

placed at the four points along the

center of the beam on the underside

opposite the fiber. Luna’s optical

backscatter reflectometer was used to

measure the strain along the fiber

sensor, while the MTS data acquisition

component of the actuation

equipment was used to acquire the

strain data. The figure below

illustrates the test configuration.

Data was collected with the fiber

sensor and the foil gages with the beam

fully deflected 4in up at the tip

(maximum strain) and deflected 2in

LEFT: Test

configuration

showing fiber

layout and strain

gauge locations

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SHOWCASE 2013

AEROSPACETESTINGINTERNATIONAL.COM

“ONE COMMON CONCERN WITH

AN EMERGING SENSING

TECHNOLOGY IS VALIDATING ITS

PERFORMANCE AGAINST

KNOWN TECHNIQUES”

sensor was a commercially available

polyimide-coated, low-bend loss fiber.

In subsequent tests, similar

fiberglass coupons were loaded in a

four-point bend configuration and

cycled to ±4,250µε. While foil gages

bonded to the coupons typically failed

after a few hundred cycles under this

high-strain fatigue cycling, the fiber

sensor continued to make accurate

measurements over several thousand

cycles. In the maximum tested case,

the fiber sensor was unaffected after

28,000 cycles.

These results demonstrate the

advantage of optical fiber sensors over

foil gages for high-load fatigue testing,

which is commonly needed for

composite aerospace structural and

material testing.

NRC said in its report, issued in

March 2012, that: “It is evident that

the use of Luna’s fiber optic system

compares very well with the strain

gages applied by NRC to SHM

Platform 1A for static loading

conditions. The Luna fiber optic

system has several advantages over

strain gages, such as their immunity

to electromagnetic interference. The

values obtained from Luna’s fiber

optic system are accurate and

repeatable as shown in this report.

This technology shows promising

results for static and quasi-static

loading conditions, making it a

promising technology for full-scale

tests of aerospace structures in a

laboratory environment.”

As part of future work, Luna will

apply its sensing system on additional

testing platforms to evaluate

capabilities on a more complex

structure, as well as in the outer wing

of a fighter jet testbed. z

Dawn K. Gifford, PhD, is director of technology

development at Luna Innovations Inc, based in

Virginia, USA

percent difference in this data point is

high only because the overall strain

value at the beam tip was low.

Tables 1 and 2 (above) show the

comparative results.

FATIGUE LIFE COMPARISON

Composite structures used in

aerospace are often tested at strain

levels higher than a few thousand

microstrain, a point where metal foil

gages are known to drift and eventually

fail by fatigue

10,11

(of electrical circuitry,

bonding, connectors, etc.). Optical

fiber, however, is made of fused silica,

which has a high fatigue life.

Separate from the Canadian

research, Luna recently performed

tests demonstrating the fatigue life of

fiber sensors versus foil gages for high-

strain fatigue life testing. A fiber sensor

was bonded along the length of a

fiberglass coupon

⁄16 in thick and ¾in

wide. A foil gage was bonded

immediately beside the fiber at the root

of the coupon when placed in a simple

cantilever configuration.

Vishay M-Bond 200 adhesive was

used to bond both sensor types to the

coupon. After placement in the test

configuration, the cantilevered length

of the coupon was 3.7in. The tip of the

coupon was displaced cyclically by

0.65in to produce strains at the root of

±4,000µε. Measurements were

recorded with both types of sensors at

the maximum, minimum, and zero

load conditions after every 50 cycles.

As expected, the foil gage deviated

from the expected strain value with

increasing number of cycles.

The fiber sensor, however,

continued to make accurate strain

readings throughout the fatigue

testing, deviating by less than 2%.

These results are illustrated in Figure

4. The foil gage used in this testing was

a Vishay EA series gage in a quarter-

bridge configuration. The optical fiber

down at the tip (minimum strain) and

compared at four locations in the fiber

corresponding to the placement of the

foil gages.

The figure above shows the plot of

the maximum and minimum bending

strains measured with both sensor

types at each of the four stations

along the span of the beam. The

displayed strains are absolute values

as the foil gage and fiber sensor were

on opposite sides of the beam and

therefore experienced strains with

opposite signs.

The results showed good correlation

between the two sensor types, with

strain measurements differing typically

by less than 10 microstrain. The

highest difference of 32 microstrain

occurred in the maximum loading

condition at station 1 near the root of

the beam. The percent difference was

lower than 6% in all cases except for

the minimum strain at station 4. The

ABOVE:

Comparison of foil

gauge results with

results from Luna

fiber sensor at four

locations along the

test article. The

max strains were

taken with the

beam deflected

upward 4in at its

tip, while the

minimum strains

were recorded

when the beam was

deflected

downward by 2in

z Fiber sensors

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