zonta et al. • photonic crystals for distributed strain measurements

SPIE Smart Structures/NDE • San Diego, Mar 10, 2010

Photonic crystals Photonic crystals for distributed strain for distributed strain

measurementsmeasurements

Daniele ZontaDaniele Zonta11, Andrea Chiappini, Andrea Chiappini22, Alessandro Chiasera, Alessandro Chiasera22, Maurizio , Maurizio

FerrariFerrari22, ,

Matteo PozziMatteo Pozzi11, Lorenzo Battisti, Lorenzo Battisti11, Matteo Benedetti, Matteo Benedetti33

1-DIMS - University of Trento – Italy1-DIMS - University of Trento – Italy

2-CNR-IFN, Istituto di Fotonica e Nanotecnologie, CSMFO Lab2-CNR-IFN, Istituto di Fotonica e Nanotecnologie, CSMFO Lab..

3-DIMTI, University of Trento3-DIMTI, University of Trento

zonta et al. • photonic crystals for distributed strain measurements

outline

motivationmotivation what is a Photonic Crystal? what is a Photonic Crystal? producing a Photonic Crystalproducing a Photonic Crystal experimental validationexperimental validation conclusionsconclusions

zonta et al. • photonic crystals for distributed strain measurements

motivation

Drewry and Georgiou "A review of NDT techniques for wind turbines" NDT.net (2007).

the strong drive to exploit wind the strong drive to exploit wind energy has recently led to new types energy has recently led to new types of location for wind turbine of location for wind turbine installations being considered, installations being considered, including including mountain regionsmountain regions

such locations are bound to be more such locations are bound to be more critical, because of the more critical, because of the more severe severe weather and wind conditionsweather and wind conditions

zonta et al. • photonic crystals for distributed strain measurements

motivation

Connor and Hodgson, [Field Instrumentation and Testing of High-Mast

Lighting Towers in the State of Iowa], (2006).

a major concern is the a major concern is the reliability of the reliability of the supporting structuressupporting structures in the life-long in the life-long periodperiod

recent recent failures of lighting towersfailures of lighting towers due to due to fatiguefatigue have raised questions as to the have raised questions as to the robustness and safety of the existing robustness and safety of the existing inventory of similar structuresinventory of similar structures

small cracks gradually develop due to small cracks gradually develop due to wind-induced cyclic loads and eventually wind-induced cyclic loads and eventually the cracks with highest stress the cracks with highest stress concentration propagate, occasionally concentration propagate, occasionally resulting in a catastrophic failureresulting in a catastrophic failure

zonta et al. • photonic crystals for distributed strain measurements

motivation

the reliability of the structures supporting the wind turbines the reliability of the structures supporting the wind turbines can be can be enhanced enhanced via the adoption of via the adoption of permanent instrumented permanent instrumented monitoringmonitoring

an efficient sensing system provides the owner with a an efficient sensing system provides the owner with a real-real-time damage estimatetime damage estimate and allows easy maintenance in order and allows easy maintenance in order to prevent failureto prevent failure

the system should allow the system should allow early recognitionearly recognition of the micro and of the micro and macro macro crackcrack patterns induced by patterns induced by fatigue loadfatigue load

zonta et al. • photonic crystals for distributed strain measurements

case study

prototype mini wind turbine prototype mini wind turbine erected at the Trento Experimental erected at the Trento Experimental

Wind FarmWind Farm suitable for installation in mountain suitable for installation in mountain

regionsregions two bladed downwind turbinetwo bladed downwind turbine rated power 11kWrated power 11kW diameter of the rotor 13mdiameter of the rotor 13m tower is 18m tall, made in three tower is 18m tall, made in three

segments connected with bolted jointssegments connected with bolted joints connection flanges are welded to connection flanges are welded to

tubular profilestubular profiles

zonta et al. • photonic crystals for distributed strain measurements

case-study

18.1

0 m

13.0

0 m

turbinewind

velocity

fondation

1.20 m4.50 m

steel tower

diameter:0.55 m

to1.10 m

prototype mini wind turbine prototype mini wind turbine erected at the Trento Experimental erected at the Trento Experimental

Wind FarmWind Farm suitable for installation in mountain suitable for installation in mountain

regionsregions two bladed downwind turbinetwo bladed downwind turbine rated power 11kWrated power 11kW diameter of the rotor 13mdiameter of the rotor 13m tower is 18m tall, made in three tower is 18m tall, made in three

segments connected with bolted segments connected with bolted jointsjoints

connection flanges are welded to connection flanges are welded to tubular profilestubular profiles

zonta et al. • photonic crystals for distributed strain measurements

numerical models

A first FEM predict the global response of the base of the tower under various environmental conditions

The model accounts for the gust wind loads dimamic effect effects on the blades

and the rotor and on the tower itself.

zonta et al. • photonic crystals for distributed strain measurements

numerical models of fatigue cracks

a second FEM details the a second FEM details the behaviour of the tower about the behaviour of the tower about the base connectionbase connection

3D elements used3D elements used

the strain field calculated with the the strain field calculated with the global model is assigned as inputs global model is assigned as inputs to the local FEMto the local FEM

various crack scenarios consideredvarious crack scenarios considered

zonta et al. • photonic crystals for distributed strain measurements

strain field around a fatigue crack

the strain field due to a crack is inhomogeneous high concentrations around the crack tips and a specific pattern of

stress increment and release we need a distributed strain sensor capable measure the strain

field!

zonta et al. • photonic crystals for distributed strain measurements

outline

motivationmotivation what is a Photonic Crystal?what is a Photonic Crystal? producing a Photonic Crystalproducing a Photonic Crystal experimental validationexperimental validation conclusionsconclusions

zonta et al. • photonic crystals for distributed strain measurements

what is a photonic crystal?

produced of produced of colloidal spherescolloidal spheres in solution, organized into a 3- in solution, organized into a 3-dimensional crystalline latticedimensional crystalline lattice

the size of the spheres and their polydispersivity are such that the the size of the spheres and their polydispersivity are such that the crystal acts crystal acts as a narrow-band mirroras a narrow-band mirror (it reflects or transmits only a (it reflects or transmits only a specific wavelength range)specific wavelength range)

by an appropriate selection of the optical properties, by an appropriate selection of the optical properties, the stop bands the stop bands can be set in the visiblecan be set in the visible range. range.

d0

light source light detector

d()

axial elongation

tra

nsve

rsa

l co

ntra

ctio

n

Initial configuration:

Strained configuration:

polystyrenesphere

elastometer

l

support

l

x=l

d0

light source light detector

d()

axial elongation

tra

nsve

rsa

l co

ntra

ctio

n

Initial configuration:

Strained configuration:

polystyrenesphere

elastometer

l

support

l

x=l

zonta et al. • photonic crystals for distributed strain measurements

Bragg’s law

the reflecting behavior of the PC can be express by the modified form of the reflecting behavior of the PC can be express by the modified form of Bragg's lawBragg's law..

wherewhere λλ is the free-space wavelength of the lightis the free-space wavelength of the light dd is the inter-planar spacingis the inter-planar spacing nneffeff is the effective refractive index is the effective refractive index

dneff2

d0

light source light detector

d()

axial elongation

tra

nsve

rsa

l co

ntra

ctio

n

Initial configuration:

Strained configuration:

polystyrenesphere

elastometer

l

support

l

x=l

d0

light source light detector

d()

axial elongation

tra

nsve

rsa

l co

ntra

ctio

n

Initial configuration:

Strained configuration:

polystyrenesphere

elastometer

l

support

l

x=l

zonta et al. • photonic crystals for distributed strain measurements

theoretical sensitivity to strain

d0

light source light detector

d()

axial elongation

tran

sver

sal

cont

ract

ion

Initial configuration:

Strained configuration:

polystyrene sphere

elastometer

l

support

l

x=l

When an axial strain is applied to the PC, the inter-planar spacing is When an axial strain is applied to the PC, the inter-planar spacing is modified by the transversal contraction. Hence the initial value dmodified by the transversal contraction. Hence the initial value d00 is is reduced to d(reduced to d() depending on the applied strain value ) depending on the applied strain value . .

This effect changes the reflectance properties of the PC, and This effect changes the reflectance properties of the PC, and particularly the wavelength peak in reflection.particularly the wavelength peak in reflection.

zonta et al. • photonic crystals for distributed strain measurements

theoretical sensitivity to transversal strain

zeff dn 02

dneff2

x

z

y

yxz

1

Bragg’s law:

sensitivity to transversal strain:

transversal strain of crystal to in-plane strain of the support:

crystal

support s

zonta et al. • photonic crystals for distributed strain measurements

theoretical sensitivity to strain

xsy

x

z

y

xeff dn 02

xxs

z

1

1

applying an axial load:

sensitivity to axial strain:

z

x

xcrystal

support s

zonta et al. • photonic crystals for distributed strain measurements

outline

motivationmotivation what is a Photonic Crystal? what is a Photonic Crystal? producing a Photonic Crystalproducing a Photonic Crystal experimental validationexperimental validation conclusionsconclusions

zonta et al. • photonic crystals for distributed strain measurements

production of PC

we produced crystal with polystyrene spheres (PSs) embedded we produced crystal with polystyrene spheres (PSs) embedded in a poly-dimethylsiloxane (PDMS) elastomerin a poly-dimethylsiloxane (PDMS) elastomer

The fabrication process includes 3 steps:The fabrication process includes 3 steps:

1- production of the polystyrene spheres will form the opal 1- production of the polystyrene spheres will form the opal structure; structure;

2- deposition of the spheres in a lattice structure on the support; 2- deposition of the spheres in a lattice structure on the support;

3- infiltration of the crystal with an elastomer.3- infiltration of the crystal with an elastomer.

zonta et al. • photonic crystals for distributed strain measurements

step 1: production of the nano-spheres

mix of styrene water surfactant (SDS) a polymerization initiator

(K2S2O8) controlled temperature and

stirring speed the PSs produced with a

diameter of about 230nm to obtain a suitable PC color;

the size can be modified by applying a different fabrication protocol

stirrer

condenser

temperature controller

zonta et al. • photonic crystals for distributed strain measurements

step 2: deposition on the substrate

A highly mono-dispersed PS A highly mono-dispersed PS suspension is deposited on suspension is deposited on a Viton substrate of size a Viton substrate of size 50x15x1mm.50x15x1mm.

the substrate is placed the substrate is placed vertically into the water vertically into the water suspension suspension

in oven at constant in oven at constant temperature of 50°C for two temperature of 50°C for two daysdays

zonta et al. • photonic crystals for distributed strain measurements

step 2: deposition on the substrate

a)

stirrer

condenser

temperature controller

b) c)

crystallization is initially crystallization is initially driven driven by strong attractive capillary by strong attractive capillary forcesforces acting between particles acting between particles at the drying frontat the drying front

This effect drives particles to This effect drives particles to aggregate in layersaggregate in layers and and creates a porous structure with creates a porous structure with high specific surface area. high specific surface area.

The large surface area The large surface area facilitates solvent evaporation. facilitates solvent evaporation. The thickness of the PC is in The thickness of the PC is in the order of the order of 1010mm..

zonta et al. • photonic crystals for distributed strain measurements

step 3: infiltration

the lattice is the lattice is infiltrated with infiltrated with PDMSPDMS (poly-dimethylsiloxane) (poly-dimethylsiloxane)

after infiltration the PDMS is after infiltration the PDMS is cured for 4 hours at 65°Ccured for 4 hours at 65°C

then, the excess elastomer is then, the excess elastomer is peeled-off from the crystalpeeled-off from the crystal

several infiltration cyclesseveral infiltration cycles were were necessary to completely fill necessary to completely fill the voids between the PS the voids between the PS spheresspheres

before any re-infiltration step before any re-infiltration step the sample is the sample is swollenswollen in a in a silicone fluid.silicone fluid.

zonta et al. • photonic crystals for distributed strain measurements

SEM image of crystal lattice

zonta et al. • photonic crystals for distributed strain measurements

result

assuming that the PSs are in assuming that the PSs are in contact, theory predicts a value of contact, theory predicts a value of reflected peak reflected peak =550nm (=green)=550nm (=green)

The unstressed PC appears red The unstressed PC appears red

while it is not stressed (while it is not stressed ( = = 650nm), and turns green as a 650nm), and turns green as a tensile strain is applied.tensile strain is applied.

the actual color of the PC shows the actual color of the PC shows that the PSs are separated by the that the PSs are separated by the elastomer, (d is larger than their elastomer, (d is larger than their diameter)diameter)

zonta et al. • photonic crystals for distributed strain measurements

outline

motivationmotivation what is a Photonic Crystal? what is a Photonic Crystal? producing a Photonic Crystalproducing a Photonic Crystal experimental validationexperimental validation conclusionsconclusions

zonta et al. • photonic crystals for distributed strain measurements

experimental validation

spectrometer

linear stage

photonic crystal

light source light detector

anchorage

the strip is fastened to two micrometric linear stagesthe strip is fastened to two micrometric linear stages the diffraction characteristic of the PC is recorded using a the diffraction characteristic of the PC is recorded using a

spectrometer, composed by a light source, a light receiver and a spectrometer, composed by a light source, a light receiver and a spectrum analyzerspectrum analyzer

the probe is kept perpendicular to the strip the probe is kept perpendicular to the strip the area of the PC inspected by the device is about 4mmthe area of the PC inspected by the device is about 4mm22

zonta et al. • photonic crystals for distributed strain measurements

set-up

linear stage

probe for lightsource and detector

photonic crystal

anchorage

zonta et al. • photonic crystals for distributed strain measurements

0 50 100 150 200540

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[nm

]

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zonta et al. • photonic crystals for distributed strain measurements

specimen 1

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zonta et al. • photonic crystals for distributed strain measurements

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zonta et al. • photonic crystals for distributed strain measurements

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zonta et al. • photonic crystals for distributed strain measurements

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zonta et al. • photonic crystals for distributed strain measurements

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zonta et al. • photonic crystals for distributed strain measurements

specimen 1

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20

30

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ctan

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abs.

uni

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wav

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peak

[nm

]

[m]

zonta et al. • photonic crystals for distributed strain measurements

experimental performances

sensitivity to strain: sensitivity to strain: -288 pm/m-288 pm/m inverse sensitivityinverse sensitivity 3.47 3.47 /pm/pm resolution of the reflected peakresolution of the reflected peak ~~100 pm100 pm instrumental resolutioninstrumental resolution ~~350 350

max measurable elongation:max measurable elongation: >150 m>150 m= 15%]= 15%]

to appreciate a change in color:to appreciate a change in color: >10 nm>10 nm

>35 m>35 m [= 3.5%] [= 3.5%]

these performance can be improved!these performance can be improved!

zonta et al. • photonic crystals for distributed strain measurements

PCs vs. FBGsPhotonic crystalPhotonic crystal

resolution of the reflected peak resolution of the reflected peak in the order of ~100pmin the order of ~100pm

(inspected area of 4mm(inspected area of 4mm22))

reflected band depends on the reflected band depends on the transversal straintransversal strain

works in the visible works in the visible ((~400-600 nm)~400-600 nm)

FBGFBG

width of the reflected peak in width of the reflected peak in the order of ~20pm the order of ~20pm (for a 20mm-long FBG) (for a 20mm-long FBG)

the grating is deformed directly the grating is deformed directly by the strain of the supportby the strain of the support

normally works in the infrared normally works in the infrared ((~1550 nm)~1550 nm)

xeff dn 02 xeff dn 02

zonta et al. • photonic crystals for distributed strain measurements

concluding remarks

the laboratory experiment demonstrates that the fabrication the laboratory experiment demonstrates that the fabrication of a distributed strain sensor based on PC is of a distributed strain sensor based on PC is feasiblefeasible

the resolution obtained so far is the resolution obtained so far is insufficient for civil insufficient for civil applicationsapplications

appropriate selection of the diameter of the spheres and of appropriate selection of the diameter of the spheres and of the swelling process the swelling process can improve the resolutioncan improve the resolution

compared with Fiber Bragg Grating, the instrumental compared with Fiber Bragg Grating, the instrumental resolution of PCs is much lowerresolution of PCs is much lower

however, in order to recognize fatigue damage it is essential however, in order to recognize fatigue damage it is essential to identify the planar to identify the planar strain fieldstrain field around cracks around cracks

PC can be used as a PC can be used as a distributed sensordistributed sensor to reconstruct the to reconstruct the strain fieldstrain field

zonta et al. • photonic crystals for distributed strain measurements

acknowledgments

Thank you for your attention!Thank you for your attention!

This work was carried out within the follwing projects: This work was carried out within the follwing projects: PAT-CRS2007 PAT-CRS2007 PAT FaStFal 2007-2010PAT FaStFal 2007-2010 COST MP0702COST MP0702

Special thanks to: Special thanks to: Cristina Armellini (CNR-IFN)Cristina Armellini (CNR-IFN) Roberto Fedrizzi (DIMS-UniTN) Roberto Fedrizzi (DIMS-UniTN) Vigilio Fortanari and Daniele Laner (DIMTI-UniTN)Vigilio Fortanari and Daniele Laner (DIMTI-UniTN)