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FATIGUE DETECTION OF FIBRES

REINFORCED COMPOSITE MATERIALS

BY LASER’S

SPECKLE-SHEAR INTERFEROMETRY

(SHEAROGRAPHY)

Ventseslav Sainov

Bulgarian Academy of Sciences

Applications for NDT in avionics,

spacecraft’s and rocket’s industries

ABSTRACT

Fatigue detection by speckle-shear interferometry (shearography)

of subjected to cycling loading fibers reinforced composite

materials is presented. Shearography nondestructive testing is

providing a better and faster means to nondestructively inspecting

new aircraft both during manufacturing and in the field. The mainnew aircraft both during manufacturing and in the field. The main

advantages of the used technique is their possible application in a

wide dynamic range and working conditions. The experimentally

obtained results for non-cycled and cycled specimens are presented

together with the results from the pure tensile test and from the

cyclic test. Fatigue detection of subjected to cycling loading

(pressure) composite vessel has been obtained by lateral and 2D-

folding speckle shear interferometry. The results confirm the non-

linear mechanical behavior and fatigue of composite materials.

The calculation of the material quantities requires to measure the whole three-dimensional

displacement vector field

The interferometer consists of an optimized arrangement with 4 illumination directions and 1

observation direction to measure the 3D displacements and coordinates precisely,

Introduction

BIAS digital holographic interferometry set-up with four illumination

directions and its practical implementation.

Macro/micro measurements by speckle-shear interferometry

(SHEAROGRAPHY)(SHEAROGRAPHY)

Normal displacement macro-measurments by lateral shear inteferometry

Micro-measurement by 2D folding

shear interferometry

The electronic laser shearography imaging interferometer was

pioneered in the early 1980’s by three researchers, Dr. John Butters at

Loughborough University in the UK, Dr. S. Nakadate in Japan and Dr.

Mike Hung at Oakland University in the USA.

Shearography nondestructive testing has evolved considerably since

first used on a production aircraft program in the USA in 1986.

Shearography laser interferometric imaging methods measure test

Applications of shearography for NDT in

avionics, spacecraft and rocket’s industries

Shearography laser interferometric imaging methods measure test

structure deformation due to an applied engineered change in stress.

The resulting changes in Z-Axis strain component reveal images of

subsurface defects such as disbonds, delaminations, core defects and

impact damage in aerospace structures. Shearography NDT provides

high thru-put, cost-effective productivity enhancements, improved

manufacturing processes and quality. Development of digital CCD

cameras, the PC and small, high power solid-state lasers have led to

dramatic performance improvements in shearography instruments and

systems.

In the quest to maximize fuel economy and performance, engineers

have turned from riveted and bonded aluminum structures to solid

composite laminates, composite sandwich panels with honeycomb or

foam cores and tape wound composite structures such as fuselages.



The traditional methods for nondestructive testing, such as ultrasonic

(UT) C-Scan, may not provide the best defect detection capability for

these new materials and geometries and are slow with a typical

through-put of just 10 sq. ft./hour. Further, the process of

manufacturing complex composite structures requires a means for fast

inspection to provide a process control feedback and to ensure quality

and reliability at the lowest possible cost. In many aerospace programs

today, laser shearography is providing a large part of the solution.

Shearography is a mature and cost effective NDT technology for

many aerospace applications. Shearography provides very rapid

inspection allowing immediate feedback for process controls as well

as field inspection capability.

Composite aircraft manufacturing requires 100% inspection of all

bonded surfaces to verify structural integrity and compliance with

design.



These NDT instruments can be used on-aircraft, even on the tarmac or

in a hangar environment and offer excellent inspection capability for a

wide variety of defect types including non-visible impact damage,

disbands, voids, delamination, water entrapment and porosity in

composite repairs.

as field inspection capability.

Shearography is currently in use on a wide variety of aircraft

including F-22, F-35 JSF, Airbus, Cessna Citation X, Raytheon

Premier I and the NASA Space Shuttle.

In the last twenty years more than 1,200 shearography systems

have been integrated into the manufacturing process for aircraft

composites, tires and high-reliability electronics. As with all NDT

methods and technologies, shearography’s strengths and weakness

must be completely understood, and applications qualified



must be completely understood, and applications qualified

through Probability of detection (PoD) verification with written

procedures and rigorous training for operators and engineers

alike. Once qualified, however, shearography systems can operate

with extraordinary efficiency reaching through-puts from 25 to

1200 sq. ft per hour, 2.5 to 120 times the typical 10 sq. ft./hour

inspection rate for ultrasonic C-Scan.

1 foot =30.48 centimeters

Unlike UT C-Scan, which uses a single transducer that requires

a raster scan over the part to build up an image, Shearography

is a whole field, real-time imaging technique that reveals out of-

plane deformation derivatives in response to and applied stress.

Using a slight pressure reduction in a shearography test

chamber, critical defects are imaged and measured in seconds.

The shearography camera detects surface bumps as small as 3


avionics, spacecraft’s and rocket’s industries

The shearography camera detects surface bumps as small as 3

nanometers caused by local strain changes around subsurface

defects as the pressure is reduced on the part. Vacuum

shearography is highly effective for image disbonds,

delaminations, core damage and core splice-joint separations.

Other Shearography NDT techniques that are frequently used

include thermal pulse shearography for non-visible impact

damage, pressure shearography for damage to composite

wrapped pressure vessels. Vibration shearography has been

highly developed in the last several years to inspect the foam on

the external tank of NASA’s Space Shuttle.

Loading by:

Partial vacuum(from -0,14 to -49 kPa

differential)

Applied for testing:

ElastomersCoating, rubber and plastic voids, disbonds, tires, solidrocket motor liners, rubber-to-substrate bond, cork-tosubstrate bond

Sandwich panels-to-honeycomb, foam coresImpact damage, voids, disbonds, aircraft controlsurfaces, flaps, air brakes, helicopter blades, turbineengine ducts, laminated wood structures



Vibration(from 0,5 to 200 kHz,

90 to 125 dB)

engine ducts, laminated wood structures

Composite overwrap pressure vesselsFiber bridging, liner disbands

Foam rocket thermal protection systemsDamage, disbonds, delamination, cracks

Light weight honeycombSpacecraft solar panels, solar cell bond

Metal honeycombTurbine fan blade erosion strip bond, metal-to metalbonded panels and honeycomb

Metal brazed bonded and plasma sprayedDisbonds

Thermal loading

(from 0,5 to 48 deg C)

Laminated panelsImpact damage, delamination, wrinkled fibers, porosity, inclusions, embedded foreign materials, repairs

Sandwich panel honeycomb, foam coreImpact damage, skin-to-core disbonds, damage core, foam-tofoam disbonds, metal core-to-skin disbands, repairs

Resin transfer molded compositesResin lean areas, porosity, damage

Engine stators, vanes, composite fan blades Errosion strip bonds, voids, resin lean areas, damage, foreignobjects

Steel, aluminum, ceramics, compositesSurface breaking or near-surface breaking cracks

Pressure

(from 0,07 to 3500 kPa)

Surface breaking or near-surface breaking cracks

Composite overwrap pressure vessels with metal linersDisbonds at the liner-to-composite bond, fiber bridging

Composite overwrap pressure vessels and composite

roket motorsImpact damage, composite cracks, broken fibers, fiber bridging,porosity

Pressure vessels and heat transfer structuresMetal pressure tanks, liquid propellant rocket exit cones, thrustramps, piping, space vented core hoheycomb

Experimental setup for shearography of

composite vessel under pressure

Macro measurements

Micro measurements

Operation programs

User friendly interface for system operation and data processing

Fourier transform technique for phase retrieval

0fr

( ) ( ) ( ) ( )[ ]( ) ( ) ( )[ ]{ }∑∞

=⋅++

=⋅++=

10

0

2cos

2

p

pVB

VB

rfrpArIrI

rfrfrIrIrI

rrrrr

rrrrrr

πϕ

πϕ

- carrier frequency

( ) ( ) rftrtr orrrr ⋅+= 02,, πϕϕ

∑=1p

0f

Mitsuo Takeda, Hideki Ina, and Seiji Kobayashi, “Fourier-transform method of

fringe-pattern analysis for computer-based topography and interferometry”,

JOSA, Vol. 72, Issue 1, pp. 156-160 (1982)

Phase stepping techniques for phase retrieval

All measurements are performed in static conditions. Five steps

algorithm is used for phase calculation. Initial five intencity’s frames with

consecutive π/2 phase shifts are recorded. Phase distribution ϕ0 iscalculated from the recorded light intensities

Ii, (i = 0, 1, 2, 3, 4), as:

2 ( / 2) 2 ( / 2)I Iϕ − π − ϕ + π0

2 ( / 2) 2 ( / 2)

2 ( ) ( ) ( )

I Iarctg

I I I

ϕ − π − ϕ + πϕ =ϕ − ϕ − π − ϕ + π

The next five frames with the same π/2 phase shifts are recorded afterapplying the normal displacement of the loaded sample. Components of

the displacement vector and their derivatives are calculated from the

phase differences.

Experiment Loading F = 0 N

Φ0,h,+θ (F = 0 N)

→

→

Loading F = 2 N

Φ2,h,+θ − Φ0,h,+θ = ∆Φ2-0,h,+θ → ∆Φ2-0,h,+θ,f

-2∆φ -∆φ 0 +∆φ +2∆φ

−

Φ2,h,+θ (F = 2 N)

=

Subsequent steps in the automated FFT filtering

technique. Experimental phase fringe pattern (upper - left) and filtered phase fringe pattern (upper - right)

Phase-stepping technique for phase retrieval

Algorithm (5,1) for measurement in real time in digital ESPI

The idea – recording five phase shifted at 90 deg intensities maps for undeformed state of the object and a single intensity map at for the deformed.

( ) ( )[ ] ( )[ ]2 =∆Φ−=Φ−Φ

( ) ( )[ ] ( )[ ] 220011 cos14 AHO IIIII =−∆Φ−=−Φ−Φ ππ( ) ( )[ ] ( )[ ] 220011 2/cos142/ BHO IIIII =−∆Φ−=−Φ−Φ ππ

(α = π/2)(α = 0)

( ) ( )[ ] ( )[ ] 220011 cos14 CHO IIIII =∆Φ−=Φ−Φ( ) ( )[ ] ( )[ ] 220011 2/cos142/ DHO IIIII =+∆Φ−=+Φ−Φ ππ( ) ( )[ ] ( )[ ] 220011 cos14 EHO IIIII =+∆Φ−=+Φ−Φ ππ

( )( )222

22

2

2

EAC

DB

III

IIarctg

−−−=∆Φ

Chih-Cheng Kao, Gym-Bin Yeh, Shu-Sheng Lee, Chih-Kung Lee, Ching-Sang Yang,

and Kuang-Chong Wu, “Phase-shifting algorithms for electronic specklepattern interferometry”, APPLIED OPTICS Vol. 41, No. 1 1 January 2002.

Loading F=0 N

Loading F=2 N

∆Φ,h,+θ

Experiment

Five steps algorithm versus 5.1 for measurement in real time

- 2∆φ - ∆φ ∆φ = 0 ∆φ 2∆φ

- 2∆φ - ∆φ ∆φ = 0 ∆φ 2∆φ

Loading F=2 N

∆Φ,h,+θ

(5.1 algorithm)

(5 steps algorithm)

Sensitivity for different shear techniques

The two times higher sensitivity of 2D folding shear interferometry in measurement of one and the same object (glass flask at 60 kPa pressure) is illustrated bellow:

Lateral shear along X direction:

Folding shear about Y direction:

( ) ( )4 , ,w x y w x x yπ ∆ϕ = − + ∆ λ

( ) ( )4 , ,w x y w x yπ ∆ϕ = − − λ

The two times higher sensitivity of 2D folding shear interferometry is presented bellow:

Sensitivity for different shear techniques

Folding shear about X direction:

Two dimensional folding shear about X and Y directions:

( ) ( )4 , ,w x y w x yπ ∆ϕ = − − λ

( ) ( )4 , ,w x y w x yπ ∆ϕ = − − − λ

In-plane and out-of the plane derivatives of displacements measurement by speckle-shear interferometry

For in-plane and out-of the

plane derivatives of

displacements measurement

in static loading (static

pressure), lateral speckle-

shear interferometry has

been applied onto the same

100×100 mm area of the

object.

Setup for measurement by two

beams symmetrical illumination

The first results for derivatives of in-plane and out-of the plane

displacements are presented at 200 kPa loading at 30 pxls lateral shear

(5% over the object). The phase differences, obtained at sequence

illuminations through two arms at angles ± 40 deg to the normaldirection are:

for x direction


( ) ( ) ( )1, 22

1 cos , sin ,w u

x x y x x y xx x

π ∂ ∂ ∆ϕ = + θ + ∆ ± θ + ∆ ∆ λ ∂ ∂

( ) ( ) ( )1, 22

1 cos , sin ,w v

x y y x y y yy y

π ∂ ∂∆ϕ = + θ + ∆ ± θ + ∆ ∆ λ ∂ ∂

for x direction

for y direction

w∂≈

u∂≈


w

x

∂≈

∂u

x

∂≈

∂

w

y

∂≈

∂v

y

∂≈

∂

TESTING OF FIBRES TESTING OF FIBRES

REINFORCED COMPOSITE REINFORCED COMPOSITE

SAMPLESSAMPLESSAMPLESSAMPLES

Tensile and Cyclic TestsThe tested samples are plates with dimensions 200 × 30 × 3 mm,cut from unidirectional glass/epoxy fibres reinforced compositewith eight layers. All layers are reinforced with unidirectional glass

fibres. The stacking sequence used is [+45/−45]_2s.

Cyclic Test [+45/–45]_2s UD glass fibresTensile Test [+45/–45]_2s UD glass fibres

0 2 4 6 8 10 12 14

0

2

4

6

8

10

12

14

Cyclic Test [+45/–45]_2s UD glass fibres

load

[kN

]

displacement [mm]

0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5

0

2

4

6

8

10

loa

d [

kN

]

displacement [mm]

Tensile Test [+45/–45]_2s UD glass fibres

Three-points bending test

Three-points bending tests of fabric composite materials

a) normal displacement of cycled sample

(phase map – sample’s back side) b) normal displacement of cycled sample

a), b) influence of surface damage at 1 kN loading and 1.5 mm Z (normal) displacement – sample’s back side

c) results for non-cycled sample at 5 kN loading and 1.5 mm Z (normal) displacementd) influence of the material fatigue after cycling test

(at 5 kN loading and 1.5 mm Z (normal) displacement – sample’s front side)

c) normal displacement of non-cycled sample d) normal displacement of cycled sample

The object was subjected to cyclic loading and derivatives ofnormal displacements are periodically measured in staticcondition. The applied loading is near to the sinusoidal with0.2 Hz frequency from 300 to 500 kPa. The initial, interimsand final macro measurements are performed by lateralshear interferometry along x direction (1% over the central

Fatigue detection of fibers reinforced composite vessel after cycling loading

by speckle-shear interferometry

shear interferometry along x direction (1% over the centralpart sized 100×100 mm of the object)

at ~∆200 kPa static loading (static pressure), as well as micromeasurement using two dimensional folding shearinterferometry.

Experimental results

modulus 2π before cyclic loading

a) macro-measurement by lateral shear interferometry (1% over the object) at ∆200 kPa static loading;

b) micro-measurement by 2D folding shear interferometry at ∆500 kPa static loading and magnification 16× of the selected zone, indicated by a circle in a).

a) b)


modulus 2π obtained by lateral shear interferometry (1% over the object) at ∆200 kPa static loading

a) after 200 cycles (from 300 to 500 kPa loading)

b) after 400 cycles(from 300 to 500 kPa loading)

a) b)


modulus 2π after 600 cycles (from 300 to 500 kPa loading)

a) macro-measurement by lateral shear interferometry (1% over the object) at ∆200 kPa static loading;

b) micro-measurement by 2D folding shear interferometry at ∆500 kPa static loading and magnification 16× of the selected zone

a) b)

( )2 cos ;i O R O R iH I I I I α= + + Φ +( )2 cosR O R O RH I I I I= + + Φ

∆Φ ∆Φ

0α =( )2 cosO O R O RH I I I I= + + Φ + ∆Φ

at

i = 1÷5

( ) ( )4 1 cos 2 1 cos ;O RH I I= − Φ + ∆Φ − ∆Φ

( )2 2 216 sin sin2 2

O R O RH H H I I

∆Φ ∆Φ = − = Φ +

2 1sin2 2

∆Φ + Φ ≅

Phase retrieval 5.1 algorithm for “real time” interferometry with arbitrary phase steps

( ) ( )

( ) ( )

( ) ( )

( ) ( )

2

1 3 1

2

2 3 2

2

3 3 3

2

4 1 cos 2

4 1 cos

4 1 cos

O R O R

O R O R

O R O R

H H H I I

H H H I I

H H H I I

α

α

α

= − = − ∆Φ −

= − = − ∆Φ −

= − = − ∆Φ

= − = − ∆Φ + ( ) ( )

( ) ( )

2

4 3 4

2

5 3 5

4 1 cos

4 1 cos 2

O R O R

O R O R

H H H I I

H H H I I

α

α

= − = − ∆Φ +

= − = − ∆Φ +

4 2

1 3 5

( )1 cos(2 )tan ;

sin( ) 2

H Ha

H H H

αα

−−∆Φ = − + 5 1

4 2

( )cos

2( )

H Ha

H Hα

−= −

Loading F=0 N

Loading F=2 N

Experiment

Five steps algorithm versus 5.1 for measurement in real time

- 2∆φ - ∆φ ∆φ = 0 ∆φ 2∆φ

- 2∆φ - ∆φ ∆φ = 0 ∆φ 2∆φ

Loading F=2 N

∆Φ,h,+θ

(5.1 algorithm)

∆Φ,h,+θ

(5 steps algorithm)

12

3

4

5

metal plateobject

x

Experimental set-up for void's detection by shearography with thermal loading of the object

The objectis a sheet of copper laminated composite layer

with dimensions (x,y,z) 290x190x~0.2 mm

+-

α β

6

78 9

10

l

+-

Holographic table

to vacuum pump

Peltier element

metal base

Styropor

x

Optical arrangement for two dimensional lateral shear

interferometry, when 1 is a laser, 2- interferometer with

CCD camera, 3 micro objective, 4- pinhole, 5- objective

Voids detection by phase stepped two dimensional lateral shear interferometry

Two dimensional later shears over the detection area is 20%

along x and 2% along y directions. Thermal loading (∆T~10

deg C) is applied with the incorporated in the experimental

device Peltier element 30 x 30 x 3.5 mm. As the illumination

and observation angles are small, the contribution of in-plane

displacement could be neglected. and phase difference along

x direction after loading could be expressed as

where w(x,y) is the normal component of the displacement

vector

( ) ( )

∆+∂∂α+

λπ≈ϕ∆ yxx

x

w,cos1

2


Phase map of the phase

difference due to thermal

loading (∆T~10 deg C)3D presentation of the phase map


3D presentation of normal displacement due to thermal loading (∆T~100C) after integration along x axis

In the present work the possibility of speckle shear, fringes and projection

interferometry for fatigue detection of fibers reinforced composite

materials is presented.

The presentation of the results as first difference (derivatives) of normal

The results for normal displacements and their first derivatives for cycled

and non-cycled specimens are obtained. The fatigue of the tested

composite material as well as local damage are clearly identified.

The tensile, cyclic and 3-points bending tests have been applied on plates

with dimensions 200 × 30 × 3 mm, cut from unidirectional glass/epoxy fibresreinforced composite with eight layers.

CCOONNCCLLUU The presentation of the results as first difference (derivatives) of normal

displacement is more informative due to the higher sensitivity, that allow

fatigue detection of composite materials and machine parts to be performed

at low-levels loadings.

Full-field displacement’s derivatives by speckle-shear interferomety of real

3D composite vessel were performed. For the first time 2D folding-shear

interferometry has been applied for measurements with 16× magnification.

The obtained results confirm non-linear mechanical behavior of composite

materials. The possibility for measurement and testing of such objects in

“real” time operation mode and working conditions by speckle shear

interferometry is shown.

UUSSIIOONNSS

Acknowledgements:

This report is dedicated to the memory of Prof. Pierre Boone(1941-2010) from Gent University, Belgium, for the friendship andthe common pioneer’s works in digital holographic, patternprojection, and speckle shear interferometry.

THANK YOU!THANK YOU!

Documents

FATIGUE DETECTION OF FIBRES REINFORCED COMPOSITE … · surfaces, flaps, air brakes, helicopter blades, turbine engine ducts, laminated wood structures ... roket motors Impact damage,