Shearography

Shearography

1.Introduction:

Because of high strength to weight ratio, the usage of composite material in load

resisting structure is increasing at rapid rate. A composite material is a combination of two

or more material and thus, the likelihood of having flaws in composite material generally is

higher than that in metal. Consequently, there is a need to monitor the integrity of composite

structure during and after fabrication. Since flaw and damage may develop during service,

non destructive inspections are also required in service.

Shearography is an optical nondestructive testing method that provides fast

information about the inside quality of different materials. Shearography is being extensively

used in production and development within aerospace, space, wind rotor blades, automotive

and materials research areas. Main advantages of shearography are the large area testing

capabilities (up to 1 m² per minute), non-contact properties and its good performance on

honey-comb materials, which is a big challenge for traditional NDT methods.

1.1History of Shearography NDT:

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. The author’s team

at Laser Technology Inc. led the development of the shearography camera as a tool for non-

destructive testing, delivering the world’s first production shearography NDT system to

Northrop Grumman in 1987 for the manufacturing of the USAF B2 Stealth Bomber.

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 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.

[ The USAF B-2 stealth bomber was the first aircraft to incorporate

Shearography NDT technology in the manufacturing of complex composite]

Basic principle:

The very basic idea with shearography is to take images of a test specimen's

surface with a special shearography camera. The camera acquires an interferometric

image of the surface and stores it in a computer. This image can be thought of as a

unique footprint of this surface, at this state, including surface roughness and shape.

The material is now stressed with a small amount of load, for example with heat. The

material wants to expand when heated up, and if it has weak spots it will be allowed to

expand more. At the loaded state one more interferometric image is taken. Now we

also have an interferometric footprint of the area at the deformed state. To extract

information about the difference between the two states, with appropriate software in

the computer, we subtract the two images and a shearogram is created. This

shearogram is in fact a map of the strains the surface has undergone due to the applied

heat, in other words the gradients (slopes) of the expansions on the surface were

measured, not the surface's expansion. The sensitivity of measurement normal to the

surface (out of plane) is about one half the wavelength of the laser light used in

illuminating the surface (about 30.0 nanometers in the case of HeNe laser). Using a

Phase Stepping Shearography Sensor will however give a much higher sensitivity to

fractions of the wavelength, normally 20 nm is a good rule of thumb. The defects will

be seen as fringe patterns resembling a pair of “hills” or a pair of "bulls-eyes"

superimposed on the surface's image. The size of the defects (in plane) can be

quantified by measuring how large this fringe pattern is.

Shearing function:

[ Fig. 1A primitive shearography setup ]

[ Fig. 1 : Two physical points on test object will be projected on to one point on the CCD ship to

record a interferometric footprint. Up to 1 square meter from a test object can be projected to a

high-resolution CCD chip. The tested surface is illuminated with a monochromatic light, typical 650

nm.]

[Fig. 2 The primitive shearography principle]

[Fig.2 : A shearography image is recorded at unloaded state and one image is recorded in the

loaded state. Thereafter they are subtracted and in the result defects can be detected.]

When a surface area is illuminated with a highly coherent laser light a stochastical

interference patterns is created. This interference pattern is called a speckle and is projected

on a rigid camera's CCD chip. Analogous with ESPI, Electronic speckle pattern

interferometry, to obtain results from the speckle we need to compare it with a known

reference light. Shearography uses the test object itself as the known reference, it shears the

image so a double image is created. The superposition of the two images, a shear image,

represents the surface of the test object at this unloaded state. This makes the method much

more insensible to external vibrations and noise. By applying a small load; heat or vacuum,

the material will deform. A non-uniform material quality will generate a non uniform

movement of the surface of the test object. A new shearing image is recorded at the loaded

state and will be compared with the sheared image before load. If a flaw is present it will be

seen in this result.

Due to the latest development of efficient laser diodes that are suitable for

interferometry, the illumination of the components with laser light is much simpler. A

homogeneous illumination of the measurement area can be realized with an array of laser

diodes. Since laser diodes are quite small and easy to handle, maximum practical use is

guaranteed. During measurement, an image of the object which is not under load is recorded

and stored. Then, the object to be measured is put under strain. In this condition, a second

image of the object is recorded and stored. By subtraction of these two images areas of the

object will be clearly visible where the surface is deformed. This can be watched in real time

while the component is put under strain. Faulty areas show a specific deformation pattern in

contrast to areas without defects.

As a means of strain, heat and pressure differences are especially suitable. An object

is put under heat excitation with heating lamps. Even if the surface is only heated up by few

Kelvin, the material deforms in such a way that this can be measured with Shearography.

Faulty parts of a component which lead to an inhomogeneous mechanical stiffness can be

seen in the deformation image as inhomogeneous.

For low-pressure strain, the sample to be measured is placed inside a pressure chamber.

Closed component faults, e.g., faults which are separated air tightly from the surrounding

parts lead to deformations due to pressure difference. Even slight pressure difference in the

range of a few ten millibar is enough to make the faults visible.

Phase-shift technology:

Fig.3

A modified Michelson cube is here used where a double breaking mirror as a beam splitter.

One mirror is for adjustment of shear properties and the other one is the phase stepper

Fig.4

The phase stepper moves through its four positions with an internal difference of 1/4

wavelength; at each position an image is recorded and sent through the software processor to

evaluate the be phase relationship with a best fit algorithm. To increase the sensitivity of the

measurement method, a real-time phase shift process is used in the sensor. This contains a

stepping mirror that shifts the reference beam, which is then processed with a best fit

algorithm and presents the information in real time.

Portable Thermal Shearography System

Model LTI 6200S

Description

The LTI-6200S is a compact, portable thermal shearography system designed for the

nondestructive inspection of aerospace composite repairs, structures and components. The

LTI-6200S has a vacuum attach feature to allow operation in any orientation on-aircraft, on

panels in the shop or on the bench. The cantilevered design (Patent applied) allows inspection

up to edges and corners of flaps, control surfaces, wing panels or cut-outs. The system

features automatic operation with easily programmed NDE Procedure Macros, automatic

exposure and image storage. The LTI-6200S includes the Inspection Head with built-in

digital shearography camera and Transit Case with all electronics and image processing

computer built-in.

System Features

Self contained

Light weight

Vacuum attach

Test macros

Defect measurement tools

Simple image download

Material Applications:

Composite Laminate

Composite Repairs

Metal and Composite Honeycomb

Metal to Metal Bonds

Specifications:

Dimensions : (L x W x H)

o Inspection Head : 15 x 12 x 12 inches, 38 x 30 x 30 cm

o Transit Case : 22 x 18 x 16 inches, 56 x 46 x 41 cm

Weight

o Inspection Head : 10 lbs., 4.5 kg.

o Transit Case : 25 lbs., 11.3 kg.

Power : 100 to 240 VAC, 50/60 Hz., 15 amps max.

Field of View (H x W) : 4 x 6 inches, 10.3 x 15.4 cm

Displays (2) (H x W) : 4 x 6 inches, 10.3 x 15.4 cm, 1200 x 800

pixels, 32bit color,

16 inch/40cm, 1200 x 800 pixels, 32bit color

Operation Modes : Time Resolved Thermal Shearography Analysis

and Measurement Image Overlay

System Software : Thermal Shear 2.0

Options : Vacuum Window and Generator VW-10

External Hard Drive for Data Transport

(40GB)

Types of Results

Fig. 5: Tool drop impact damage to Fig. 6: Disbands on composite repair composite wing panel

Fig. 7: Impact damage to aircraft composite fig. 8: Metal honeycomb cells honeycomb fairing

Shearography:

The optical set-up for the speckle shearing interferometer is shown in

Figure

Fig. A schematic diagram of the shearing interferometer.

A single mode and single frequency HeNe laser of 60 mW output power with an

emission wavelength of 632 nm is used as the coherent source in the set-up. The laser light

illuminates the object to be measured via a single mode fibre. The diffusely scattered light

from the object passes through a beam splitter and is imaged at the plane of the CCD camera

by the two mirrors, which are orthogonally placed at an equal distance of 15 mm from the

beam splitter. The mirror 1 acts as the shearing mirror and the amount of shear can be

adjusted by shifting the angle of the mirror. After passing through the prism, the two laterally

sheared wave fronts interfere and overlap each other at the plane of the camera and produce

the resultant speckle pattern. The light intensity of the speckle pattern is converted to an

electric video signal and this is sent ot frame grabber board where it is sampled to yield a

digital image. The whole optical set-up was mounted on a vibration isolation table.

Damage area measurement using laser shearography system

Laser shearography inspection method belongs to optical methods working with laser

beam and is based on concept of optical holography and Electronic Speckle Pattern

Interferometry (ESPI). The specimen tested is put under low strain generated either by

heating, vibrations or by pressure, and a change in surface strain caused by the presence of a

fault in the material is detected by the shearographic camera. The shearing device brings the

light waves from two points on the object surface into one point on the image plane, which

results in an interference phenomenon, i.e. so-called speckle interferogram, without using an

additional reference beam as holography and ESPI does. By comparing interferograms before

and after loading a fringe pattern is produced and displayed in real-time. It can be

recalculated to the gradient of deformation. Defects are typically indicated as ‘butterfly

pattern’. Surface deformations of a few microns can be observed. To improve image quality

as well as defect visibility, the ‘phase shifting technique’ is used. The image quality can also

be improved by image post-processing like a noise filtering etc.

Moreover, a rigid-body motion does not produce strain; thus shearography is relative insensitive to such motion. This is a significant advantage of shearography, which is thus predetermined for use in a typical industrial environment.

The measurement presented in this paper was performed with the Dantec Ettemeyer Q-800 portable shearography system with two shearing directions and software package ISTRA for analysis, visualization and storage of the measured data. The heat loading was used within all measurements.

The shearography images for all selected panels are shown in Fig. 8(a)–(f). These figures show measured and smoothed phase maps recalculated to deformation gradient.

(a)Shearography image of damage in panel number 4 impacted with 1 in.

diameter impactor and 10 J impact energy level under thermal load.

(b) Shearography image of damage in panel number 12 impacted with 1 in.


(c) Shearography image of damage in panel number 24 impacted with 1 in.


(d) Shearography image of damage in panel number 30 impacted with 1 in.


(e) Shearography image of damage in panel number 31 impacted with 1 in.


(f) Shearography image of damage in panel number 33 impacted with 1 in.


The measured area with a dent and delamination in panel number 4 has a diameter 26

mm. Note that it is not possible to determine, the exact depth of the flaw because the response

intensity depends on the load magnitude that in case of the thermal one decrease with time as

the sample gets colder. From the deformation gradient profile it is possible to determine the

boundary of the dent.

[Fig. 9. Measured and smoothed planar phase map recalculated to deformation gradient (in

the middle) with two selected profiles of deformation gradient through delaminations (up and

down) for panel No. 4.]

The derivative is zero on the dent's border as in the maximal depth where is the global

extreme. This also helps us to explain the butterfly pattern of the defect response with

positive and negative branch showing where the deformation is increasing or decreasing.

Whether the surface is dented or bulged out, is usually, clear from the performed test,

generally, one had to investigate the second derivative of deformation, i.e. differentiate

numerically the measured deformation gradient data. Fig. 9 shows the example of two

profiles of deformation gradient through the areas of founded delaminations in the panel

number 4. Defects are indicated as typical butterfly patterns.

The facesheet of panel number 12 was completely penetrated and it is seen on the

response. The impact spot is not distinguishable, due to two cracks bounding the facesheet

perforation. Their length is about 40 mm. For this type of damage isolines of deformation

gradient are plotted in Fig. 10. Thick lines mark the cracks in the facesheet.

[Fig. 10. Isolines of deformation gradient. Thick lines mark the cracks in the facesheet (panel No. 12)]

The shearogram of panel number 24 shows a dent of diameter 25 mm approximately in

the middle of the plate. This is a typical result of the impact damage measurement. Measured

damaged area in panel number 30 from impact caused penetration of the facesheet is about

25×30 mm, in panel number 33 is about 20×28 mm. The shearography of the panel number

31 revealed a dent of diameter 24 mm.

Engineered Composite Laminate repair with Thermal Shearography

The various defects in the composite laminate material is shown in figure. This defects

are detected in thermal shearography.

In this type of testing, the object is radiated with heat between the exposure. The

temperature gradient developed induces stresses in the object. This stressing mode is

particularly suited to the evaluation of the of bonding between two different materials. The

difference in the co-efficient of thermal expansion between the materials gives rise to a quasi

bi-metallic strip effect. The debond area is not constrained and is there fore free to deform

away from the interface. This in turn produces a strain analogy on the surface.

In the case, where there is trapped air in the debond region, the heat will cause, the

trapped air to expand, causing the material above the flaw to bulge out. Usually the steady

state thermal deformation may not be easily maintained. In this case, real time shearography

should be employed to observe the transient thermal deformation.

STEP 1:

Capture initial shearography image of repair at ambient temperature.Warm with Infrared Radiation.

[Fig.]

STEP 2:

Heat diffuses through composite material.Uniform material thermally expands (U).Areas with discontinuities, voids, porosity and disbonds have greater thermal expansion (D).

[Fig.]

Examples:

Repaired Aluminum HoneycombAircraft Control Surfaces

Extensive repairs make conventional UT difficult or impossible

Shearography tests shows all disbonds and core damage in 7 min.

AWACS Rotodome Shearography NDE

Detects and measures Impact Damage & DisbondsDifferentiates between damage and repairs

Advantages:

Easy inspection of large and flat surfaces.

Simpel setup, no special safety regulations.

Advanced inspection documentation.

Constant results, independent from operator.

Inspection without loading the component.

Ests parts 3-100 times faster than UT C-Scan.

Disadvantages:

Measuring device is bound to type of material.

Limited possibilities on strongly shaped surfaces.

Classification of defects is subjective.

Often highlighting of the surface necessary.

Application :

Industries where Shearography is used are

Application

Aerospace :Raytheon Aircraft Premier 1

Cessna Aircraft- Citation X, Mustang

Helicopter Blades

Concorde

AWACS, E2 Rotodomes

Boeing Delta IV Rocket

NASA Space Shuttle

Space

Boat

Wind power

Automotive

Tire

The non-destructive testing industry is controlled by Inspection Standard Documents & Codes. Shearography is incorporated in following standard documents:

NAS 410, 2008 Rev 3

ASNT SNT-TC-1A, 2006 edition

ASNT CP-105, 2006 edition

ASTM E2581 -07, "Standard Practice for Shearography on Polymer Matrix

Composites, Sandwich Core Materials and Filament Wound Pressure Wessel’s in

Aerospace Applications"