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  LASER SHEAROGRAPHY TECHNOLOGY

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Abstract
Laser NDT methods based on interferometric imaging, primarily holography and shearography, have seen growing acceptance since the mid 1980's. With the large increase in the use of composite materials and sandwich structures, the need for high speed, large area inspection for fracture critical, sub-surface defects such as disbonds, delaminations, sheared core or non-visible damage in aircraft, missiles and marine composites led to broad acceptance of laser based NDT methods. Laser NDT Methods employing holography and shearography imaging interferometers compliment UT, Thermography and other NDT methods as highly developed, mature and cost effective technology.


Typical shearography camera systems,
include a built-in laser illumination
source, the shearing interferometer,
image processing computer and remote
controls. These systems may be used
alone with thermal or vibration stress or
in test chambers with vacuum stress
shearography techniques.


As with all NDT methods, strengths and weakness must be completely understood, applications qualified through PoD verification with written procedures and rigorous training for operators and engineers alike. Once qualified for a particular application, holography and shearography systems can operate with extraordinary efficiency reaching through-puts from 25 to 1200 sq. ft per hour, 2.5 to 120 times the inspection rate for ultrasonic C-Scan. As these technologies become more widely known, commercial applications in aerospace, electronics, marine composites, high-performance tires and medical devices have greatly increased. In 2005, Laser Methods have reached a fundamental milestone with inclusion of Holography and Shearography in ASNT TC-1A for Level III Certification. This paper will present an over view of Laser NDT Method applications in aerospace, electronics and marine composites where they serve as highly effective, fully integrated industrial process controls, improving manufacturing quality while reducing costs.


Background
Laser interferometric imaging NDT techniques such as holography and shearography have seen dramatic performance improvements in the last decade and wide acceptance in industry as a means for high-speed, cost effective inspection and manufacturing process control. These performance gains have been made possible by the development of the personal computer, high resolution CCD and digital video cameras, high performance solid-state lasers and the development of phase stepping algorithms. System output images show qualitatively pictures of structural features and surface and subsurface anomalies as well as quantitative data such as defect size, area, depth, material deformation vs. load change and material properties. Both holography and shearography have been implemented in important aerospace programs providing cost effective, high-speed defect detection.

Holography images test part responses to changes in load showing the, as well as part movement. Holography using continuous wave lasers and video frame rate data acquisition require vibration isolation usually in the form of air supported isolation tables. Coupled with ultrasonic vibration excitation of the test part, holographic systems in production provide very high-resolution images of disbonds in small complex shaped components, such as turbine aircraft components and medical devices.

Shearography NDT systems use a common path interferometer to image the first derivative of the out-of-plane deformation of the test part surface in response to a change in load. This important distinction is responsible to two key phenomena. First, shearography is less sensitive to the image degrading effect of environmental vibration. Shearography systems may be built as portable units or into gantry systems, similar to UT C-Scan systems, for scanning large structures. Second, the changes in the applied load required to reveal subsurface anomalies frequently induce gross deformation or rotation of the test part. With holography, several important test part stressing techniques, such as thermal and vacuum stress, create gross part deformation. Defect indications may be completed obscured by these translation fringe lines. Shearography, on the other hand is sensitive only to the deformation derivatives and tend to show only the local deformation on the target surface due to the presence of a surface or subsurface flaw.

Shearography, in particular offers unique and proven defect detection capabilities in aerospace composites manufacturing. Shearography images show changes in surface slope, in response to a change in applied load. Shearography whole field, real-time imaging of the out-of-plane deformation derivatives is sensitive to subsurface disbonds, delaminations, core damage, core splice joint separations as well as surface damage. Secondary aircraft structures have long used composite materials. The drive for better vehicle performance, lower fuel consumption and maintainability are pushing the application of composites and sandwich designs for primary structures as well. Faster and less expensive inspection tools are necessary to reduce manufacturing costs and ensure consistent quality.

 

 


Fig.1. Schematic diagram of a Michelson Type
shearography interferometer observing a flat metal
plate with a 4-inch diameter machined flat-bottomed
hole. A center load change on the plate provides a
variable deformation, observed on the computer
monitor in real-time.

 

 

 


Fig.2 A phase map shearogram with horizontal shear
vector yields a fringe pattern showing the first
derivative of the out-of-plane deformation, ?w/ ?x.
Using an unwrapping algorithm, the image at right
shows the positive (white) and negative (black)
slope change. The metal plate with a 4.0-inch
diameter flat-bottomed hole was deformed by 7.0
microns.

Shearography
The concept of using a common path interferometer to image test part deformation derivatives to overcome the effects of environment vibration and loss of the defect signal due to gross part deformation , as seen with thermal stress holography, was first introduced by Butters et al (1971) and reduced to practice by Nakadate et al(1985).

Shearography cameras generally use a Michelson type interferometer with two essential modifications. First, one mirror may be precisely tilted to induce an offset, or sheared image, of the test part with respect to a second image of the part. The sheared amount is a vector with an angle and a displacement amount. The shear vector, among other factors, determines the sensitivity of the interferometer to surface displacement derivatives, Fig 1.

The two laser speckle images of the test part, offset by the shear vector, interfere at every paired point over the surface in the field of view. The single frequency laser light from the two sheared images of the part is focused onto the CCD camera array of photosensitive pixels. Light from pairs of points in each sheared image interfere. Each video frame, comprised of the complex addition of these two sheared images can be subtracted from a stored reference image. The absolute difference yields a fringe pattern observed on the monitor. The second mirror in the Michelson interferometer may be phase stepped using a piezoelectric device and the images combined to create a phase map. Further processing using any number of unwrapping algorithms may be used to generate fringe free images of local surface deformation derivates, Fig 2.

In practice each step in creating a shearogram is performed automatically using image-processing macros constructed by combining each processing function in a sequence. Shearography system operators perform a test with a single keystroke. Portable shearography systems using voice recognition commands have been built further freeing the operator from system functional operations.

 


Fig. 3 The integrated image of the shearogram in Fig 2. shows the out-of-plane deformation.
Integrated images of deformations derived from shearography data are free errors due
to gross object deformation or translation.

Quantitative Shearography Measurements
Precision calibration of the shearogram image scale (pixels/inch) and the shear vector allow further processing of shearography data to determine defect indication dimensions, area and the deformation of the material. The digital measurement of the deformation derivative may be integrated to show the shape of the target surface deformation as well as the magnitude of the deformation at any location, as in Fig 3. Shearography can be used to measure the deformation response of a structure to an applied load and as a means for deriving material properties.

 

Shearography NDT Systems
Shearography NDT systems are either portable, for on vehicle or structure inspection or fixed production systems using gantries to scan large panels or structures. As with all laser devices, exposure of the operator to laser emissions must be tightly controlled and in compliance with State and Federal laws. Laser interlocked to test cell doors or vacuum attachment features are an


Portable Shearography NDT Systems

Portable shearography systems generally are either tripod mounted or attached in some manner to the test object (Fig.5).

Portable system use laser diodes and various means such as vacuum changes, thermal flux or vibration to stress the object surface to detect subsurface anomalies.

Shearography techniques using portable systems are excellent for engineered repairs in composite laminates. Fig. 6 shows a repair to an aircraft laminate with far side, bonded stringers (diagonal linear features). The repair uses scarf plies built up thicker than the original material; hence the signal from the stringers appears to disappear under the repair. Visible also are areas of porosity (circled in white). Test time is 15 seconds.

Portable shearography systems have seen extensive use in aerospace and marine composite inspection since the introduction of the first systems in 1989. More than 170 composite boats and ships, including the Swedish Visby Naval Corvette ships measuring 73 meters. Portable shearography systems also recently were used to inspect the composite wind fairings covering the full 2,200 ft. length of the Bronx Whitestone.



Fig. 5. Shearography inspection of
composite honeycomb engine reversers
on the Airbus A330 using the LTI-4200.
The structure is GRP face sheets with
aluminum core. The inner surfaces are
coated with a foam fire retardant material.
Shearography indications in the sandwich
structure, through the foam are routine.
Defects indications are verified with
secondary UT measurements requiring
spot removal of the foam.

 



Fig. 6 Shearography image of
engineered repair to a solid laminate
aircraft structure with far side bonded
stringers. Porosity is circled.

 

Tripod mounted shearography cameras, are used frequently with thermal stress shearography techniques. While Thermography is sensitive to changes in surface temperature (or the derivatives of the temperature change), thermal shearography images changes in the thermal expansion of a structure. Damage, disbonds, FOD or delaminations produce local changes in the coefficient of thermal expansion.

Thermal shearography is not generally effected by variations in emissivity or paint on the test part surface.


Fig. 7. Thermal shearogram of a Global Hawk aircraft fairing showing
lay-up of the composite material. Thermal shearography is used for
detection of disbonds and face sheet delaminations.


Fixed Production Shearography Systems

First introduced on the USAF B-2 production program, gantry mounted shearography systems share many operational features with UT C-scan systems. These include: teach/learn part scan programming, electronic image of the entire part, image analysis and defect measurement tools, automated operation. Shearography system however operate at throughputs typically in the range of 100 to 500 sq. feet/hour compared to a typical throughput of 10 sq. ft./ hour for UT C-Scan systems. In addition, gantries are considerable less expensive since precision part contour following is unnecessary. Currently dozens of these systems are in operation on aerospace manufacturing programs


Conclusions

Shearography and holography NDT methods are mature and cost effective production NDT methods for many aerospace applications. Shearography provides very rapid inspection allowing immediate feedback for process controls. Recent inclusion in ASNT TC-1A will help further the development of new applications and methods.

 

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