Laser Shearography

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Laser Shearography of Aerospace and Marine
Composites
Roger Gregory Laser Testing Instruments Ltd
Presentation for CPD 4 AEA Technology, Culham,
June 2000
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Laser Shearography

• Many years experience in Aerospace composite
  sandwich inspections ie honeycombs
• Proven ability to consistently find sub surface
  defects by analyzing the surface strain pattern
  in response to an applied stress.
• Discriminates between cosmetic and relevant flaws
  by analyzing the materials reaction to stress
• Realistic fault reference samples required
• Single sided technique
• Rapid coverage

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Northrop B-2
Corporate Executive Jet
35 of Stealth Surface Area Inspected by Laser
Shearography
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Selection of Plane Strain Orientation

• Out-of-Plane
• Idea for laminar bonded composites
• Plays on the weakest vector of the laminar bond
• Active NDT, bond strength and delaminations
• In-Plane Strain Mapping
• Separation of Vectors
• X or Y directional strain
• incorporation of X and Y to form complete plane
  strain maps
• Proofing, current work indicates up to 35 error
  for 3D types

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Out-of-Plane System Layout
Laser
CCD TV Camera
CCD camera
Shearing lens
Polarizing correction Lens
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Creation of Speckle
Surface deformation causes incoherence causes
positive or negative interference on image plane
To Camera
Coherent Monochromatic light from Laser
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Interference at the Image Plane
Each pixel acts as a strain gauge minimum of
262,144
Each pixel receives light from 2 points on the
surface
The intensity value of each pixel is a
combination of the waves reflected of each point
on the surface directed by the shearing lens
Light from the surface via the shearing optics
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Method of Inspection Vacuum Hood

• Uniform Stressing of the Surface by vacuum
• 1st stage Vacuum locks hood to structure and
  reference image of surface stored
• Vacuum Hood Locked to surface and moves in unison
• Ramping to the 2nd stage Vacuum pressure
  reference image subtracted from new images at
  video refresh rate
• Results presented and updated every 1/30th Second
• Provides smooth formation of fringes
• Final result frozen on screen for analysis

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Defect Image Interpretation

• Each disbond Imaged
• Vertical Shearing
• Fringe Density dependent on size, depth and
  stress applied

Holography
Shearography
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Affect of Stress on Fringes
Dimensions of the defect indication remain
constant however the outward deformation
increases in response to increasing vacuum stress
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Portable Vacuum Equipment

• Image Processor
• Vacuum Generator
• Vacuum Hood for Flat surfaces
• Curved Vacuum hood for sharper radii
• Inspection area 420cm2 in 0.6 seconds

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Laser Shearography

• High POD level Single Sided Inspection for
  Un/Disbonds -
• Within FRP Skin
• At skin to core Interface
• At Core to core Interfaces and lower
• Other features-
• Qualification of Bond Integrity
• Qualification of Repairs
• Measures to Micron level displacements
• Insensitive to vibration - allows construction
  work to carry on

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Large Area Scanning
GRP-Foam fast Super yachts
Wave Piercing fast patrol Boats
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Inspection during Manufacture
Immediately after joining of the 2 hull sides -
cost effective to find and repair faults before
internal bulkheads and equipment are fitted.
Over 35 boats have been inspected
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Hull Topsides Construction
Hull topsides constructed of FRP over multiple
sheets of closed cell PVC Foam core
Multiple thin sheets used to follow shape of
hull
Disbonds and delaminations at any of the
interfaces
Hard Chine
Single skin keel
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Built Problems

• Rigid foams facet shaped to take-up curves
• Large areas to vacuum bag and cure in one
  operation
• Disbonds and Unbonds created
• Size, cost and low throughput prevent automation
• Human element in the build quality
• Out-gassing of the foam core can collect in
  disbonds
• Disbond propagation under Thermal and Operating
  stresses.
• Permanent Blistering and distortion of Hull

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Non Destructive Testing Techniques
General NDT techniques for composites still rely
on point measuring equipment

• Ultrasonics -
• Unable to cope with high gas content of foams
• Impedance Resonance Testing MIA Ults
• large area for point contact
• Unable to cope with variable build state
• High time Penalty
• Coin Tapping
• limited to skin to core bondline
• Subjective - no 2 inspectors can agree on size
  shape or position
• Easily fooled by internal structure
• health and safety hazard

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Application

• Each Placing covers 900cm2
• Examination area covers 450cm2 within the placing
• Each Placing inspected within 5 seconds
• Overlapping Scans Ensure Complete Coverage
• Coverage of 5 m2/hour on Uniform Surfaces
• Laser is Safety Interlocked to Vacuum Pressure
• Classified as Class 1 laser product
• Safer than laser pointer
• Sc5100 at Class 2 safety level

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Inspection Technique

• Technique parameters established by realistic
  reference standards
• Dynamic interpretation of results prevents false
  calls
• Defects recorded as TIF files
• Actual size and shape of fault is marked on the
  surface.
• Defect case history recorded for each boat or
  structure
• Defect map reported to owners for repair schemes
• Follow up inspection after repair if required

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Base Line Inspection
Locating Feet

• Normal evenly well bonded structure
• Surface is deformed by applied vacuum
• Each fringe approx 40 micro strain
• Fringes formed by vacuum acting around the 3 feet

Locating Feet
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Structural Aspects
Core Splice
25 Reduction in thickness
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Internal Structure Inspection
Normal Core splice - little overall change to
fringe Pattern - Kink in the fringes is regular
and even
Weak Core Splice - large shift in Fringe Pattern
- with disbond indication attached
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Typical Fault Indications

• Faults appear as Double Bulls Eye
• Shearing angle45º
• Fringes depict lines of Iso-Strain
• Not all defect has to seen to find centre of
  defect

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Fault location and Repair

• Fault located,
• Sized and mapped on the inspected surface
• Weak area cut out -
• Ready for repair

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Case History - Super Yacht Fault Indications
No evidence of catalyst in bond - glue is wet
after 30 months Bulwark is 115mm thick Unbond
found at mid thickness 67mm below surface
Multiple disbonds at mid thickness to cockpit
side due to Thermal and Operating stresses
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Aero Space Applications
Space Shuttle
X33
Delta IV
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Aero Engine Fan Cases
Disbond between the skin and the metal honeycomb
core Inspection time 1 second
F100 fan case
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Raytheon Premier 1 Fuselage
Carbon Fibre to nomex core Fuselage inspected
automatically in 6 hours replaces 330 man-hours
of ultrasonics Inspected Internally and
Externally under cyclic vacuum stressing
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Vacuum Chamber Stressing
Boeing composite repair standard 6 second test
Boeing Repair Panel R-9 (Jeff Kollgaard)
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Aero Engine Seal Inspection
Felt metal seals of Pratt Witney F100 inspected
by Digital Holography
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New Developments

• SC5100
• Phase stepping Laser
• Shearography
• Remote control -
• Zoom Focus
• Laser Zoom
• Laser alignment
• Tilt Turn
• Enhanced Image capabilities-
• Phase wrapped
• Unwrapped
• Integrated
• 3D analysis
• Profiling

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Phase Stepping Advances Sensitivity
Phase Stepping and the unwrapping of phase data
allows sub-fringe defects to be detected Whole
body fringes can be filtered to highlight only
the fault indications
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Aerospace Applications
Wrapped, unwrapped Integrated and Analysis images
of impact damage
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Inspection Applications

• AWACS Rotordome - RAF
• Large Area
• Rapid Inspection
• Complex Double Honeycomb
• Neoprene Covered Composite

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AWACs Radome
Neoprene coating
FRP
Nomex honeycomb
Subtraction correlation with integration
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Aerospace Applications AWACS
Phase Stepping enables strength of bonds to be
assessed Difference between disbonds and stronger
repairs easily established
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Composite Pressure Vessels
Axial strain Inspection at 1 of normal operating
pressure delaminations shown in 4 90 degree
scans Uniform response from the undamaged
material
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Results Panel 6/1/C
Partial good bond across upper right-hand corner
on par with parent material
Variable bond strength across bond - however each
ply is visible with original defect apparent
CPD 4A Project PERA
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Results Panel 6/1/C
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In-Plane Strain Imaging

• Ability to extract in-plane from all other data
• Effective use for structural and proofing of
  Finite element modelling on the actual component
• Stressing by
• Thermal- 0.1 to 3 degrees Centigrade
• Mechanical to microstrain resolution
• Visualisation of Torque
• Tensile
• Real life load cycle regimes
• Fatigue rigs

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In-Plane Analysis
Separate the Out-of-Plane from the In-Plane
In-Plane strain Analysis is able to visualise and
measure the strain
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Visualizing Torque
Good Bond- rivet tight-transmits strain _at_ 12 Nm
torque
Bond Failure puts rivet in shear _at_ same torque
Suspect Bond between inner and outer tubes
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Tyre Inspection
System takes 9 shots (40 degree) under cyclic
vacuum load Crown, sidewall or complete bead to
bead inspection -automatically
Large Proportion of the worlds Aircraft and Motor
racing Tyres are inspected by laser shearography
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Conclusion

• Laser Shearography provides a rapid inspection
  tool for the inspection of advanced marine
  composites
• Sensitive to Micron level Displacements while
  relatively insensitive to vibration - therefore
• Other work or repairs are not hampered by
  inspection
• Digital Measurement of Flaw Shape and size.
• Exact size, position and shape plotted on the
  surface
• Integrity of Repairs can be readily Assessed