Transcript Laser Shearography

Laser Shearography of Aerospace
and Marine Composites
Roger Gregory
Laser Testing
Instruments Ltd
LTI
June 2000
1 2000
PresentationAEA
forTechnology
CPD 4- AEA
Technology, Culham, June
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 material’s 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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LT I
Selection of Plane Strain
Orientation
 Out-of-Plane
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Idea for laminar bonded composites
Plays on the weakest vector of the laminar bond
Active NDT, bond strength and delaminations
 In-Plane Strain Mapping
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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
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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 receives light from
2 points on the surface
Each pixel acts as a strain
gauge minimum of 262,144
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
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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
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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 :
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Within FRP Skin
At skin to core Interface
At Core to core Interfaces and lower
 Other features:
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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
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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
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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
•
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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
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IFollow up inspection
after- June
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
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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
angle=45º
 Fringes depict
lines of IsoStrain
 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
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Delta IV
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Aero Engine Fan Cases
F100 fan case
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Disbond
between the
skin and the
metal
honeycomb
core
Inspection time
1 second
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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
L TLIT I
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
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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 @ 12 Nm torque
Bond Failure puts rivet in shear
@ 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
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