university of cape town mechanical engineering introducing project caliper a sub-set of ledger...
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UNIVERSITY OF CAPE TOWNMECHANICAL ENGINEERING
Introducing project Caliper
A sub-set of
Ledger
Jasson Gryzagoridis and Dirk Findeis Department of Mechanical EngineeringUniversity of Cape Town, South Africa
Phone: +27 21 650 3229Fax: +27 21 650 3240
E-mail : [email protected]
CSIR – Ledger Conference Pretoria 29-30 Nov 2007
UNIVERSITY OF CAPE TOWNMECHANICAL ENGINEERING
Caliper
a project at theUniversity of Cape Town
About
Non Destructive Testing (NDT) TechnologyExcellence in Aeronautical NDT
Jasson Gryzagoridis and Dirk Findeis Department of Mechanical EngineeringUniversity of Cape Town, South Africa
UNIVERSITY OF CAPE TOWNMECHANICAL ENGINEERING
Caliper
has been operating for a decade or morein the following categories:
SANDF-NDT support
NDT Training - Tertiary education students
Development of laser based techniques
International contact and benchmarking
UNIVERSITY OF CAPE TOWNMECHANICAL ENGINEERING
We specialize in Optical NDT
Holographic Interferometry
ESPI – Electronic Speckle Pattern Interferometry
Digital Shearography
All are non contact, Whole Field , Real Time, Optical Interference
techniques
UNIVERSITY OF CAPE TOWNMECHANICAL ENGINEERING
Optical Interference NDT techniques that can be applied to inspect objects for:
De-laminations in composites.
Ferrous and Non-ferrous corrosion. Surface and sub-surface cracks.
Foreign inclusions. Discontinuities.
De-bonds.
UNIVERSITY OF CAPE TOWNMECHANICAL ENGINEERING
Currently the main researchers are:
Jasson Gryzagoridis and Dirk Findeis
(Emeritus Professor and Senior Lecturer in the Mech. Eng. Dept.)
There are 5 postgraduates
(2 are finished - 1 PhD & 1 MSc.)
(3 MSc.’s in their 1st year)
Also
Between 4 to 8 honours theses/year
UNIVERSITY OF CAPE TOWNMECHANICAL ENGINEERING
Shearography- an NDT method for composite structures and components
Jasson Gryzagoridis and Dirk Findeis Department of Mechanical EngineeringUniversity of Cape Town, South Africa
Phone: +27 21 650 3229Fax: +27 21 650 3240
E-mail : [email protected]
CSIR – Ledger Conference Pretoria 29-30 Nov 2007
Historical background
• Originally developed as a strain measurement technique• Y Y Hung (1974)• Digital Shearography as a NDT technique. • YY Hung (early report 1982)
• Wide range of applications reported by work at research labs.
• Large industrial systems for:- Tire industry- Eurocopter SA helicopter blades- Heat protection parts for Ariane launcher
• Light weight portable systems- UCT’s portable digital shearography- Pennsylvania State University- Dantec-Ettemeyer
UNIVERSITY OF CAPE TOWNMECHANICAL ENGINEERING
UNIVERSITY OF CAPE TOWNMECHANICAL ENGINEERING
Digital Shearography:Employs a digital camera and a computer to capture the laser illuminated object’s image.
Optics shear the image into two overlapping images : which generates a particular speckled image.
Two overlapped sheared images (one before and one after mild stressing), when combined (subtracted from each other) create a fringe pattern.
This fringe pattern indicates the gradients of surface displacements (or strains) on the object.The presence of defects is revealed by a localized disturbance in the fringe pattern.
The technique produces results in almost real-time. Relatively insensitive to environmental disturbances. Tests are easily repeatable.
UNIVERSITY OF CAPE TOWNMechanical Engineering Department
Laser
Mirror
Object
CCD
Shearing device
Beam expander
Typical Laboratory Digital Shearography set-up
UNIVERSITY OF CAPE TOWNMechanical Engineering Department
** Typical Shearing device** The Michelson Interferometer
*P
Mirror
Mirror
Object Wave-front
Small angle
Sheared image plane
Separation distance
Mathematical background
UNIVERSITY OF CAPE TOWNMechanical Engineering Department
The resultant Intensity of an object’s image after the subtraction of the two stored images is
Ir = Ii – If = 4√I1I2 sin[ (θ1 – θ2)+ 1/2Δφ]sin 1/2Δφ
Maximum correlation occurs when the two images are identical therefore
Ir = 0… black fringes are formed therefore … Δφ = 2Nπ N= 0,1,2,3….
an expression for the out-of-plane strain:
S
N
x
p
2
• Quantification as a Non Destructive testing technique
The size of the defect … near enough to the size of the disturbance in the fringe pattern (normally appears as a double bulls-eye)
With phase stepping ….
The depth of the flaw is a function of the number of fringes of the localized fringe disturbance
And …. in addition information to whether the surface is displaced away or toward the observer
UNIVERSITY OF CAPE TOWNMechanical Engineering Department
• Currently UCT’s Digital Shearography System
Capable of inspecting a surface of…
Size: -variable depending on choice of operator and intensity of light reflected from surface
Example : approx. 750 mm Dia
(at a distance of 1.0 metre )
Phase stepping, colour enhance, filter – Yes,yes,yes
Automatic fringe quantification – yes
3D visualization of disturbance (finishing touch)
UNIVERSITY OF CAPE TOWNMechanical Engineering Department
UNIVERSITY OF CAPE TOWNMechanical Engineering Department
Test sample from the UAV wing section
Impact Damage invisible to naked eye
(a) (b)
UCT”s Dig. Shearography Head Phase filtered
UNIVERSITY OF CAPE TOWNMechanical Engineering Department
Typical examples of shearographic image enhancement depicting defects below the surface of a composite of GFRP skin and Monex core :
(a) and (d) intensity images, (b) and (e) phase stepped images , (c) phase stepped with colour image and (f) phase stepped filtered image.
UNIVERSITY OF CAPE TOWNMechanical Engineering Department
Phase map (a) and unwrapped image (b) of defect.
(a) (b)
Matlab 3D visualization of the unwrapped phase image
Typical example
of UCT’s D S 03
capability
UNIVERSITY OF CAPE TOWNMechanical Engineering Department
Reconstructed Matlab 3D visualization of displacement image. reconstructed image.
UNIVERSITY OF CAPE TOWNMechanical Engineering Department
Advantages of the techniqueas an NDT tool
• It is non-contacting. • It is performed in real time mode• Requires none or minimal object surface preparation. • Provides graphical whole field information. • It is easily repeatable.• Particularly suitable for composite materials (from experience)
Successful applications
• Wide range of applications from research labs• Eurocopter SA – Composite Helicopter rotor blades• Ariane launcher – Thermal protection parts• Tire industry –New and re-treaded tires (routine tests)
Surprising that validation or even better an International Standard like ISO is not available !
Where do we go from here?
UNIVERSITY OF CAPE TOWNMechanical Engineering Department
perhaps Benchmarking ?
is a powerful management tool because it overcomes ‘paradigm blindness’….. “The way we do it is the best because this is the way we‘ve always done it”
we need to measure and compare the technique against other techniques, in testing several products and applications
• implies competitive benchmarking with industry accepted norm hence it is a slow process but on the long run well worth it.
UNIVERSITY OF CAPE TOWNMechanical Engineering Department