UNIVERSITY OF CAPE TOWN MECHANICAL ENGINEERING Introducing project Caliper A sub-set of Ledger Jasson Gryzagoridis and Dirk Findeis Department of Mechanical Engineering University of Cape Town, South Africa Phone: Fax: CSIR – Ledger Conference Pretoria Nov 2007

UNIVERSITY OF CAPE TOWN MECHANICAL ENGINEERING Caliper a project at the University of Cape Town About Non Destructive Testing (NDT) Technology Excellence in Aeronautical NDT Jasson Gryzagoridis and Dirk Findeis Department of Mechanical Engineering University of Cape Town, South Africa

UNIVERSITY OF CAPE TOWN MECHANICAL ENGINEERING Caliper has been operating for a decade or more in the following categories: SANDF-NDT support NDT Training - Tertiary education students Development of laser based techniques International contact and benchmarking

UNIVERSITY OF CAPE TOWN MECHANICAL 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 TOWN MECHANICAL 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 TOWN MECHANICAL 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 1 st year) Also Between 4 to 8 honours theses/year

UNIVERSITY OF CAPE TOWN MECHANICAL ENGINEERING Shearography- an NDT method for composite structures and components Jasson Gryzagoridis and Dirk Findeis Department of Mechanical Engineering University of Cape Town, South Africa Phone: Fax: CSIR – Ledger Conference Pretoria 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 TOWN MECHANICAL ENGINEERING

UNIVERSITY OF CAPE TOWN MECHANICAL 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 TOWN Mechanical Engineering Department Laser Mirror Object CCD Shearing device Beam expander Typical Laboratory Digital Shearography set-up

UNIVERSITY OF CAPE TOWN Mechanical Engineering Department ** Typical Shearing device** The Michelson Interferometer *P Mirror Object Wave-front Small angle Sheared image plane Separation distance

Mathematical background UNIVERSITY OF CAPE TOWN Mechanical Engineering Department The resultant Intensity of an object’s image after the subtraction of the two stored images is I r = I i – I f = 4√I 1 I 2 sin[ (θ 1 – θ 2 )+ 1/2Δφ]sin 1/2Δφ Maximum correlation occurs when the two images are identical therefore I r = 0… black fringes are formed therefore … Δφ = 2Nπ N= 0,1,2,3…. an expression for the out-of-plane strain:

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 TOWN Mechanical 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 TOWN Mechanical Engineering Department

UNIVERSITY OF CAPE TOWN Mechanical 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 TOWN Mechanical 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 TOWN Mechanical 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 TOWN Mechanical Engineering Department Reconstructed Matlab 3D visualization of displacement image. reconstructed image.

UNIVERSITY OF CAPE TOWN Mechanical Engineering Department Advantages of the technique as 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 TOWN Mechanical 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 TOWN Mechanical Engineering Department