2. Introduction…..
• Originally developed as a strain measurement technique Y Y Hung (1974)
• Shearography is a laser based interferometric technique that is sensitive to
the out-of-plane deformation of a surface. Under the action of a small load, the
structure is deformed and the presence of defects are revealed by local strain
singularities.
6. Interferometry
Interferometry is determination of fractional relative phase difference between light waves traversing
different paths. Phenomena which can be measured are those which influence the phase of the light beams. In
practice this is e. g. surface deformation
7. Digital Image Correlation
Digital Image Correlation is a data analysis method, which uses a proprietary mathematical correlation method to
analyse digital image data taken while samples are subjected to mechanical stresses. Consecutive image captures taken
during the testing phase will “show” a change in surface characteristics as the specimen is effected by the mechanical
stresses imposed upon it.
9. Fringe Formation
•
•
•
A shearographic image of an object may be mathematically represented as:
After deformed
I=I0 [1+ γ cos∅ ]
I’ =I0 [1+γ cos(∅+∆)
Computing the difference of the two speckle patterns before and after deformation yields
Id= 2I0[ γsin(∅+∆/2) sin(∆/2)]
•
•
Dark fringe lines corresponds to ∆=2(pie)n, n is fringe order.
The computerized shearography allows ∆, the phase change due to deformation, to be automatically determined
using a phase determination technique.
10. Fringe interpretation
•
The phase ∆ is induced by the relative optical path length change between the light scattered from two
neighboring points, P(x,y,z) and P’(x+∆x, y,z) on the object surface.
•
In this case the shearing direction is assumed parallel to x axis and the amount is δx. It can be shown that ∆
is related to the relative displacement δu, δv, δw of two neighboring points separated by
∆ = 2(pie)/λ [A δu + B δv +C δw]
Where u,v,w are displacement vectors , λ wavelength and A,B,C are sensitivity factors related to the position of
the illumination point and the camera position.
11. Applications
The use of Shearograph allows Non-Destructive-Inspection of:
•
•
•
•
•
•
Disbondings
Delaminations
Dents
Impacts
Cracks
Water & air inclusions
12.
13.
14.
15. Advantages:
•
•
•
•
No contact with the surface
Global method (able to inspect surfaces up to 50x50 cm2 in one shot)
High-resolution detection method (fractions of micrometers).
Reduce inspection time and costs
•
•
•
•
whole component inspection viable
surface preparation requirements eliminated
Produce real-time data during inspection
•
•
permanent computer based records
“Fitness for purpose information”
•
•
“real-time”, in-situ, in service inspections of components (measurement of gradients makes the system less sensitive to
environmental noise)
.
Identify significant defects
16. Optical NDT
Benefits
•Reduce inspection time and costs
•whole component inspection viable
•“real-time”, in-situ, in service inspections of components
•surface preparation requirements eliminated
•Produce real-time data during inspection
•permanent computer based records
•“Fitness for purpose information”
•Identify significant defects
