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Shadow moiré and the use of flexible gratings to adapt to curved surfaces
J.C. Martínez-Antón, H. Canabal, J.A. Quiroga, E. Bernabeu
Dpto. Óptica, Universidad Complutense de Madrid, 28040 Madrid, SPAIN
M. Álvaro Labajo, V. Cortés Testillano
Subdirección de Investigación y Desarrollo de Tecnología y Materiales
Dirección de Proyectos y Sistemas
Construcciones Aeronáuticas, Avda. John Lennon s/n
Getafe 28906 Madrid, SPAIN
We have extended the use of the well know shadow moire technique to be
implemented in curved surfaces (particularly convex cylindrical and conical). The
prototype is based on a tripod support spring loaded. A polyester foil with a printed
Ronchi grating is stretched between three points. This allows the definition of a perfect
plane surface and further the elastic load permits to adapt the foil to any cylindrical or
conical surface, no matter the relative orientation of the support to the surface and with
an optimum distribution of tensile stress. Some experimental results have been
obtained which are quite satisfactory in comparison with the flat grating approach. This
work is under the INDUCE European project.
The shadow moiré is a well-known technique for defect inspection and profile
measurements covering a wide range of resolution . We are interested in the
detection of surface deviations related to some types of defects in aeronautic surfaces
(under an European project ). In particular we focus our attention to detect surface
deviations due to impact damages (or indentations) or due to hidden corrosion
(protuberances) (see Fig. 1). The main applications would be the in-service inspection
of large aeronautical structures manufactured in composite materials looking for impact
damage and, on the other hand, the inspection of lap-joints in aeronautical metallic
structures looking for corrosion.
This technique is half way between the simple naked-eye visual inspection and
the conventional non-destructive techniques. In this sense, we consider moiré
techniques as aided visual inspection tools and their main objective is to locate, in
more precise way than naked-eye visual inspection, a series of small zones in a large
structure to be investigated deeply using conventional NDT techniques. The expected.benefit is a dramatic reduction of in-service inspection times, then the related costs,
keeping a high level of reliability.
Fig 1: Scheme of the Shadow moiré technique|
Our goal is to make a prototype ready for aided visual inspection of aeronautic
surfaces capable of detecting reliably surface deviations greater than 50 µm in depth
(and around of 1 cm 2 wide). Current state of the art of shadow moiré for defect
inspection works well only with quasi-flat surfaces and not so easy with curved ones.
An explanation of this statement is shown in Fig. 2.
The lack of adaptation of the grating to the overall surface shape to test leads to
a bad detectability of the surface deviation. Although flat profile gradient produce
ordered fringes (Fig. 2c) that do not hide the surface deviation, this is not the case
when we want to explore general curved surfaces where the global moiré pattern
generally difficult the detection of surface deviations of interest. Furthermore, we want
to explore a big area (around 10 x 10 inches 2 ). Therefore, slightly curved surfaces
produce a quick separation over a flat grating probe in the order of several millimeters,
so serious contrast decrease is also expected due to diffraction in such exploration
window. These two pointed out problems make rather impossible the reliable detection
of surface deviations with the current state of the art in shadow moiré.
Fig 2: a) Direct observation of a flat surface test (made of composite) with an indentation of
350 µm in depth. b) The same with an interposed Ronchi grating of 10 l/mm making clearly
apparent an indentation. c) When the grating is tilted few millimeters over the test it appears
several ordered fringes with decreasing contrast due to diffraction. Curved surfaces produces
disordered packed moiré patterns with low contrast spoiling the apparent detectability of Fig. 2b.|
Several authors have approached the problem of curved surfaces with rigid curved gratings in a way that they adapt themselves to the surface . The main disadvantage is that we need a specific tool for every different surface we want to explore. On the other hand, to adapt their curvatures appropriately we need a right alignment of the two surfaces. Notice that alignment is irrelevant when dealing with only flat surfaces.
Our intention is to try a flexible grating adaptation that, in principle, does not depend on the surface to explore. Besides, we wish to develop a device with a constant performance over its relative orientation to the surface, i.e. free or independent of any alignment.
Adaptable gratings for curved surfaces. Results
As a consequence of a practical use in real hangar conditions we have notice that our
prototype can be also used to detect underneath structural details. This is of interest
especially to locate the substructure (stiffeners, reinforcements, etc.) in large composite
structures when necessary.
Difficulties encountered and solutions
One of the goals is to develop a prototype ready for a reliable visual inspection. In
practice we have found several difficulties, which are not associated to the tripod
adaptable solution but to the problem of surface deviations detection itself. Dust and
dirty or bubble inclusions are the main limitation because they compete with the real
deviation in producing a surface deviation appearance where we only have an external
Due to the nature of the flexible tripod approach, the prototype can be dragged
along the surface to explore. This introduces a novel factor difficult to illustrate with a
static picture but which in fact is the key to distinguish relevant surface deviations from
inclusions. As the grating is moving along the surface, the inclusions are normally
dragged too (no matter which kind of inclusion) while the real surface deviation
associated to the surface remain in fixed points. An approximate description of the
dynamic process of detection is that while moving the device along the surface there
we appreciate fixed moiré stains and others moving.
- O. Kafri, I. Glatt, The Physics of Moiré Metrology, J. Wiley and Sons, New York,
- European project INDUCE: (Advanced Integrated NDT Concepts for Unified Life-Cycle),
- J. Marasco, "Use of a curved grating in shadow Moiré", Experimental mechanics,
vol. 15, no. 12, Dec. 1975; p.464-70.