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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 Contact

Abstract

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.

Introduction

The shadow moiré is a well-known technique for defect inspection and profile measurements covering a wide range of resolution [1]. We are interested in the detection of surface deviations related to some types of defects in aeronautic surfaces (under an European project [2]). 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.

Fig 1: Scheme of the Shadow moiré technique

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.
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.

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.

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.
Several authors have approached the problem of curved surfaces with rigid curved gratings in a way that they adapt themselves to the surface [3]. 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

We have tried two different designs for grating adaptation to curved surfaces. We make different tests on a curved panel (Fig. 3) with several indentations in the order of 70 µm measured and labeled by mechanical means. Probably, the real depth is less due to certain recovery of composite materials after an impact, which may last several days. The first essay consisted in using a Ronchi grating printed on a semi-rigid plastic plate of around 2 mm thick and adapted to the curved panel by means of elastic arms (see Fig. 4). However, not fully satisfactory results were obtained in relation with visual detectability and this setup has the problem of proper alignment.

Fig 3: Curved panel used to make the tests on curved surfaces. This is a retail made of composite material typically used in aeronautic applications.

Fig 4: First prototype in use over a curved panel. In this case the adaptation needs a right alignment and the tensile stress on the foil is somewhat inhomogeneous. It gives poorer results that the tripod approach. We may appreciate moiré fringes in the central area due to a bad adaptation. This disturbs a reliable detection of surface deviations.

The solution proposed consists of a plastic foil of ~0.1 mm thick made of PET (Polyethylenterephthalate) with a printed Ronchi grating stretched between three points in a tripod support (see Fig. 5). We have tested different periods from 4 l/mm to 10 l/mm with satisfactory results in both cases. Similar resolution is attainable by changing the illumination angle.

Fig 5: Tripod solution to the adaptation of flexible gratings to curved surfaces. The flexible grating is stretched between three contacting points. The tensile stress is given by the spring situated at the middle. This setup allows a good adaptation to convex conical and cylindrical surfaces, no matter the relative orientation of the tripod to the surface. The uniform gray appearance of the grating is a signal of good adaptation to the surface. Surface deviations produce a slightly more dark stain type appearance.

The tripod approach permits to completely adapt flexible gratings to convex conical and cylindrical surfaces independently of the relative orientation of the device to the surface. From a digital camera installed in the middle of the tripod we have taken different pictures in the same exploration area with a flat grating and a flexible grating (Fig. 6) were the difference among them in noticeable. The flexible approach improves significantly the detectability over the flat solution on curved surfaces. Anyhow, it is worthwhile to mention that commonly visual detection is made dynamically, i.e. moving the grating along the surface. This characterizes the detectability in a way that favors the tripod approach.

Fig 6: Left: shadow moiré over the curved panel with a flat grating. Right: shadow moiré over the same area (left) with a flexible grating mounted on the tripod support. Better detectability is obtained in this last case. Indentations are detected left to the labels 65 and 70 µm. A protuberance can be appreciated right between the labels due to a particle dust originated in the paint with a different appearance (notice a white small point in the middle).

Other 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 inclusion.
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.

References

1. O. Kafri, I. Glatt, The Physics of Moiré Metrology, J. Wiley and Sons, New York, 1990.
2. European project INDUCE: (Advanced Integrated NDT Concepts for Unified Life-Cycle), ref: BRPR-CT98-0805.
3. J. Marasco, "Use of a curved grating in shadow Moiré", Experimental mechanics, vol. 15, no. 12, Dec. 1975; p.464-70.

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