Process and instrument for checking the bonding of the...

Optics: measuring and testing – Inspection of flaws or impurities

Reexamination Certificate

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C356S237100

Reexamination Certificate

active

06452670

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to laminated honeycomb structures, and more especially to the checking of the bonding of the cellular core of the honeycomb to a skin. In this regard the invention proposes a checking process and a specially designed device for implementing the present process.
2. Summary of the Prior Art
In what follows, a laminated honeycomb structure will simply be referred to as a “honeycomb”. Honeycombs are well known in industry, and especially in aeronautics for making in particular turbomachine seal packings, acoustic panels or for constructing thin and rigid panels such as nozzle flaps or aircraft structures. Honeycombs usually take the form of flat or shaped plates, the form of frustoconical monobloc ferrules or the form of sectors of these same ferrules. The term monobloc should be understood to mean one-piece. At this stage of manufacture the honeycomb comprises a cellular core bonded via one of its sides to a skin. The skin is a flat or bowed plate which can be of the same material as the cellular core or of a different material. The cellular core takes the form of adjacent cells separated by partitions extending in the direction of the thickness of the honeycomb, the ends of the partitions on one side of the cellular core being bonded to the skin, the cells opening out on the other side, these cells customarily being hexagonal, but sometimes rectangular, these partitions customarily being substantially perpendicular to the surface of the honeycomb, and more rarely inclined.
The making of the honeycomb comprises a tricky operation, i.e. the bonding of the cellular core to the skin, this bonding being done for example by gluing or brazing. The defects in bonding the partitions to the skin appear in an isolated manner or over an expanse grouping together several mutually adjacent cells. These defects can be an absence or a localized insufficiency of binding agent or an incipient melting of the partitions of the cells when using a binder such as a high-resistance brazing whose melting point is near that of the metallic alloy constituting the honeycomb. Such defects are manifested by the appearance on the bottom of the cells of spaces between the partitions of the cells and the skin, these spaces placing the cells in communication with one another.
In the case of hexagonal cells, the cellular core can be made from strips whose width is equal to the thickness of the cellular core, said strips each being folded to form a succession of half-hexagons, each fold constituting one of the six partitions of each hexagonal cell, said strips thereafter being disposed side by side and assembled together, each cell thereby comprising two double opposite partitions. In the case of braised honeycombs, part of the brazing used to bond the cellular core to the skin rises up by capillarity between the two partitions constituting a double partition, and effects the bonding thereof. In any event and regardless of the mode of bonding used, this bonding together of the double partitions must also be checked.
The checking of the brazing runs up against three sorts of difficulties industrially, i.e.:
a honeycomb must be able to be checked rapidly, despite the high number of its cells,
the defects are situated essentially on the bottom of the cells whilst the depth of said cells often exceeds ten times their mean diameter,
the cells may be of small sizes, with a width of less than a millimeter,
it must be possible to detect point defects involving only two adjacent cells, and not merely expanses of defects extending over several cells, or even a large number of cells.
In turbomachines, especially turboengines for aircraft, the honeycombs are mainly used as abradable seal packings between the rotating parts and the fixed parts. Cells are customarily hexagonal with a major diameter of less than 3 mm, typically 1.8 mm. Diameters dropping to 0.6 mm are envisaged. For example, a 45×145 mm frustoconical sector comprises around 3000 cells of {fraction (1/16)}th of an inch, i.e. around 1.6 mm, and a monobloc ferrule of width 45 mm and diameter 800 mm comprises around 53,000 thereof. In aircraft structures and pods, the diameter of the cells may reach 36 mm, this again representing a density of around 1200 cells per square meter.
The surface of the cellular core of the honeycomb which is opposite the skin will be referred to as the “free surface”. To simplify the language and unless specified to the contrary, the expressions “on the free surface”, “above” etc. will be used to specify that which is outside the honeycomb on the free surface side, and the expressions “under the free surface”, “below” etc. will be used to specify that which is depthwise in the honeycomb, without prejudging the actual orientation of said free surface in space.
A widespread so-called “capillarity-based” checking process consists in filling the cells of the honeycomb with a solvent such as fluorinated trichloroethane by dipping in a vessel, in inclining the honeycomb and in visually examining its surface under black light. When cells are placed in communication via defects of bonding, liquid flows from one cell to another, and the openings of the cells concerned show up with a different intensity. This process is rapid, but nevertheless has three drawbacks:
it emits polluting vapors, especially with regard to the operator,
it demands large vessels when the panels to be checked are of large size,
it demands an additional operation of removing the liquid from the cells, this becoming problematic when the cells are of small size or else when the honeycomb is an annulus and the openings of the cells are pointing inward.
A process is also known which consists in heating the honeycomb through the side of the skin and in examining the emission of infrared from the other side, this emission being weaker in the expanses exhibiting brazing defects, since the thermal conductivity in the thickness direction of the honeycomb is lower there. This process is however reserved for honeycombs of large size, since the edge effects are important. Moreover, this edge effect masks point defects. Consequently, this process only allows detection of expanses of defects extending over numerous cells.
The patent FR-2,716,260 also granted in the United States under number U.S. Pat. No. 5,548,400 discloses a checking process consisting in strongly illuminating a cell by bringing in front of its opening an optical fiber connected to a light source, and in detecting the weak light passing through a bonding defect into the adjacent cells by bringing above said adjacent cells optical fibers connected to optoelectronic detection means, the optical fibers being held by a solid support laid on the surface of the honeycomb, this support being positioned with respect to the openings of the cells by pegs penetrating into neighboring cells. This process nevertheless has the drawback of being very low, since the instrument must be positioned in succession in front of each cell to be examined, and the use of a machine of the robot arm type would merely reduce this drawback without eliminating it. Moreover, the inaccuracy in the geometry of the cells precludes the assembling of a sizable number of instruments for checking several cells simultaneously.
Hand-held scanners which can be connected to microcomputers and make it possible to digitize a document with the aid of specialized software are also known. Such scanners comprise a light source illuminating a line on the document, an objective constructing a real image of the illuminated line, an array of photoelectric receptors disposed on the real image of the illuminated line, a sampling analog/digital converter producing a digital signal consisting of a series of doublets indicating the intensity of the signal received by each photoelectric receptor as well as the position of the receptor in the array, and means for measuring the displacement of the scanner on the surface of the document and for producing a digital signal indicating the displacement.

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