GB2091530A - A method for performing non- destructive tests on work-pieces of composite material - Google Patents

Abstract

A method for performing non- destructive tests on workpieces of composite material such as synthetic resin reinforced with glass fibres provides for the interposition of a workpiece (7) under test between an X-ray source (4) and a plate (1) covered with a semi-conductor layer (3) of preliminarily polarised amorphous selenium and the formation on said plate (1) of a latent image obtained by charge migration within the semi-conductor layer (3) as a result of the residual energy of the radiation incident on the said plate (1) after having traversed the workpiece (7) under test, said latent image being made visible by sprinkling the plate (1) with powder. After traversing the workpiece the radiation strikes the plate (1) with an intensity related to the density and thickness of the workpiece. It interacts with the semi-conductor layer (3) to generate thereon an electrostatic image of the internal structure of the workpiece. <IMAGE>

Description

SPECIFICATION A method for performing non destructive tests on workpieces of composite material The present invention relates to a method for performing non destructive tests on workpieces of composite material such as, in particular, synthetic resins reinforced with glass fibres, which find a particularly advantageous use in the aeronautical industry because of their lightness and strength.

Above all, in the aeronautical industry there is particularly felt the necessity to subject the produced workpieces to non destructive tests from the point of view that, on aeroplanes, the failure of any structural workpiece whatsoever can easily involve catastrophic consequences.

Until now, in the aeronautical industry, non destructive tests performed on structural elements of aeroplanes such as, for example panels, wing structures etc., are normally performed by using two different investigation systems the first of which includes a normal internal inspection of the workpieces with X-rays, whilst the second involves the ultrasonic examination of the workpieces.

When the workpieces under test are constituted by composite materials such as synthetic resins reinforced with fibres, both the investigation systems mentioned above have given scarcely satisfactory results.

As far as the ultrasonic system is concerned, this consists in immersing a workpiece under test in a liquid and bombarding it with ultrasonic waves and detecting the residual energy after it has traversed the workpiece, or the energy reflected from the workpiece; the information obtained about the internal structure of workpieces of composite material is made apparent only by extremely difficult interpretations because of destructive interference between the ultrasonic waves due to the anisotropy of the composite material of the workpieces under tesl.

Moreover, some workpieces such as, for example, blades made using honeycomb structures, cannot be immersed in a liquid and therefore cannot be examined by the ultrasonic system.

As far as the X-ray system is concerned, it is normally impossible to examine the internal structure of the workpiece when the workpiece itself is constituted by materials having significantly different permeabilities to X-rays. For example, it is not possible to obtain with a single plate a complete view of the internal structure of a helicopter blade because this normally includes a friction reducing metal casing the presence of which makes it necessary to expose two plates, the first relating to the part of the blade covered by the casing and the second to the exposed part.

Moreover, non destructive tests performed on workpieces of composite material by the X-ray system do not permit the easy identification on the exposed plate of the lines of discontinuity in the thickness of the workpiece examined.

The object of the present invention is that of providing a method for the performance of non destructive tests on workpieces made of composite material such as, in particular, synthetic resin reinforced with fibres, which permits all the structural characteristics of the workpieces examined to be clearly apparent on a single plate in an easily recognisable and interpretable manner and with a relatively high power of resolution independently of the distribution of materials and without requiring the immersion of the workpieces themselves in a liquid.

The said object is achieved by the present invention in that it relates to a method for the performance of non destructive tests on workpieces of composite material, in particular of synthetic resin reinforced with fibres, the method being characterised by the fact that it comprises the steps of providing a plate consisting of a support layer of metal material covered with a semiconductor layer of amorphous selenium; polarising the plate in such a way as to generate, on the free surface of the said semiconductor layer, a uniform electrostatic field; exposing the said semiconductor layer to irradiation by X-rays emitted from an X-ray source after having disposed a workpiece to be examined between the said source and the said plate in such a way as to form on this latter a latent electrostatic image of the workpiece obtained by migration of electric charges to the interior of the said semiconductor layer due to the residual energy of the incident radiation on the said plate after having traversed the said workpiece to be examined; and making the said latent image visible by means of an electrostatic process.

Further characteristics and advantages of the present invention will become apparent from the following description with reference to the attached drawings, which illustrate a non limitative embodiment thereof, and in which: Figures 1, 2 and 3 illustrate in a perspective and schematic manner the successive steps of a method according to the principles of the present invention for the performance of non destructive tests on composite materials; and Figure 4 is a perspective view of a plate exposed using the method illustrated in Figures 1 to 3.

In Figure 1 there is illustrated a plate generally indicated 1 and comprising a metal support layer 2, preferably, made of aluminium, covered with a semiconductor layer 3 of amorphous selenium.

The plate 1, before being exposed to the action of the X-rays emitted by an X-ray source 4 (Figure 2) is subjected to the action of a polarising element 5 the effect of which is to generate on the layer 3 a uniform electrostatic potential by concentrating the positive charges on the free surface of the layer 3, an equal number of negative charges being concentrated on the other surface of the layer 3.

Once polarised, the plate 1 is positioned beneath a support element 6 for workpieces 7 to be examined, which, in the case described, is constituted by a root or anchorage element of a helicopter blade made of synthetic resin reinforced with glass fibres and including two tubular couplings 8.

In practice, the support element 6 is constituted by the cover of a casing (not illustrated) within which the plate 1 is disposed and which can in this way be handled without excessive precautions and difficulty.

The radiation, after having traversed the workpiece 7, impinges on the plate 1 with an intensity which depends on the characteristics of density and thickness of the workpiece 7 itself, and interacts with the semiconductor layer 3 producing within this free charges which migrate towards the surface charges of opposite sign artificially created by means of the polarisation, cancelling them out and generating, in this way, on the layer 3 a latent electrostatic image of the internal structure of the workpiece 7.

As illustrated in Figure 3, the exposed plate 1 is subjected to the action of a mist of powder emitted by a nozzle 9. The powder is attracted to the residual electric field of the plate 1 making an image 10 of the internal structure of the workpiece 7 corresponding to the said latent image visible thereon, the level of resolution of which is solely dependent on the grain size of the powder utilised.

Further to what has been discussed above, it is appropriate to add that it is always possible to give the layer 3 a uniform electric potential in addition to the said residual electric potential, in this way increasing the capacity of the layer 3 to attract the powder. In particular, the said additional uniform potential can be of the same sign as that of the said residual potential or of opposite sign thereby making possible the formation of positive or negative images.

On the basis of the laws which regulate electric fields, the concentration of the residual charges and, therefore the concentration of the powder on the plate 1 varies not only with the variation in the density of the material of the workpiece 7, but also in dependence on its thickness. In particular, there will be a concentration of powder accompanied by a rarefaction of the powder in an immediately adjacent region along all the lines which delimit a variation in the thickness of the workpiece under examination. Consequently, the image 10 of this latter will have, at each variation in thickness of the workpiece, an immediately perceptible and comprehensible iight and shade effect.

Obviously, once formed, the image 10 can be fixed by any electrostatic process onto an auxiliary support. In general, powder constituting the image 10 is transferred from the plate 1 onto a sheet (not illustrated) of previously polarised plastic coated paper. Subsequently, the said sheet is subjected to heat and partially fused in such a way as to incorporate the powder and stabilise the image 10.

Naturally, the principle of the invention remaining the same, it would be possible to introduce numerous modifications to the method described purely as non limitative example without by this departing from the scope of the present invention.

Claims (4)

1. A method for performing non destructive tests on workpieces made of composite material, in particular synthetic resin reinforced with fibres, the method being characterised by the fact that it comprises the steps of: providing a plate (1) consisting of a metal support layer (2) covered with a semiconductor layer (3) of amorphous selenium; -polarising the plate (1) in such a way as to generate a uniform electric field on the free surface of the said semiconductor layer (3);; -exposing the said semiconductor layer (3) to irradiation by X-rays emitted from an X-ray source (4) after having positioned a workpiece (7) to be examined between the said source and the said plate (1) in such a way as to form on this latter a latent electrostatic image of the workpiece obtained by migration of electric charges within the said semiconductor layer (3) due to the residual energy of the radiation incident on the said plate after having traversed the said workpiece (7) under examination; and -rendering the said latent image visible by means of an electrostatic process.

2. A method according to Claim 1, characterised by the fact that the said electrostatic process consists in subjecting the exposed plate to the action of a mist of powder.

3. A method according to Claim 1 or Claim 2, characterised by the fact that it comprises the further step of conferring an additional uniform electric potential to the said semiconductor layer (3) in addition to the residual electric potential defining the said latent image.

4. A method for performing non destructive tests on workpieces of composite material, in particular resin reinforced with fibres, substantially as described with reference to the attached drawings.