RU2794392C1 - Method for determining the area of damage to the skin of an aircraft - Google Patents

Abstract

FIELD: aviation.

SUBSTANCE: invention relates to a method for determining the area of damage to the skin of an aircraft. To determine the skin area with residual plastic deformations that reduce the strength properties of the skin, the paintwork in the area under study is chemically removed, cleaned of contaminants and dried, the contact potential difference of the skin surface is measured, the damage zone is determined by increasing the contact potential difference relative to the measuring electrode of the dynamic capacitor of the device measurements of the contact potential difference compared to measurements in the undamaged area of the aircraft skin.

EFFECT: reduction of time and labour intensity required during the repair process to determine the skin area around the hole with residual plastic deformations.

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Description

The present invention relates to the field of non-destructive testing of metal parts of aircraft during their repair.

Aluminum-based alloys are widely used in the aircraft industry. Aircraft skins are made from duralumin. The operation of the aircraft may be accompanied by damage to the skin in the form of holes. Holes in the aircraft skin can be repaired with riveting with inserts and overlays. In this case, the damaged section of the skin around the hole has reduced strength properties and is removed before riveting [1]. However, even the edges of the skin without cracks at the site of the hole may have plastic deformations invisible to the naked eye, accompanied by a change in the strength properties of the skin material and contributing to the development of cracks in the skin during further operation of the aircraft. Such areas of damage to the aircraft skin should also be removed. Thus, the determination of the skin damage zone around its hole is a very important task for ensuring the quality of the aircraft repair.

There is a known method of recording the deformation of solids [2], in which a brittle strain-sensitive coating is applied to the surface of the investigated solid, the solid is deformed, and the magnitude of the deformation is estimated by changing the parameters of the strain-sensitive coating. The disadvantage of this method is the need to deform the investigated solid.

A known method for determining the plastic deformation of samples [3], which consists in the fact that a coordinate grid is applied to the surface of the sample under study, the sample is deformed, and the value of plastic deformation is judged by the distortion of the cells of the coordinate grid. This method has the disadvantage of requiring sample deformation.

A device for measuring deformations [4] is known, which makes it possible to determine the linear dimensions of residual deformations, containing two parallel plates, racks placed between them and rigidly connected to the latter, and a strain gauge. In order to improve accuracy, the device is equipped with an additional plate located between the main plates with holes through which the racks are passed, and rods installed at an angle to the racks and connected to them at one end, and at the other with an additional plate, and the strain gauge is connected with additional plate. The disadvantage of this device is the large size, the inability to use on an aircraft without dismantling the damaged skin.

A device for measuring the deformation of a structural element (options) [5] is known, which makes it possible to measure forces on a deformable structural element. The device consists of a transmission link and one or two sensors. The sensor is made as a sensor having a measuring surface and supplying an electrical signal dependent on the deformation of the measuring surface. The transmission link rests on the one hand on the sensor, and on the other hand on the structural element or another sensor. When the structural element is deformed, the distance between the bearing points of the transmission link changes. This change in distance produces a deformation of the measuring surface of the sensor, which is evaluated as a measure of the deformation of the structural element.

The disadvantage of this device is the need to deform the test sample, the difficulty in applying in order to determine the area of damage to the skin of the aircraft.

There is a known method and device for monitoring the state of the aircraft structure [6], which allows for constant monitoring and contains piezoelectric sensors. The signals emitted by the sensors are recorded continuously. From them, a conclusion is drawn about the fatigue that critical parts of the aircraft are subject to. The disadvantage of this invention is that piezoelectric sensors are installed in critical parts of the aircraft, but the skin of the aircraft can be damaged anywhere.

The closest to the proposed invention is a method for determining the surface deformations of samples [7], which consists in the following: electrically insulating and electrically conductive polymer coatings are applied to the sample surface in series, an orthogonal system of four electrodes is mounted on the latter, which, at each loading of the sample, are connected in turn with opposite pairs to a current source and at each connection using two probes register the distribution of electrical potential on the surface of the electrically conductive coating between the electrodes.

The disadvantages of this method is the high complexity.

The aim of the present invention is to improve the quality of aircraft repair, as well as to reduce the time and labor required during the repair process to determine the skin area around the hole with residual plastic deformations.

The goal is achieved by the fact that in the claimed method for determining the zone of damage to the skin of an aircraft, which consists in determining the zone of the skin with residual plastic deformations that reduce the strength properties of the skin, in the studied area of the skin, the paintwork is first chemically removed, then the surface of the skin is thoroughly cleaned of contaminants and dried. . After that, the contact potential difference of the skin surface is measured. The area of damage to the skin is determined by the increase in the contact potential difference relative to the measuring electrode of the dynamic capacitor of the device for measuring the contact potential difference compared to measurements in the undamaged area of the skin of the aircraft.

In the case of using duralumin aircraft skin, the damage zone is determined by an increase in the contact potential difference by more than 0.25 V relative to the nickel measuring electrode of the dynamic capacitor of the device for measuring the contact potential difference compared to the undamaged aircraft skin area.

Thus, the goal is achieved by using the method of contact potential difference (CDP) to determine the plastically deformed area of the skin of the aircraft.

The CFC method refers to the electrical type of non-destructive testing and is based on a comparison of the work function of an electron from the area of the controlled metal plating of an aircraft and the previously known work function of an electron from the surface of the measuring electrode of the sensor of the CFC measurement device according to the formula:

U _k \u003d (φ _ie -φ _o ) / e ,

where U _to - KRP, V;

φ _o is the work function of an electron from the skin surface, J;

φ _ie is the work function of an electron from the surface of the measuring electrode of the CRP measurement device, J;

e is the unit charge of the electron, C.

The CRP method is sensitive to the state of the surface of electrically conductive materials, including duralumin aircraft skin. The distortions of the crystal lattice formed during plastic deformations in the surface layers of the cladding material and an increase in the dislocation density on the cladding surface lead to a decrease in the work function of the electron from the cladding. This is fixed by an increase in the measured CRP.

Measurement of CRP in order to determine the area of damage to the skin of an aircraft can be carried out, for example, with a device described in patent RU 2717747 dated March 25, 2020 [8], the application diagram of which is shown in Fig. 1.

In FIG. 1 the numbers indicate:

1 - aircraft skin;

2 - measuring electrode;

3 - surface of the skin on which the CRP is measured;

4 - oscillatory circuit;

5 - preamplifier;

6 - sensor;

7 - digital portable oscilloscope with the ability to record measurement results in its memory.

Before measuring the CRP on the surface of the skin 3 of the aircraft in order to determine the zone of its damage, the paint and varnish coating from the surface of the skin 3 is removed chemically (for example, with AFT-1 wash and solvents) 5 cm around the hole. The surface of the casing 3 and the measuring electrode 2 of the sensor 6 of the CRP measuring device is thoroughly cleaned, for example, according to the method described in patent RU 2488093 dated July 20, 2013 [9] and dried. To dry the skin of 1 aircraft, heated air with a temperature of not more than 60 °C can be used.

To improve the accuracy of measurements on the surface of the skin 3, it is recommended to calibrate the CRP measurement device, for example, by the method proposed in patent RU 2758272 dated October 27, 2021 [10].

When determining the area of damage to the skin 1 of the aircraft, the sensor 6 of the FDC measurement device is applied to the skin 1 and measurements of the CRF around the hole are carried out. The damaged zone of the duralumin skin, which has residual plastic deformations, is determined by the excess of the CRC by at least 0.25 V compared to the defect-free zones of the aircraft skin.

Comparative tests carried out, the results of applying the proposed method for determining the damage zone of the aircraft skin, were confirmed by the eddy current method of non-destructive testing using the VDL-5.2 device and by measuring the microhardness of the D19 alloy skin with the PMT-3 device. The measurement results are presented in the table.

Table - Average values of comparative measurements of the lining of the alloy D19

Sheathing area KRP, V Microhardness, MPa Indications of the VDL-5.2 device Intact 0.671 105 48 plastically deformed 0.924 68 93, alarm triggered

The sheathing zone with residual plastic deformations determined by the method proposed by us is marked out and removed before repair with riveting (Fig. 2). In FIG. 2 numbers indicate:

8 - hole;

9 — damaged skin zone with residual plastic deformations (highlighted by dashes);

10 - a skin cutout made during the repair.

An increase in the CPD measured on the surface of a plastically deformed skin occurs due to a decrease in the work function of an electron from the skin due to an increase in the density of dislocations around which the interatomic distances decrease and the density of electron distributions increases.

After the repair, the paintwork of the skin is restored.

The proposed method allows us to quickly, with satisfactory accuracy, determine the areas of damage to the aircraft skin during its repair, improve the quality of aircraft repair.

Information sources

1. Oleshko V.S. Preparation of military aviation equipment for repair. - M .: MAI Publishing House, 2016. - 72 p.

2. Copyright certificate SU 970082 dated October 30, 1982.

3. Author's certificate SU 977925 dated 12/02/1982

4. Copyright certificate SU 998846 dated February 23, 1983.

5. Patent RU 2116614 dated July 27, 1998

6. Patent RU 2385456 dated March 27, 2010

7. Copyright certificate SU 1516759 dated 10/23/1989.

8. Patent RU 2717747 dated March 25, 2020

9. Patent RU 2488093 dated July 20, 2013

10. Patent RU 2758272 dated October 27, 2021

Claims (2)

1. A method for determining the area of damage to the skin of an aircraft, which consists in determining the area of the skin with residual plastic deformations that reduce the strength properties of the skin, for which the paint and varnish coating is first chemically removed in the investigated area of the skin, then the surface of the skin is thoroughly cleaned of contaminants and dried, after that measuring the contact potential difference of the skin surface; the area of damage to the skin is determined by the increase in the contact potential difference relative to the measuring electrode of the dynamic capacitor of the device for measuring the contact potential difference compared to measurements in the undamaged area of the skin of the aircraft. 2. The method for determining the damage zone of the aircraft skin according to claim 1, characterized in that for the duralumin skin of the aircraft, the damage zone is determined by an increase in the contact potential difference by more than 0.25 V relative to the nickel measuring electrode of the dynamic capacitor of the device for measuring the contact potential difference according to compared to the undamaged aircraft skin area.

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