RU2446392C1 - Method to install crack detection sensors - Google Patents

Abstract

FIELD: electricity.

SUBSTANCE: method to install graphical crack detectors (GCD) consists of making an electroconductive paste on the basis of dispersed silver and an epoxide binder of cold hardening, serving as a sensitive current-conducting element of a sensor. GCD installation areas are prepared, afterwards the paste is applied by the graphical method onto the prepared surface in the form of paths, and electric outputs are installed at the edges of the paths with the help of the same current-conducting paste. After its polymerisation electric parameters are measured, and installed GCD are adjusted.

EFFECT: increased reliability of a crack detector, expansion of functional capabilities, expansion of the field of application of crack detectors in case of resource tests.

4 dwg

Description

The present invention relates to the field of technical diagnostics of aeronautical engineering (AT), namely, to register cracks during fatigue tests of AT, and can be used in strength testing laboratories of all types of equipment.

Graphic crack detection sensors are installed in the proposed cracking sites during the life and strength tests of the equipment.

Typically, for this purpose, narrow foil strips made in the form of a sectional sensor (1) are glued at the supposed site of fatigue crack formation, perpendicular to the expected direction of crack propagation. Then the sensor is included in the electrical circuit of the recorder.

When the crack reaches the location of the sensing element (conductive path), the latter breaks along the crack and an open circuit is fixed by the equipment - the recorder.

Structurally, the crack detection sensor is a sensing element made of constantan foil on a varnish or paper insulating base, glued to the test surface with cyanoacrylate adhesive. The width of the sensor element is within 0.015-0.5 mm, the thickness is 0.01-0.015 mm, the electrical outputs are wire, soldered with tin.

The main disadvantage of the foil crack detection sensor is its limited life relative to the resource of the tested product, which forces the sensors to be changed several times during the life test.

The closest technical solution, selected as a prototype, is the method of installing sensors (strain gages) (2), based on which a sensitive conductive element made of constantan foil, on an insulating substrate, is glued to the surface of the test product in places of the alleged formation of cracks.

Constantan foil with a thickness of 0.01-0.015, as a rule, is produced by the photochemical method. An adhesive film of phenol-formaldehyde adhesive or heat-resistant paper impregnated with UVS-10T glue is used as an insulating substrate of the sensor (strain gauge).

According to the shape of the sensitive conductive element, the sensors are single, sockets, chains with the base (length) of the sensitive element up to 50 mm.

The sensors are glued to the surface of the test product by cold-hardened cyanocrylate adhesives such as “Cyarin EO”.

Complete cure time 48 hours. Operating temperature range from -80 to + 80 ° С.

UVS glue - 10T (one-component phenolic, hot curing - 5 hours at a temperature of 180 ° C, under a pressure of 0.1 - 0.3 MPa, then polymerization at a temperature of 215 ° C for 5 hours).

Technical characteristics of foil strain gages (crack sensors) are as follows:

- electrical resistance up to 2000 Ohms;

- sensitivity not more than 0.05 mm;

- temperature range -70 ... + 100 ° С;

- failure-free operation of at least 0.98 for 2 × 10 ⁵ loading cycles.

The foil crack detection sensor has significant disadvantages:

- the adhesive used to fasten the sensor to the test surface of the product reduces the reliability and, in general, the resource of the foil crack detection sensor;

- limiting the length and application of the sensor on complex embossed surfaces of the test product;

- decrease in reliability and sensitivity when approaching the crack formed not perpendicular to the axis of the sensor;

- requires a special coating of the sensor to prevent oxide corrosion damage;

- Deliveries of sensors for detecting cracks are carried out in batches, and some of the sensors remain unclaimed.

The objectives of the invention are:

1. improving the reliability of the crack detection sensor by increasing the fatigue strength to the level of fatigue strength of the tested structure and more;

2. expansion of functionality due to the installation of the sensor on complex relief and long sections of the tested structures and reliability at any angles of the crack approach;

3. expanding the scope of crack detection sensors for life tests.

The method for installing graphical crack sensors (GDTs) consists of the following basic operations: making an electrically conductive paste (for example, based on dispersed silver and cold-cured epoxy binder), preparing the places for installing GDTs and making electrical leads, drawing conductive paths, and labeling electrical leads for GDTs.

A characteristic feature of gas turbine engines is the joint process of their manufacture and installation. Hydraulic motors can be installed on the tested structures, welding units and joints of any configuration in horizontal, vertical and ceiling versions at an angle from 5 ° to 175 ° to the axis of the proposed crack.

GDT has the additional possibility of using it after working off the formed crack as a crack development sensor by applying a layer of conductive paste to the conductive path of the sensor in the area of the formed crack.

Electric parameters of gas turbine engines:

- insulation resistance of at least 20 kOhm;

- the resistance of the sensor is set by its cross section and long;

- GTD sensitivity - 0.003-0.06 mm;

- failure-free operation of at least 2 · 10 ⁶ loading cycles;

- working temperature range -50 + 2000 ° С.

The implementation of the manufacturing technology of gas turbine engines during its installation imposes certain increased requirements on the quality of the materials used and the completeness of technological operations.

Particular requirements apply to the paintwork (LCP) of the test product. It must be intact at the place of installation of the gas turbine engine, without sagging and sinks, and have an insulation resistance of at least 1 megaohm (to be measured with a wet working electrode). When measuring the insulation resistance with a wet electrode, the working probe of the megohmmeter is wrapped with a wet swab. LCP should be resistant to ethyl alcohol. If these conditions do not meet, the paintwork is applied again (the best tested paintwork is primer EP-0215 for aluminum alloys and EP-076 for steel). Application is carried out with an airbrush or airbrush in two layers on a prepared and degreased surface.

The next critical operation is the manufacture of dispersed silver. As a rule, this operation is performed by a chemical laboratory: use reagents - silver nitrate, chemically pure, GOST 1277-75; chemically pure medical glucose; ammonia, GOST 3760-79 (1: 1 solution).

Dispersed silver is made at the rate of 3 g per 10 sensors.

The conductive paste for HDT is prepared immediately before its use, its viability is not more than 1 hour (the rational volume of the paste before full use with 1 brush is obtained from 3 g of dispersed silver).

A conductive composite paste of cold curing is prepared in a ratio of 1-3-1, where one part of KDA epoxy with a PEPA hardener, 3 parts of dispersed silver and one part of ethyl alcohol are mixed in the following sequence: 1 part of epoxide is mixed with 1 part of alcohol, then into a solution By constantly mixing, 3 parts of dispersed silver are gradually added. The paste is thoroughly mixed in one direction until a homogeneous (oily) mass without grains is obtained.

In the process of using the paste, it is also necessary to mix it in the same direction, when thickening, add a few drops of alcohol, maintaining the necessary consistency (flowability). Optimum flowability - when the applied paste on a vertical surface does not leak.

Electrical outputs for gas turbine engines are made of tinned copper foil with dimensions of 0.1 × 1.0 × 40 mm.

The application of the conductive paths of the GDT is carried out with an art brush No. 1 or with a special dispenser, it can be applied on a stencil.

Before applying the paste, the prepared surfaces are wiped with alcohol. The dimensions of the conductive path of the sensor can be:

- thickness 0.1-0.15 mm (adjustable spreadability of the paste);

- across the width of 2.0-4.5 mm.

In length - restriction of the electrical resistance of the sensor, ensuring the operation of the recorder (up to 200 Ohms).

At the edges of the conductive track of the sensor, the electrical leads are glued with an additional drop of conductive paste with an art brush.

For vertical and ceiling installation of HDT, electrical outlets must first be fixed with segments (4 × 12 mm) of masking tape. The polymerization of conductive paste on installed gas turbine engines for at least 12 hours at a temperature of from 20 ° C to 25 ° C. At the end of the polymerization of the conductive paste, the electrical characteristics of the installed gas turbine are measured: the insulation resistance should be at least 20 kOhm, with a lower insulation resistance, the sensor is cut off with a mini-chisel according to the paintwork and changed to a new one.

The electrical resistance of the gas turbine engine is checked for stability along the entire length of the sensor. It should gradually increase as the ohmmeter probe moves along the sensor, places with an abrupt increase in resistance are marked with a marker (as a rule, this is a local thinning of the conductive track).

The final operation of the GDT installation is to correct them - align the sensor geometry and eliminate the detected thinning of the conductive track in previously marked places by applying an additional layer of paste, then the sensor resistance measurement is repeated.

The essence of the method for installing crack detection sensors is illustrated in the photo:

Photo 1 shows the options for installed hydraulic fracturing on specimens and crack simulators when testing the functional capabilities of hydraulic fracturing.

Photos 2,3 reflect the use of gas turbine engines in fatigue tests of Be-200RI aircraft.

Photo 4 depicts the life tests of the gas turbine engines themselves on a special bench, where a sample of a beam of equal bending resistance with six gas turbine engines installed is cyclically loaded with a force of 8 to 12 kg / m ² at a frequency of 10 Hz. All gas-turbine engines passed the test until the beam sample broke when the operating time was about 2 · 10 ⁶ cycles.

The technical result is achieved by combining three operations: manufacturing, surface preparation and installation of crack detection sensors.

Information sources

1. A.N. Seryoznov “Measurements during testing of aircraft structures for strength”. Ed. M .: "Engineering", 1976, S. 98-101.

2. "WWW.VEDA.COM.UA", "Tensoresistors".

Claims (1)

A method of installing a crack detection sensor containing a sensitive conductive element with electrical leads, comprising installing it on the prepared surface of the test product, characterized in that the conductive composite paste is used as the sensitive conductive element of the sensor, which is prefabricated on the basis of dispersed silver and cold-cured epoxy binder , applied graphically to the prepared surface in the form of tracks, then along the edges the tracks are electrically connected using the same conductive paste, and after its polymerization, electrical parameters are measured and the installed HDT (graphic crack sensors) are edited.

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