RU2633288C1 - Diagnostic method of reliability and limited life of multi-layer structures made from composite materials operation - Google Patents

Abstract

FIELD: measuring equipment.

SUBSTANCE: method and device, implementing this method includes placing of optical fibers with Bragg gratings layer by layer in the structure layers from polymer composite materials in the manufacturing process. The gratings coordinates of different optical fibers are set one above the other with the error of not more than half of the grating length. The temperature and deformation are measured, the values of the strains on the gratings due to deformation and temperature are determined by solving the systems of equations, describing the corresponding mathematical models. During the product loading process, the deformation values and the temperature are compared with the maximum permissible deformation and temperature. Measure the dependence of the voltage and temperature values on the gratings from the depth of its occurrence in the structure. Measure the difference between the measured and reference dependencies and, based on the comparison, form the conclusion on the structure reliability under the action of power loads and the maximum service life. Based on the results of the temperature measured along the optical fibers, the areas of the stress concentrators position are localized.

EFFECT: increasing the detection reliability of the local reduced strength areas, increasing the result reliability of evaluating the technical and operational status of complex structures and its elements made of composite materials.

6 cl, 9 dwg

Description

Technical field

The invention relates to the field of measuring technology and can be used to assess the reliability of complex spatial structures made of composite materials (CM), including with metal layers, based on the results of monitoring the magnitude of the deformation when they are loaded with a static or dynamic load.

The invention can be used to control the reliability of complex spatial multilayer structures from CM both in the production process and in the operation process: spatial mesh structures: compartments of spacecraft, rocket engines, pipelines, pressurized vessels, etc.

Especially effective is the application of the invention in testing potentially hazardous and expensive structures to manufacture, which, on the one hand, have high requirements for reliable operation, and on the other hand, they are quite expensive and time-consuming to manufacture so that a sufficiently large number of structures can was tested using destructive testing methods, i.e. to destroy. In this case, it is necessary to identify potentially dangerous places (structural units) that can primarily collapse (due to defects, reduced strength, or other reasons) under loads, which can lead to accidents and which may need to be strengthened without bringing the product to failure to predict the ultimate load level that will cause structural resolution.

This invention can be used in structures made of polymer composite materials manufactured by the method of winding, in which it is impossible to lay sufficiently large state sensors in the material due to a violation of the strength characteristics of the structure.

State of the art

A promising direction in modern technology is the use of composite materials, including polymer composite materials, which have several advantages over traditional materials - metals, especially in the aerospace industries, engineering, energy, etc. Such materials require a special approach, new solutions in the development and creation of methods and means of assessing the reliability of their operation. This is due to the wide variety of types of such materials, the specific features of the structures made of them and the manufacturing technology and the random change in the physicomechanical and strength characteristics, a large variety of types of defects that arise during the manufacturing process.

In addition, these materials in most industries operate under static and dynamic loads.

Improving the quality of structures is impossible without a reliable assessment of their reliability. Accordingly, it is impossible to develop measures and technologies to improve the reliability of structures. One of the signs of the reliability of structures is the magnitude of the deformation of the structure over the entire package of material when it is loaded and the absence (or) the presence of stress concentrators, which, as a rule, are formed in places of reduced strength, or in a material having continuity violations.

Considering that such designs are, as a rule, quite expensive both in cost terms and in the complexity of manufacturing, it is necessary, on the one hand, to test each design for compliance with its reliability required, and on the other hand, these tests should be minimally “ to injure ”the design with the maximum information content of the test results.

Depreciation of fixed assets and technical equipment, a decrease in the quality of the material and other similar causes leads to a decrease in the reliability of operation of structures made of CM.

For example, KM fatigue, features of their manufacturing technology, etc. lead to a change in the deformation characteristics of both the outer layer and the inner layers, the appearance of residual internal stresses that cause discontinuity and, ultimately, lead to the destruction of the material and structure. This phenomenon is widely described in the literature. Recently, a number of programs have been adopted aimed at correcting the situation: modernization of production, improving the quality of materials, etc. However, the full solution of these problems is currently difficult, including for financial reasons.

In this regard, non-destructive methods for monitoring and diagnosing such structures are of great importance. They allow you to objectively determine the actual state of the structure, evaluate the reliability of their operation, without bringing them to destruction and give recommendations for its repair or restoration. In addition, the methods should make it possible to control the quality of the structure both during its manufacturing under production conditions and during operation, where real power loads act on the structure.

A known method for determining the residual stresses in the plates (ed. Certificate of the USSR No. 1543259), according to which the control object is illuminated with coherent light, records a hologram of the surface, removes part of the material, creates a local zone of deformations by means of a point load in the zone of displacements caused by material removal, records the hologram surface secondary. The magnitude and sign of the residual stresses is determined by the number of interference fringes and their distortion. This method is applicable exclusively to flat parts, is associated with the destruction of the material and is used for scientific research in laboratories.

A known method for determining residual stresses according to RF patent No. 2032162, according to which a pyramidal indenter is statically pressed into the test material until an imprint with developing brittle cracks is formed, stress and crack parameters are measured, crack topology is evaluated, the equilibrium and effective values of fracture toughness are determined, and the value of residual stresses calculated according to known ratios, taking into account the linear dimensions of the actual grain in the coating.

The method is difficult to implement and is applicable only for laboratory purposes.

There is also a method of non-destructive testing of the physicomechanical properties of a polymeric material or structure made of a polymeric material: patent BY 10472. It is based on the force acting on the material and analysis of the reaction of the material. The disadvantage of this method is similar to the disadvantages of the method according to the previous patent.

A known method of thermal control of residual stresses and structural defects and the system that implements it (RF patent No. 2383009). Known technical solution allows thermal control of the reliability of structures. The known method includes the force on the controlled products and the registration of the temperature field, the analysis of which is used to judge the state, incl. about reliability, products. The system includes a thermogram recording device, a visualization unit, and a processing device.

A disadvantage of the known technical solution is as follows.

The method allows to determine the location of stress concentrators by recording the temperature field arising from the rupture of the internal fibers. However, this information does not give a complete picture of the reliability of the product, since it does not allow to evaluate its deformability of both the surface and the inner layers.

Moreover, information on the deformability of the inner layers is often more important for assessing the reliability of multilayer products from CM than the data on the outer layer. This is due to the fact that the reliability of products - the ability to withstand the applied internal and external power loads - is largely determined by the inner layers, including their location, the presence of a binder, technological modes of winding, etc.

Therefore, this technical solution is applicable only to control a limited range of products.

There is a method of determining the deformation of a glued structure made of composite material and, based on the data obtained, the identification of non-glue in the place of gluing, achieved through the use of fiber-optic Bragg gratings and information about the reflected and transmitted spectrum taking into account Brillouin scattering (US application No. 20088008385).

The disadvantage of this method is the use of two recording spectrometers and the absence of calculation of the effects of temperature gradients, since the information on Brillouin scattering in an optical fiber in a short section makes it possible to record only the integral temperature characteristic.

A known method for determining the deformation of parts through the use of fiber-optic Bragg grating, mounted on a special design made in the form of an external strain gauge for placement on the studied surfaces (application US No. 20090126501).

The disadvantage of this method is the inability to use it to determine changes in the deformation inside the structure of the composite material.

A known method for determining structural defects in a composite material due to sounding by acoustic waves generated by piezoelectric transducers inside the composite and fiber-optic Bragg gratings that register acoustic waves (US application No. 2008156971).

This method is intended to determine structural defects in the composite material, but is not intended to simultaneously determine the deformation and temperature of the composite material.

A known method of thermal compensation in determining deformation using a single fiber-optic Bragg grating, which consists in creating a special design that provides transmission of mechanical deformation in the absence of thermal contact of the surface to be monitored with a fiber-optic Bragg grating (Canadian patent No. 2348037).

The disadvantage of this method is the inability to use it to determine the strain inside structures made of composite material.

A known method for determining the deformation of cylindrical structures through the use of a special removable shell with integrated fiber optic Bragg gratings. This method allows you to determine the deformation-stress state of the structure with temperature compensation by using an additional fiber-optic Bragg grating to record the temperature outside the zone of exposure to mechanical deformation (US application No. 2009052832).

The disadvantage of this method is the impossibility of its use inside structures made of composite material having a shape other than cylindrical, and the determination of internal defects.

There is a method of determining the shape of a tube by measuring its deformation along its axis by spiraling through an optical fiber with an array of fiber-optic Bragg gratings, which allows one to determine compression / tension, bending, and torsion of the tube. This method involves the use of axial and axial projections of strains recorded by an array of fiber optic Bragg gratings. The separation into axial and axial projections is based on the known angles of the sensors in a spiral direction on the tube (publication No. WO 2009068907).

This method is intended only for determining the shape of tubes or other cylindrical surfaces and cannot be used in flat and complex profiles made of composite materials. The calculations used do not allow differentiating the presence of a temperature gradient in the tube.

A known method of using a network of fiber-optic Bragg gratings in the surface layer of a composite material between two sections of reinforcing stiffeners for organizing a network of built-in structural inspection, including aircraft, in the manufacturing process of composite material. This method involves the use of an array of Bragg gratings on two optical lines. Fiber optic Bragg gratings are used with only two periods. Registration of strains in the composite material is achieved by using a straight and transverse line with Bragg gratings in such a way that the deviation from the set wavelength of the Bragg grating determines the deformation, and the joint deviation of the Bragg gratings at the intersection of the fiber lines determines the location of the applied load (French patent No. 2865539) .

The disadvantage of this method is the use of a large number of fiber-optic Bragg gratings, the impossibility of simultaneously recording the occurrence of several loads, especially the distribution of loads, the absence of temperature compensation, which leads to false registration of mechanical deformations.

A known method of solving the tasks according to US application No. 20090092352.

The disadvantage of this method is that it does not allow to measure the deformation of a structure made of composite material during operation, since it is not possible to simultaneously take into account the effects of various types of loads. This method leads to a false registration of mechanical deformation in the event of a temperature gradient inside the composite material due to the fact that an optical fiber with a Bragg grating is located simultaneously between several monolayers of the composite material and has a large length. In the case of tensile (or compressive) strains, the spectral position of the peak changes, which can be interpreted as a false temperature effect, and in the case of a simultaneous negative temperature effect and tensile strain, it can lead to the absence of changes in the recorded spectrum of the Bragg grating.

A known method of determining the deformability of the product under the action of power loads, described in the patent for invention No. 2216684. It includes the installation of strain gauge strain gauges on the surface of the controlled object, measurement of the strain for some time and, based on the measurement results, the development of a conclusion on the magnitude of the stress-strain state (VAT) of the control object and, accordingly, a conclusion on its reliability of operation.

The disadvantages of this approach are obvious: deformation is determined only on the surface of the controlled object, which is completely insufficient to develop a reliable conclusion of a multilayer object, where each layer carries its own specific load to counteract the applied destructive loads. It is not always possible to lay strain gauges currently used in practice in multilayer structures, because These sensors will be artificial internal voltage concentrators and will create additional dangerous foci of destruction.

The closest in technical essence and purpose and adopted as a prototype is a method for determining the deformation of a structure made of composite material (KM) according to the patent for the invention No. 2427795.

This invention relates to the field of diagnostics of the mechanical properties of structures made of polymer and metal-polymer composite materials and can be used to determine the deformation of structures. According to the method, in the manufacturing process of the composite material, an optical fiber structure with Bragg gratings is placed in it. The spectral position of the peaks of the Bragg gratings is measured after the structure is made of composite material and the distribution of mechanical and thermal strains inside the structure of the composite material is determined by solving the corresponding system of equations describing the mathematical relationship between the optical characteristics of optical fibers with Bragg gratings and the deformation of the product.

The disadvantage of this method is that it does not provide diagnostics of the reliability and ultimate service life of multilayer structures made of composite materials.

To date, there is an urgent need to create a method and device for diagnosing the technical condition of real complex multilayer spatial structures from CM, including their reliability of operation and their ultimate service life (residual resource), which can be applied in practice, incl. for a wide range of objects using simple and accurate equipment.

The most significant results have appeared in the last decade.

SUMMARY OF THE INVENTION

The invention is aimed at solving the problem of increasing the reliability of diagnostics of the technical condition of real complex multilayer spatial structures from CM, including their reliability of operation and their ultimate service life (residual resource), which can be applied in practice, incl. for a wide range of objects using simple and accurate equipment.

At the same time, control should be carried out both in the production process and in actual operating conditions, including under load conditions, determining areas of reduced strength, defective areas (areas that do not comply with regulatory documents), developing recommendations for eliminating defects or restoring the structure.

Those. ultimately, the invention is aimed at improving the safety of the operation of complex potentially hazardous structures under continuous or cyclic loads (mechanical, internal pressure, etc.).

The technical result achieved by using the claimed group is to increase the reliability of detection of local areas of reduced strength, increase the reliability of the results of assessing the technical and operational status of complex structures and their elements from CM.

The technical result is achieved due to the fact that in the method for diagnosing the reliability and ultimate service life of multilayer structures made of composite materials, including placing in a composite material in the process of manufacturing an optical fiber with Bragg gratings, determining the magnitude of the mechanical and temperature effects on it by measuring the spectral position of the Bragg peaks gratings

before placing an optical fiber with Bragg gratings in the composite material, the initial spectral position of the peaks of the Bragg gratings is determined,

- the optical fiber is placed along the entire structure between the monolayers of the composite material susceptible to mechanical deformation,

- after manufacturing the structure from a composite material, the spectral position of the peaks of the Bragg gratings is measured,

- and then determine the distribution of mechanical and thermal strains inside the composite material structure by solving a system of equations describing the mathematical model of the structure and functional dependencies, taking into account the values of real mechanical and thermal strains and total strains obtained from the displacement of the peaks of the Bragg gratings,

- optical fibers with Bragg gratings are placed layer-by-layer in the structural layers of polymer composite materials during the manufacturing process and the formation of the product structure and material in one technological cycle in such a way that the coordinates of the Bragg gratings of different optical fibers are on top of each other with an error of no more than half the length:

where X _ij - coordinate Bragg grating,

δ is half the length of the Bragg grating,

i = 1 ... n is the number of the optical fiber located in the i-th layer,

j = 1 ... k is the number of the Bragg grating on the i-th optical fiber,

n is the number of layers in the structure in which the optical fibers are embedded,

k is the number of Bragg gratings on optical fibers,

- in addition to the measured Bragg gratings in the process of loading the design values of strain U ^DEF _s (i, j, t) is measured temperature T _s (i, j, t), where t - current time of loading the structure,

- determine the magnitude of the stresses on the Bragg gratings due to deformation of the structure due to the power load ^voltage U _s (i, j, t) changes due to temperature and ^voltage s T (i, j, t), by solving the system of equations describing the corresponding mathematical model,

- during loading products compared the amount of deformation U ^DEF _s (i, j, t) and the temperature T _s (i, j, t) with a maximum deformation quantity of controlled items U layer ^DEF _ovmax and, accordingly, the maximum permissible temperature value, due to the rupture of the fibers in the places of stress concentrators T _{o max} ,

- when the condition U ^def _ov (i, j, t)> U ^def _ovmax Λ (T _ov (i, j, t))> T _ovmax the product loading process is stopped,

- measure the dependence of the voltage and temperature on the Bragg gratings on the depth of their occurrence in the structure:

^Voltage U _s (i, j, t) = f (h i, j),

T ^{for example} (i, j, t) = f ₁ (h _{i, j} ),

where h _{i, j} is the depth of the j-th Bragg grating on the i-th optical fiber in the controlled structure

- measure the difference between the measured dependencies and reference dependencies and, based on the comparison, form a conclusion on the reliability of the structure under the action of power loads and the ultimate service life,

- additionally, according to the results of temperature measurement along the embedded optical fiber lines, the locations of the stress concentrators are localized.

The technical result is enhanced due to the fact that they determine the interval of placement of Bragg gratings on the optical fiber to record the strain and temperature field based on the detection of the minimum in size of the deformation and temperature anomalies with a spatial period Δa, determined by the size of the minimum deformation and temperature anomaly:

where Δx ^def _min - the geometric dimensions of the minimum deformation anomaly,

Δx ^t _min - the geometric dimensions of the minimum temperature anomaly due to the presence of an internal voltage concentrator,

Δa ^def is the location step of the Bragg grating for reliable registration of deformation sections,

Δa ^t is the location step of the Bragg grating for reliable registration of temperature areas due to the presence of an internal voltage concentrator,

Δa is the common (single) step of the Bragg grating of an optical fiber line embedded in a controlled product for measuring strain and temperature.

The geometric dimensions of the minimum temperature anomaly due to the presence of an internal voltage concentrator Δx ^t _min are determined as follows:

- measure the size of all temperature anomalies contained on the surface identified as a result of preliminary registration of the temperature field: Δх ^t _i ,

- determine the size of the minimum anomaly Δx ^t _min , solving the equation:

where δ is the probability that (Δx _i ) ≥ (Δx ^T _min ),

p (ΔX ^T _i) - function of distribution Δx ^T _i.

The geometric dimensions of the minimum deformation anomaly Δx ^def _min is determined as follows:

- measure the dimensions of all deformation anomalies contained on the surface, identified as a result of preliminary: Δx ^def _i ,

- determine the size of the minimum anomaly Δx ^def _min , solving the equation:

where δ is the probability that (Δx _i ) ≥ (Δx ^def _min ),

p (ΔX ^def _i ) is the distribution function of the quantities Δ x ^def _i .

The optimal interval for sequential measurement of τ _ISM during loading of the controlled product is determined by solving the system of equations:

where f (U ^DEF _s), U _s ^DEF - density distribution in duration time values (information signal) U _s and U ^{DEF DEF} _s, respectively,

τ _ISM - time interval of measurement,

P ^def _s and P ^T _s - the probability of missing values (information signal) U ^def _s and U ^def _s , respectively,

τ ₀ - temporal resolution encoders,

η is the integration parameter,

^DEF τ _s - an optimal time interval of the strain amount measuring ^DEF U _s,

τ ^T _s - the optimal time interval for measuring the magnitude of the strain T _s .

The technical result in terms of the reliability diagnostics device and the ultimate service life of multilayer structures made of composite materials is achieved due to the fact that the device includes optical fibers (5) with Bragg gratings (6) placed in the layers of the structure of polymer composite materials

- loading system (2) of the controlled product,

- loading system control unit (3),

- a strain measurement unit (7) along the optical fibers (5) on the Bragg gratings (6),

- block recording results (18),

while the output of the control unit of the loading system (2) is connected to the input of the loading system (2) of the controlled product,

- the inputs of the strain measurement unit (7) along the optical fibers (5) on the Bragg gratings (6) are connected to the corresponding outputs of the optical fibers (5),

a unit for measuring stress by strain values (4),

- optical fibers (5) with Bragg gratings located between the layers of material,

- temperature measuring unit (8) on the Bragg gratings,

- the first and second threshold devices (9, 10),

- unit for measuring voltage on Bragg gratings by temperature (11),

- voltage measurement unit, depending on the depth structure in ^voltage U _s (i, j, t) = f (h i, j) ( 12)

- a temperature measuring unit depending on the depth of ^eg T _s (i, j, t) = f ₁ (h _{i, j)} (13)

- a database block of the reference values of voltages and and temperatures (14),

- the first and second adders (15, 16) and

- a unit for forming a conclusion on the reliability of the design and the ultimate service life (17),

the outputs of the optical fibers (1) with Bragg gratings (6) are connected to the corresponding inputs of the temperature measuring unit (8) on the Bragg gratings (6),

- the first output of the temperature measurement unit (8) on the Bragg gratings is connected to the input of the second threshold device (10),

- the first output of the strain measurement block (7) on the Bragg gratings is connected to the input of the voltage measurement block (4) by the strain values,

- the output of the unit (4) for measuring stress by the strain values is connected to the input of the unit (12) for measuring the dependence of stress on the depth

^Voltage U _s (i, j, t) = f (h i, j),

- the second output of the strain measurement block (7) on the Bragg gratings is connected to the input of the first threshold device (9),

- the first output of the first threshold device (9) and the first output of the second threshold device (10) are connected to the first input of the unit (3) for controlling the product loading system,

- the second output of the first threshold device (9) and the second output of the second threshold device (10) are connected to the second input of the unit (3) for controlling the product loading system,

- the second output of the temperature measurement unit (8) on the Bragg gratings is connected to the input of the temperature measurement unit (11),

- the output unit (11) for temperature measurement voltage connected to the input unit (13) measuring the ^voltage dependence of T _s (i, j, t) = f ₁ (h _{i, j),}

- the output unit (13) measuring the ^voltage dependence of T _s (i, j, t) = f ₁ (h _{i, j)} is connected to the input of the first adder (15),

- the output unit (12) depending on the measurement ^voltage U _s (i, j, t) = f (h i, j) is connected to the input of the second adder (16),

- the first and second outputs of the block (14) of the database of the reference values of voltages and temperatures are connected to the second inputs of the adders (15) and (16),

- the outputs of the adders (15) and (16) are connected to the inputs of the block (17) of forming a conclusion about the reliability of the design and the ultimate service life,

- and the output of the block (17) of the formation of the conclusion on the reliability of the design and the ultimate service life is connected to the input of the block (18) of recording the results.

Brief Description of the Drawings

The invention and the possibility of achieving a technical result will be more clear from the following description with reference to the position of the drawings, where:

FIG. 1 shows a structural diagram of a device

FIG. 2 shows a photograph of the manufacturing process (winding) of a product from polymer composite materials,

FIG. Figure 3 shows a photograph of the placement of optical fibers in a product made of polymer composite materials during its winding (manufacturing) - the optical fiber (5) is laid along the fiber carrying the power load,

FIG. 4 shows a photograph of the claimed device,

FIG. Figure 5 shows, as an example, some measurement results on Bragg gratings,

FIG. 6 is a graph of calculating the reliability of the structure,

FIG. 7 - loading system,

Fig 8 is a photograph of the product brought to destruction,

FIG. 9 results of measurements on Bragg gratings.

In the drawings, the following notation:

1 - controlled product

1-1, 1-2, 1-3, 1-n - 1st, 2nd, 3rd, nth layers of the controlled product,

2 - product loading system,

3 - control unit loading system of the product,

4 - unit for measuring stress by strain,

5 - optical fibers between the layers,

6 - Bragg gratings on optical fibers,

7 - a unit for measuring strain on Bragg gratings,

8 - block temperature measurement on the Bragg gratings,

9, 10 - threshold devices,

11 is a block for measuring voltage by temperature,

12 - measuring unit depending ^voltage U _s (i, j, t) = f (h i, j),

13 - measuring unit depending ^eg T _s (i, j, t) = f ₁ (h _{i, j),}

14 is a block database of the reference values of voltages and temperatures,

15, 16 - adders,

17 - block forming conclusions on the reliability of the design and the ultimate resource of operation,

18 - block recording results

19 - a bundle of threads from which the products are made,

20 - winding machine,

21 is a spiral layer,

i = 1 ... n is the number of the optical fiber located in the i-th layer,

j = 1 ... k is the number of the Bragg grating on the i-th optical fiber,

n is the number of layers in the structure in which the optical fibers are embedded,

k is the number of Bragg gratings on optical fibers,

X _ij - location coordinates of the Bragg gratings,

δ is the error of the location of the Bragg gratings on the optical fiber,

U ^def _s (i, j, t) is the value of the measured strain,

T _s (i, j, t) is the value of the measured temperature,

t is the current load time of the structure.

Preferred Embodiment

All electronic components used are built on the basis of standard microprocessor circuits and microprocessor assemblies with reprogrammable memory devices, and the control system for turning off / on the loading system is built on standard relay systems (see, for example, EP Ugryumov. Digital circuitry: textbook for universities . - 3rd ed., Revised and add. - St. Petersburg: BHV-Petersburg, 2010).

The implementation of the method is as follows.

Optical fibers (5) with Bragg gratings (6) are placed layer-by-layer in the layers of a structure (1) of polymer composite materials (composite materials) during its manufacture and simultaneous formation of the structure and material of the product from in one technological cycle so that the coordinates of the Bragg gratings (6) optical fibers (5) coincided in height location on different layers (1-1, 1-2, ... 1-n (located one above the other) with an error of no more than half the length of the Bragg grating:

i = 1 ... n is the number of the optical fiber located in the i-th layer,

j = 1 ... k is the number of the Bragg grating on the i-th optical fiber,

n is the number of layers in the structure in which the optical fibers are embedded,

k is the number of Bragg gratings on optical fibers.

In FIG. Figure 2 shows a photograph of the manufacturing process (winding) of a product from polymer composite materials.

In FIG. Figure 3 shows a photograph of the placement of optical fibers in a product made of polymer composite materials during its winding (manufacturing) - the optical fiber (5) is laid along the fiber that carries the power load.

In addition to measured on Bragg gratings (6) unit (7) measure strain on the Bragg grating during loading structure deformation values (U ^DEF _s (i, j, t)) measured temperature (T _s (i, j, t)) block (8) temperature measurements on the Bragg gratings, where t is the current load time of the structure.

In FIG. 4 shows a photograph of the claimed device.

In FIG. Figure 5 shows, as an example, some results of measurements on Bragg gratings.

21 - the values of the deformation value (U ^DEF _s (i, j, t)) (T _s (i, j, t)) for a time of loading the product on different optical fibers (5) and at different Bragg grating (6).

During continuous record variables (U ^DEF _s (i, j, t)) (T _s (i, j, t)) is made and compared with the maximum permissible relevant limit values for the investigated structure (1) U ^DEF _ovmax and T _ovmax in threshold devices (9) and (10). At the first and second outputs of the threshold devices (9) and (10), the corresponding initiative signals (0 or 1) are generated, which indicate the excess of the current values (U ^def _s (i, j, t)) and (T _s (i, j, t).

The outputs of the threshold device (9) generate proactive signals:

From the outputs of block (10), initiative signals are generated:

Proactive signals I (U ^DEF _s) and I (T _s) applied to two inputs of the block (3) Control product loading system. On admission to the input unit (3) of the action signals I (U _s ^DEF) or I (T _s) from the output unit (3) on the input unit (2) loading the product loading process system products terminated in order to avoid its injury or destruction. Product loading cessation time - t _max .

During the loading article (1) in the block (4) determine the value of the stress on the Bragg gratings due to deformation of the structure due to power loads (U ^eg _s (i, j, t)) , a control unit (11) determining the stress value at the Bragg gratings due temperature changes (T ^ex (i, j, t)) by solving systems of equations describing the corresponding mathematical models.

In block (12), the dependence of the voltage on the Bragg gratings on their depth in the structure is measured:

^Voltage U _s (i, j, t) = f (h i, j).

In block (13), the dependence of the voltage across the Bragg gratings on their depth in the structure is measured:

^Eg T _s (i, j, t) = f ₁ (h _{i, j),}

where h _{i, j} is the depth of the j-th Bragg grating on the i-th optical fiber in the controlled structure.

Before the control unit (14), - a database reference values of voltage and temperature - forming reference according stresses due to deformation (U ^eg _s (i, j, t) _e) and temperature (T ^eg _OB (i, j, t) _e ), from the depth (h _{i, j} ) of the Bragg gratings:

^Voltage U _s (i, j, t) _{e = f (h i, j)} ,

T ^{for example} _o (i, j, t) _e = f ₁ (h _{i, j} ).

In the adders (15) and (16), the formed stress dependencies are compared with the reference values:

Further, the signals from blocks (15) and (16) are sent to the block (17) to form a conclusion on the reliability of the structure and the ultimate service life, where, on the basis of special equations that implement mathematical models, the reliability of the structure with respect to the applied loads and the maximum service life is determined.

The conclusion on the reliability of operation of the controlled product is formed using two models:

1. Model of thermomechanical deformation of a composite material product with accumulation of damage

The cyclic (re-static) deformation of a product made of composite material can be described by equilibrium equations, kinematic equations of a continuous medium, which defines the thermoelasticity equation taking into account the accumulated irreversible deformation and accumulated damage, as well as the boundary conditions and the initial condition - the absence of irreversible deformation.

Equilibrium equations:

divσ + p = 0,

where σ is the stress tensor,

p - volumetric forces.

In the case of high-frequency cyclic loading, the d'Alembert inertia forces should be taken into account as bulk forces in the equilibrium equation:

where ρ is the density of the material,

t is time.

Kinematic equations:

where ε is the strain tensor,

u is the displacement vector,

∇ is the Hamilton differential operator.

Boundary conditions in displacements and stresses:

where G ₁ and G ₂ - respectively, a fixed and loaded surface,

n is the unit normal vector,

u * - given displacements,

s _n - given surface forces.

The defining equation:

σ = D (χ) (ε-ε ₀ -α⋅ΔT),

where D is the elasticity matrix,

ε ₀ - accumulated deformations,

α is the tensor of the coefficients of thermal deformations,

ΔТ - temperature increment,

χ is the damage parameter.

The temperature in the product during the test is described by the heat balance equation:

where a is the coefficient of thermal conductivity,

s is the specific heat

q is the heat release intensity.

Border conditions:

where n is the external normal to the surface,

T _∞ - ambient temperature,

h is the convective heat transfer coefficient.

The intensity of heat during irreversible deformation is proportional to the dissipation power of the work of external forces.

The above equations are solved numerically, for example, using finite element analysis. The damage parameter and the accumulated deformation are parameters fixed at each moment of time, which are refined according to the measurement data during loading.

Damage parameter and accumulated deformation are tuning parameters of the model. Based on the design parameters by calculation, sensitivity coefficients equal to the ratios of the increments of the calculated strains to the increments of the tuning parameters can be obtained.

2. Measurement system model

The input variables of the model are the data on the shift of the peaks of the Bragg gratings. The output variables are optical fiber strains. The model should take into account controlled parameters - temperature measurement data of optical fibers.

The total deformation at the point where the Bragg grating is located is determined by the equality:

where k is the sensitivity coefficient of the sensor,

λ is the length of the reflected wave,

Δλ is the change in the length of the reflected wave upon deformation.

Fiber optic sensors are highly sensitive to thermal deformations. In this regard, from the total deformation, it is necessary to subtract the temperature deformation:

ε _t = α⋅ΔT,

which can be calculated by solving the heat balance equation, taking into account the data of temperature measurement on the surface.

Thus, as a result of measurements at successive instants of time t _n, we obtain a vector of measured values (i is the Bragg grating index):

and the vector of measured surface temperatures at the selected points of the thermogram (k is the index of the point):

The model of the measuring system, considered together with the model of thermomechanical behavior, allows you to determine the sensitivity coefficients of the measured parameters to the variation of the tuning parameters: damage and accumulated deformation.

The strain growth (strain amplitude) during the test is a random process that can be predicted from retrospective data. When the forecast result exceeds the threshold deviation from the normal data corresponding to the design parameters of the product, a conclusion is drawn on approaching the limit state. The point in time at which the predicted deviation reaches its limit value is an estimate of the product’s life.

Additionally, in block (17), according to the results of temperature measurement along the embedded optical fiber lines, the locations of the stress concentrators are localized.

The mathematical model for the diagnosis of stress concentrators, with which work was carried out, is described in the article: Budadin O.N., Kaledin V.O., Kulkov A.A., Pichugin A.N., Nagaitseva N.V. Diagnostics of the quality of structures made of composite materials in the process of their force loading by the analysis of dynamic temperature fields. - The control. Diagnostics, 2014, No. 7 (193), p. 52-56.

During the control, the spatial frequency of information recording, of which the step of arranging the Bragg gratings on the optical fiber.

Obviously, a too small step, firstly, will create redundant information during monitoring, reduce performance and may cause the device to malfunction.

Therefore, it is important to determine the optimal location step of the Bragg gratings in order to, on the one hand, ensure the required accuracy of positioning of information recording points on the surface of the object, and on the other hand, the necessary speed of the monitoring system.

To solve this problem, the spatial resolution of the placement of Bragg gratings on the optical fiber to determine the deformation and temperature field is determined based on the detection of the minimum in size deformation and temperature anomalies with a spatial period Δа determined by the sizes of the minimum deformation and temperature anomaly:

where Δx ^def _min - the geometric dimensions of the minimum deformation anomaly,

Δx ^t _min - the geometric dimensions of the minimum temperature anomaly due to the presence of an internal voltage concentrator,

Δa ^def is the location step of the Bragg grating for reliable registration of deformation sections,

Δa ^t is the location step of the Bragg grating for reliable registration of temperature areas due to the presence of an internal voltage concentrator,

Δa is the common (single) step of the Bragg grating of an optical fiber line embedded in a controlled product for measuring strain and temperature.

The geometric dimensions of the minimum temperature anomaly due to the presence of an internal voltage concentrator Δx ^t _min are determined as follows:

- measure the size of all temperature anomalies contained on the surface identified as a result of preliminary registration of the temperature field: Δх ^t _i ,

- determine the size of the minimum anomaly Δx ^t _min , solving the equation:

where δ is the probability that (Δx _i ) ≥ (Δx ^T _min ),

p (ΔX ^T _i ) is the distribution function of the quantities Δx ^T _i .

The geometric dimensions of the minimum deformation anomaly Δx ^def _min is determined as follows:

- measure the dimensions of all deformation anomalies contained on the surface, identified as a result of preliminary: Δx ^def _i ,

- determine the size of the minimum anomaly Δx ^def _min , solving the equation:

where δ is the probability that (Δx _i ) ≥ (Δx ^def _min ),

p (ΔX ^def _i ) is the distribution function of the quantities Δ x ^def _i .

When conducting monitoring, the temporal frequency of recording information is of great importance.

Obviously, a too short time period for registration, firstly, will create redundant information during monitoring, reduce performance and may cause the device to malfunction.

The optimal interval for sequential measurement of τ _ISM during loading of the controlled product is determined by solving the system of equations:

where ƒ (U ^DEF _s), U _s ^DEF - density distribution in duration time values (information signal) U _s and U ^{DEF DEF} _s, respectively,

τ _ISM - time interval of measurement,

P ^def _s and P ^T _s - the probability of missing values (information signal) U ^def _s and U ^def _s , respectively,

τ ₀ - temporary resolution of the measuring sensors,

η is the integration parameter,

τ ^def _ov - the optimal time interval for measuring the magnitude of the strain U ^def _ov ,

τ ^T _s - the optimal time interval for measuring the magnitude of the strain T _s .

Experimental studies were carried out on multilayer structures (cylinders) made of polymer composite materials. The purpose of the cylinders is to maintain internal pressure.

Theoretical and experimental studies of the proposed method and the device that implements it were carried out on a real product (Fig. 2) made of carbon fiber made from coal fiber by wet winding. In the process of winding, tangential and radial layers alternated. In the process of winding, optical fiber with Bragg gratings (BR) was laid. The optical fiber was laid in accordance with the proposed method along the tangential and radial layers (Fig. 3). In FIG. 3, the spiral winding layer is clearly visible (21).

After winding, the product passed the heat treatment stage and entered the test site.

The product was tested for breaking by internal pressure.

The test program was as follows:

1. The product was filled with water.

2. The product was installed in a testing machine (Fig. 7) and was loaded with internal pressure from the initial pressure - P ₀ .

3. The internal pressure was brought to the nominal value (working load) - P _slave. . The condition of the product (appearance, cracks, etc.) was recorded.

4. After this, the product by increasing internal pressure was brought to failure (Fig. 8) - P _{bit (exp)} = 83 MPa.

5. In the process of loading the product with pressure from P ₀ = 0 MPa to P _slave = 5 MPa, information about the deformation of the layers from the Bragg gratings came into the device (Fig. 1, Fig. 8), where the information was processed in accordance with the description in the proposed application method using the described mathematical models. In FIG. 5 and FIG. Figure 9 shows some current measurements on various Bragg gratings.

As a result of data processing, the reliability (ultimate resource) of operation of the multilayer design of the test product made of composite material was determined (predicted). This value was expressed as the ultimate pressure that the product under test can withstand - fracture pressure: P _{bit (calculation)} = 80 MPa.

In FIG. Figure 6 shows graphs of the experimental and theoretical dependences of the deformation on the loading pressure.

The experimentally obtained pressure fracture P _{bit (exp)} = 83 MPa.

The error of the proposed method for diagnosing the reliability of multilayer composite materials is, for this experimental study:

ξ = ((P _{bit (exp)} -P _{bit (exp)} / P _{bit (exp)} ) × 100% = 3.6%.

Conducted similar experimental studies on a similar design showed that the error of the proposed method lies in the range ξ = 3 ... 4.2%.

This error is quite acceptable for the practical use of the proposed method.

The proposed method allows you to:

1. To diagnose the reliability of operation of multilayer structures made of composite materials, not leading to destruction. This allows you to save both material and financial resources.

2. To improve the design and manufacturing technology of multilayer structures from composite materials, because reliable information is available on the internal volume of materials and construction.

3. Diagnose structures in the field under the influence of natural destructive factors and timely report the occurrence of emergency (dangerous) situations, etc.

Claims (92)

1. A method for diagnosing the reliability and ultimate service life of multilayer structures made of composite materials, including - placement in a composite material in the process of manufacturing an optical fiber with Bragg gratings, - determining the magnitude of the mechanical and temperature effects on it by measuring the spectral position of the peaks of the Bragg gratings, in this case, before placing an optical fiber with Bragg gratings in the composite material, the initial spectral position of the peaks of the Bragg gratings is determined, - the optical fiber is placed along the entire structure between the monolayers of the composite material susceptible to mechanical deformation, - after manufacturing the structure from a composite material, the spectral position of the peaks of the Bragg gratings is measured, - and then determine the distribution of mechanical and thermal strains inside the composite material structure by solving a system of equations describing the mathematical model of the structure and functional dependencies, taking into account the values of real mechanical and thermal strains and total strains obtained from the displacement of the peaks of the Bragg gratings, - optical fibers with Bragg gratings are placed layer-by-layer in the structural layers of polymer composite materials during the manufacturing process and the formation of the product structure and material in one technological cycle in such a way that the coordinates of the Bragg gratings of different optical fibers are on top of each other with an error of no more than half the length: X _ij ∈ (X _ij ± δ), where X _ij is the coordinate of the Bragg lattice, δ is half the length of the Bragg grating, i = 1 ... n is the number of the optical fiber located in the i-th layer, j = 1 ... k is the number of the Bragg grating on the i-th optical fiber, n is the number of layers in the structure in which the optical fibers are embedded, k is the number of Bragg gratings on optical fibers, - in addition to the measured Bragg gratings in the process of loading the design values of strain U ^DEF _s (i, j, t) is measured temperature T _s (i, j, t), where t - current time of loading the structure, - determine the magnitude of the stresses on the Bragg gratings due to deformation of the structure due to the power load ^voltage U _s (i, j, t) and T the temperature changes due to ^voltage _s (i, j, t), by solving the system of equations describing the corresponding mathematical model, - during loading products compared the amount of deformation U ^DEF _s (i, j, t) and the temperature T _s (i, j, t) with a maximum deformation quantity of controlled items U layer ^DEF _ovmax and, accordingly, the maximum permissible temperature value, due to the rupture of the fibers in the places of stress concentrators T _{o max} , - when the condition U ^def _ov (i, j, t)> U ^def _ovmax Λ (T _ov (i, j, t))> T _ovmax the product loading process is stopped, - measure the dependence of the voltage and temperature on the Bragg gratings on the depth of their occurrence in the structure: ^Voltage U _s (i, j, t) = f (h i, j), ^Eg T _s (i, j, t) = f ₁ (h _{i, j),} where h _{i, j} is the depth of the j-th Bragg grating on the i-th optical fiber in the controlled structure - measure the difference between the measured dependencies and reference dependencies and, based on the comparison, form a conclusion on the reliability of the structure under the action of power loads and the ultimate service life, - additionally, according to the results of temperature measurement along the embedded optical fiber lines, the locations of the stress concentrators are localized. 2. The method according to p. 1, characterized in that determine the interval of placement of Bragg gratings on the optical fiber to record the deformation and temperature field based on the detection of the minimum in size deformation and temperature anomalies with a spatial period Δa determined by the size of the minimum deformation and temperature anomaly:

where Δx ^def _min - the geometric dimensions of the minimum deformation anomaly, Δx ^t _min - the geometric dimensions of the minimum temperature anomaly due to the presence of an internal voltage concentrator, Δa ^def is the location step of the Bragg grating for reliable registration of deformation sections, Δa ^t is the location step of the Bragg grating for reliable registration of temperature areas due to the presence of an internal voltage concentrator, Δa is the common (single) step of the Bragg grating of an optical fiber line embedded in a controlled product for measuring strain and temperature. 3. The method according to p. 1, characterized in that the geometric dimensions of the minimum temperature anomaly due to the presence of an internal voltage concentrator Δx ^t _min are determined as follows: - measure the size of all temperature anomalies contained on the surface identified as a result of preliminary registration of the temperature field: Δх ^t _i , - determine the size of the minimum anomaly Δx ^t _min , solving the equation:

where δ is the probability that (Δx _i ) ≥ (Δx ^T _min ), p (ΔX ^T _i) - function of distribution Δx ^T _i. 4. The method according to p. 1, characterized in that the geometric dimensions of the minimum deformation anomaly Δx ^def _min is determined as follows: - measure the dimensions of all deformation anomalies contained on the surface, identified as a result of preliminary: Δx ^def _i , - determine the size of the minimum anomaly Δx ^def _min , solving the equation:

where δ is the probability that (Δx _i ) ≥ (Δx ^def _min ), p (ΔX ^def _i ) is the distribution function of the quantities Δx ^def _i . 5. The method according to p. 1, characterized in that the optimal interval for sequential measurement of τ _ISM during loading of the controlled product is determined by solving a system of equations:

where f (U ^DEF _s), U _s ^DEF - density distribution in duration time values (information signal) U _s and U ^{DEF DEF} _s, respectively, τ _ISM - time interval of measurement, P ^def _s and P ^T _s - the probability of missing values (information signal) U ^def _s and U ^def _s , respectively, τ ₀ - temporal resolution encoders, η is the integration parameter, ^DEF τ _s - an optimal time interval of the strain amount measuring ^DEF U _s, τ ^T _s - the optimal time interval for measuring the magnitude of the strain T _s . 6. Diagnostic device for reliability and ultimate service life of multilayer structures made of composite materials, including - layer-by-layer optical fibers (1) with Bragg gratings (6) placed in the layers of the structure of polymer composite materials - loading system (2) of the controlled product, - loading system control unit (3), - a strain measurement unit (7) along the optical fibers (5) on the Bragg gratings (6), - block recording results (18), while the output of the control unit of the loading system (2) is connected to the input of the loading system (2) of the controlled product, - the inputs of the strain measurement unit (7) along the optical fibers (5) on the Bragg gratings (6) are connected to the corresponding outputs of the optical fibers (5), characterized in that it includes: a unit for measuring stress by strain values (4), - optical fibers (5) with Bragg gratings located between the layers of material, - temperature measuring unit (8) on the Bragg gratings, - the first and second threshold devices (9, 10), - unit for measuring voltage on Bragg gratings by temperature (11), - voltage measurement unit, depending on the depth structure in ^voltage U _s (i, j, t) = f (h i, j) ( 12) - a temperature measuring unit depending on the depth of ^eg T _s (i, j, t) = f ₁ (h _{i, j)} (13) - a database block of the reference values of voltages and temperatures (14), - the first and second adders (15, 16) and - a unit for forming a conclusion on the reliability of the design and the ultimate service life (17), wherein - the outputs of the optical fibers (1) with Bragg gratings (6) are connected to the corresponding inputs of the temperature measuring unit (8) on the Bragg gratings (6), - the first output of the temperature measurement unit (8) on the Bragg gratings is connected to the input of the second threshold device (10), - the first output of the strain measurement block (7) on the Bragg gratings is connected to the input of the voltage measurement block (4) by the strain values, - the output unit (4) for measuring the strain amount of voltage is connected to the input unit (12) measuring the voltage depending on the depth of occurrence of ^voltage U _s (i, j, t) = f (h i, j), - the second output of the strain measurement block (7) on the Bragg gratings is connected to the input of the first threshold device (9), - the first output of the first threshold device (9) and the first output of the second threshold device (10) are connected to the first input of the unit (3) for controlling the product loading system, - the second output of the first threshold device (9) and the second output of the second threshold device (10) are connected to the second input of the unit (3) for controlling the product loading system, - the second output of the temperature measurement unit (8) on the Bragg gratings is connected to the input of the temperature measurement unit (11), - the output unit (11) for temperature measurement voltage connected to the input unit (13) measuring the ^voltage dependence of T _s (i, j, t) = f ₁ (h _{i, j),} - the output unit (13) measuring the ^voltage dependence of T _s (i, j, t) = f ₁ (h _{i, j)} is connected to the input of the first adder (15), - the output unit (12) depending on the measurement ^voltage U _s (i, j, t) = f (h i, j) is connected to the input of the second adder (16), - the first and second outputs of the block (14) of the database of the reference values of voltages and temperatures are connected to the second inputs of the adders (15) and (16), - the outputs of the adders (15) and (16) are connected to the inputs of the block (17) of forming a conclusion about the reliability of the design and the ultimate service life, - and the output of the block (17) of the formation of the conclusion on the reliability of the design and the ultimate service life is connected to the input of the block (18) of recording the results.

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