RU2687939C1 - Method of reinforcing reinforced with carbon fiber polymer composite materials - Google Patents

Abstract

FIELD: manufacturing technology.SUBSTANCE: invention relates to production of articles from carbon-fiber reinforced polymer composite materials, specifically to electrophysical hardening of finally formed articles of different complexity and can be used in making parts of transport vehicles, in particular – aircrafts, to strength and endurance of which are high requirements. Method includes impregnation of fibrous filler with an epoxy binder, shaping and hardening of the workpiece under action of a magnetic field. After final hardening and shaping of the article, the final product is placed under a horn radiating antenna of the microwave process installation at a distance from the plane of the antenna equal to 190–210 mm, and is exposed to an electromagnetic field with frequency of 433–2450 MHz for a period of time at which the surface temperature of the sample reaches level (28–30) °C. In case of large surface area of article (for example – elements of fuselage skin or trusses of planes and stabilizer, etc.), scanning of radiating antenna on surface is provided, providing uniform coverage by spot irradiation of all required sections, wherein antenna offset to next position is carried out after reaching previous position of surface temperature equal to (28–30) °C.EFFECT: increased strength characteristics as per interlayer shear stresses by (40–48) % of finally formed structures from hardened multilayer composite materials reinforced with carbon fiber, due to application of additional finishing operation of scanning effect of UHF electromagnetic field on final formed and processed item.1 cl, 3 tbl, 5 dwg

Description

The invention relates to the technology of manufacturing products made of carbon fiber-reinforced polymer composite materials, namely, electrophysical hardening of finally formed products of varying complexity and can be used in the manufacture of parts of transport machines, in particular - aircraft, to the strength and endurance of which increased demands.

A method of obtaining multilayer substrates of thermoplastic synthetic resinous material (US patent for the invention №5338611 A), according to which strips containing a thermoplastic polymer with soot particles are formed and reinforced with fiberglass in an amount by weight from 5 to 60% and carbon fiber in an amount by weight from 1 to 20%. The formed block of reinforced substrates is placed in an electromagnetic field with a frequency of 0.5 to 10 GHz with a power sufficient to heat to a temperature higher than the glass transition temperature but lower than the melting point, which creates a connection between the layers.

The disadvantages of this method are thermal stresses that occur at the interfaces of the layers and the fiber-matrix boundaries. The occurrence of stresses is associated with different coefficients of thermal expansion in reinforcing fibers made of a dissimilar material and polymer matrix, which causes significant deformation of the fibers, which do not relax as the matrix cools due to its hardening. This prevents the contraction of elongated fibers. Accordingly, the resulting stresses decrease the strength characteristics of the material. Additionally, there is a stress concentration during the molding of a product made of this material, causing heterogeneity of the stress-strain state (VAT), which increases the risk of fracture under alternating loads that occur, for example, during the evolution of objects flying with large accelerations. The heterogeneity of the SSS is promoted by the introduction of soot particles into the matrix, which are concentrators of the release of thermal energy when interacting with a microwave electromagnetic field, but cannot be evenly distributed in the matrix volume when introduced into it by known technological methods. The material contains a small amount of carbon fiber, which reduces its strength properties. Regarding the applicability of the method to the processing of predominantly carbon reinforcing elements, there is no information.

There is also known a method of producing monovinyl aromatic polymers heated by microwave radiation (SN patent for invention No. 2438867 dated 01/10/2012, IPC V29C), which includes placing impact polystyrene as a layer in a multilayer composite that has one or more layers that are not susceptible to microwave energy radiation, heating of impact polystyrene in the volume by means of microwave radiation energy and molding the material from the melt.

The disadvantages of this method are thermal stresses arising at the interfaces of layers of different thermal characteristics of materials, inapplicability to the production of carbon fiber-reinforced materials that are most promising for modern transport equipment due to low mass and high strength, impact on the performance of the formed product technological heredity previous heat treatment and dimensional molding. As a result, the product has low strength and operational reliability.

There is also known a method of stabilizing carbon-containing fiber, in which a fiber placed in a gaseous medium is subjected to microwave radiation treatment with simultaneous heating of the gaseous medium (RU patent for invention No. 2416682, IPC D01F 9/22, D01F 9/16, D01F 9/12, D01F 11/16, D01F 11/10). The method is implemented as follows.

Natural or synthetic carbon-containing fibers, such as polyacrylonitrile, rayon, etc., can be used as the initial fiber. In the first stage of processing — stabilization — the initial fiber (precursor) is placed in a working chamber containing a working gas medium, as which can be used well-known in this area, working gases, such as molecular oxygen, air, ozone, etc., the microwaves are fed into the chamber so that they are directed to the fiber treatment zone. For these purposes, as a working chamber, any known devices can be used in which microwave radiation acts on the material being processed, for example, waveguides, applicators, resonant and nonresonant volumes, etc. At the same time, the working chamber is heated using any heat sources, as which, without loss of generality, electric heating devices can be used, for example, an electric coil, an inductor, ceramic infrared (IR) emitters, etc. One or several heats Firs (heat sources) can be installed outside the working chamber in such a way that the heat they generate is directed to the working chamber. The operating frequency can be selected from the known range of 300-30000 MHz, microwave radiation with a power of 10-1000 W is supplied to the fiber processing zone, which provides heating of the material in the temperature range of 50-500 ° С,

The disadvantage of this method is that it is implemented in relation to the source component of a composite material, namely, carbon fiber at the stage of its production, which does not eliminate the negative impact of subsequent operations of obtaining a composite and molding its article on the strength and durability of finally formed objects, which due to thermal processes of stabilization and curing inevitably lead to residual stresses and their concentration in hazardous areas yka components.

Thus, the described methods are not applicable to increase the strength of complex-shaped products made of carbon fiber-reinforced composite materials. At the same time, despite the noted shortcomings, the analysis of the described analogs allows us to conclude that the use of microwave radiation (microwave electromagnetic field) is promising for modifying composite materials reinforced with carbon fiber in order to increase their strength.

The closest analogue to the claimed invention is a method for producing reinforced polymeric materials (RU patent for invention No. 2135530 C08L 63/02, C08J 5/24, C08J 5/06, C08G 59/56, published on August 27, 1999). The method includes operations: impregnation of a filler with a resin, heat treatment, impregnation with a curing system. For reinforcement use a nylon thread treated with a magnetic field before impregnation with its curing system, a protective polymer is introduced into the curing system: styrene-butadiene latex or CMC glue, with the following mass ratio of components in the curing system: water, curing agent, protective polymer 1.7-2.3 : 0.5-1.5: 0: 7-1.3. The technical result is an increase in destructive stresses during static bending and an increase in the specific viscosity of polymer composites with their simultaneous cheapening.

The disadvantages of the method are the following:

1. The modes described in the description of the method may not be applicable to the processing of materials reinforced with carbon fiber;

2. The effect on the performance of the formed product of technological heredity of the preceding heat treatment and dimensional molding, as a result, the effect of increasing destructive stresses indicated during static bending, the toughness is reduced by the subsequent processing of the material by dimensional processing;

3. Additionally, stress concentration occurs during the molding of a product made of this material and during its subsequent dimensional processing, which causes non-uniformity of the VAT, increases the risk of destruction under alternating loads, such as those evolving objects evolving with large accelerations.

4. The method cannot be applied to large-sized lengthy products such as structural power structures and plating elements of aircraft and other transport systems due to the significant unevenness of the electromagnetic field in the microwave chamber. Also, the creation of a microwave chamber of considerable size (of the order of several meters) with the electromagnetic field intensity distributed according to the required law is technically difficult to implement.

In the end, the reason for the lack of effectiveness of this method for hardening carbon fiber-reinforced composite materials are thermal stresses that occur at the interfaces of layers of different thermal characteristics of the reinforcing components, namely carbon fibers, and the matrix. Therefore, it is advisable to use modifying electrophysical effects that do not cause materials to heat up to a temperature conducive to the occurrence of internal thermal stresses, but at the same time stimulating interaction of the matrix components, as well as between the matrix and the border of the reinforcing fibers. Such an effect may be the processing in the microwave electromagnetic field of low power density, carried out after the completion of all formative operations, as a finishing process.

The technical problem of the present invention is the need to create a method for improving the strength characteristics of products made of composite polymer materials reinforced with carbon fiber by processing them in a microwave electromagnetic field after final shaping and dimensional processing in modes that provide rational heating of the composition.

The problem is solved by the fact that in the method of hardening carbon fiber-reinforced polymer composite materials based on epoxy binder, including the operation of impregnating a fibrous filler with an epoxy binder, shaping and curing the workpiece when exposed to a magnetic field after the final curing, carry out an additional effect of a microwave electromagnetic field at a frequency of 433-2450 MHz depending on the thickness of the product, have a horn radiating antenna at a distance of 190-210 mm from the irradiation products and carry out continuous contactless control of the surface temperature of the product. The process is stopped when the temperature reaches 28-30 ° C.

The technical result of the proposed solution is to change the microstructure of the composite material, which consists in increasing the fractal dimension of the elements of the matrix, the formation of a larger number of small fragments with a large number of active contact surfaces with reinforcing fibers. Additionally, due to the conductive properties of carbon fibers on their surface in the electromagnetic field of ultra-high frequency, local heat generation occurs, distributed along the fibers and corresponding to their orientation in the product. The combination of these two mechanisms leads to the formation of additional bonds of fibers and elements of the matrix, their additional cross-linking, which forms a hardened frame. At the same time, there is no significant volume heating of the material, and high thermal stresses, which can lead to the appearance of microcracks in the cured matrix and reduce the strength of the material, are eliminated. Thus, increase the strength characteristics of the product and their uniformity in its volume. In the end, the described mechanisms cause an increase in the resistance of the product to various types of loading that may occur during its operation.

The most appropriate can be considered the effect of microwave electromagnetic field on a fully formed product due to the significant penetration depth of the electromagnetic wave, which is at industrial frequencies (433-2450) MHz 5-20 mm, depending on the dielectric properties of the material. In the inventions-analogues, the positive effects of various electrophysical influences (microwave radiation, magnetic field, electric current, etc.) are manifested exclusively in the process of manufacturing the components of the composite material, namely, fibers, or during thermal stabilization of the polymer matrix. It does not take into account the processes of changing the structure of the material during its final curing and the final shaping or dimensional processing, which pass randomly and can lead to anisotropy of properties, disruption of the formed structural bonds, disruption of the integrity of the structure and other phenomena that can cause softening or unevenness of the strength characteristics . The design features of the formed products, creating stress concentrators, can also cause a decrease in strength in hazardous areas, which can no longer be compensated by improving the properties of the initial components of the material. Using the processing of the microwave electromagnetic field at a rational power input applied to the final product will reduce the results of the impact on the structure and strength of the material of the shaping finishing operations, increase the stability of the entire technological process due to the preservation of rather complex but well-developed chemical technologies for obtaining the original components, manage the strength of any constructive products difficulties.

The method is as follows.

Form the compositional structure of the product by laying the required amount of the required manner of oriented layers of carbon fiber reinforcement impregnated with layers of epoxy or other resin. This is followed by shaping the product in accordance with the requirements of the drawing by squeezing through a special mold and curing the matrix by introducing a hardener into its composition or heating it to a specific for each composition and concentration of temperature until the required mechanical characteristics are obtained. The finally formed product is placed under the horn radiating antenna of the microwave installation at a distance from the antenna plane equal to 190-210 mm and is applied to it by an electromagnetic field with a frequency of 433-2450 MHz, during the time at which the surface temperature reaches the level (28-30) ° C. The level of temperature is controlled using an electronic recording device: a pyrometer or a thermal imager, whose readings are given to the installation operator or, via an analog-to-digital converter, are entered into the automation system ovarian control. The process of microwave treatment is stopped when the temperature reaches (28-30) ° C. The frequency of 2450 MHz is used with a thickness of no more than 5-7 mm, 915 MHz - no more than 15-20 mm, 433 MHz - no more than 30 mm to obtain the wave penetration depth, providing minimal power loss and maximum uniformity of impact. In the case of a large surface area of the product (for example, fuselage skin elements or truss structures of planes and a stabilizer, etc.), scanning of the radiating antenna over the surface is used, ensuring uniform coverage of the irradiation spot of all necessary sections. In this case, the antenna is shifted to the next position after reaching the above surface temperature at the previous position.

An example implementation of the method.

For the implementation of the method used technological microwave installation of the type "Zhuk-2-02" produced by LLC "AgroEkoTech" (Obninsk, Kaluga region) with a radiation frequency of 2450 MHz and a magnetron power of 1200 watts.

The results of the practical implementation of the method are illustrated by the graphs of FIG. 1 to FIG. five.

FIG. 1. The dependence of the heating temperature of 100 ml of water in the microwave electromagnetic field on the distance to the plane of the radiating antenna with an irradiation time of 2 minutes.

FIG. 2. Dependence of the heating temperature of 100 ml of water in the microwave electromagnetic field on the distance to the plane of the radiating antenna with an irradiation time of 1 minute.

FIG. 3. The effect of the microwave electromagnetic field on the limiting stresses of the interlayer shear of 3.3 mm thick samples depending on the processing time.

FIG. 4. The effect of the microwave electromagnetic field on the limiting stresses of the interlayer shear of samples with a thickness of 5.0 mm depending on the processing time.

FIG. 5. The effect of the temperature of the microwave dielectric heating of samples from carbon fiber-reinforced polymer composite material corresponding to the process conditions, to increase the limit stresses of interlayer shear

To identify the dependence of the thermal effect of the microwave electromagnetic field on the distance from the antenna plane to the object of exposure, the temperature dependence of heating 100 ml of water on the distance was preliminarily determined, which was set to 50, 100, 150, 200, 250 mm. The exposure time was set equal to 1 and 2 minutes. The dependences obtained are presented in the graphs of FIG. 1 and FIG. 2. Based on the analysis of the graphs, it was concluded that the range of variation of the distance to the antenna plane should be 100 to 200 mm, since as the distance increases, the temperature of the object practically does not differ from the ambient air temperature, and it is impractical to expect any noticeable changes in the structure of the cured material. At smaller distances, the temperature of the object exceeds 40 ° C, which, according to our research and literature data, can cause the formation of internal stresses and destructive changes in the material. When processing samples, the radiating antenna was placed at a distance of 100 and 200 mm, the processing was carried out for 0.5; one; 2; 3 and 4 minutes, since with a longer time a significant increase in temperature is expected in all modes, and less time with a low microwave power does not cause a noticeable change. Samples of cured carbon fiber with a quasi-isotropic structure with a length and width of 70 and 9.5 mm, respectively, were used. The thickness of the samples was 3.3 and 5 mm. Simultaneously treated with 3 samples.

In the course of processing, the temperature of the samples was monitored and the temperature was distributed over the thickness by continuously recording thermograms in the frontal and flank zones using a FLIR E40 thermal imager (USA). Preliminary readings at reference points were calibrated using a Testo 830-T1 pyrometer (Germany).

Samples were tested for interlayer shear, as most often used to evaluate the performance characteristics of layered carbon and fiberglass. Used the installation, equipped with a strain gauge force and worm loading mechanism. The signals from the sensors were transmitted through an analog-to-digital converter (ADC) to a computer. Processing the results of measuring the increase in the load applied to the sample using the special LabVIEW program installed in the installation (Orel), allowed us to obtain load graphs (bending moment) in the dynamics from application to sample destruction. The distance between the supports of the equipment on which the test sample was placed was 60 mm. Internal stresses were calculated according to the standard, adopted in the resistance of materials, method through the loading force and the moment of resistance of the cross-section of the beam sample.

Accordingly, the values of the loading moment were read from the monitor screen of the installation. Measurements were stopped after the sample lost integrity. The maximum load was determined as the average value of several values of the loading moment according to the obtained schedule from the moment of the termination of a stable increase of its value to the time of the fall of not less than 15%.

As a result of the tests, it was established that with an increase in the exposure time of the microwave electromagnetic field from 0.5 to 2 minutes, a stable increase in the determined strength parameters is observed. The maximum effect is achieved at a time of 1-2 minutes. With a further increase in time to 4 minutes or more, the growth of the parameter becomes insignificant (3-5%) or some decrease is observed. Thus, an increase in the microwave processing time of more than 2 minutes is impractical.

The test results of the samples are presented in table. 1-3 and FIG. 3-5

Data analysis table. 1 and 2, as well as the graphs of FIG. 3 and 4 shows that the highest interlayer shear stresses withstand the samples subjected to the microwave electromagnetic field for 2 minutes at a distance between the plane of the horn radiating antenna and the sample surface equal to 200 mm. At shorter distances, which corresponds to a higher electromagnetic field strength and, accordingly, a higher level of microwave power supplied, the limit voltages are lower for both samples of 3.3 mm thick and 5 mm, respectively, by 27% and 18%. On the other hand, with an exposure time of 1 minute, the greater strength of samples 5 mm thick in terms of the interlayer shear stress is ensured with a larger applied power, i.e. at a distance of 100 mm, at (7-8)%. For samples with a thickness of 3.3 mm, the processing efficiency at a distance of 200 mm is reduced to 4%, which can be considered unimportant. Obviously, with a small level of microwave power, which corresponds to a greater distance and a shorter exposure time, in the structure of the matrix and the interfacial zones, the microstructure restructuring and the formation of additional adhesive bonds do not occur. With a higher level of power (reduced to 100 mm distance) such changes occur, and the material becomes more durable. As time increases, bond formation processes grow at a low power level, while dielectric heating proceeds less intensively due to heat removal to the environment and the deep regions of the material. In the case of a high level of microwave power, the heat released by the dielectric heating of the matrix does not have time to dissipate into the environment and the depth of the material. Therefore, the process is accompanied by an increase in temperature and the formation of new adhesive bonds occurs simultaneously with the occurrence of thermal stresses and the actions of different thermal deformations of the matrix and reinforcing fibers of various magnitudes. This leads to cracking at various levels and, as a result, to a decrease in strength due to interlayer shear stresses. This statement is confirmed by the data of Table 3 and the graphs of FIG. 5 and 6. It can be seen that the greatest effect on increasing the strength of the carbon fiber-reinforced polymer composite material is observed for thicker specimens, but the absolute value of stresses for specimens 3.3 mm thick is much higher. This fact may be associated with a larger relative fraction of the matrix in the volume of samples 5 mm thick, the structure of which is largely affected by the microwave electromagnetic field. At the same time, the lower strength of the matrix determines the lower values of the limit stresses of the interlayer shear in the samples, with its greater content.

For the samples of the two cross sections studied (9.5 × 3.3 and 9.5 × 5 mm), the surface temperature at the modes corresponding to the maximum strengthening effect was (28-30) ° C, which may be an objective criterion for the operational control of the process duration processing in the microwave electromagnetic field. At the same time, for samples with a lower thickness, the dependence is clearer (“sharp”); for samples of greater thickness, the temperature range can be somewhat extended.

Thus, it has been established experimentally that the processing of finally formed samples of carbon fiber-reinforced polymer composite material in the microwave electromagnetic field with a frequency of 2450 MHz at a distance of 190-210 mm for 2 minutes provides, in comparison with known methods, an increase in strength in interlayer shear stresses (40 48)% depending on the sample thickness.

Thus, the problem posed is solved by increasing the strength of carbon fiber-reinforced polymer composite materials in the composition of the finished and processed products.

Claims (2)

1. The method of hardening products made of carbon fiber-reinforced polymer composite materials based on epoxy binder, including the operation of impregnating fiber filler with epoxy binder, shaping and curing the workpiece when exposed to a magnetic field, characterized in that the finally formed product is placed under the radiator antenna of the microwave process unit the distance from the antenna plane, equal to 190-210 mm, and is influenced by an electromagnetic field with a frequency of 433-2450 MHz in those ix time at which the temperature reaches the level of the sample surface (28-30) ° C. 2. The method according to p. 1, characterized in that in the case of a large surface area of the product - the fuselage skin elements or the truss structures of the planes and the stabilizer, use scanning the radiating antenna over the surface, ensuring uniform coverage of the irradiation spot of all necessary sections, while shifting the antenna on the next position is carried out after reaching at the previous position a surface temperature of (28-30) ° C.

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