rus
Issue #5/2021
E. V. Kuznetsov, P. Yu. Lobanov, I. S. Manuylovich, M. N. Meshkov, O. E. Sidoryuk, L. A. Skvortsov
Non-Destructive Control of Plastic Products by Means of Active Thermography With Pulse Laser Heating
Non-Destructive Control of Plastic Products by Means of Active Thermography With Pulse Laser Heating
DOI: 10.22184/1993-7296.FRos.2021.15.5.428.442
Теги: active thermography laser heating non-destructive testing subsurface structure активная термография лазерный нагрев неразрушающий контроль подповерхностная структура
Non-Destructive Control of Plastic Products by Means of Active Thermography With Pulse Laser Heating
E. V. Kuznetsov, P. Yu. Lobanov, I. S. Manuylovich, M. N. Meshkov, O. E. Sidoryuk, L. A. Skvortsov M. F. Stelmakh Research Institute «Polyus» JSC, Moscow, Russia
The article shows the possibilities of non-destructive testing of the subsurface structure of various objects by means of pulse thermography technique. A number of examples are presented and their structure is analyzed. The technique includes heating the surface of an object with laser radiation and pyrometric investigation of local changes in surface temperature. Information is extracted from thermography results obtained both during forced heating and during subsequent cooling of the sample. The main attention in the article is paid to parts made of plastics used as structural materials in a wide range of products.
Key words: non-destructive testing, subsurface structure, laser heating, active thermography
Received on: 15.06.2021
Accepted on: 19.08.2021
INTRODUCTION
In numerous researches on non-destructive testing of the subsurface structure of various objects, noticeable attention is paid to the pulse thermography technique, which consists in pyrometric recording of surface temperature changes under conditions of forced heating and during subsequent cooling [1, 2]. Differences in thermal diffusion in different parts of the sample due to the peculiarities of the structure and its inhomogeneities form a picture of the object, which reflects the nature, composition and location of individual components of the inner subsurface layer.
Modern possibilities of mathematical modeling of heating samples with built-in inhomogeneity make it possible to quantitatively analyze the response of a surface subjected to pulsed heating.
The results of calculations for objects with different structures of the surface layer can be taken as a basis for interpreting thermographic images obtained experimentally. The progress in the field of numerical methods for the analysis of non-stationary heat transfer processes provides the possibility of extending pulse thermography to the study of a wide range of objects, different in properties and purpose.
At the same time, the development of this technique is facilitated by advances in the production of infrared video cameras, increasing their sensitivity, resolution and speed. The decline in the prices of these products makes the use of thermographic research available in increasingly wide areas.
The emergence of new technical means of pulse heating of samples also opens up new possibilities of IR thermography in solving applied problems of non-destructive testing. In particular, the rapid growth in the number of proposed semiconductor light sources and a significant increase in their power deserve attention. Today, the radiation intensity of commercial LEDs and semiconductor lasers is sufficient for efficient heating of the irradiated surface in various applications of the active thermography technique. In this case, additional advantages are the possibility of remote exposure and its spatial localization, temporal modulation and the necessary synchronization of pulses, and the choice of the optimal spectral range of radiation.
In this paper, we consider examples of practical applications of the active thermography technique for analyzing the hidden structure of objects using pulse heating by laser radiation. The main attention is paid to samples of plastics used as structural materials in mechanical engineering, chemical industry, medicine, etc.
MATHEMATICAL MODELING
Techniques for calculating the surface temperature of an object under study in conditions of its heating by external radiation are well known to those skilled in the art [2]. In this case, the specific features of the internal structure of the sample and the set of its thermophysical characteristics determine the characteristic pattern of the thermal field of the surface and the dynamics of its change in time both during the period of exposure to radiation and during cooling. The task of mathematical modeling is to calculate the reactions of an object to an external influence for the entire spectrum of possible variants of its internal structure.
Comparison of the results of such calculations with the experimental data of IR thermography is the basis for conclusions about the internal structure of the studied parts.
When considering a sample with a flat frontal surface, the coordinate system is selected in such a way that the laser radiation acts perpendicular to it along the z axis in the positive direction, and the plane coordinate is z = 0.
The temperature distribution T(x, y, z, t) in the sample obeys the heat conduction equation, which has the form:
, (1)
where ρ is the density of the material, C is the specific heat of the material, k is the coefficient of thermal conductivity. The initial condition in the case under consideration corresponds to a constant temperature value:
. (2)
Laser heating is simulated by a given heat flux through the «front» surface, which corresponds to the boundary condition of the second kind:
, (3)
where q(t) is the power density function of the absorbed laser radiation, which has the form
,
where W is the power density of laser radiation, α is the absorption coefficient, and τ is the time of laser exposure. On the remaining surfaces of the sample, a boundary condition of the second kind is set, corresponding to thermal insulation:
, (4)
where n is the normal to the surface S of the sample.
Equation (1) together with the initial condition (2) and boundary conditions (3,4) is the Neumann problem, which is solved numerically using a finite element numerical scheme on a tetrahedral computational grid. The numerical solution uses a standard library for solving sparse systems of linear algebraic equations.
Fig. 1 shows, as examples, the calculated distributions of surface temperatures for two parts made of ABS plastic, a copolymer of acrylonitrile with butadiene and styrene (ρ = 1 040 kg / m3; k = 0.258 J / m ∙ s ∙ K; C = 1 720 J / kg ∙ K), which is widely used in the design of objects of complex shapes using additive technologies. The first sample is a 30 mm thick plate with 12 mm wide slots and varying depths from 29 to 25 mm (in sequence at 1 mm intervals for positions a to e), and the second is a block of the same thickness with 12 mm diameter holes and similar distribution over their depth.
The figures reflect the dynamics of the cooling processes of parts after preliminary heating of the surface by a laser pulse with a duration of 60 s. In this case, for the first sample, fragments (from 1 to 10) are captured, following each other with an interval of 60 s, and for the second, a similar sequence of thermograms is shown every 40 s. It is essential that the observed modulations of the temperature field for each fragment are normalized independently. If at each i-th moment of time the local temperatures are in the range from the minimum values to the maximum ones , then the values of the relative inhomogeneities of their distribution can be expressed as
(5)
and shown using the color scale shown on the right in Fig. 1.
Thinner surface layers (positions 1a) are initially heated to higher temperatures (displayed in red), but they cool faster, which is reflected in Fig. 1 by a shift in color to the blue area of the spectrum. Over time, a redistribution of relative temperature changes in local zones is observed. Indistinguishable, initially farther from the surface, violations of the integrity of the material can be detected when recording thermograms with a time delay, the optimal value of which is determined by its thermophysical characteristics.
Although Fig. 1 illustrates the result of modeling active thermography processes for one of the materials, in particular, ABS plastic, similar results are typical for other cases with a corresponding adjustment to the time scale of the phenomena under consideration. For example, for low density polyethylene (LDPE), the cooling of the samples of the geometry described above proceeds on average 20% faster due to the higher thermal conductivity.
We should also note that in general case the differences of thermal diffusion in different areas can form a more complex picture of the investigated object. The use of mathematical modeling tools allows us to select the conditions for its maximum contrast, followed by decoding of the composition and location of individual components of the inner subsurface layer.
TECHNICAL MEANS OF LASER THERMOGRAPHY
A general view of the experimental setup used for non-destructive testing of plastic products by means of active thermography is shown schematically in Fig. 2.
The sample under study (1) is heated using laser sources with fiber-optic radiation outputs (2). The temperature distributions of its surface are recorded by means of a thermal imager (3) both during exposure to laser radiation and during cooling of the observed object. In this case, a continuous analysis of thermograms and their temporal dynamics is carried out by means of a computer (4), which simultaneously controls an electronic unit (5), which sets the parameters of laser radiation and the duration of heating.
In this paper, we used modules based on laser diode assemblies with a total power of up to 300 W, emitting in the 915 nm area. The fiber optic cables associated with them had optical blocks at the output, which form the radiation field in the plane of the sample under study. In order to ensure uniform heating of large objects, laser radiation sources with several fiber-optic outputs were used. In this case, the alignment achieved uniformity of illumination by observing with a video camera in the near infrared range (not shown in the figure). It is essential that the fiber-optic cable simultaneously served as an effective filter against thermal radiation from the active areas of laser diodes, which undergo natural heating during operation.
Thermal radiation from the sample was recorded using a FLIR Tau2 digital video camera with a sensitivity of 0.05 °C and a frame rate of 30 Hz.
A special computer program was used to analyze the thermograms and present the results of their processing.
INNER FINS AND CAVITIES CONTROL
Studies based on thermography of the surface of a part when it is heated by radiation and during the period of subsequent cooling, make it possible to analyze the subsurface structure, detect the presence of fins and hollow areas inside the object, and determine their main geometric characteristics. The relevance of such control is increasing due to the expansion of the range of plastic products, the complication of their designs due to the development of technologies for the production of polymeric materials and parts based on them.
Great opportunities for new applications of active thermography are associated with the progress in the field of additive technologies, the shaping of various plastic parts through 3D printing.
Among products with a complex internal structure, a prominent place is occupied by various acoustic parts with many hollow resonators [3], antennas and passive elements in the microwave range of electromagnetic radiation [4], impellers of pumps for pumping liquids and air fans [5], etc.
Examples of analysis of similar products are shown in Fig. 3. and Fig. 4.
The detail in Fig. 3 has internal fins, which are reflected in the photograph of its reverse side (Fig. 3a). At the same time, its front side is a flat, flat surface. The material is black LDPE plastic.
If it is necessary to control the structure of the sample from the front surface, when the internal structure is inaccessible for observation, the solution to the problem can be ensured by using the active thermography technique. The result is illustrated in Fig. 3b., Which shows the frontal plane of the part in the infrared range after it has been exposed to short-term exposure to laser radiation.
The optical system formed a field of uniform illumination of the investigated area with a power density of up to 0.3 W / cm2. Registration of thermal radiation from the sample was carried out using a digital video camera at the end of the laser exposure lasting 7 seconds.
On the surface of the areas conjugated with the inner fins, due to the more efficient heat transfer, the temperature turns out to be lower. According to the accepted color scale (see Fig. 1), they are blue in color and stand out in contrast against the background of red zones, which are devoid of heat removal into the material due to the presence of extensive cavities.
And Fig. 4 shows a similar result of monitoring the impeller blades made of ABS plastic for a water pump. They are part of an internal structure that is closed to visual observation. In this case, the technical means of active thermography, exposure modes and analysis parameters were used the same as in the previous example.
Naturally, the question of interpreting the obtained thermograms does not cause difficulties only with a relatively simple design of the investigated product. In more complex cases, the decoding of experimental data, as indicated above, can be performed by means of a multistage comparison of thermograms for computational models with a predicted structure and a sequence of possible deviations from it. However, in particular, an approach based on the assessment of the controlled object by comparing its thermograms (Fig. 5a) with similar ones for the control sample (Fig. 5b) obtained under the same conditions of laser exposure may turn out to be more rational. The result of the differential analysis of digital images is shown in Fig. 5c and reveals the differences characteristic of the investigated part.
In particular, the example in Fig. 5 reflects the detection of a structural defect (bubble) in the near-surface area. Such diagnostics can be in demand in the conditions of production of the same type of parts to identify defects.
INSPECTION OF WELDED CONNECTIONS AND REPAIR WORKS
Techniques for inspecting products made of polymeric materials based on the use of laser active thermography can be successfully used to check the quality of welded joints.
In particular, they prove to be effective in evaluating the results of ultrasonic spot welding. Such an example is shown in Fig. 6. The thermogram represents a fragment of the sample, where two sheets of LDPE 2.5 mm thick are overlapped. The average diameter of an individual welded joint is 5 mm.
The result with the highest contrast was recorded 5 seconds after the cessation of exposure to the product surface for 10 seconds with laser radiation with a wavelength of 915 nm and a power density of 0.3 W / cm2. At the same time, the thermogram reveals differences in the quality of the material connection in different sections of the weld under study: defective zones (5 and 8 in Fig. 6) turn out to be heated to higher temperatures in comparison with similar ones (1–4, 6–7) under conditions of good contact.
Laser active thermography can also be used to register hidden seams on plastic parts used in repair work to eliminate cracks by surfacing a filler rod of the same material. Their manifestation on the front side of the part is similar to that observed for products with the presence of internal fins and has already been described above.
The discussed technique is an effective tool for control and a number of other types of repair work performed on parts made of polymer materials. One example is shown in Fig. 7. As a sample, a plastic rear bumper of a car with a dent in the left corner after an accident serves as a sample (Fig. 7a). After repair, the part has no defects that could be detected visually (Fig. 7b), but they are detected by active laser thermography. Figure 7c shows an image recorded by a thermal imager 20 seconds after the termination of laser exposure with a power density of 0.2 W / cm2 for 8 seconds. Hidden defects are clearly visible. Their registration reveals the emergency history of the investigated part.
Using the described technique increases the diagnostic capabilities of vehicle body components. Its advantage will become more significant as the range of non-metallic parts in automobiles expands. In addition, it can be similarly effective in the analysis of similar parts of the hulls of air and water transport, and other plastic products.
CONCLUSION
This paper provides a number of examples of using active thermography to analyze the internal structure of plastic parts. Within the framework of well-known and widely used approaches [1, 2], the emphasis is placed on demonstrating techniques using laser surface heating. It is essential that the appearance of efficient sources of laser radiation of relatively low cost gives new impulses to the development of this direction of non-destructive testing. In this regard, the technique has good prospects for widespread use in industry and other industries.
Additional possibilities are opened up with the use of remote control, which is equally unattainable with other types of active thermography.
Coupling of radiation sources with fiber-optic elements makes it possible to completely exclude interference from reflections of secondary sources of thermal radiation, the presence of which must be taken into account in active thermography with other means of surface heating. The achievement of the same goal is also facilitated by the localization of the impact on the sample under study, which is necessary in some cases, in order to exclude interference from third-party surrounding objects. And the additional equipment of the laser source with a radiation scanning system is able to ensure the spatial uniformity of illumination over the surface of a sample of any shape.
The rapid development of laser technology with the advent of new efficient emitters [6] makes it possible to predict a further increase in the potential of laser thermography.
REFERENCES
Skvortsov L. A. Osnovi fototermicheskoi radiometrii i lazernoi termografii. – М.: Technosphera. 2017. 218 p. (In Russ.). ISBN 978-5-94836-493-3.
Vavilov V. P. Infrakrasnaya termografiya i teplovoi control. – М.: Spektr. 2009. 544 p. (In Russ.). ISBN 978-5-904270-05-6.
Kumar S., Lee H. P. Recent advances in acoustic metamaterials for simultaneous sound attenuation and air ventilation performances. Crystals. 2020; 10(8):1–22. DOI:10.3390/cryst10080686.
Kharalgin S. V., Kulikov G. V., Kotelnikov A. B., Snastin M. V., Dobychina E. M. Prototyping of microwave devices with specified electrodynamic characteristics using additive 3D printing technology. Rossiyskiy tekhnologicheskiy zhurnal (Russian Technological Journal). 2019; 7(1): 80–101. (in Russ.). DOI: 10.32362 / 2500-316X‑2019-7-1-80-101.
Prabha K. A., Rohit P. S., Nitturi1 S. Ch., Nithin B. Manufacturing of 3D shrouded impeller of a centrifugal compressor on 3D-printing machine using FDM technology. IOP Conference Series: Materials Science and Engineering. 2021; 1012(012039):1–7. DOI:10.1088/1757-899X/1012/1/012039.
Skvortsov L. A. Primenenie kvantovo-kaskadnih lazerov: sostoyanie i perspektivi. – М.: Technosphera. 2017. 270 p. (In Russ.). ISBN 978-5-94836-608-1.
About the authors:
E. V. Kuznetsov, General Director of Polyus Research Institute of M. F. Stelmakh JSC, Moscow, Russia.
ORCID: 0000-0003-3489-6805
P. Yu. Lobanov, Leading Engineer of Polyus Research Institute of M. F. Stelmakh JSC, Moscow, Russia.
ORCID: 0000-0001-8034-7773
I. S. Manuylovich, Senior Researcher of Polyus Research Institute of M. F. Stelmakh JSC, Moscow, Russia.
ORCID: 0000-0002-1737-3554
M. N. Meshkov, First Cat. Engineer of Polyus Research Institute of M. F. Stelmakh JSC, Moscow, Russia.
ORCID: 0000-0003-1708-416X
O. E. Sidoryuk, Head of the Laboratory of Polyus Research Institute of M. F. Stelmakh JSC, Moscow, Russia.
ORCID: 0000-0002-9641-4667
L. A. Skvortsov, Head of the Laboratory of Polyus Research Institute of M. F. Stelmakh JSC, Moscow, Russia.
ORCID: 0000-0001-7504-4778
Contribution of the members of author’s team
The article was prepared on the basis of the work of all members of the team of authors: E. V. Kuznetsov – organization of work, discussion of results; P. Yu. Lobanov – conducting experiments, processing the results; I. S. Manuylovich – mathematical modeling, processing of results; M. N. Meshkov – mathematical modeling, processing of results; O. E. Sidoryuk – conducting experiments, processing the results; L. A. Skvortsov – organization of work, processing and discussion of results.
Development and research were carried out at the expense of Polyus Research Institute of M. F. Stelmakh JSC.
Conflict of interests
The authors declare that they have no conflict of interests. All authors took part in the writing of the manuscript in terms of the contribution of each of them to the paper and agree with the full text of the manuscript.
E. V. Kuznetsov, P. Yu. Lobanov, I. S. Manuylovich, M. N. Meshkov, O. E. Sidoryuk, L. A. Skvortsov M. F. Stelmakh Research Institute «Polyus» JSC, Moscow, Russia
The article shows the possibilities of non-destructive testing of the subsurface structure of various objects by means of pulse thermography technique. A number of examples are presented and their structure is analyzed. The technique includes heating the surface of an object with laser radiation and pyrometric investigation of local changes in surface temperature. Information is extracted from thermography results obtained both during forced heating and during subsequent cooling of the sample. The main attention in the article is paid to parts made of plastics used as structural materials in a wide range of products.
Key words: non-destructive testing, subsurface structure, laser heating, active thermography
Received on: 15.06.2021
Accepted on: 19.08.2021
INTRODUCTION
In numerous researches on non-destructive testing of the subsurface structure of various objects, noticeable attention is paid to the pulse thermography technique, which consists in pyrometric recording of surface temperature changes under conditions of forced heating and during subsequent cooling [1, 2]. Differences in thermal diffusion in different parts of the sample due to the peculiarities of the structure and its inhomogeneities form a picture of the object, which reflects the nature, composition and location of individual components of the inner subsurface layer.
Modern possibilities of mathematical modeling of heating samples with built-in inhomogeneity make it possible to quantitatively analyze the response of a surface subjected to pulsed heating.
The results of calculations for objects with different structures of the surface layer can be taken as a basis for interpreting thermographic images obtained experimentally. The progress in the field of numerical methods for the analysis of non-stationary heat transfer processes provides the possibility of extending pulse thermography to the study of a wide range of objects, different in properties and purpose.
At the same time, the development of this technique is facilitated by advances in the production of infrared video cameras, increasing their sensitivity, resolution and speed. The decline in the prices of these products makes the use of thermographic research available in increasingly wide areas.
The emergence of new technical means of pulse heating of samples also opens up new possibilities of IR thermography in solving applied problems of non-destructive testing. In particular, the rapid growth in the number of proposed semiconductor light sources and a significant increase in their power deserve attention. Today, the radiation intensity of commercial LEDs and semiconductor lasers is sufficient for efficient heating of the irradiated surface in various applications of the active thermography technique. In this case, additional advantages are the possibility of remote exposure and its spatial localization, temporal modulation and the necessary synchronization of pulses, and the choice of the optimal spectral range of radiation.
In this paper, we consider examples of practical applications of the active thermography technique for analyzing the hidden structure of objects using pulse heating by laser radiation. The main attention is paid to samples of plastics used as structural materials in mechanical engineering, chemical industry, medicine, etc.
MATHEMATICAL MODELING
Techniques for calculating the surface temperature of an object under study in conditions of its heating by external radiation are well known to those skilled in the art [2]. In this case, the specific features of the internal structure of the sample and the set of its thermophysical characteristics determine the characteristic pattern of the thermal field of the surface and the dynamics of its change in time both during the period of exposure to radiation and during cooling. The task of mathematical modeling is to calculate the reactions of an object to an external influence for the entire spectrum of possible variants of its internal structure.
Comparison of the results of such calculations with the experimental data of IR thermography is the basis for conclusions about the internal structure of the studied parts.
When considering a sample with a flat frontal surface, the coordinate system is selected in such a way that the laser radiation acts perpendicular to it along the z axis in the positive direction, and the plane coordinate is z = 0.
The temperature distribution T(x, y, z, t) in the sample obeys the heat conduction equation, which has the form:
, (1)
where ρ is the density of the material, C is the specific heat of the material, k is the coefficient of thermal conductivity. The initial condition in the case under consideration corresponds to a constant temperature value:
. (2)
Laser heating is simulated by a given heat flux through the «front» surface, which corresponds to the boundary condition of the second kind:
, (3)
where q(t) is the power density function of the absorbed laser radiation, which has the form
,
where W is the power density of laser radiation, α is the absorption coefficient, and τ is the time of laser exposure. On the remaining surfaces of the sample, a boundary condition of the second kind is set, corresponding to thermal insulation:
, (4)
where n is the normal to the surface S of the sample.
Equation (1) together with the initial condition (2) and boundary conditions (3,4) is the Neumann problem, which is solved numerically using a finite element numerical scheme on a tetrahedral computational grid. The numerical solution uses a standard library for solving sparse systems of linear algebraic equations.
Fig. 1 shows, as examples, the calculated distributions of surface temperatures for two parts made of ABS plastic, a copolymer of acrylonitrile with butadiene and styrene (ρ = 1 040 kg / m3; k = 0.258 J / m ∙ s ∙ K; C = 1 720 J / kg ∙ K), which is widely used in the design of objects of complex shapes using additive technologies. The first sample is a 30 mm thick plate with 12 mm wide slots and varying depths from 29 to 25 mm (in sequence at 1 mm intervals for positions a to e), and the second is a block of the same thickness with 12 mm diameter holes and similar distribution over their depth.
The figures reflect the dynamics of the cooling processes of parts after preliminary heating of the surface by a laser pulse with a duration of 60 s. In this case, for the first sample, fragments (from 1 to 10) are captured, following each other with an interval of 60 s, and for the second, a similar sequence of thermograms is shown every 40 s. It is essential that the observed modulations of the temperature field for each fragment are normalized independently. If at each i-th moment of time the local temperatures are in the range from the minimum values to the maximum ones , then the values of the relative inhomogeneities of their distribution can be expressed as
(5)
and shown using the color scale shown on the right in Fig. 1.
Thinner surface layers (positions 1a) are initially heated to higher temperatures (displayed in red), but they cool faster, which is reflected in Fig. 1 by a shift in color to the blue area of the spectrum. Over time, a redistribution of relative temperature changes in local zones is observed. Indistinguishable, initially farther from the surface, violations of the integrity of the material can be detected when recording thermograms with a time delay, the optimal value of which is determined by its thermophysical characteristics.
Although Fig. 1 illustrates the result of modeling active thermography processes for one of the materials, in particular, ABS plastic, similar results are typical for other cases with a corresponding adjustment to the time scale of the phenomena under consideration. For example, for low density polyethylene (LDPE), the cooling of the samples of the geometry described above proceeds on average 20% faster due to the higher thermal conductivity.
We should also note that in general case the differences of thermal diffusion in different areas can form a more complex picture of the investigated object. The use of mathematical modeling tools allows us to select the conditions for its maximum contrast, followed by decoding of the composition and location of individual components of the inner subsurface layer.
TECHNICAL MEANS OF LASER THERMOGRAPHY
A general view of the experimental setup used for non-destructive testing of plastic products by means of active thermography is shown schematically in Fig. 2.
The sample under study (1) is heated using laser sources with fiber-optic radiation outputs (2). The temperature distributions of its surface are recorded by means of a thermal imager (3) both during exposure to laser radiation and during cooling of the observed object. In this case, a continuous analysis of thermograms and their temporal dynamics is carried out by means of a computer (4), which simultaneously controls an electronic unit (5), which sets the parameters of laser radiation and the duration of heating.
In this paper, we used modules based on laser diode assemblies with a total power of up to 300 W, emitting in the 915 nm area. The fiber optic cables associated with them had optical blocks at the output, which form the radiation field in the plane of the sample under study. In order to ensure uniform heating of large objects, laser radiation sources with several fiber-optic outputs were used. In this case, the alignment achieved uniformity of illumination by observing with a video camera in the near infrared range (not shown in the figure). It is essential that the fiber-optic cable simultaneously served as an effective filter against thermal radiation from the active areas of laser diodes, which undergo natural heating during operation.
Thermal radiation from the sample was recorded using a FLIR Tau2 digital video camera with a sensitivity of 0.05 °C and a frame rate of 30 Hz.
A special computer program was used to analyze the thermograms and present the results of their processing.
INNER FINS AND CAVITIES CONTROL
Studies based on thermography of the surface of a part when it is heated by radiation and during the period of subsequent cooling, make it possible to analyze the subsurface structure, detect the presence of fins and hollow areas inside the object, and determine their main geometric characteristics. The relevance of such control is increasing due to the expansion of the range of plastic products, the complication of their designs due to the development of technologies for the production of polymeric materials and parts based on them.
Great opportunities for new applications of active thermography are associated with the progress in the field of additive technologies, the shaping of various plastic parts through 3D printing.
Among products with a complex internal structure, a prominent place is occupied by various acoustic parts with many hollow resonators [3], antennas and passive elements in the microwave range of electromagnetic radiation [4], impellers of pumps for pumping liquids and air fans [5], etc.
Examples of analysis of similar products are shown in Fig. 3. and Fig. 4.
The detail in Fig. 3 has internal fins, which are reflected in the photograph of its reverse side (Fig. 3a). At the same time, its front side is a flat, flat surface. The material is black LDPE plastic.
If it is necessary to control the structure of the sample from the front surface, when the internal structure is inaccessible for observation, the solution to the problem can be ensured by using the active thermography technique. The result is illustrated in Fig. 3b., Which shows the frontal plane of the part in the infrared range after it has been exposed to short-term exposure to laser radiation.
The optical system formed a field of uniform illumination of the investigated area with a power density of up to 0.3 W / cm2. Registration of thermal radiation from the sample was carried out using a digital video camera at the end of the laser exposure lasting 7 seconds.
On the surface of the areas conjugated with the inner fins, due to the more efficient heat transfer, the temperature turns out to be lower. According to the accepted color scale (see Fig. 1), they are blue in color and stand out in contrast against the background of red zones, which are devoid of heat removal into the material due to the presence of extensive cavities.
And Fig. 4 shows a similar result of monitoring the impeller blades made of ABS plastic for a water pump. They are part of an internal structure that is closed to visual observation. In this case, the technical means of active thermography, exposure modes and analysis parameters were used the same as in the previous example.
Naturally, the question of interpreting the obtained thermograms does not cause difficulties only with a relatively simple design of the investigated product. In more complex cases, the decoding of experimental data, as indicated above, can be performed by means of a multistage comparison of thermograms for computational models with a predicted structure and a sequence of possible deviations from it. However, in particular, an approach based on the assessment of the controlled object by comparing its thermograms (Fig. 5a) with similar ones for the control sample (Fig. 5b) obtained under the same conditions of laser exposure may turn out to be more rational. The result of the differential analysis of digital images is shown in Fig. 5c and reveals the differences characteristic of the investigated part.
In particular, the example in Fig. 5 reflects the detection of a structural defect (bubble) in the near-surface area. Such diagnostics can be in demand in the conditions of production of the same type of parts to identify defects.
INSPECTION OF WELDED CONNECTIONS AND REPAIR WORKS
Techniques for inspecting products made of polymeric materials based on the use of laser active thermography can be successfully used to check the quality of welded joints.
In particular, they prove to be effective in evaluating the results of ultrasonic spot welding. Such an example is shown in Fig. 6. The thermogram represents a fragment of the sample, where two sheets of LDPE 2.5 mm thick are overlapped. The average diameter of an individual welded joint is 5 mm.
The result with the highest contrast was recorded 5 seconds after the cessation of exposure to the product surface for 10 seconds with laser radiation with a wavelength of 915 nm and a power density of 0.3 W / cm2. At the same time, the thermogram reveals differences in the quality of the material connection in different sections of the weld under study: defective zones (5 and 8 in Fig. 6) turn out to be heated to higher temperatures in comparison with similar ones (1–4, 6–7) under conditions of good contact.
Laser active thermography can also be used to register hidden seams on plastic parts used in repair work to eliminate cracks by surfacing a filler rod of the same material. Their manifestation on the front side of the part is similar to that observed for products with the presence of internal fins and has already been described above.
The discussed technique is an effective tool for control and a number of other types of repair work performed on parts made of polymer materials. One example is shown in Fig. 7. As a sample, a plastic rear bumper of a car with a dent in the left corner after an accident serves as a sample (Fig. 7a). After repair, the part has no defects that could be detected visually (Fig. 7b), but they are detected by active laser thermography. Figure 7c shows an image recorded by a thermal imager 20 seconds after the termination of laser exposure with a power density of 0.2 W / cm2 for 8 seconds. Hidden defects are clearly visible. Their registration reveals the emergency history of the investigated part.
Using the described technique increases the diagnostic capabilities of vehicle body components. Its advantage will become more significant as the range of non-metallic parts in automobiles expands. In addition, it can be similarly effective in the analysis of similar parts of the hulls of air and water transport, and other plastic products.
CONCLUSION
This paper provides a number of examples of using active thermography to analyze the internal structure of plastic parts. Within the framework of well-known and widely used approaches [1, 2], the emphasis is placed on demonstrating techniques using laser surface heating. It is essential that the appearance of efficient sources of laser radiation of relatively low cost gives new impulses to the development of this direction of non-destructive testing. In this regard, the technique has good prospects for widespread use in industry and other industries.
Additional possibilities are opened up with the use of remote control, which is equally unattainable with other types of active thermography.
Coupling of radiation sources with fiber-optic elements makes it possible to completely exclude interference from reflections of secondary sources of thermal radiation, the presence of which must be taken into account in active thermography with other means of surface heating. The achievement of the same goal is also facilitated by the localization of the impact on the sample under study, which is necessary in some cases, in order to exclude interference from third-party surrounding objects. And the additional equipment of the laser source with a radiation scanning system is able to ensure the spatial uniformity of illumination over the surface of a sample of any shape.
The rapid development of laser technology with the advent of new efficient emitters [6] makes it possible to predict a further increase in the potential of laser thermography.
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About the authors:
E. V. Kuznetsov, General Director of Polyus Research Institute of M. F. Stelmakh JSC, Moscow, Russia.
ORCID: 0000-0003-3489-6805
P. Yu. Lobanov, Leading Engineer of Polyus Research Institute of M. F. Stelmakh JSC, Moscow, Russia.
ORCID: 0000-0001-8034-7773
I. S. Manuylovich, Senior Researcher of Polyus Research Institute of M. F. Stelmakh JSC, Moscow, Russia.
ORCID: 0000-0002-1737-3554
M. N. Meshkov, First Cat. Engineer of Polyus Research Institute of M. F. Stelmakh JSC, Moscow, Russia.
ORCID: 0000-0003-1708-416X
O. E. Sidoryuk, Head of the Laboratory of Polyus Research Institute of M. F. Stelmakh JSC, Moscow, Russia.
ORCID: 0000-0002-9641-4667
L. A. Skvortsov, Head of the Laboratory of Polyus Research Institute of M. F. Stelmakh JSC, Moscow, Russia.
ORCID: 0000-0001-7504-4778
Contribution of the members of author’s team
The article was prepared on the basis of the work of all members of the team of authors: E. V. Kuznetsov – organization of work, discussion of results; P. Yu. Lobanov – conducting experiments, processing the results; I. S. Manuylovich – mathematical modeling, processing of results; M. N. Meshkov – mathematical modeling, processing of results; O. E. Sidoryuk – conducting experiments, processing the results; L. A. Skvortsov – organization of work, processing and discussion of results.
Development and research were carried out at the expense of Polyus Research Institute of M. F. Stelmakh JSC.
Conflict of interests
The authors declare that they have no conflict of interests. All authors took part in the writing of the manuscript in terms of the contribution of each of them to the paper and agree with the full text of the manuscript.
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