Non-destructive testing of plastics and composites

The research and development project at HIDRA Adult Education Centre Ltd started with a literature search, as polymer technology, plastics and composites for industrial applications is an uncharted and continuously developing field.
our partner's R&D results and experience

Our partner is HIDRA Adult Education Centre Ltd. in the context of an R&D grant, has done gap-filling work and gained considerable experience in the field of testing plastics and composite materials. We are pleased to accept their invitation and share the results of their work:

Introduction

The research and development project at HIDRA Adult Education Centre Ltd. started with a literature search, as polymer technology, plastics and composites for industrial applications is an uncharted and continuously developing field. In each case, it is essential to have a detailed knowledge of the production technology of the material under investigation in order to be aware of its specificities and possible material continuity deviations.
The first step in the literature search was therefore to familiarise ourselves with the production technologies of the products under investigation. Since the subject of non-destructive testing is essentially welded metals and castings, the relevant standards were one of the main bases for the R&D process. After the production technology of typical metal products, we were introduced to the production methods of plastic and composite products. There are a number of plastic and composite production methods, such as injection moulding, extrusion, centrifugal moulding, laminating, vacuum forming and other additive manufacturing technologies. It is also important to mention manufacturing processes similar to welding of metals, such as adhesive bonding or welding, which are technologies regularly used for plastics, albeit according to different physical laws. Knowledge of these processes has helped us to become familiar with the critical points of manufacturing and possible sources of manufacturing failure, so that we could work much more carefully during the investigation part of the R&D proposal.

In addition to learning about raw materials and manufacturing technologies, we have conducted detailed research into non-destructive testing tools. As a result, we requested a tool modification in the application.

This modification was necessary because since the submission of the tender, more modern, more technically advanced and more versatile material testing equipment has appeared on the market. The previously planned phosphor plate radiography system has been replaced by the latest state-of-the-art inspection technique and procedure, a pulse radiography system with an integral digital detector.

The system is fully computer-controlled and has much lower safety requirements than a conventional X-ray machine, which emits continuous radiation rather than pulses during exposure. In the time between the submission of the application and its implementation, there has been a technological change in the ultrasound examination equipment. In addition to the two instruments mentioned above, a digital microscope was acquired. This opens up new possibilities for non-destructive and destructive testing. In addition to the inspection activities, these instruments can also be used for training purposes, thus making a major contribution to the successful implementation of the R&D project.

It was important for us to get a more complex, complete and comprehensive picture of each area of non-destructive testing, so we looked in detail at the different areas, procedures and methods of non-destructive testing. This knowledge is important both for research and development purposes and also because we intend to use the equipment acquired in the course of the project for further materials testing activities and, as mentioned above, for educational purposes. We have focused on radiography, ultrasonics, phased array ultrasonics, but also, due to possible changes in the tender and to gain a broad knowledge of the field, we have also explored other non-destructive material testing techniques. The starting point for the knowledge gained is, of course, the field of metals. For metals and alloys, these procedures have been used for decades, the methods are standardised and the evaluation criteria are based on detailed fracture mechanics analyses and practical experience.

For plastics and multi-material products, such a wide range of theoretical and practical knowledge, standardised methods and general standards are not available. In addition to radiographic, ultrasonic, phased array ultrasonic and microscopic materials testing, other methods were also studied. Visual inspection looks for large free deviations, the presence of which may lead to the product being released for further examination. The fluid penetration test can be used to detect surface deviations in any material, whether metal, plastic or composite. The magnetic materials test is only applicable to ferromagnetic materials, such as metal matrix composites. Its advantage over the penetration method is that it requires less equipment, less cleaning and post-cleaning. A more specialised test for typically pressure equipment and fabrications, the compaction test, was introduced. It is an excellent non-destructive material testing method, but less relevant to the aim and objectives of the present R&D project.

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An important part of the manufacturing technologies for plastics and composites are the various additive manufacturing processes. These so-called additive manufacturing technologies are increasingly used in a growing number of fields and therefore more detailed research was needed.

Today, plastic products are increasingly produced using additive processes and current trends suggest that their market share will continue to grow. In particular, FDM (fibre deposition) and SLA (stereolithography) processes have been discussed in more detail, as these are the two most commonly used manufacturing processes. As we wanted to investigate products manufactured by these processes in the subsequent material testing part, we were familiarised with the specificities of 3D modelling. We modelled products typical of the manufacturing process, which were subsequently fabricated for the test research.

3D modellezes

Examinations

After an extensive literature search, the "practical" part of the competition followed, during which the purchased test products were used to learn about the possibilities and limitations of the test system and the optimal use of the test system. Three non-destructive material testing procedures were implemented within the framework of the tender. A phased array ultrasonic process, implemented by a Sonatest VEO+ system; a digital pulsed radiography system, implemented by a Vidisco RayzorX system; and last but not least, microscopic inspection, provided by an ASH OmniCore digital magnification system. The three photos below show the equipment purchased.
Sonatest VEO plus
Sonatest VEO plus
Vidisco RayzorX Pro
Vidisco RayzorX Pro
ASH OmniCore
ASH OmniCore
During the implementation of the research and development proposal, in parallel with the research, the materials we planned to test were outlined, and the limitations and limits of the test system and test method, as well as their suitability and application, were tested and verified. As mentioned before, destructive testing is essentially concerned with different metals and their alloys, and therefore the first step of the test part started with the testing of different metal products. The purpose of this part is to familiarise ourselves with the testing equipment we have purchased, and to learn and master the testing methods and options.

Welded metals were examined by radiographic and ultrasonic instruments. As the wider application of the phased array technique is not yet widespread, learning, interpreting and performing the tests was a new challenge. Thanks to the modernity of the instrument, the results of the products tested can be evaluated using the instrument's software. The applicable and valid material testing standards for metals provided the starting point here and for subsequent material testing tasks. From the phased array ultrasonic instrument, the test data can be exported. Not only the test results are stored, but also all settings, so that tests can be repeated. During periodic review, not only the results but also the test parameters can be compared. The most comprehensive and applicable visualisations we have seen are shown below:

  • sectoral display (S-scan)
  • linear display (L-scan)
  • Top view
  • End view
  • A-scan view

With the help of the pathfinder used in the tests, the individual deviations in the direction of the sweep are accurately measured and scaled, so that their position and size are correct. In the case of metal-based products, we have tried to gain as detailed and complex knowledge as possible, so that we can apply this knowledge later to special plastic and composite products.

Within the phased array equipment, a number of functions and parameters can be adjusted to best match the test file to the characteristics of the test object, material quality, test temperature, test head or feed wedge used, or the shape of the weld seam on the product. This will also ensure that the recorded test results, the size and nature of the recorded deviations, are as close as possible to the shape and size of the actual defect.

It is also essential to calibrate the equipment before the test task. This is when we set the sound propagation velocity that can be measured in the given material, the sensitivity of the equipment, taking into account the smallest continuity gap or deviation that we want to detect in the given test task. Thanks to the modernity of the instrument, the results of the products tested can be evaluated using the instrument's software. The software can also be installed on a computer, so that testing and evaluation can be carried out in parallel and separately, which will greatly facilitate the actual work, as it will shorten the duration of a testing task. The completed test file can be saved and recalled later, so that the file only needs to be created once for a given task.

Vidisco rendszer

Our previous experience in materials testing and the relevant standards have helped us in the examination of radiographic blunt patterns in metals. This made it easy to learn about Vidisco's pulsed radiography system. Today, a large proportion of materials testing still uses analogue equipment, compared to which digital equipment offers a number of advantages. Their time and therefore cost efficiency is due to the fact that their relatively low radiation levels do not require a complex design of the exposure area. Once exposed, the image is immediately visible, no developer is required, no developer room is needed and there is essentially no development time. Thus, if a shot is not properly taken, it can be repeated immediately. With analogue X-ray equipment, where no digital imaging equipment is used, it takes at best 10-15 minutes to redo an image that has not been taken properly. The digital system has a few disadvantages compared to the conventional method, which can be avoided with a little care. For example, the machine does not draw the voltage it needs to operate from the mains, but from a battery. The device emits a beep if it runs out of power, so you only need to make sure you have a charged spare battery. Another weakness of the system is the software control. It is advisable to dedicate the computer used to control the device to this task only, so that general work on the computer does not "use up" the computer itself, either in hardware or software. An additional advantage of the system is the various functions of the management software. The material continuity defect to be detected can be made more detectable in this way, depending on its size, environment and other characteristics.

Vidisco RTG gép

In the radiographic examination of welds in metals, "detection" was the filter that best revealed defects. For plastic tubes, the "revelation" and "sharpening" functions were also used for finer deviations. These features did not significantly improve the detectability of metallic tubes. The aim of testing composite materials and plastics was to gain a better understanding of an unknown material with no relevant standards for materials testing. The range of materials used in industry is expanding and changing, and materials testing needs to keep pace with this. For a given size and type of material, a different degree of variation in material consistency may be a source of failure for a metal and for a material with a significantly lower density, such as polyethylene. 

The idea of composite materials is that different materials are combined to form a material with favourable physical properties, and in this case, defects in those properties may be of different significance than in metals. This is why it is useful that the Vidisco device system can be used to accurately determine the locations and dimensions of even very small material discontinuities and defects.

Based on the standards for radiographic testing of metals, we excluded a number of metal-specific defect types and collected data on the frequency of material continuity defects when plastic pipes are joined by mirror welding. Consequently, it is considered feasible to investigate the various industrial uses of plastics and their typical material continuity defects with a view to developing a relevant standard.

After metals, the next test task was ultrasonic testing of plastic pipes. At this stage of the research, we already had a good knowledge of the handling and operation of the machines. We were familiar with the potential of the equipment and the application methods.

With the knowledge of the evaluation of the prepared recordings and measurement files, there are many possibilities to carry out the evaluation and to present the results as clearly as possible. The testing of plastic pipes is feasible with limitations. The first and most important limitation is the lack of standardisation. For conventional ultrasonic materials testing, there is a test standard (MSZ EN 13100-3) for non-destructive testing of welded joints of thermoplastic semi-finished products. However, the standard does not provide any information on the assessment, only on how to perform the test. And as we have just written, the instructions in the standard refer to conventional ultrasonic testing methods.

DRT

Based on the standard and practical experience, pipes can be tested if there is a gap of up to 0.5 mm between the contact surfaces. The tests could be carried out for diameters of 160 mm and above. The next difficulty was to determine the shape, size and nature of the weld. In the case of metals, most weld shapes and sizes can be specified and parameterised in the system, but in the case of mirror welding, different weld shapes from those typical for metals will be obtained, which is always a function of the task and the pieces and process parameters used. But the test method itself is still applicable, the test can be performed, the location of the defects can be determined, the defect itself can be scaled. A third limitation or difficulty, whether it is non-destructive testing of any plastic or composite product, is the lack of an evaluation standard or specification. A further development of the project could be a research on fracture mechanics combining non-destructive testing and evaluation. This research could compare the deviations that can be detected by non-destructive testing with the mechanical damage, fracture mechanics factors and failure probabilities caused by the deviations.

fázisvezérelt

The following products are made using additive manufacturing technology, 3D printing. These products were first examined by radiography. The use of phase-controlled ultrasound equipment presents a number of difficulties for products with shaped, fractured surfaces, simply because the probe cannot be placed on the surface of the test object. 3D printed products using FDM and SLA techniques were investigated. FDM is a filament-drawing process, where products are made by melting a plastic filament and building them layer by layer. SLA stands for stereolithography, a process in which a light source hardens the material layer by layer to form the product. The base material is a liquid resin, in which cross-linking is formed by light, laser or UV radiation, creating a cross-linked structure. Testing the finished products was more challenging than with the previous test pieces, as this is a completely new area of materials testing. There is no exact, generally accepted method or guideline for testing or evaluation. During the evaluation, we have tried to describe what we have seen as accurately and accurately as possible, bearing in mind the characteristics and difficulties specific to the production of the product in question. The two very different manufacturing processes produced different experiences. In the case of products printed by FDM, the manufacturing process is characterised by the fact that the products are not completely filled, but contain an infill of a specific shape and design. This can partly complicate, or in worse cases completely prevent, the evaluation of the radiographic images produced. Another characteristic of this is that there is only a thinner wall, which is a continuous material for X-rays, making it more difficult to find gaps in the material continuity of the boundary surfaces. However, it is also important to note the positive experiences. By having fill inside the products, any defects or deviations in the fill can be very nicely highlighted. Layer separation between larger sizes and layers was also noted in several places. It is possible to accurately observe and determine the deviations between the intended and actual shape. Joining points of indented products consisting of several parts but manufactured with one print can be checked, and fusions due to improper manufacturing can be observed.

The SLA procedure is slightly different, as the production process is completely different. Products with fully filled, non-hollow interiors are typical. We have been able to better apply the knowledge gained from the metal products and literature research, because the testing of SLA products is more similar to the testing of cast steel products. As can be observed in the images shown here, any deviations can be found with a high degree of certainty, and their size and location can be identified and detected. The deviations can be detected with equal ease either on the surface or under the surface, meaning that the radiographic method can be used for non-destructive testing of 3D printed products manufactured using SLA.

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The next group of products we examined is drawing stones used in the production of copper or other fibres. The radiographic examination of the drawing stones did not lead to any results. The internal "funnel" section appears to be a different size on the radiograph than it is in reality. This is due to the laws of blackening. In none of the cases was a lack of material continuity found in the stone material. However, we have been able to examine the reducing cone with the digital magnification system and have observed varying degrees of wear due to normal use.

húzókő
húzókő

In most cases, artificial defects could be detected by simple visual inspection. However, there were some small artificial defects that required magnification. Both the manufacture and the use of traction stones are carried out with a high degree of precision. Detection of any defects requires an equally precise tool, which the digital magnifying equipment is capable of providing. We do not consider it justified to establish a standard for the use of tensile test specimens, but the process method provides an excellent basis for quality control and for identifying the causes of possible defects.

Printed circuit boards may be tested by visual inspection, electronic measuring instruments or, where appropriate, the manufacturer's own test system. Work with digital magnification equipment may be hampered by contamination on the circuits, as removal of this contamination may damage the product. In extreme cases, it may not be possible to carry out an instrumented or test system inspection. Radiographic examination has been found to be a suitable alternative. Even in the case of multilayer printed circuit boards, possible defects in the conductive strips can be clearly detected.

The detectability of the error is due to the difference in density of the materials used. The 0.2 mm thick copper conductive band is still perfectly visible at a thickness of 1.2 mm for the 1.8 g/cm3 glass fibre epoxy composite, which has a significantly higher density of 8.96 g/cm³, five times that of the glass fibre epoxy. It is practically possible to detect very small metal inclusions in composite materials. We were looking for very small variations, and the features of Vidisco's control software, which had previously been little or not used, proved useful this time. Unfortunately, the manufacturer provides only generous descriptions in the user manual of how the filter functions of the control software make the details of the image more detectable.
chip 2
anyagvizsgálat chip

The "detection" function used for larger defects did not provide adequate images, and "refining" was of particular importance for such small objects. In conclusion, radiography is perfectly suited for the inspection of printed circuits, and we envisage that, as with plastics, future research could be carried out to regulate the quality of composite materials and products made of composite materials, even in the form of standards for different methods of materials testing.

Radiographic examination of the products was followed by a phased array ultrasound scan. If ultrasonic non-destructive material testing is to be performed, a calibration block is required.

ET2

A calibration block of known size and shape can be used, for example, to calibrate the test range, determine the sensitivity and verify it. In the case of the present products, we have taken the standardised knowledge of metals as a starting point and have tried to apply this knowledge to the testing of products manufactured by the FDM process. Once the test file has been prepared, it is possible to start calibrating the equipment. The first step in calibration is always the validation of the 0 point and the measuring range, i.e. the geometric validation. It has not been possible to produce echo signals using the test standards, hence unfortunately there is nothing to illustrate with a graph. We have tried several methods and other practices in vain. One reason for this may be the technology itself. The individual layers do not solidify at the same time, so that a completely homogeneous material structure is not formed, and microscopically small amounts of air or other substances can be introduced between the individual fibres, which act as a resistance to the propagation of ultrasound. It may be possible to change technological parameters to improve the filling, but the effect of each parameter on the filling and the homogeneous contact between the fibres and layers should be investigated separately.

The most important experience in examining the lenses was the differences between microscopic and radiographic images. The difference is due to the circumstances in which the defects were created. In the case of abrasions and scratches, we could see that the material missing from a given point can be curled up along the defect, which in the radiographic image appears as a kind of excess material. The advantage of the examination was that we could "see" the material with the naked eye, and with the digital magnifier we could record any deviations and thus scale them. The X-ray images "perceived" the deviations differently.

The difference between radiographic and microscopic images is also due to the angular difference in level between the surface of the material and the starting point of the defect. In other words, the steeper the "wall" of the deviation, the more accurately the extent of the defect can be determined radiographically, since this steepness defines the contour.

meredekség
The question arises, when testing metals, of the actual size of a given defect, whether porosity or bonding defect. It also raises the question of whether it is necessary to deal with such precision of sizing in a practical standard.

Our last product category is acid-proof ball valves made of different materials. Two different materials - metal or plastic - are used in the same structure, with different material qualities but serving the same function.

Radiographic examination of the welds on acid-resistant steel ball studs produced better images than we had hoped, given the thickness of the material and the complexity of the product surface. The "pipe" section of the ball stud has a thick wall, small diameter, short, and a variety of surface areas of different sizes and dimensions (ball stud structure, flange, etc.) in its surroundings. Given all the attributes of the ball spigot, it was thought that, among other things, its pack levelling effect would allow the best images to be taken with isotopes. Thanks to the so-called "averaging" function in the control software of the Vidisco instrument, it is possible to screen thicknesses that would normally be performed with an isotope. However, radiographic examination of the internal structure of the ball valve did not yield satisfactory results, as its complexity could not be overcome with the available tools. The defects on the surface of the sphere can be detected at most for very extreme defect sizes. The artificial defects on the spherical structure of the products we have examined are not of a size that can be detected radiographically.

In the case of the ball valves in plastic housings, the radiographic images show that in most cases we were able to detect artificial and natural defects and continuity gaps in the products. In the cases where the artificial indication could not be detected, the problem was in the size of the continuity defect, not in the inspection system or the position of the defect (for example, a similar but deeper crack could be detected without any problems).

The advantage of this method is the ease of testing for both types of ball valves. The individual components would have to be tested one by one in a disassembled state. Thanks to the modernity of the radiographic system, the technology used and its portability, the products can be inspected even when installed. In other words, it is possible to obtain adequate information on the condition of the components without dismantling them.

The mobile examination platform

roncsolásmentes vizsgáló kocsi

The R&D project has therefore succeeded in creating a mobile platform for materials testing and materials testing education that meets the needs, principles and methods set out in the objectives. The platform is perfectly suited for non-destructive testing of metal, plastic and composite materials. Thanks to its mobility, it can be used to reach test sites in difficult environmental conditions. Using the chosen equipment and methods, complete surface and volumetric material testing tasks can be performed, ensuring 100% inspection of the products and components under test. A testing method has been developed for the inspection of innovative products that our students have not had the opportunity to encounter in materials testing. Examples include printed circuit boards and additive manufacturing products. Test methods are easy to learn and teach.

The platform is also perfect for educational purposes. Thanks to the trailer design, it is particularly suitable for external courses and presentations, as one of the side surfaces can be fully opened. The equipment chosen also contributes greatly to the effective transfer of the most innovative and forward-looking technical knowledge. Some of the equipment is computer-controlled. Others are stand-alone systems that can be connected to a computer, monitor or projection screen. In both cases, information on tasks performed, settings and processes can be shared very easily with several people. The safety requirements of the radiography system are also designed for effective training, as the X-ray tubes we have chosen have the least stringent radiation protection requirements of any known materials testing X-ray tube. Overall, the mobile non-destructive materials testing platform has been implemented as planned.

Bibliography: Balder Rai, C.V. Subramanion, T. Jayakumar: Non desructive testing of welds (2004); Chawla, K. K. Ceramic Matrix Composites. London: Chapman and Hall; Hegedűs Sándor: Erőművi hegesztett kötések minősítő rendszere; I. Gibson, D.W. Rosen, B. Stucker: Additive Manufacturing Technologies, Springer, New York; J. Krautkrmer, H. Krautkrmer, Ultrasonic Testing of Materials, 3rd Edition, Springer Berlin Heidelberg; J. L. Rose, Ultrasonic Waves in Solid Media, Cambridge University Press, Cambridge; Morgan, Peter. Carbon Fibers and the Composites. Boca Raton, FL: Taylor & Francis; Olympus Corporation, Panametrics-NDT, Ultrasonic Transducer Technical Notes (2006); Radigrafhy In Modern Industry (KODAK, 2009); Radiografiai vizsgálatok (Fülesi Lajos 2009, befejetlen kézirat);Schwartz, Mel M. Composites Materials. Upper Saddle River, NJ: Prentice-Hall, Inc; Szalay Ferenc: Roncsolásmentes vizsgálatok a GDHD–ban;stb)
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