Introduction and motivation For numerous activities in or with soils, the A BR value in accordance with NF P18-579 [1] is required to characterise the abrasiveness of the soil (cf. among others DIN 18312 [2]). The LCPC test required for this involves the use of a soil sample with a grain size distribution curve from 4.0 mm to 6.3 mm, which corresponds to the grain fraction of fine gravel. A steel disc (test wing) rotates in this sample for 5 minutes. The mass loss of the test wing is measured and the abrasiveness, the so called A BR value, is calculated. It is given in gram (abraded) material per tonne of soil material (g/ t). The standard specifies that the test wings must be made of rolled unalloyed quality steel C15 (1.0401), have a hardness between 60 and 75 HRB (Rockwell-B) and their surfaces must be descaled by sandblasting [1]. Furthermore, in [3] it is recommended that the surface of the test wings must be metallically bright and free of adhesions, in particular oxide layers, and that the hardness of the test wings must be verified by random batch testing. In addition to the grain size described above, investigations can be carried out with soils that have a grain size between 0.0 mm and 6.3 mm (cf. [4]) or 2.0 mm to 8.0 mm (cf. [5]). Previous work (including [6], [7], [8] in particular) do not take into account the material pro- Science and Research 45 Tribologie + Schmierungstechnik · volume 71 · issue 5-6/ 2024 DOI 10.24053/ TuS-2024-0038 Materials Investigation of Test Wings from LCPC Abrasiveness Test with Different Soils and Varying Test Duration Danka Katrakova-Krüger, Jonas Kotscha, Christoph Budach, Peter Erdmann* submitted: 20.09.2024 accepted: 24.01.2025 (peer review) Presented at GfT Conference 2024 For numerous activities in or with soils, information on the abrasiveness of the soil is required, as this is an important parameter for the economic efficiency of a construction project. To determine the abrasiveness of loose soil, the LCPC test is carried out in accordance with the French standard NF P 18-579. In this test, a test wing rotates for five minutes in a soil sample with a grain size between 4.0 mm and 6.3 mm. The mass loss is determined, from which the A BR value is calculated. The test wings used for the tests must have defined material properties so as not to falsify the abrasiveness results. This article presents material characterization of test wings from different test series using the LCPC test. In contrast to the French standard, the test wings are not only examined with regard to their mass loss, but their hardness is measured and a metallographic characterization of the metal microstructure is also carried out. In the tests, the grain fraction, the test duration and the fraction of broken grains in the soil sample were specifically varied. On the one hand, the metallographic investigations support and confirm the geotechnical results, on the other hand, they provide important insights into the wear of the test wings in the LCPC test. In the series of tests with a grain fraction that deviated from the standard, the influence of different grain sizes on the metal structure was also clearly demonstrated. The progression of wear on the test wings over time can be visualised in the tests with different test times. In addition, differences in the microstructure and surface properties of test wings, which can affect the resulting A BR values, were identified and analysed. Recommendations for the manufacturing of the test wings for the standardised LCPC test are derived. Keywords LCPC-test, abrasiveness, test wings, metallography, soil, soil grain size distribution curve, test duration Abstract * Prof. Dr. Danka Katrakova-Krüger (corresponding author) Jonas Kotscha M.Sc. Faculty of Computer Science and Engineering Science, Institute for General Mechanical Engineering, Materials Laboratory, University of Applied Sciences (TH) Cologne, Gummersbach, Germany Prof. Dr. Christoph Budach Faculty of Civil Engineering and Environmental Technology, Geotechnical Engineering and Tunneling, University of Applied Sciences (TH) Cologne, Cologne, Germany Prof. Dr. Peter Erdmann Faculty of Process Engineering, Energy and Mechanical Systems, Cologne Institute of Construction Machinery and Agricultural Engineering, University of Applied Sciences (TH) Cologne, Cologne, Germany As part of this study, the grain size distribution and the proportion of broken grains in the soil as well as the duration of the test were systematically varied. In addition to investigating the mass loss of the test wings, their hardness was determined and also a detailed characterization of the metal microstructure was also carried out. First results of these investigations - in particular the geotechnical aspects - were shown in [14] and [15]. In this contribution the focus is set on the microstructure investigation of the test wings in relation to the observed weight loss and the respective A BR value. Materials and methods Three test campaigns were carried out, in which the standardised parameters of the LCPC test were specifically varied. In test series A1 and A2 (test campaign A), the test duration was changed for two different soils. Science and Research 46 Tribologie + Schmierungstechnik · volume 71 · issue 5-6/ 2024 DOI 10.24053/ TuS-2024-0038 perties mentioned in [1] and [5] or have focussed on the properties of the test wings without specific variation of the grain size of coarse-grained soils. Also, the development of the wear progression over the time was not considered in combination with materials characterization. Other studies, including [9], [10], [11], [12] and [13], deal with LCPC tests under variation of soil parameters ( [9] and [10] ) as well as with the adjustment of test parameters, such as rotation speed [12]. In [13], a correlation analysis is carried out between the abrasiveness, hardness and mechanical properties of rocks exposed to different freeze-thaw cycles. In addition, [11] provides an overview of various laboratory tests for assessing abrasiveness. In contrast to these studies, the present work includes a detailed material characterisation of each test wing in addition to geotechnical analyses in order to provide a more comprehensive understanding of the influencing factors. Table 1: Boundary conditions and results of the LCPC tests carried out based on [16], [17] and [18] Figure 1: A BR value as a function of the test duration 0 - 60 minutes and the grain fraction Figure 2: A BR value as a function of the test duration 0 - 10 minutes and the grain fraction The aim of test campaign B was to investigate the influence of different grain fractions, i.e. grains of different sizes in the soil samples. The boundary conditions and results of the LCPC tests of test campaigns A and B as well as the hardness values of the test wings are shown in Table 1. The sample A1.5 was tested according to the standard LCPC conditions. In Figure 1 the abrasiveness results of the test series A1 and A2 are visualized. The A BR value increases as a function of the test duration. At the beginning, in the first ten minutes, the A BR value increases significantly more than later. Figure 2 shows an enlarged version of the curves for the first ten minutes. From the higher slope of the curves in the first ten minutes, especially the first five minutes, it can be concluded that the wear process is strongest at the beginning. From the tenth minute onwards, the wear rate decreases slightly and remains more or less constant for the rest of the time. Comparable results were obtained in the studies [6] and [7]. It can also be seen that the abrasiveness is higher for soils containing coarser grains (series A2). The results of test campaign C are listed in Table 2. In this test series, the fraction of broken grains in the soil sample was gradually increased by 20 % (C0 0 % - C5 100 %). Sample C0 serves as a reference sample and in this case corresponds to sample A1.5. Both designations refer to one and the same sample. The diagram (Figure 3) shows the A BR value as a function of the fraction of broken grains. It can be seen from the graph that the A BR value increases almost linearly with increasing fracture grain content in the soil sample. After completion of the LCPC tests and the geotechnical evaluation, the metallographic investigation was carried out using the following methodology. First of all, the test wings were visually inspected, their appearance described, and any special features documented. The hardness according to Rockwell B (HRB) was then recorded for all test wings. Three measurements were carried out in each case, from which the average value was determined. The hardness measurements were carried out after the LCPC tests. In order to avoid the influence of the test, the hardness indentations were placed in the area of the clamping of the test wing (cf. Figure 4). This area is unaffected by collisions with the soil grains during the LCPC test, which is why the material is expected to remain unchanged there and can be used for the hardness measurements on the wing material. Science and Research 47 Tribologie + Schmierungstechnik · volume 71 · issue 5-6/ 2024 DOI 10.24053/ TuS-2024-0038 Table 2: Boundary conditions and results of the LCPC tests carried out based on [18] Figure 3: A BR -value as a function of the proportion of broken grains including linear regression 10 minutes). In addition, tests with significantly longer test durations of 20, 40 and 60 minutes were carried out in test series A1. The progress of wear is shown in Figure 6 (for the shorter and the regular 5-minutes test duration) and in Figure 7 (for longer test duration). On the test wings, which were tested less than the regular 5 min, it can be observed that the material is initially compressed and strongly deformed. At both corners, areas form in which the test wing material is “smeared” over the edge. In Figure 8 and Figure 9 the samples with a test duration of one minute for both investigated soil grain fractions are given with more detail. With longer test durations, such as four minutes, these areas are increasingly eroded (see Figure 10 and Figure 11 at the upper edge). The heavily deformed areas lose their connection to the base material and become smaller. In addition, when comparing the samples from test series A1 and A2, it can be seen that the samples from series A2 wear faster. The heavily deformed areas become smaller more quickly than in the samples from test series A1. The reason for this lies in the soil sample used in each case. The soil sample from test series A2 Science and Research 48 Tribologie + Schmierungstechnik · volume 71 · issue 5-6/ 2024 DOI 10.24053/ TuS-2024-0038 To create micrographs, one of the opposing, heavily worn corners of the test wing was cut out and hot-embedded. Figure 5 shows how the test wing was cut. The small triangular piece at the bottom right of the image has been used. The sample is embedded so that the longest side (diagonal section / red line in Figure 5) can be observed and metallographically evaluated. After hot embedding, the sample was ground and polished in several passes. The finest polishing agent had a grain size of 1 µm. The sample was etched in three per cent nitric acid. The sample preparation and analysis correspond to the one described in [9]. The test wing shown in Figure 4 and Figure 5 has been stored in the normal laboratory environment after the LCPC test, which is why it has a slight oxidised layer. Results and discussion Variation of the test duration In test campaign A, various LCPC tests were carried out with different test durations for the grain fractions 4.0 mm - 6.3 mm and 2.0 mm - 8.0 mm. The test duration varied from the standardised five-minute test duration, being shorter (1, 2, 3 and 4 minutes) and longer (7.5 and Figure 4: Test wing after hardness test Figure 6: Wear progress LCPC test duration 1, 2, 3, 4 and 5 minutes (from left to right) grain fraction 4.0 mm - 6.3 mm Figure 5: Test wing separated has larger maximum grain size (8.0 mm) than that from test series A1 (6.3 mm), which is in accordance with the determined higher A BR values (cf. Table 1 and Figure 2). When analysing sample A1.60, which was tested for 60 minutes in the LCPC test, it can be clearly seen that the entire sample is very heavily worn, and a lot of material has been removed. In the area of the left edge, near Science and Research 49 Tribologie + Schmierungstechnik · volume 71 · issue 5-6/ 2024 DOI 10.24053/ TuS-2024-0038 Figure 7: Wear progress LCPC test duration 7.5, 20 and 60 minutes (from left to right) grain fraction 4.0 mm - 6.3 mm Figure 8: Sample A1.1 Test duration 1 minute grain fraction 4.0 mm - 6.3 mm Figure 9: Sample A2.1 Test duration 1 minute grain fraction 2.0 mm - 8.0 mm Figure 10: Sample A1.4 Test duration 4 minutes grain fraction 4.0 - 6.3 mm Figure 11: Sample A2.4 Test duration 4 minutes grain fraction 2.0 - 8.0 mm produce a large deformation layer on the test wing. This phenomenon is investigated and described in more detail in test campaign B. The tests with varying test durations are needed to validate a numerical simulation. The LCPC test was digitally modelled. A discrete element method (DEM) simulation of the LCPC test was carried out using the EDEM software. The digital replacement model is shown in Figure 15. When evaluating the simulation results, both the wear pattern on the test blade and the overall wear progress (cf. Figure 6 and Figure 7) as well as the changes in the soil sample (see Figure 15) can be analysed and compared. The experimentally obtained test results, including the micrographs and soil samples, serve as a reference. Science and Research 50 Tribologie + Schmierungstechnik · volume 71 · issue 5-6/ 2024 DOI 10.24053/ TuS-2024-0038 the outermost point in Figure 12 the test wing has a thickness of approx. 2.5 mm. Compared to the unworn sample, half of the material thickness has been removed. Overall, the test wing has taken on an aerodynamic shape. Sample A1.60 also has a different surface which is much finer and softer to the touch. A change in the deformed surface layer can be seen in the micrograph. The surface layer of sample A1.60 is approx. 19.37 µm thick (cf. Figure 13). If the surface layers of samples A1.1 to A1.60 are compared, it is noticeable that the deformation layer generally decreases over the test duration. The surface layer is increasingly removed as the LCPC test progresses. Furthermore, apparently no new surface layer is built up. This can be attributed to the fact that the bigger grains of a soil sample are crushed during the LCPC test (cf. Figure 14) and the remaining smaller grains do not Figure 12: Sample A1.60 Test duration 60 minutes grain fraction 4.0 mm - 6.3 mm Figure 13: Surface layer sample A1.60 grain fraction 4.0 mm - 6.3 mm Figure 14: Example soil sample before (left) and after (right) the LCPC test [19] Variation of the grain size distribution of the soil samples In test campaign B, a soil comparable to test campaign A in terms of its mineral composition was used for the LCPC trials. In the tests of test campaign B, the grain size distribution curve of the soils was deliberately varied further by also taking sand fractions into account. In principle, the results of the geotechnical investigations can also be observed and confirmed here metallographically. The increased wear of samples that were tested in a larger grain fraction (larger maximum soil grain size) can be clearly recognised on the micrographs. This can be seen in the comparison of sample B1 (Figure 16) and sample B2 (Figure 17). Sample B2 is significantly more rounded over the entire height of the test wing. With sample B1, rounding can only be recognised in the area of the corners. Further conclusions can be drawn from the micrographs. Due to the mechanical impact of the soil grains on the test wing during the LCPC test, cold deformation occurs on the surface of the test wing. Deformed material can be seen in the edge area of the heavily worn sample B2. The crystallites near the surface show a clear compression, in fact the material has been work-hardened (see Figure 19). Looking at the only slightly worn sample B1 (Figure 18), it is noticeable that the deformed edge layer is not so well pronounced here, or cannot be recognised at all. This finding also confirms that soil samples with a smaller maximum grain size (grain fraction) are less abrasive than soil samples with a larger grain fraction (cf. among others [15]). Further findings from test campaign B relate to the production of the test wings. When analysing the micrographs of sample B4, special features emerged that were not observed in the other samples of both test campaigns. Science and Research 51 Tribologie + Schmierungstechnik · volume 71 · issue 5-6/ 2024 DOI 10.24053/ TuS-2024-0038 Figure 15: Digital replacement model of the LCPC test [20] Figure 16: Micrograph sample B1 grain fraction 0.063 mm - 2.0 mm after 5 minutes - A BR = 468 g/ t Figure 17: Micrograph sample B2 grain fraction 0.25 mm - 6.3 mm after 5 minutes - A BR = 1138 g/ t wing was done in an oxygen-containing atmosphere. Normally, nitrogen atmosphere or vacuum are used for heat treatment in order to prevent possible surface layer decarburisation. The extent to which these deviating microstructural findings influence the results of the LCPC test was not investigated in depth yet. Taking into account the findings from [8] it can be assumed that the wear in the decarburised areas will be significantly higher, as there are almost no hard phases (pearlite, containing cementite Fe 3 C) in the microstructure, but only soft ferrite. This could not be suspected when looking only at the material hardness which in this case is within the specified range and also comparable with the other tested wings. But the comparability of the abrasiveness results is no longer given, at least from a material point of view. Based on [8] and on the results of own investi- Science and Research 52 Tribologie + Schmierungstechnik · volume 71 · issue 5-6/ 2024 DOI 10.24053/ TuS-2024-0038 When comparing sample B4 (Figure 20 and Figure 21) with the samples B1 and B2 (Figure 16 and Figure 17), it is noticeable that the perlite lines of sample B4 run vertically and not horizontally, i.e. the rolling direction is different. This leads to the conclusion that sample B4 was manufactured from a different semi-finished product. On the other hand, there is an approx. 0.5 mm thick layer in the upper and lower area, which is different. The microstructure there is considerably coarser-grained than the rest of the sample. This might be due to longer time or higher temperature during the heat treatment. In addition, almost no perlite can be recognised. This means that the carbon content is much lower in those areas. All these observations point to surface layer decarburisation during the annealing process. Surface layer decarburisation can occur if the heat treatment of the test Figure 18: Micrograph sample B1 enlarged 500 times grain fraction 0.063 mm - 2.0 mm Figure 19: Micrograph sample B2 enlarged 500 times grain fraction 0.25 mm - 6.3 mm Figure 20: Micrograph sample B4 grain fraction 0.063 mm - 8.0 mm Figure 21: Micrograph sample B4 enlarged 50 times grain fraction 0.063 mm - 8.0 mm gations, it is recommended to include the production processes (same semi-finished product, same rolling direction, cooled CNC milling, heat treatment in inert gas atmosphere or vacuum) of the test wings in the standard of the LCPC test [1] or the recommendation [3] and clearly define them. It is also advisable to specify the storage conditions of the test wings to prevent corrosion on the surface. This can be ensured by lightly oiling the surface and storing in a dry place. Random micrographs of the test wings before they are used for abrasiveness testing should be taken to detect possible deviation in the microstructure which might be a reason for misleading results as this cannot be predicted by the hardness alone. Change in the fraction of broken grains In test campaign C (change in fraction of broken grains), the geotechnical observation of increasing abrasiveness is difficult to confirm in terms of material characterisation. The purely visual inspection of the micrographs of samples C1, C3 and C5 (Figure 22, Figure 23 and Figure 24) makes it difficult to prove the increasing abrasiveness. For a clear confirmation of the geotechnical results, the increase in abrasiveness due to the increasing fraction of broken grains in this test campaign is too small. Overall, the micrographs show similar findings. Science and Research 53 Tribologie + Schmierungstechnik · volume 71 · issue 5-6/ 2024 DOI 10.24053/ TuS-2024-0038 Figure 22: Micrograph of sample C1 Figure 23: Micrograph of sample C3 Figure 24: Micrograph of sample C5 Figure 25: Thickness of the surface layer test campaign C ! "#$"# % $' ' $#! #? $%! ? $ ' Table 3: Thickness of the surface layer test campaign C information on the wear progression of the test wings. The tests show that the test duration and the grain fraction have a significant influence on wear. After an initial high abrasion, the wear rate diminishes and remains nearly constant. Higher wear is observed for soils with higher maximum grain size. Also, an influence was found on the deformation layer built on the surface during the test. With increasing proportion of broken grains, the abrasivity of the soil is increasing. The thickness of the deformation layer seems to follow correspondingly but is significantly lower for 100 % broken grains. The reason for this is not ultimately clarified yet and further investigation is needed. Significant differences were found in the surface quality and the microstructure of the test wings, which can affect the comparability of the A BR values. Based on these findings, recommendations were derived for the production of standardised test wings and their storage in order to achieve more reliable results on the abrasiveness of soils. Currently, a round robin test is being carried out in which the LCPC tests of around 20 laboratories and testing facilities are being compared. The laboratories are provided with controlled soil samples and test wings for the LCPC tests. In addition, the participating labs are using own test wings for the LCPC tests. The results of the round robin test can be used to evaluate the deviation between the test devices as well as the scattering of the LCPC tests with different test wings. Also, effort is put on the Discrete Element Method (DEM) simulation of the abrasiveness test to be able to understand the phenomena as well as to predict results and significantly reduce experimental costs. Acknowledgements This work was supported by the Ministry of Culture and Science of North-Rhine Westphalia, Germany within the program HAW Kooperation. Science and Research 54 Tribologie + Schmierungstechnik · volume 71 · issue 5-6/ 2024 DOI 10.24053/ TuS-2024-0038 The analysis of the surface layer shows a special feature. Basically, it was observed that a deformed and presumably work-hardened surface layer forms on the test wings. This surface layer initially increases proportionally with the increasing fraction of broken grains. The measured values are listed and illustrated in Table 3 and Figure 25. This development runs parallel to the increase in abrasiveness (A BR value). However, sample C5 represents an anomaly. Whereas the thickness of the surface layer increases continuously in samples C0 to C4, it drops suddenly and rapidly in sample C5. The sharply decreasing thickness of the surface layer from sample C4 to C5 is shown in Figure 26 (C4) and Figure 27 (C5). The origin of this deviation is not finally clarified up to now. One possible reason is that the deformed layer may have been more pronounced but already abraded. To check this, tests with varying time with this type of soil must be performed. Further investigations into this are planned but have not yet been realised as the compilation of a defined soil sample with an exact fraction of broken grains is very time-consuming. The major time expenditure is in the manual sorting and characterisation of the soil sample’s aggregates which is also dependent on the person doing this sorting. Summary and outlook This paper describes materials characterization of test wings after investigations of the abrasiveness of soil using the LCPC test according to the French standard NF P 18-579. The LCPC test determines the mass loss of a rotating steel test wing in a soil sample in order to calculate the A BR value. In the test campaigns carried out, the test durations and the soil grain size distribution curves were varied in order to investigate their influence on the abrasiveness and wear of the test wings. The results of the metallographic analyses fit well to the geotechnical results and provide Figure 26: Thickness of the surface layer sample C4 Figure 27: Thickness of the surface layer sample C5 Remarque Part of this research was presented at the 65th German Tribology Conference 2024 taking place 23.-25-09.2024 in Göttingen, Germany [21]. 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Katrakova-Krüger and B. Siebert, “Untersuchungen zur Bestimmung der Abrasivität von grobkörnigen Böden im maschinellen Erd- und Tunnelbau,” 9. Fachtagung Baumaschinentechnik, 2022. [21] D. Katrakova-Krüger, J. Kotscha, C. Budach and P. Erdmann, “Werkstoffuntersuchungen an Prüfflügeln aus dem LCPC-Versuch zur Bestimmung der Abrasivität von Böden,” Tagungsband der 65. Tribologie Fachtagung, ISBN: 978-3-9817451-9-1, 45/ 1-11, September 2024. Science and Research 55 Tribologie + Schmierungstechnik · volume 71 · issue 5-6/ 2024 DOI 10.24053/ TuS-2024-0038