Acta Polytechnica CTU Proceedings doi:10.14311/APP.2017.7.0072 Acta Polytechnica CTU Proceedings 7:72–75, 2017 © Czech Technical University in Prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app SEMI–AUTOMATED ASSESSMENT OF MICROMECHANICAL PROPERTIES OF THE METAL FOAMS ON THE CELL-WALL LEVEL Nela Krčmářováa, b, ∗, Jan Šleichrta, Tomáš Doktora, Daniel Kytýřa, b, Ondřej Jiroušeka a Czech Technical University in Prague, Faculty of Transportation Sciences, Konviktská 20, Prague 1, 155 00, Czech republic b The Institute of Theoretical and Applied Mechanics AS CR, Prosecká 76, Prague 9, 190 00, Czech republic ∗ corresponding author: krcmarova@fd.cvut.cz Abstract. Metal foams are innovative porous material used for wide range of application such as deformation energy or sound absorption, filter material, or microbiological incubation carrier. To predict mechanical properties of the metal foam is necessary to precisely describe elasto–plastic properties of the foam on cell–wall level. Indentation with low load is suitable tool for this purpose. In this paper custom designed instrumented microindentation device was used for measurement of cell-wall characteristics of two different aluminium foams (ALPORAS and ALCORAS). To demonstrate the possibility of automated statistical estimation of measured characteristics the device had been enhanced by semi-automatic indent positioning and evaluation procedures based on user-defined grid. Vickers hardness was measured on two samples made from ALPORAS aluminium foam and one sample from ALCORAS aluminium foam. Average Vickers hardness of ALPORAS foam was 24.465 HV1.019 and average Vickers hardness of ALCORAS was 36.585 HV1.019. Keywords: metal foam, Vickers hardness, cell-wall, indentation under low loads. 1. Introduction Metal foams are biomimetic porous materials with cellular inner structure that find wide range of ap- plications from deformation energy absorption to noise attenuation, where their very high specific stiff- ness greatly improves overall effectiveness of construc- tions [1, 2]. Homogenization approach has been pro- posed as a method for prediction of their mechanical properties on both cell-wall level and at macroscale [3– 6]. However for calculation of macroscopic (effective) mechanical properties by homogenization, mechanical characteristics at the lower level of the foam’s hier- archical microstructure (i.e. cell-wall level) have to be assessed with high precision and reliability. Here microindentation is a suitable tool for assessment of required elasto-plastic material properties (for cell- wall thicknesses from few hundreds of microns) with the possibility for extension to statistical estimation when automated indents’ positioning and evaluation procedures are introduced. 2. Materials and methods 2.1. Specimen description and preparation Closed-cell aluminium foams with similar compound Al 97.0 %, Ca 1.5 %, Ti 1.5 % (measured using energy- dispersive X-ray spectroscopy) pore size 2 − 4 mm and wall thickness 100 − 200 µm sales denominated as ALPORAS® (Shinko Wire Co., Ltd., Japan) and Sample 1 Sample 2 Sample 3 material ALPORAS ALCORAS ALPORAS width 48 mm 41 mm 44 mm height 21 mm 21 mm 23 mm thickness 10 mm 11 mm 13 mm Table 1. Dimensions of the specimen ALCORAS® (AlCarbon, Germany) were subjected for the testing [7]. Inner structure of aluminium foam is depicted in Fig. 1. Figure was obtained using scan- ning electron microscopy MIRA II (TESCAN, Czech Republic). Region of interest was chosen according to avoid large pores and structural defect. The structural defects were mainly connected with ALCORAS sam- ple. From the delivered slabs cuboids with minimal thickness of 12 mm (to ensure sample integrity) were sectioned using water cooled oscillating diamond saw (Isomet 1000, Buehler GmbH, Germany). Dimensions of each specimen are listed in Tab. 1. Low cutting speed 3 mm · min−1 minimised surface damage. Samples were embedded into mounting com- pound (VariKleer, Buehler GmbH, Germany). Grind- ing and polishing procedure employing silicon carbide grinding discs (320, 800, 1200, 4000 grains per square inch) and diamond suspension (1 µm) was performed to remove 1 mm surface layer which could be influ- enced by sectioning and to obtain plan-parallel faces 72 http://dx.doi.org/10.14311/APP.2017.7.0072 http://ojs.cvut.cz/ojs/index.php/app vol. 7/2017 Assessment of Micromechanical Properties of Metal Foams Figure 1. Inner structure of aluminium foam ob- tained by scanning electron microscopy. with minimal surface roughness necessary for proper indentation. 2.2. Indentation testing 2.2.1. Indentation device description Indentation testing was performed using custom de- signed indentation device developed and constructed on Department of mechanics and material FTS CTU. Indentation device is suitable for hardness measure- ment by low loads from 10 N up to 100 N. Limitation of the device by applying lower loads are: i) inac- curacy of the load recorded by load–cell and ii) in- sufficient accuracy of the control of the indentation axis. Device consists of three independent motorised axes. Two of them are designed for precise position- ing of the specimen with accuracy 10 µm. Third axis is indentation axis equipped by load–cell with posi- tioning accuracy 1.5 µm. Device is controlled by load or displacement using GNU/Linux software system LinuxCNC with custom made graphical user interface. Device is equipped with CCD camera (Manta G–504B, AVT, Germany) with a resolution of 2452 × 2056 px attached to a light microscope (Navitar Imaging, Inc., USA) that provided a magnification of up to 24 ×. The acquisition of the projections was controlled by in-house-built OpenCV based plug-in integrated to control software [8]. Indentation device is depicted on Fig. 2. This camera is used for the indent place estima- tion as well as to make a photo of the indent that is than used for hardness measurement. Due to high resolution CCD camera, calibrated indentation tip alignment and precise positioning of the specimen, indentation of the selected location of the specimen’s surface is allowed with high accuracy. For verification of the device accuracy of hardness measurement in- dentation in calibration hardness plate was performed. Device overall error estimated by calibration mea- Figure 2. Experimental setup. surement was 1.3 %. Maximal measurement error for indentation devices is according to standard set to 3 %. It can be advantageously used for measuring hard- ness of metal foams because indentation is possible only in places with a sufficient width and depth of the specimen, which means joints of the walls. 2.2.2. Experiment procedure description Suitable places for indentation on the sample surface were identified using CCD camera and subsequently indentation was automatically performed at these se- lected locations. After the indentation image data of each indent were captured. The detail of an indent is depicted on Fig. 3. Indentation load was set to 10 N and this value was reached in 10 s. First indent was used as a testing indent to find indentation speed to respect the condition of reaching maximal force value in 10 s. This indent was removed from data set and hardness value of this indent was not evaluated. All other indents were displacement controlled with given speed identified by the testing indentation. Course of the indentation was therefore: i) 10 s loading up to 10 N, ii) 10 s holding on the indentation load and iii) unloading with the same speed as the loading phase. On each specimen was created a series of about 50 indents on the interconnections of the walls. The foam microarchitecture with poresize 2 − 4 mm ensure avoiding that the plastic zones of the indents don’t affect each other. Because of size of interconnection and limited minimal indentation force normative 2.5× diameter distance from sample edge can’t be meet during experimental procedure. Size of thus generated imprints of the indenter was about 270 µm and their 73 N. Krčmářová, J. Šleichrt, T. Doktor et al. Acta Polytechnica CTU Proceedings Figure 3. Right: Marked area of the imprint obtained by semi–automatic evaluation method. depth was about 39 µm. 2.2.3. Evaluation of the experimental data Positioning accuracy of the indentation axis does not allow automated evaluation of the hardness from the full position-load curve using Oliver–Pharr method [9]. Therefore hardness was assessed using traditional method by measuring the imprint dimensions and evaluated as Vickers hardness: HV = F A ≈ 0.1819F d2 , (1) where F is indentation load in N, A is resulting indentation area of the imprint of the indenter in mm2 and d is average length of the diagonal of the imprint of the indenter in mm. As indentation load is taken the maximal force value registered by the load– cell. Length of the diagonals in pixels was measured from the image of each imprint of the indenter using semi–automatic method in Matlab software. Image of the calibration pattern was used for conversion of the diagonal length in pixels to millimetres. Highlighted imprint of the indenter is depicted on Fig. 3 3. Results Elasto-plastic material properties of two types of alu- minium foam was measured by Vickers hardness. In- dentation was performed on cell walls and their con- nections to ensure sufficient place for indentation. In order to get precise information about micromechan- ical properties of the foam about 50 indents were carried out at each of the three specimens. Due to foam nature of specimens the under surface area can be formed by cavity which is not visible on the sur- face and thus some indents were deformed and these Vickers hardness values are omitted from the data set. Those indents are usually easily recognised by highly deformed shape of indents. Success of the indention was about 60 %. Hardness values calculated using equation 1 are listed in Tab. 2. Sample 1 Sample 2 Sample 3 material ALPORAS ALCORAS ALPORAS Indent 1 24.630 41.507 18.149 Indent 2 33.008 35.490 29.839 Indent 3 25.204 36.360 24.817 Indent 4 23.027 32.990 23.672 Indent 5 21.665 31.514 20.633 Indent 6 22.818 34.447 22.967 Indent 7 22.211 30.213 23.779 Indent 8 24.547 30.254 16.826 Indent 9 35.845 49.110 15.233 Indent 10 33.856 47.130 13.301 Indent 11 27.067 45.138 15.093 Indent 12 33.404 28.928 24.319 Indent 13 25.699 38.507 33.361 Indent 14 21.559 24.547 13.418 Indent 15 25.510 41.085 25.044 Indent 16 27.996 37.927 16.167 Indent 17 32.651 42.484 18.410 Indent 18 29.958 30.323 19.584 Indent 19 31.472 32.506 24.322 Indent 20 21.663 43.721 21.050 Indent 21 23.802 34.975 24.192 Indent 22 30.054 32.557 31.487 Indent 23 34.450 28.754 15.765 Indent 24 29.011 45.065 26.474 Indent 25 21.114 33.645 15.346 Indent 26 40.693 24.999 Indent 27 39.462 26.053 Indent 28 35.017 Indent 29 39.386 Indent 30 33.801 Avarage 21.641 36.585 27.289 Table 2. Values of measured Vickers hardness by indentation load 10 N As can be seen from the hardness values summarised in Tab. 2 Vickers hardness of ALPORAS aluminium foam is about 30 % lower than Vickers hardness of ALCORAS aluminium foam. 3.1. Comparative measurement To ensure reliability of the indentation process with non-standarded indents size digital microscope VHX- 74 vol. 7/2017 Assessment of Micromechanical Properties of Metal Foams Figure 4. Indent surface inspection and indent profile reconstruction equipped by digital microscope. 5000 (Keyence, Japan) was employed for indented surface inspection and indent profile reconstruction (see Fig. 4). Indentation depth ≈ 40 µm was obtained from indent profile using Keyence proprietary soft- ware. This value corresponds to results of the analysis presented in 2.2.2. The profile is not significantly effected by surrounding embedding resin. 4. Conclusions Vickers hardness measurement was performed on three samples of aluminium foam on cell-wall level. One sample was aluminium foam ALCORAS and two sam- ples were made of aluminium foam ALPORAS. About 50 indents were created on each sample on the cell– walls or their interconnections with indentation load 10 N. Success of the indentation was about 60 % other indents were deformed due to wrong position on the sample surface what was not possible to avoid prior the indentation. Indentation was carried out using custom designed indentation device equipped with load–cell and CCD camera. Camera was used for identification of appropriate place for indentation and for acquisition of image data of each imprint. Vick- ers hardness was evaluated from image data using semi–automatic procedure in software Matlab. Average Vickers hardness of the sample 1 (ALPO- RAS foam) was 21.641 ± 5.496HV1.019, where HV is denotation for Vickers hardness and value 1.019 indicates indentation load in kgf. Average Vickers hardness for sample 3, which was also made from AL- PORAS, was 27.289 ± 4.731HV1.019. Average Vickers hardness of the third sample, which was made from ALCORAS foam, was 36.585 ± 6.061HV1.019. Vickers hardness of ALCORAS aluminium foam is about 30 % higher than Vickers hardness of ALPORAS aluminium foam. Information about elasto–plastic properties of the aluminium foams ALPORAS and ALCORAS on cell–wall level can be used for calculation of macro- scopic mechanical properties by homogenization. Acknowledgements The research was supported by the Czech Science Foun- dation (research project No. 15-15480S), by Czech technical university in Prague (research project No. SGS15/225/OHK2/3T/16) and by institutional support RVO: 68378297. 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Journal of Materials Research 7(06):1564–1583, 1992. 75 http://dx.doi.org/10.1002/(SICI)1527-2648(200004)2:4<179::AID-ADEM179>3.0.CO;2-G http://dx.doi.org/10.1002/(SICI)1527-2648(200004)2:4<179::AID-ADEM179>3.0.CO;2-G http://dx.doi.org/http://dx.doi.org/10.1016/j.msea.2009.03.067 http://dx.doi.org/10.1016/j.probengmech.2014.06.006 http://dx.doi.org/10.4028/www.scientific.net/KEM.586.120 Acta Polytechnica CTU Proceedings 7:70–73, 2017 1 Introduction 2 Materials and methods 2.1 Specimen description and preparation 2.2 Indentation testing 2.2.1 Indentation device description 2.2.2 Experiment procedure description 2.2.3 Evaluation of the experimental data 3 Results 3.1 Comparative measurement 4 Conclusions Acknowledgements References