PORTABLE HARDNESS TESTER FOR TIMBER CLASSIFICATION

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1 PORTABLE HARDNESS TESTER FOR TIMBER CLASSIFICATION Adriano Ballarin 1, Pedro Almeida 2, Hernando Lara Palma 3, Roberto Colenci 4 ABSTRACT: Hardness is a property largely used in material specifications, mechanical and metallurgical research and quality control of several materials. Specifically for timber, Janka hardness is a simple, quick and easy test, with good correlations with the compression parallel to grain strength, a strong reference in structural classification for this material. More recently, international studies have reported the use of Brinell hardness for timber assessment which resumes the advantages previously mentioned for Janka hardness and make it easier to be performed in the field, especially because of the lower magnitude of the involved loads. A first generation of an equipment for field evaluation of hardness in wood Portable Hardness tester for wood based on Brinell hardness has already been developed by the Research Group on Forest Products from FCA/UNESP, Brazil, with very good correlations between the evaluated hardness and several other mechanical properties of the material when performing tests with different species of native and reforested wood (traditionally used as ties - sleepers - in railways). This paper presents results obtained in the experimental program with the first generation of this equipment and preliminary tests with its second generation, which uses accelerometers to substitute the indentation measurements in wood. For the first generation of the equipment functional and calibration tests were carried out using 16 native and reforestation timber lots, among there E. citriodora, E. tereticornis, E. saligna, E. urophylla, E. grandis, Goupia glabra and Bagassa guianenses, with different origins and ages. The results obtained confirm its potential in the classification of specimens, with inclusion errors varying from 4.5% to 16.6%. KEYWORDS: timber, sleepers, Janka hardness, Brinell hardness, portable equipment, accelerometers. 1 INTRODUCTION 1234 The majority of Brazilian railroads companies use wooden sleepers (ties). More than 8% of the 296 km [1] of the Brazilian railroad mesh employs wooden sleepers. During the early implantation period of these railways (first half of last century) there was a large availability of native wood species for this purpose (Astronium urundeuva, Tecoma sp., Hymenaea stilbocarpa, Myrocarpus sp.). Soon after, however, these native species reached prohibited prices, demanding new alternatives to reduce the costs of road maintenance. Additionally, the majority of railroad companies is facing a very difficult situation, with increasing deficits of sleepers. Moreover, 1 Adriano Ballarin, Agronomic Sciences College, Sao Paulo State University, P.O. Box 237, Botucatu-SP, Brazil. awballarin@fca.unesp.br 2 Pedro Almeida, Polytechnic School University of Sao Paulo, Sao Paulo-SP, Brazil. palmeida@usp.br 3 Hernando Lara Palma, Agronomic Sciences College, Sao Paulo State University, P.O. Box 237, Botucatu-SP, Brazil. larapalma@fca.unesp.br 4 Roberto Colenci, Faculty of Technology, Botucatu-SP, Brazil. colenci@uol.com.br environmental regulations and their requirements of conformity are forcing the use of sustainable resources. The alternative, at short and medium term, has been the use of reforestation species to produce sleepers. In this new scenario, the great difficulty, in field condition, is the classification of species (most of then very similar in a visual analysis) and the evaluation of additional characteristics (age, e.g.) that are of major importance to define its suitability to sleeper production. Usually, the analysis is even harder, because it is performed in the storage patio of preserved or not preserved sleepers. Among the major strength properties of wood, hardness reveals its potentialities (good correlation to other mechanical properties and quickness of results) and can be used as a non-destructive tool in the characterization of species from reforestation [2,3]. General methods to evaluate hardness in materials are gathered in two major groups. In the first one, hardness is evaluated by the required force to promote a defined surface indentation (penetration) of a piece (a metal sphere, e.g.). The well known Janka hardness uses this principle. In the second group hardness is evaluated in terms of the indentation (deformation) promoted by a particular piece that is forced at the surface of the

2 material with a pre-defined force. The dimensions and shapes of the pieces and the magnitude of the force are determined by the method adopted. For evaluation of hardness in wood, considering its viscoelastic properties, especially its resilience, one can easily note greater appropriation of Janka hardness when compared to Brinell hardness. When the force that promoted the indentation is removed, the material automatically recovers the elastic part of the total deformation applied, and, for Brinell hardness, the evaluation of the indentation (usually the diameter of the spherical shape at the surface of the material) maybe imprecise, unless the measurement is performed immediately before the force is removed. Nevertheless, even with this limitation, several researchers have suggested Brinell method for the evaluation of hardness in wood in field condition, considering, especially, the lower magnitude of the forces involved in the indentation and the additional difficulty to control, the depth of the metal sphere indentation required on Janka hardness method [4,5]. Bektas et al. [6] and Colenci [5] studied the association between Janka and Brinell hardness in wood specimens, obtaining determination coefficients for linear regression analysis from.83 to.87. By continuing this research program, this paper reports test results performed to develop and calibrate a portable hardness tester. Subsequently, it will be coupled with accelerometers to evaluate the hardness of wood. 2 MATERIAL AND METHODS Twelve specimens of sixteen groups of native and reforested wood were used to perform the experimental tests, totalizing 192 specimens. Table 1 presents the major characteristics of the wood species analyzed. The portable hardness tester uses the Brinell hardness principles (fixed force to promote the indentation of a metal sphere). The indentation of the metal sphere (1 mm diameter) was promoted by the free fall of a mass (energy of indention varying from.98 to 3.92 J). Comparing to other possible energy source systems (hydraulic pressure, spring, e.g.), this system provided a lighter and more reliable equipment. Hardness was evaluated in wood only in the normal direction, considering mainly the results obtained by Dal Pogetto [7], about the statistical equality between parallel to grain (f H ) and normal to grain (f H9 ) under several conditions and the facility to evaluate hardness in this direction, in field condition. Hardness tests were performed with the Portable Hardness Tester (Figure 1) developed by the Material Tests Laboratory of the Agronomic Sciences College Sao Paulo State University - in four distinct situations, as follows: H1 H2 H3 H4 free fall of 1 kg mass from 1 mm height Energy =,98 J free fall of 1 kg mass from 2 mm height Energy = 1.96 J free fall of 2 kg mass from 1 mm height Energy = 1.96 J free fall of 2 kg mass from 1 mm height Energy = 3.92 J Table 1: Main characteristics of the groups of native and reforestation wood Group Specie Group particular characteristics Goupia 1 glabra 2 E.citriodora 3 E.citriodora 4 E. saligna 5 E. citriodora 6 E. grandis 7 E. tereticornis 8 E. urophylla 9 E. citriodora 1 E. citriodora 11 E. citriodora 12 E. citriodora 13 Bagassa guianenses 14 E.grandis 15 E.grandis 16 E.grandis Wood from native specie commonly used for structural purposes Wood from reforestation. Trees with more than 4 years old Wood from reforestation. Species commercially sold to sleeper production. Age of plantation unknown. Wood from reforestation with high predominance of heartwood Wood from reforestation. Samples with high predominance of sapwood Native wood with high density and strong dimensional stability. Specie specially indicated to sleeper production. Wood from reforestation. Specie hardly used for sleeper production. High predominance of sapwood. Trees with 6 years old. Hardness strength (H) was evaluated considering the energy at the impact of the metal sphere against the wood specimen, according to the expression: H 2 E = πd D D 2 d 2, d > (1) where H = hardness strength, E = energy resulted from the free fall of the mass, D = diameter of the metal sphere, d = indented diameter in the wood. Figures 1 and 2 illustrate general aspects of the equipment and typical indentation in wood, respectively.

3 Compression parallel to grain and density evaluations Figure 1: General aspects of the Portable Hardness Tester for wood. Figure 3: Specimens (5 mm x 5 mm x 5 mm) and corresponding regions of properties evaluation Figure 4: Instrumented Portable Hardness Tester Figure 2: Typical indentation in the wood reinforced by carbon paper As can be seen in Figure 3, all the specimens were also tested to obtain Janka Hardness (f H9 ), Brinell Hardness (HB) and compression parallel to grain strengths (f c ). These mechanical tests were performed in an Universal Machine EMIC, model DL 1 (1 kn load capacity). Trying to automatically evaluate indentation without measuring the indented diameter in the wood, the portable hardness tester was instrumented with piezoresistive accelerometer EMDEVCO, model 2262, as illustrated in Figure 4. The digital signal was captured with a data acquisition system LYNX, model ADS 2, using a frequency of acquisition of 3 khz. Preliminary tests to evaluate sensitivity and stability of response were performed using low density foam, neoprene rubber, aluminum board and wood from Goupia glabra 3 RESULTS AND DISCUSSION Table 2 presents results of the application of statistical Tukey s test to compare means of the strength parameters evaluated. As could be expected, groups 14 to 16 presented the lowest densities, as a consequence of specie and the age of the trees. For them, Janka and Brinell hardness strength and compression parallel to grain strength were also the lowest. Group 13 (Bagassa guianensis) revealed the highest strength in compression parallel to grain, followed by groups 5, 9 and 1. In terms of Janka hardness strength, the groups with the highest strength were 13, 9, 1 and 12. Groups 9 and 1 were representing Eucalyptus citriodora (specie potentially sold for sleeper production) with high predominance of heartwood. It was not possible to separate groups 9 and 1 from groups 11 and 12, using Janka hardness strength. In the same way, Table 3 presents results of the application of statistical Tukey s test to compare means of hardness evaluated by the Portable Hardness Tester.

4 Table 2: Results from Tukey s test in the comparison of means of several strength parameters among the groups of native and reforestation wood Density Group kg/m 3 (MPa) b (d) 3.19 cd 4.53 bc f H9 (a) HB (b) f c (c) b 2.86 bcd bc b 3.8 cd bcd a 2.33 b 36.4 b bc 3.58 defg 57.2 e bc 3.41 cde cd cd 3.53 def 34.4 b b 2.72 bc b d 4.2 fg 57.2 e d 4.28 gh de cd 3.72 efg 52.2 de cd 3.81 efgh 49. d d 4.45 h f a 1.55 a a a 1.62 a a a 1.43 a a Notes: a) Janka hardness normal to the grain b) Brinell hardness c) compression parallel to grain strength d) values in columns, followed by the same letter are not statistically different (significance level =.5) From Table 3 it can be observed that separations among groups were hardly obtained using the Portable Hardness Tester. Statistical gathering was not completely established for several groups (e.g. group 11 in H3 hardness evaluation). For all the studied situations (H1 to H4) groups 14 to 16 revealed the lowest strength results. Group 13 did not reach a top condition for these hardness tests, as observed in Janka tests (Table 2). Using the Portable Hardness Tester, the highest hardness strengths were observed in groups 9 and 1, followed by groups 11, 12 and 13. Anyway, the equipment was able to separate the best from the worst groups. Table 3: Results from Tukey s test in the comparison of means of hardness - evaluated with the portable equipment - among the groups of native and refore station wood H1 (a) H2 H3 H4 Group (kj/m 2 ) cd (b) cd bc 68.4 bcd bc 5.86 cd bc 73.5 cde cd cd bc bc ab 43.9 bc 4.57 ab de de 6.96 def 54. cd 78.5 de cd def cd de de 63.9 fg 58.8de de cd cde c bc e g 66.5 e 9.55 gh e 72.32g de h de fg cde efg de 61.5 def 58.9 de 88. fgh de ef de 8.75 ef a a a a a ab a a a a a a Notes: a) H1 to H4 hardness evaluated with the portable equipment b) values in columns, followed by the same letter are not statistically different (significance level -.5) Figure 5 shows the correlations obtained between hardness H1, H2, H3 and H4 and Janka hardness, as a reference. In the four situations the coefficient of determination obtained exceeded.8, fairly acceptable for this purposes. The regression equation presented in Figure 5b was used to calculate hardness H2 values corresponding to Janka hardness of 5, 4 and 3 MPa. These distinct levels of Janka hardness strength are assumed by the Brazilian standards for sleepers production [8] in the classification of specimens (1 st, 2 nd, 3 rd and 4 th class - discard) for this purpose. Corresponding to the presented hardness strength, regression equation resulted in hardness H2 of, respectively, 52.65, and 39.5 kj/m 2.

5 Hardness Janka (MPa) Hardness Janka (MPa) Janka Hardness (MPa) Janka Hardness (MPa) y =,1155x 1,697 2 R 2 =, Hardness H1 (kj/m 2 ) 4 y =,438x 1, R 2 =, Hardness H2 (kj/m 2 ) 4 y =,44x 1,816 2 R 2 =, a) b) c) d) Hardness H3 (kj/m 2 ) 4 y =,324x 1, R 2 =, Hardness H4 (kj/m 2 ) Figure 5: Regression analysis between hardness evaluated with Portable Hardness Tester and Janka Hardness. a) H1 x Janka; b) H2 x Janka; c) H3 x Janka; d) H4 x Janka. Table 4 presents bands of variation of H2 hardness that correspond to the Janka Hardness strength classes and number of specimens that reached this strength, separated in columns according to their respective Janka Hardness. With this arrangement, one can observe, for example, that that in the band from to kj/m 2 (4 MPa minimum Janka hardness strength), 3 specimens were classified. From them, 26 reached the expected strength of 4 MPa. Four specimens that did not reach the expected strength lead to an inclusion error of 13.3%. The omission error was obtained in a similar analysis, by evaluating the percentage of specimens that despite reaching the pretended Janka hardness (column of 4 MPa, e.g.), presented hardness H2 in the lower bands (3 out of 15 specimens). Table 4: Confusion matrix bands of variation of the H2 hardness established for each considered strength class and the corresponding classification errors Bands of variation (kj/m 2 ) Strength Classes (MPa) < H2 < 39.5 < Total error (%) 39.5 H2 < H2 < H Total Omission error (%) A similar analysis was conducted considering the hardness H3, using equation reported at Figure 5c. Results of this classification are reported in Table 5. Table 5: Confusion matrix bands of variation of the H3 hardness established for each considered strength class and the corresponding classification errors Bands of variation (kj/m 2 ) Strength Classes (MPa) < H3 < < Total error (%) H3 < , H3 < ,6 H ,1 Total Omission error (%) As one can observe, in both cases, H2 and H3 hardness evaluated using Portable Hardness Tester revealed an acceptable accuracy. For H2 and H3 hardness cases, the inclusion error (false positive specimens - wrongly assumed to reach a particular strength) was particularly low when classifying specimens with Janka hardness strength in the band exceeding 5 MPa. In a similar analysis, for the classification of specimens with Janka Hardness in the band 4 MPa - 5 MPa, assumed as class 2, the inclusion error of the equipment remained in acceptable levels, in the order of 15%. For the band 3 MPa - 4 MPa, the inclusion error was higher. Nevertheless, the portable equipment is intended

6 to be applied in the selection of higher quality specimens (upper bands of strength). In such situation, the portable equipment performed with a good level of accuracy. Omission errors (false negative specimens despite reaching an aimed strength, were not selected by the adopted criteria) observed in both cases varied from 9% to 2%. In both classification (Tables 4 and 5) the omission error observed in the band that exceeded 5 MPa, the major interest for the equipment, was around 15%. In a first analyzes, the inclusion error has a greater importance when compared to the omission error due to the structural performance required for the sleepers. Results captured by accelerometer positioned just above the indentation point with magnetic base are presented in Figure 6 for materials with different viscoelastic properties. The impact acceleration was sensitive to detect differences in all tested materials as can be seen in Figure 6. It can be clearly seen that some parameters of the digital signals, e.g. the damping, expressed by the duration of the event, were strongly affected by the nature of the materials. The first reaction of the digital signal, as an expression of the displacement of quartz crystals, was a deceleration resulting from the downward impact force over the metal box of the accelerometer, varying in magnitude among the different materials tested, followed by a quasiinstantaneous acceleration, of higher magnitude, varying from 3 to 4 g (g - gravity acceleration). The next phase of the acceleration phenomenon is reported, in terms of acceleration, as a damping. It is expected that all these main parameters, closely related to the viscoelastic parameters, may have statistical relationship. Integration of the acceleration digital signal in the time domain will result on velocity and displacement signals. In both situations, the main parameters of these digital signals will probably be associated to viscoelasticity of the material. acceleration (x 9.8m/s 2 ) acceleration (x 9.8m/s 2 ) acceleration ( x 9.8m/s 2 ) acceleration (x 9.8m/s 2 ) ,,5,1,15,2,25, time (s) ,,5,1,15,2,25, time (s) ,,5,1,15,2,25, time (s) ,,5,1,15,2,25,3-1 a) b) c) d) -2 time (s) Figure 6: Responses of acceleration in the operation of the Portable Hardness Tester in: a) low density foam; b) neoprene; c) aluminum plate; d) Goupia glabra wood 4 CONCLUSIONS From the experimental program the following main conclusions can be established: Portable Hardness Tester presented reliable response in the separation of the best from the worst wood species groups; From the various energy levels of indentation studied using Portable Hardness Tester lower

7 energies (,98 J to 1,96 J) revealed the best correlation of hardness strength to Janka hardness; The determination coefficients (R 2 ) of the regression between E1, E2, E3 and E4 hardness and Janka hardness exceeded.8, which may be considered very good for this purpose; The equipment demonstrated acceptable level of accuracy to classify wood specimens. Evaluating specimens in the upper bands of Janka hardness (f H 5 MPa and 4 MPa f H <5 MPa) the inclusion errors obtained using E2 hardness were 4.5% and 13.3%, respectively. When classification was done using E3 hardness, the inclusion errors were respectively 6.1% and 16.6% for the bands reported; The Portable Hardness Tester instrumented with piezo-resistive accelerometer was sensitive to differences in the response to an impact promoted in materials with different viscoelastic properties. Científico UNESP Faculdade de Ciências Agronômicas, Universidade Estadual Paulista Botucatu, Janeiro, 25. [8] ASSOCIAÇÃO BRASILEIRA DE NORMAS TÉCNICAS. NBR Dormente de madeira - requisitos e métodos de ensaio. Rio de Janeiro, ABNT, 25, 18 p. ACKNOWLEDGEMENTS The authors express their gratitude for the financial support given by FAPESP Sao Paulo State Agency for the development of this project (8/8414-). REFERENCES [1] Anuário estatístico dos transportes terrestres /principal.asp [2] INSTITUTO BRASILEIRO do MEIO AMBIENTE e dos RECURSOS RENOVÁVEIS. Amostragem e propriedades físico-mecanicas de madeiras amazônicas. Brasília: IBAMA, 1993, Coleção Meio Ambiente Serie estudo floresta, nº 1. [3] COLENCI, R.A. Qualificação mecânica de madeiras para uso como dormente ferroviário. Botucatu, UNESP, 22, 9 p. Dissertação (Mestrado em Agronomia Energia na Agricultura) Faculdade de Ciências Agronômicas, Universidade Estadual Paulista, 22. [4] BALLARIN, A.W.; COLENCI, R.A. Desenvolvimento de equipamento para avaliação em campo da dureza de madeiras para dormente ferroviário In: 1º Seminário sobre a aplicação de ensaios não-destrutivos em madeira e materiais à base de madeira, pages 1-6, 26 [5] COLENCI, R.A. Desenvolvimento de equipamento para avaliação em campo da dureza de madeiras para dormente ferroviário. Botucatu, UNESP, 26, 83 p. Tese (Doutorado em Agronomia Energia na Agricultura) - Faculdade de Ciências Agronômicas, Universidade Estadual Paulista, 26 [6] BEKTAS, I., ALMA, M.H., AS, N. Determination of the relationships between Brinell and Janka hardness of eastern beech (Fagus orientalis LIPSKY). Forest Products Journal, 51(11/12):.84-87, 21. [7] DAL POGETTO, M.H.F.A, Ensaios de Dureza Paralela e Normal às Fibras da Madeira, Relatório

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