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Effect of load cycle times on stiffness characteristic value of big fruit mahogany

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https://www.eduzhai.net International Journal of Materials Engineering 2015, 5(6): 143-146 DOI: 10.5923/j.ijme.20150506.01 Effect of the Number of Load Cycles on the Value of Stiffness Properties of Anadenanthera Macrocarpa Wood Reis Daniel*, Martins Andreia Institute of Architecture and Urbanism, São Carlos Engineering School, University of São Paulo (EESC/USP), São Carlos, Brazil Abstract This paper aims to study, with the aid of an analysis of variance (ANOVA), the influence of the number of load cycles on the value of stiffness properties of wood from Anadenanthera Macrocarpa (Benth.) Brenan. According to Brazilian standard ABNT NBR 7190:1997 (Design of timber structures), tests to obtain the stiffness properties of wood, should involve three loading cycles. However, the possibility of reducing the number of cycles allowing a decrease of the machine operating time and a reduction of electricity used during the tests deserves. Investigation the influence of the use preload cycles, to obtain values of the modulus of the elasticity moduli in compression parallel to the grain (Ec0), in tensile parallel to the grain (Et0), in static bending (Em) and in compression perpendicular to the grain (Ec90) of Anadenanthera Macrocarpa wood specie was studied. The above elastic moduli were measured for two, one and no preload cycle using 12 samples, totalling 48 specimens. The results of the ANOVA analysis revealed statistical equivalence of the values for stiffness properties with and without preloading tests, indicating that it is possible to carry out the tests and obtain values for the elastic moduli with a single charge cycle, allowing savings in time and energy in the operation of the equipment. Keywords Elastic moduli, Loading cycles, Anadenanthera Macrocarpa wood, Analysis of variance 1. Introduction The safe and sustainable use of timber in construction requires that the life cycle and performance of structures can be assessed and effectively planned. The performance’s analysis of any wooden structural element requires knowledge of the mechanical properties of the material used. Moreover, the correct structure dimensioning requires a rigorous analytical approach that takes into account not only the existence of three-dimensional deformation states and tension, but also the orthotropic and heterogeneous nature of wood. In this context, the correct dimensioning of any structural wooden element, as well as with any other material, requires knowledge of relevant mechanical properties, namely the elastic moduli, obtained through experimental tests, as recommended by normative documents [1]. The Brazilian standard ABNT NBR 7190:1997 [2], Annex B, specifies test methods for determination of wood properties for structural projects, with a view to complete characterization of the material. In addition, it defines the test methods for determining the other wood properties. The modulus of elasticity is a wood mechanical property, which allows obtaining other mechanical characteristics such as compression (parallel and perpendicular to the grain), tension parallel to the grain and static bending. The correlation between the modulus of elasticity and other mechanical properties allows each wood type to be classified for structural purposes [3]. In turn, the ABNT NBR 7190:1997 [1] standard recommends two cycles of preloading and final cycle, from which the strength values and corresponding elasticity moduli in compression (parallel and perpendicular to the grain), in static bending and in tensile parallel to the grain are calculated. Morales and Lahr [4] studied the statistical equivalence between the values of elastic moduli obtained with the second and the third load cycle for seven species of timber. The results showed that the elastic moduli of the second and the third loading cycles were statistically equivalent. The present research is intended to investigate the number of preloading cycles required to measure the mechanical properties of wood based on the analysis of variance (ANOVA) methodology. A secondary goal of this work is to verify the possibility to reduce the number of loading cycles as it´s prescribed in Brazilian standard 7190:1997, making it more efficient, without affecting the viability and significance of the results. * Corresponding author: dreis7@gmail.com (Reis Daniel) Published online at https://www.eduzhai.net Copyright © 2015 Scientific & Academic Publishing. All Rights Reserved 2. Materials and Methods The properties of A. Macrocarpa wood used in evaluating the influence of the number of load cycles (1;2;3) 144 Reis Daniel et al.: Effect of the Number of Load Cycles on the Value of Stiffness Properties of Anadenanthera Macrocarpa Wood were the elasticity moduli in compression parallel to grain (Ec0), in tensile parallel to the grain (Et0), in static bending (Em) and in compression perpendicular to the grain (Ec90), obtained according to the assumptions and calculation methods of the Brazilian standard ABNT NBR 7190:1997 [2]. By number of load cycles and property of stiffness investigated were fabricated 12 samples, totalling 48 specimens and were conditioned to moisture content of 12% as established with the standard. To investigate the influence of the number of charging cycles in stiffness properties, the analysis of variance (ANOVA), the Kruskal-Wallis (non-parametric) and the Student-Newman-Keuls multiple comparison test between ranks were used, with the aid of BioEstat® software, version 5.0. The significance level (α) for the ANOVA was set at 5%, and the null hypothesis (H0) formulated was the equivalence of means, implying the non-equivalence between means values as the alternative hypothesis (H1). P-value of the Kruskal-Wallis test below the significance level implies rejecting H0, accepting it otherwise. 2.1. Testing Procedure According to Brazilian Standard NBR 7190:1997, Annex B MPa is given by Equation 2. (2) A preliminary test to rupture should be done performed to estimate the tensile strength of the specimen (fc0,est). Then, the remaining specimens are tested with two cycles of preloading and final cycle. The wood stiffness in the direction parallel to the grain must be determined by its elasticity modulus, obtained in linear stretch of the stress-strain diagram. To this end, the modulus of elasticity (Ec0) in MPa, is determined by the inclination of secant to the stress-stain curve, defined by the points (????10%; ????10%) and (????50%; ????50%), corresponding respectively to 10% and 50% of the conventional strength to compression parallel to the grain, obtained during the test, and can be calculated by Equation 3. (3) As mentioned before, the Brazilian standard NBR 7190 [2] provides the test procedures to determine the physical and mechanical properties of wood. They will be presented succinctly below. 2.1.1. Compression Perpendicular to the Grain 2.1.3. Static Bending The specimens must have prismatic shape with square cross section of 5.0 cm side and length of 115 cm in the direction parallel to the grain. The bending strength (fM) in MPa is given by Equation 4. The specimens must have prismatic shape with square cross section of 5.0 cm side and a length of 10.0 cm. The tests are performed with two cycles of preloading and final cycle. The stiffness to compression perpendicular to the grain (fwc,90 or fc90), corresponds to the specific residual strain of 0.2% obtained during the test performance on the specimen. The wood strength in the direction perpendicular to the grain must be determined by its modulus of elasticity, obtained in linear stretch of the third load cycle from the stress-strain diagram. To this end, the modulus of elasticity (Ec90) in MPa, is determined by the inclination of the secant to the stress-stain curve, defined by the points (????10%; ????10%) e (????50%; ????50%), corresponding respectively to 10% and 50% of the conventional strength to compression perpendicular to the grain, obtained during the test, and can be calculated by Equation 1. (4) Again a preliminary loading test on a reference specimen should be made, and bringing it to rupture and estimate the strength of the specimen (fM,est). Then, the remaining specimens are tested with two cycles of preloading and final cycle. The wood stiffness to static bending is characterized by the modulus of elasticity, obtained in linear stretch of the load-displacement diagram. The modulus of elasticity (EM) in MPa, is calculated by the inclination of the secant to load-displacement curve at mid-span, defined by the points (F10%; V10%) and (F50%; V50%), corresponding respectively to 10% and 50% of the maxim load, obtained during the test, and can be calculated by Equation 5. (5) (1) 2.1.2. Compression Parallel to the Grain The specimens must have prismatic shape with square cross section of 5.0 cm side and length of 15.0 cm. The compressive strength parallel to the grain (fwc,0 ou fc0) in 2.1.4. Tensile Parallel to the Grain To determine the strength and the modulus of elasticity in tension parallel to the grain it must be used a specimen suitable for the effect. The tension strength parallel to grain (fwt,0 or ft0) in MPa is given by Equation 6. International Journal of Materials Engineering 2015, 5(6): 143-146 145 (6) Again a preliminary loading test on a reference specimen must be made bringing it to rupture and estimate the strength of the specimen (ft0,est). Then, the remaining specimens are tested with two cycles of preloading and a final cycle. The modulus of elasticity (Et0) is determined by the inclination of secant to the stress-stain curve, defined by the points ( ???? 10%; ???? 10%) and ( ???? 50%; ???? 50%) corresponding respectively to 10% and 50% of the strength to tension parallel to grain, obtained during the test, and can be determinate by Equation 7. (7) 3. Results and Discussion Tables 1,2 and 3 show the means ( x ), coefficients of variation (Cv) and a minimum (Min) and maximum (Max) values of the stiffness properties for the tests with one, two and three cycles of loading, respectively. Table 1. Results of stiffness properties for the test condition with one cycle only Statistics x Cv (%) Mín Máx Ec0 (MPa) 11945 13 9459 14023 Et0 (MPa) 13901 14 11497 17451 Em (MPa) 11667 10 10893 15144 Ec90 (MPa) 688 17 519 900 Table 2. Results of stiffness properties for the test condition with two cycles Statistics x Cv (%) Mín Máx Ec0 (MPa) 11188 11 9249 13128 Et0 (MPa) 12920 15 10524 16395 Em (MPa) 11457 12 9886 14471 Ec90 (MPa) 644 17 502 825 Table 3. Results of stiffness properties for the test condition with three cycles Statistics x Cv (%) Mín Máx Ec0 (MPa) 11171 11 9198 13204 Et0 (MPa) 12710 15 10620 15998 Em (MPa) 11535 11 9687 14486 Ec90 (MPa) 650 18 502 864 Table 4 shows the results of Kruskal-Wallis ANOVA and multiple comparison test between ranks of Student-Newman-Keuls for grouping the factor of number of load cycles, DF is the degrees of freedom. Equal letters imply treatment with equivalent means. Table 4. Results of Kruskal-Wallis ANOVA for the factor number of loading cycles Stiffness H DF P-valor Ec0 2,7132 2 0,2575 Et0 2,6532 2 0,2654 Em 2,4207 2 0,2446 Ec90 1,0637 2 0,5875 Comparisons of Student-Newman-Keuls 1 Cycle 2 Cycle 3 Cycle A A A A A A A A A A A A According to table 4, the P-values obtained from ANOVA for the four properties of stiffness investigated for wood from A. Macrocarpa were greater than 5%, showing for the number of load cycles investigated here, the equivalence of results. To support the approach of this study, the Brazilian standard NBR 7190:1997 was compared with the European standard CEN EN 408: 2010 [5] that replaced the previous standard EN 408:2003. After analysing the procedures for laboratory tests to determine the physical and mechanical properties of wood through the European standard, it is possible to identify, as the main difference from the Brazilian standard, the omission of preloading cycles to obtain the elastic moduli in compression (normal and parallel to grain), static bending and tension parallel to grain. Gaff and Gaborik [6] studied the influence of the number of loading cycles to obtain the modulus of elasticity to fatigue life in structural elements of solid wood and laminated beech. The results indicated that in case of laminated wood the number of loading cycles to fatigue life does not influence the value of the modulus of elasticity (P=0:20); on the other hand, in solid wood the number of loading cycles influence the determination of the modulus of elasticity (P<0.05). Therefore it appears that although the number of loading cycles does not influence the determination of the strength and stiffness properties of wood, it no longer can be applied to determination of properties in structural material fatigue, with a higher loading cycles. The secondary goal of this work is to verify the possibility of reduce the number of loading cycles as preconized in Brazilian standard. Thus, the equivalency between results from the use of 2 or 3 cycles reveals that it is possible to reduce the number of load cycles without affecting the significance of the results making the standard more efficiency, maintaining its viability. 146 Reis Daniel et al.: Effect of the Number of Load Cycles on the Value of Stiffness Properties of Anadenanthera Macrocarpa Wood 4. Conclusions REFERENCES The results of the analysis of variance revealed equivalence between the stiffness properties of wood from A. Macrocarpa species. For the three levels of load cycles, it can be concluded that the use of preload cycles to obtain the stiffness properties of wood as recommended by the Brazilian standard ABNT NBR 7190:1997, are not necessary, thus contributing to reduce the time perform the tests as well as energy costs in the operation of the equipment. As a confirmation, the results obtained in Morales and Lahr [4] were analysed, as well as the recommendations of European standard EN 408:2010 for proceeding tests to obtain the modulus of elasticity to compression and tension parallel to the grain, in static bending and compression perpendicular to the grain. [1] A. L. Christoforo, S. L. M. R. Filho, A. R. V. Wolensky, A. R. Monteiro, F. A. R. Lahr and M. A. D. Demarzo. Revista Vértice, v. 14, n.2, p. 61-70, 2012. [2] Brazilian Association of Technical Standards (ABNT) NBR 7190. Design of Timber Structures. Brazil, ABNT Press (1997), p.107. [3] A. L. Zangiacomo, A. L. Christoforo and F. A. R. Lahr. Scientia Forestalis, v.41, n. 98, p. 283-291, 2013. [4] E. A. M. Morales and F. A. R. Lahr Revista de Ciencia y Tecnologia. Vol. 13 (2002), p. 19. [5] European Committee for Standardization (CEN) EN 408. Timber Structures – Structural Timber and Glued Laminated Timber – Determination of Some Physical and Mechanical Properties. Brussels 2010. [6] M. Gaff and J. Gáborik J. Bioresources, v.09, n.3, p.4288-4296, 2014.

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