Flexural properties of four fast-growing eucalypts woods deteriorated by three different field tests

Durability is a wood characteristic determined by several factors, making it difficult to investigate the service life of pieces designated for outdoor use. In this study, the decaying of juvenile and adult woods of four fast-growing eucalypts from southern Brazil subjected to three different exposure environments was monitored through mechanical properties (flexural test). The study material was obtained from adult trees of Eucalyptus botryoides, Corymbia citriodora, Eucalyptus paniculata and Eucalyptus tereticornis. Field tests were conducted in the city of Piratini, southern Brazil, and samplings were carried out during 540 days of experiment. Comparing the four eucalypts, the decreasing order of biological resistance was: Eucalyptus tereticornis, Corymbia citriodora, Eucalyptus paniculata and Eucalyptus botryoides. The mature wood showed greater and more stable physical-mechanical properties than juvenile wood.


Introduction
The current scenario of the global forestry market has a dwindling number of mature forests, due to the growing demand for wood from fastgrowing species.Overall, assessing the culture of the latter species, Brazilian and foreign forestry producers have obtained greater financial benefits from the use of species that allow shorter crop cycles.As such, the attractiveness of plantations of species of the genera Eucalyptus and Corymbia has grown.
In addition to the above factors, eucalypts wood has numerous other advantages, such as those related to its planting: high adaptability and productivity, high resistance to pests and diseases, easy hybridization, consolidated management culture, and tolerance to stress conditions.There are also positive aspects related to the wood, exemplified by: high homogeneity properties, good mechanical properties and the possibility of using their oils in alternative industries.
However, its rapid physiological activity gives rise to a chemical composition and anatomical structure that makes it highly resistant to natural wood degrading agents.In this context, it is known that the wood degradation process is a very complex mechanism, given that until today it has not been possible to adjust mathematical models to predict this behavior effectively (Carvalho, Canilha, Ferraz, & Milagres, 2009).For this reason, several laboratory studies have been directed to evaluating and monitoring the technological properties of wood when subject to deterioration attributed to biological (Negrão et al., 2014;Thybring, 2013)  Additionally, we proposed three types of exposure environments.The first was an outdoor field, where the samples remained fully exposed to weathering agents as the ground had almost nonexistent vegetation.The second field was installed in a partially flooded area, where the specimens were partially immersed in a water slide with a height of approximately 2 cm.The last field test was conducted within a homogeneous plantation of Pinus elliottii, wherein during the test the trees were between 5 and 6 years old, with an average height of about 8 m.
A total of 864 samples were subjected to field tests, and the other 72 were kept during the experiment as control samples.To monitor the decay mechanism, three samples of each reference (combination of the factors species and wood) were taken from each field every 45 days for a total of 540 days, totaling 12 withdrawals.

Mass loss measurements
For this, we utilized the data obtained by the weighing carried out before and after the 540 days of field tests, considering the aforementioned conditioning in terms of stabilization of the moisture content (12%).Given this data, we used the equation 1 to calculate the percentage mass loss (Ml) of the tested samples.where: Ml= mass loss (%); im= initial mass (g); fm= final mass (g).

Flexural tests
Through the three point flexural test, the modulus of elasticity (E f ) and flexural strength (σ f ) were evaluated.For the tests, we used a electromechanical and informative universal testing machine, EMIC brand with nominal capacity of 300 kN.As a guideline for testing, the product software was configured in accordance with the regulatory requirements of D143-94 procedure of American Society for Testing and Materials (ASTM, 2014).
After the tests, the moisture content was determined by the gravimetric method, and the mechanical parameters of stiffness (E f ) and strength (σ f ) were corrected to standard moisture content (12%), using the equations provided in the NBR 7190 of Associação Brasileira De Normas Técnicas (ABNT, 1997).

Brittleness
After the mechanical tests, the graphs of stress vs. strain were exported from the software and analyzed according to the methodology described by Phuong, Shida, and Saito (2007).Thus, weighing up the respective areas of the elastic and plastic regions of the analyzed charts, Brittleness was calculated for each sample by the ratio between their resilience and toughness (Equation ( 2)).

Results and discussion
The results for the mechanical parameters obtained by multifactorial ANOVA indicate a similarity in the changes occurring due to the effects of the factors proposed for this study (Specie, Type of wood, Exposure environment and Exposure time).Thus, it appears that the three mechanical parameters (E f , σ f and Brittleness) were sensitive to the factors Specie, Type of wood and Exposure time.However, the effect of Type of field was not statistically significant.Finally, the mass loss was significantly affected for all the factors shown in Table 1.Confronting the statistical parameters obtained by the multifactorial ANOVA, we note that the mass loss was the most sensitive and effective parameter to express the deterioration occurred in the field tests because it was the only property able to differentiate the depreciation mechanism occurring in samples of the three different exposure environments.This statement is an endorsement of the findings of similar previous studies (Curling, Clausen, & Winandy, 2002;Delucis et al., 2016;Råberg et al., 2005).Although it does not necessarily mean that the evaluation of other technological properties of the wood would not contribute interesting additional information.
According to previous studies, from 10% mass loss onwards the wood samples exposed in the field    According with Ali et al. (2011), decreased levels of physical and mechanical properties are due to the weakening of the cell wall of the wood caused by the action of xylophagous fungus.In a study examining three fast-growing eucalypts woods subjected to field tests, Mattos et al. (2014) attributed the decrease of physical and mechanical properties of wood to the decrease in cellulose and hemicellulose levels.
As previously mentioned, according to other studies, wood decayed in a field test becomes mechanically compromised, with a mass loss of approximately 10% (Curling et al., 2002;Venäläinen et al., 2014).This statement is divergent with the results obtained in this study, since the levels of mechanical properties for the woods with higher mass loss (> 10%) were within the same range as the other samples in the study (samples with Ml < 10%).Thus, regardless of the exposure environment and the species, at the end of the 540 days of exposure the average E f and σ f levels for all the combinations of factors studied were above 7,000 and 60 MPa, respectively.
In the comparison between the four species of eucalypts, it appears that wood of C. citriodora showed the highest levels of mechanical properties when unexposed.Nevertheless, when comparing the samples in natura and those collected after 540 days of experiment, it appears that such wood depreciated more than E. tereticornis wood, and the percentage of decrease for the E f and σ f was 33.88 and 30.16% for E. tereticornis and 37.06 and 36.17% for C. citriodora, respectively.
This behavior confirms the results obtained from the analysis of mass loss, confirming the superiority of the E. tereticornis wood with respect to biological resistance, followed in decreasing order by C. citriodora, E. paniculata and E. botryoides.
In analyzing the results obtained by Santos, Cademartori, Prado, Gatto, and Labidi (2014), about the chemical properties of the same four eucalypts woods used in this study, selected according to the same methods as this study, it appears that the extractives content does not explain the durability of the E. tereticornis wood when compared to the others eucalypts.This is because the authors reported a mean of 1.1 ± 0.3% for E. tereticornis and higher levels for E. paniculata and C. citriodora (2.9 ± 0.6% and 4.4 ± 0.4%, respectively) (Santos et al., 2014).Thus, possibly the greater durability of E. tereticornis wood compared to others eucalypts is due to the chemical constitution of their extractive compounds and not the quantity thereof.
Regarding the comparison between the mechanical properties of juvenile and mature woods of the four eucalypts studied, similar to the results seen in the mass loss of the samples, when compared to the juvenile wood, the mature wood showed higher levels and chronologically more stable structural stiffness (E f ) and strength (σ f ).The variability of the Brittleness of the two mentioned woods indicates that, compared to the juvenile wood, the mature wood showed greater absorption capacity without permanent deformation.
This result, taken to a practical situation of the use of eucalypts wood, indicates that depending on the durability required for a particular piece of solid wood designated to an outdoor environment, it is necessary to use longer harvesting cycles in order to achieve adult trees, i.e., individual trees that have a significant growth of mature wood in their trunks.Another possibility is the use of grinding techniques according to the formation periods for each of the two woods, in order to suppress or enhance the growth at a particular time of planting.In this context, according to the literature, in general, fastgrowing species from Eucalyptus and Corymbia genres planted on Brazilian soil begin to form mature wood in its constitution at around 20 years of age (Lara Palma, Leonello, & Ballarin, 2010;Ramos et al., 2011).

Conclusion
E. tereticornis was the most resistant wood to decay, followed by (in decreasing order of decay resistance): C. citriodora, E. paniculata and E. botryoides.
Mature wood showed greater durability than juvenile wood.
Besides the mass loss results, the flexural properties provide highly relevant information, because they differentiate wood samples by species and types.
In general, the loss of flexural properties were double the mass loss for the E. botryoides and E. paniculata woods, and three times for the C. citriodora and E. tereticornis woods.
Based on mass loss, the flooded field enabled the most suitable environmental for wood-decaying agents.

Figure
Figure 2. Average Similarly to the Figure 2, about the mass and 5 show mechanical pro exposure perio proves the clos mechanical pro (Ali Uetiman Mattos, Cadem 2014).

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Figure 3. Aver according to the Comparing collected at the m.Technology

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Figure 4.A according to t

Table 1 .
F and p values of multifactorial ANOVA performed for the physicomechanical parameters of wood subjected to field tests.