Tested specimens presented results shown in Table 1.
Table 1
Number of cycles and resulting MOE for each of the tests performed on the specimens.
| | MOE (MPa) | | |
Nº of cyles | Peltogyne sp. 1 | Peltogyne sp. 2 | Peltogyne sp. 3 | Peltogyne sp. 4 |
0 | 22,491 | 24,878 | 25,131 | 24,864 |
100,000 | 21,408 | 24,687 | 24,607 | 24,041 |
200,000 | 20,451 | 24,045 | 24,340 | 25,127 |
300,000 | 20,421 | 24,465 | 24,774 | 25,348 |
Initially, the mean values of the reference MOE (with 0 cycles applied) were between 22,490 and 25,130 being, Peltogyne sp. 1 = 22,491, Peltogyne sp. 2 = 24,878, Peltogyne sp. 3 = 25,131 and Peltogyne sp. 4 = 24,864.
Upon reaching 100,000 cycles, the specimens showed a decrease in the MOE value, being 4.82% over the reference MOE value for the Peltogyne sp. 1 specimen, 0.77% for the Peltogyne sp. 2, 2.09% for Peltogyne sp. 3 and 3.31% for Peltogyne sp. 4.
Upon reaching 200,000 cycles, three specimens (1, 2 and 3) continued to show a decrease in the MOE value, although a little lower than before, being 4.47% over the MOE value of 100,000 cycles for the Peltogyne sp. 1 specimen, 2.60% for the Peltogyne sp. 2 and 1.09% for the Peltogyne sp. 3. In this test for 200,000 cycles, the Peltogyne sp. 4 specimen behaved in an adverse way to the other specimens showing 4.52% increase over the MOE value of this specimen when tested at 100,000 cycles.
With 300,000 cycles completed, the specimens showed the following MOE values, Peltogyne sp. 1 = 20,421, Peltogyne sp. = 24,465, Peltogyne sp. = 24,774 and Peltogyne sp. = 25,348. Two of the MOE values, Peltogyne sp. 2 and Peltogyne sp. 3, showed an increase in relation to the 200,000 cycles values and Peltogyne sp. 4 showed an increase in relation to all values analyzed since the reference test, while Peltogyne sp. 1 showed a decrease in relation to the value of 200,000 cycles and also all other values analyzed so far.
In graph 01 (Fig. 14) it is possible to notice the oscillation of the MOE values throughout the tests and all the results obtained for the specimens 1, 2, 3 and 4.
Observing the numbers in isolation, there is a tendency to conclude that the mechanical properties of wood are increasing and, therefore, fatigue does not interfere negatively in these properties. This is a wrong analysis since this increase in MOE has already been observed by other authors and at the end of the studies it is concluded that fatigue generates a reduction in the strength and stiffness properties of the material.
The effect of cyclic loading tends to reduce the void spaces within the material, generating compaction (Gardete 2006), this compaction is responsible for greater deposition of cellulose in the cell wall which leads to an increase in the stiffness coefficient (Guimarães 2012).
In Carvalho (2022), who studies fatigue applied to solid wood, it is also possible to observe an increase in MOE and MOR at the beginning of the fatigue tests for three of the five species analyzed, these three species Caixeta (Tabebuia Casinoides), Cedroarana (Cedrelinga catenaeformis) and Cambará (Erisma uncinatum). Purpleheart (Peltogyne sp.) and Tatajuba (Bagassa guianensis) were the species that did not present this increase in MOE value in any of the analysis stages. It is important to analyze that the three species that presented a behavior similar to that observed in this article have lower density and strength, which makes them suffer the action of fatigue earlier than the other two woods (Purpleheart and Tatajuba) that have greater strength. The study by Carvalho (2022) used 450, 4,500 and 45,000 cycles, a value sufficient to affect the structure of wood species with less strength to the point that their structure is compacted and presents this increase in property. It is also worth mentioning that the increase in MOE observed between 0 and 450 cycles was considered statistically insignificant since for these three wood species the MOE values for 0 and 450 cycles were classified as A in the Tukey test performed. Thus, making a broader analysis of the study, it is possible to reach some conclusions such as: woods with lower density and strength undergo compaction more quickly during the application of the cycles, while denser and resistant woods such as Peltogyne sp. also present compaction and increase in properties, but with a greater number of cycles applied.
Guimarães (2012) studied the effect of cyclic loading on flexion and ultrasound properties in wood from four forest species, named Dipteryx odorata (Aubl.) Willd, known as Cumaru (ATIBT 1982; BSI 1991) and Pouteria guianensis (Aubl.), high density and Cedrelinga catenaeformis (Ducke) Ducke, known in Brazil as Cedroarana and in other countries as Tornillo (ATIBT 1982; BSI 1991) and Tectona grandis (L. f), known in Brazil as Teca and in other countries as Teak (BSI 1991), both low density. In this study, he observed an increase in the values of MOE and MOR for 5 of the 8 analyzes performed and also an increase for wave propagation speed and for elastic constant in all analyzes performed for both samples subjected to 40,000 and 100,000 cycles. Guimarães (2012) cites the possibility of reorganizing internal structures, generating an increase in specific mass of a small region of the material. No deterioration of the fibers was observed for the applied conditions, since the mechanical properties did not present a significant difference, that is, there was no reduction in the strength of the woods, which indicates fibers in stable conditions.
The numerical analysis alone does not show the integrity of the results, it was observed throughout the tests an appearance of cracks in the lower central part, that is, in the region most tensioned during the bending request of the Peltogyne sp. 1 and Peltogyne sp. 3 specimens.
With the application of time and more stresses on the piece, the crack can increase to the point of generating rupture and collapse of the structure, so the appearance of the cracks was a determining factor to confirm the suspicions regarding the weakening of the wood due to fatigue of the material and consequently the commitment of structural parts exposed over time to the effect of fatigue.
In order to better understand specimen’s behavior, Peltogyne sp. microscopy images were made, Figs. 16 and 17, allowing the anatomical analysis of the wood structures, since it has a very complex microstructure in face of its organic constitution.
In Figs. 16 and 17 it is possible to see the fibers and rays of Peltogyne sp.. Rays are homocellular, as they are formed exclusively by procumbent cells. rays are also multiseriate, as they are formed by groups of 3 or more rows of cells positioned one on top of the other, which can also be seen in Fig. 17; the rays of this species are not very abundant (4–7 per linear mm) and small to tall (98–489µm in height); with absent stratification, that is, the rays are not aligned with each other, which can be seen in image 16. The fibers of this species are libriform, nomenclature given to its continuous and pointed format, with simple pits; fibers are thick-walled; small lumen, that is, it has a circumference of small diameter or an internal area of small proportions in the inner part of the fiber (IPT 1983; IPT 1989; Santini Junior, Florsheim and Tommasiello Filho 2021).
This wood has prismatic crystals present and abundant in crystalline series in the axial parenchyma cells and fibers, a factor that influences its properties and behavior against mechanical stresses (Santini Junior, Florsheim and Tommasiello Filho 2021). Because it grows more slowly than other species, of Peltogyne sp.'s growth rings are closely spaced and marginal parenchyma lines are percentage larger in Peltogyne sp.'s structure than in faster-growing species. This greater amount of parenchyma in the region may be one of the factors that explains the behavior observed during the tests. Another point to be considered is the fact that the wood growth layers are positioned or not in the specimen perpendicularly to the applied force, which increases the probability of slippage between layers, which causes greater loss of stiffness in the specimens.
As mentioned at the beginning of this work, the focus was to observe the behavior of a species of high density wood, but for a broader and clearer understanding of this phenomenon, it is necessary to study with the application of more request cycles to observe if the break will happen and how many cycles will it take for that. Furthermore, it is interesting to study wood species with different densities and mechanical characteristics, allowing a comparison of the microscopic structure and the behavior observed for each structure.