The Influence of Cyclic Loading on Ultimate Bending Strength of Beech Solid and Laminated Wood

This work investigates the infl uence of cyclic loading of beech solid and laminated wood of various thicknesses on ultimate bending strength (ultimate fl exural strength) σp during bending in radial direction. For identifi cation of ultimate bending strength, the static bending test with three-point loading was used. The ultimate bending strength was detected on testing samples that were not cyclically loaded and the results were compared with other results obtained from samples that were cyclically loaded. The results of this work show that the infl uence of cyclic loading on values of ultimate bending strength for solid and laminated wood is not signifi cant. Thickness infl uence is a signifi cant factor for both tested materials. The results show that the increase of thickness causes the decrease of ultimate bending strength. In our opinion, the decrease of ultimate bending strength is not caused by the sample thickness but by different sample lengths. With the increase of the sample length, the infl uence of inhomogeneous wood properties also increases.


INTRODUCTION
1. UVOD Lamination is the technology for producing a wood composite material -lamella.This technology is known mainly in the U.S., Canada and Scandinavian countries, but it is usually used in construction (Razumov et al., 2013) and not in the furniture industry.In Slovakia, the lamellae are mainly used for the production of some parts of beds.
Laminated veneer lumber is produced by combining thin wood veneers.Veneers are usually 3 mm thick and made by slicing (Barcík et al., 2011;Glos et al., 2004;Frese and Blaß, 2006).Direction of veneer fi bers of laminated veneer lumber is parallel to the length of the fi nished products.The resulting product is characterized by improved mechanical properties and good dimensional stability compared to solid wood, which is why it is increasingly applied for the production of fi nished products for various purposes.Both hard and soft woods can be used for the production of laminated wood, depending on the needs of the customer.It is also possible to combine different types of wood according to their strength -e.g.outer layers of lamellae composed of softwood for aesthetic appearance and inner layers made from hardwood to achieve higher strength (Gáborík et al., 2001).
The aim of this research was to determine the effect of cyclic loading on strength properties of beech solid and laminated wood.Durability (lifetime) of these products varies considerably, affecting the properties of the material itself.As a result of long term use, the characteristics of furniture components change and over time cease to perform the function for which they were intended (Brutovský, 2013;Gaff, 2003;Gáborík andDudas, 2006, 2008).
Our goal is to expand the knowledge related to certain products, which should largely determine the infl uence of monitored factors on the related characteristics and thus help to produce quality furniture products.

MATERIJAL I METODE
The aim of this experiment was to determine the infl uence of cyclic loading on the ultimate bending strength σ p of beech wood (Fagus sylvatica L.) -solid and laminated wood, when bending in the radial direction.The experiment investigated the changes of monitored characteristics that occurred after 0, 1000, 2000, 3000 cycles.For the purpose of detecting the infl uence of material thickness, tests were performed on samples made of 2 mm thick veneer glued together.The exact categorization of test pieces, for individual sets, is shown in Figure 1 (Maro, 2012;Brutovský, 2013).90 % proportionality limit was chosen so as not to exceed the value due to the diversity of material properties.This action will ensure that all test samples will be loaded so that the limit of elastic area is not exceeded.

Lamelirano drvo
Table 1 shows the results of variance analysis that evaluates the infl uence of the material thickness, number of cycles and their mutual interaction.According to these results, it can be concluded that the infl uence of cyclic loading has no signifi cant effect on the values of ultimate bending strength for laminated beech wood (Fig. 4).
Table 1 also shows that the second test factormaterial thickness, has a signifi cant infl uence on the values of the tested characteristics.Mutual interaction, between the number of cycles and material thickness, also has a signifi cant effect.
However, the infl uence of the test factor, number of loading cycles, on the values of ultimate bending strength can be considered as insignifi cant (Fig. 4).
If material thickness increases, then the values of ultimate bending strength go down (Fig. 5).Statistically the most signifi cant difference occurs at material thicknesses of 6 mm, 10 mm and 18 mm.
Basically the variance analysis that evaluates the infl uence of interaction between the two factors -material thickness and number of cycles on ultimate bending strength (Tab.1), shows that statistically the effect of this interaction of factors is moderately signifi cant (Fig. 6).
Graphs of 95 % confi dence intervals represent the infl uence of material thickness and number of cycles on ultimate bending strength during bending of laminated wood (Fig. 6) and show that the infl uence was proved because of expressive effect of various material thicknesses.The varying thickness of test samples had to comply with the condition, which provides a span of 20•h.Loading rate was set so that the breaking of the test sample occurred during the 1.5 ± 0.5 min from the start of loading.Flexure was measured at the center of the test sample under bending pin to the accuracy of 0.1 mm, and the measured value was recorded together with the corresponding loading measured to the accuracy of 1 % of the measured values (Fig. 2).Loading break was recorded to the accuracy of 1 % of the measured value.
The ultimate bending strength, at three-point bending, was calculated according to Equation 1 Measured and calculated values are converted to ultimate bending strength at 12 % moisture content rounded to 1 MPa according to STN 490115 and Equation 2: Where: σ 12 -ultimate bending strength of wood at 12 % moisture content, MPa, σ w -ultimate bending strength of wood at a certain moisture content, MPa, w -moisture content of the test sample during testing, %, α -moisture correction factor, which is 0.04 for all wood species.

Cyclic loading by bending 2.2. Ciklično opterećenje na savijanje
Cyclic loading of samples was performed by a cycler machine (Fig. 3), which is based on the principle of cyclic bending of test samples by uniaxial (unilateral) loading.The number of cycles was 0, 1000, 2000 and 3000.In preliminary tests, test samples were loaded by static bending in order to detect ultimate strength and proportionality limit, because the test samples were to be loaded to 90 % of proportionality limit  When evaluating the infl uence of test factors on ultimate bending strength of solid wood (Tab.2), the same results were obtained as for laminated wood (Tab.1).The infl uence of the number of cycles was statistically insignifi cant, while the infl uence of material thickness was statistically signifi cant in affecting the values of ultimate bending strength of solid wood.
Statistically, the interaction between the test factors was moderately signifi cant, similarly as with laminated wood.Statistically insignifi cant infl uence of the number of cycles on ultimate bending strength is also proved by the results shown in Figure 7.
The values of ultimate bending strength decrease with the increase of thickness of solid wood as shown in Figure 8.This fi gure also proves that the lowest values of ultimate bending strength were measured at 18 mm material thickness.
The graph of confi dence intervals represents the infl uence of the number of cycles and material thickness on ultimate bending strength during bending of solid wood (Fig. 9).This graph shows that with the increase of material thickness, the values of ultimate bending strength go down.The number of cycles has no statistically signifi cant effect on values of ultimate bending strength.Generally, laminated wood showed the lowest values of ultimate bending strength (Fig. 9).
Measurements prove that solid wood has about 30 MPa higher values of ultimate bending strength than laminated wood.These results (ultimate bending strength goes down with increasing material thickness) also correspond to the results of Stark (1997), who found out that if conditions of slenderness ratio (20 x material thickness) are kept at a level, the values of ultimate bending strength goes down because of increasing of material length.In fact, heterogeneous distribution of properties is higher with the increase of material length.
Other possibilities of modifi cation of these materials should be sought aimed at improving the properties of furniture.

ZAKLJUČAK
Based on the results of this study, the following conclusions and recommendations can be made: -number of loading cycles has no signifi cant effect on values of ultimate bending strength with either test material (solid and laminated wood), -material thickness has a strong infl uence.With the increase of material thickness, the values of ultimate bending strength of both types of material go down signifi cantly, -the interaction of material thickness and number of cycles affects the values of ultimate bending strength.However, Figure 5, 6, 9 and 10 show that this infl uence is caused by material thickness, -generally, when comparing individual results, it can be stated that the values of ultimate bending strength of laminated wood are on average about 30 MPa lower.

Table 1
Basic table of two-factor analysis of variance that evaluates the infl uence of individual factors on the change of ultimate bending strength values of laminated wood Tablica 1. Osnovna tablica analize varijance za ocjenu utjecaja pojedinačnih činitelja na graničnu čvrstoću savijanja lameliranog drva Infl uence of material thickness and number of cycles on ultimate bending strength during bending of laminated wood Slika 6. Utjecaj debljine materijala i broja ciklusa opterećenja na graničnu čvrstoću savijanja uzoraka od lamelirane bukovine

Table 2
Basic table of two-factor analysis of variance that evaluates the infl uence of individual factors on change of ultimate bending strength of solid wood Tablica 2. Osnovna tablica analize varijance za ocjenu utjecaja pojedinačnih činitelja na graničnu čvrstoću savijanja masivne bukovine