Effects of Wood Drying Temperatures on the Reduction in Mechanical Properties of Japanese Cedar (Cryptomeria japonica D. Don) Perpendicular to Grain

Abstract

Wood drying is often accompanied by changes in mechanical properties due to external thermal energy. This study examined the influences of drying temperature on the mechanical properties of Japanese cedar based on the partial compression properties and bending properties. Two types of longitudinal specimens with quarter grain on both lateral surfaces were prepared under green conditions, followed by subsequent drying under each drying temperature (20 °C, 40 °C, 60 °C, 80 °C, and 100 °C). Then, the partial compression and bending tests were performed using the respective specimens. Young’s modulus perpendicular to grain, yield stress, and compressive strength obtained by the partial compressive test were highest for wood dried at 20 °C. It was considered that the decrease in mechanical properties was attributed to the thermal influences during drying at 100 °C and other factors such as compressive residual stress and cell walls collapsing at lower temperatures. The strain energy suggested that the effects of drying temperature became apparent, especially in the plastic region during loading in the direction perpendicular to the wood fiber. Bending properties showed little influence on drying temperatures compared to the partial compressive properties, whereas the fractures found under the loading point of the wood dried at 100 °C suggested a reduction in resistance to shear forces.

1. Introduction

Wood, formed by growing trees, has long been used as various components in wooden construction [1]. Coniferous species, such as cedar and cypress, with a straight tree shape and ease of woodworking, are traditionally used for the main components of wooden construction, such as beams and columns, which support their own weight and external stresses. It is well known that living trees contain a considerable amount of sap as water [2]. This condition with high moisture content is essential for maintaining the life of trees; however, it causes wood-specific defects such as rotting and warping in the case of timber production from felled trees. Therefore, moisture in wood must be appropriately removed before use as building materials to avoid unexpected defects.

Drying, an inevitable process of using wood resources for timber productions, often accompanies some defects caused by the loss of moisture content in wood cell walls, such as the surface check, internal check, and discoloration [3,4,5,6,7]. These defects, often caused by improper drying operations, spoil the appearance and reduce yields and product value. Thus, appropriate methods for moisture reduction are adopted based on each intended use of wood, e.g., structural timber, furniture, and so forth [1,2].

Moisture in wood, which is closely related to drying, is also known to affect the physical and mechanical properties of wood, particularly below fiber-saturated conditions [8]. This relationship has long been one of the concerns in wood science, and many researchers have focused on the influences of moisture on the wood properties [9], as well as the influences of heating, which changes the properties of wood [10,11]. In timber production, kiln drying is widely performed based on the recommended drying schedule to effectively remove moisture from wood and reduce drying time [12,13], and conventional fossil fuel-based drying methods account for more than 80% of wood production [14,15]. Therefore, understanding the effects of heating is essential in wood drying, and many studies have been performed on the effects of heating temperature on the mechanical properties of wood [10,11,12,16]. It should be noted here that most of the previous studies have focused on the longitudinal rather than the lateral properties of wood exhibiting anisotropic behavior, and few studies provided detailed information about the influences of drying temperature on the mechanical properties of wood. Since timber production is often subjected to lateral mechanical stresses, such as the jointing part between components, the contact region to the metal fastener, and the surface of floor materials, further investigation on the effects of drying conditions is required. Thus, this study aimed to understand the effects of drying temperature on the mechanical properties of wood perpendicular to the wood grain using Japanese cedar (Cryptomeria japonica D. Don), which is the most widely planted and used tree species in Japan and is characterized by its straight grain [17]. The dried wood, conditioned at various temperatures, was examined through mechanical tests.

2. Materials and Methods

2.1. Materials

The specimens studied were prepared from the sapwood of Japanese cedar (Cryptomeria japonica D. Don) from Ibaraki Prefecture, Japan. Two longitudinal specimens with quarter grain on both lateral surfaces were sawn under green conditions. The dimensions were 10 × 10 × 150 mm (radial × tangential × longitudinal) for the partial compression test and 5 × 5 × 110 mm (radial × tangential × longitudinal) for the bending test. The oven-dried density was 0.38 ± 0.03 g/cm³ (mean ± SD). The average width of the annual ring was 1.5 ± 0.3 mm. Then, all specimens were soaked in water before moisture reduction to ensure that specimens for each drying temperature had the same initial moisture condition.

2.2. Drying Conditions

Figure 1 shows the appearance of wood specimens during drying. Twenty specimens for each material test were stacked in an airtight container (TLO-60, Asvel Co., Ltd., Yamatokoriyama, Japan). The container with specimens was placed in a temperature-controlled room at 20 °C or an electric oven at 40 °C, 60 °C, 80 °C, and 100 °C to keep the temperature constant. A saturated salt solution was also placed in the container to ensure constant relative humidity until the specimen reached the equilibrium moisture content at each temperature.

Table 1. Experimental conditions for drying specimens.

Drying Temperature (°C)	Saturated Solution	Relative Humidity (%)	Drying Period (Days)
20	NaBr	58	132
40	KI	67	52
60	NaCl	75	11
80	KCl	80	10
100	Na2CO3	82	9

shows the experimental conditions for drying the specimens in this study. Saturated solutions of NaBr, KI, NaCl, KCl, and Na₂CO₃ were used under each temperature of 20 °C, 40 °C, 60 °C, 80 °C, and 100 °C, respectively. Relative humidity was retained at approximately 58, 67, 75, 80, and 82% under each temperature condition, respectively [8,18]. If these conditions persist for a long time, the moisture contents will eventually become constant at approximately 9–11% as the equilibrium moisture content, respectively [19]. The weight of the specimens was measured once a day to estimate moisture content and determine the end of drying. Drying under each condition took 132, 52, 11, 10, and 9 days, respectively.

The specimens were then taken out of the container after confirming that the weight of each specimen became almost constant. Subsequently, all specimens were stored in another container with a saturated salt solution of NH₄NO₃ and placed in a temperature-controlled room at 20 °C (±1 °C), which enabled the relative humidity to be kept at approximately 66% [8,18]. The moisture content of the specimens will eventually become constant at approximately 12% as the equilibrium moisture content [19]. All specimens remained in this state for a month and then were placed in an experimental laboratory with the apparatus of the material tester. The average moisture content of the specimens was approximately 13% before each material test in this study.

2.3. Partial Compression Test

The partial compression test was performed based on the conditions specified below by the test methods for wood [20] using twenty pieces of wood specimens conditioned at each drying temperature. This standard follows the guidelines specified by ISO 13061-6:2014 [21]. In this test, wood specimens were placed on the surface of a universal material testing machine (SV-201NA, Imada Seisakusho Co., Toyohashi, Japan) with the bark side up. A block of stainless steel (SUS304) with a 10 mm width was then placed on the tangential section of the specimen to apply a partial compressive load to the contact area of 100 mm². The compressive load was applied through a jig with a free-end attached to the load cell (TCLZ-2KNA, Tokyo Measuring Instruments Laboratory Co., Ltd., Tokyo, Japan), and a displacement rate of 1 mm/min was applied to the specimen. A data logger (PCD-320 A, Kyowa Electronic Instruments Co., Ltd., Tokyo, Japan) was used to collect and output data on temporal changes in load and displacement. Data collections were performed every second until the stainless-steel block indented the specimen up to 1.5 mm. Young’s modulus in partial compression perpendicular to grain, yield stress, and compressive strength were obtained based on the data. Figure 2 shows examples of each value determined in this study. The yield stress was determined based on the intersection of two tangent lines of the stress–strain curve [22,23], one in the elastic region and the other in the plateau region, because of the difficulty in finding a clear yield point in the curve during partial compression of wood [24]. This study also evaluated compressive strength at 5% strain because most specimens presented an increasing trend with strain progress, as shown in Figure 2a. This is because in porous materials, including wood, compression perpendicular to the wood fiber direction usually involves the densification of the cell wall structure [25]. In addition, the amount of mechanical work shown in Figure 2b was also evaluated based on integrated values of the load–displacement curve as the strain energy [26]. The integrated value for strain energy was calculated based on the load–displacement curve ranging from the starting point to each strain condition: 5% strain and maximum strain.

After the partial compression test, 2 cm from the end of all specimens was cut to determine the equilibrium moisture content of the specimens. All specimens were then conditioned in an airtight container with a saturated salt solution of NaCl and placed in a temperature-controlled room at 20 °C to keep the relative humidity at approximately 76% [8,18]. The moisture content of the specimens will eventually become constant at approximately 14%, which is the equilibrium moisture content [19]. All specimens remained in this state for a month, and then the weight of all small specimens was measured using an electric balance (AP124W, Shimadzu Corp., Kyoto, Japan). The equilibrium moisture content of the specimens dried at each temperature was calculated based on respective weights before and after oven drying at 105 °C for 24 h as follows:

$$U_{\mathrm{eq}}={\displaystyle \frac{W_{\mathrm{eq}}-W_0}{W_0}}\times 100 (\%),$$

where, U_eq, W_eq, and W₀ are equilibrium moisture content, constant weight after conditioning, and oven-dried weight, respectively.

2.4. Bending Test

The bending test, often adopted to evaluate the longitudinal mechanical properties of wood, was performed by the manual for the strength test of structural timber to examine the effects of drying temperature using the abovementioned apparatus [27]. In this study, a four-point bending test was performed using eighteen pieces of wood specimens conditioned at each drying temperature to obtain the modulus of elasticity (MOE) and modulus of rupture (MOR) based on the same test methods for wood mentioned above [20,21]. In this test, wood specimens were placed on the universal material testing machine mentioned above with the bark side up. The span and distance between loading points were 90 and 30 mm, respectively. The displacement rate and data collection conditions were the same as for the partial compression test: 1 mm/min and every second. The strain energy was calculated based on the integrated value of the load-deflection curve from the starting point to the maximum deflection. In the bending test, the maximum deflection was up to 15 mm because of the shape of the jig. Thus, some specimens without fracture were found after 15 mm deflection. The fracture site of the specimens was also classified into three types after the bending test: unbroken, fracture under the load point, where the shearing force became evident, and fracture among load points with only bending stress.

3. Results and Discussion

3.1. Effects of Drying Temperature on Partial Compressive Properties

Figure 3 shows the results of the partial compression test. The specimens dried at 20 °C showed higher values than the other drying temperatures except strain energy. Comparing the results dried at 100 °C with those of 20 °C, Young’s modules perpendicular to grain, yield stress, and compressive strength decreased by approximately 20–30%, as shown in Figure 3a–c. Significant differences were found in each comparison (p < 0.001). Figure 4 shows the results of the equilibrium moisture content of the specimens dried at each temperature. Specimens dried at 20 °C had slightly lower values than at other temperatures, but the difference was at most 1%. Previous studies suggested that the strength performance of wood increases in proportion to the decrease in moisture content below the fiber-saturated point [28]; a 1% decrease in moisture content results in a 1.3% increase in the case of Young’s modulus perpendicular to the grain. In this study, the average values of Young’s modulus perpendicular to grain dried at 20 °C and 100 °C were 2.1 GPa and 1.7 GPa, respectively, with a difference of about 20%. Therefore, the reduction in mechanical properties caused by the increases in drying temperature shown in Figure 3a–c cannot be explained solely by the influence of differences in the moisture content. The effect of excessive heating on wood is also known to be one factor that reduces the mechanical properties of wood [10]. This can roughly explain the decreases in mechanical properties dried at 80 °C and 100 °C, but the results dried at lower temperatures remain unclear. In this study, two other factors can potentially change the mechanical properties of dried wood. The first is that drying causes residual stresses in the wood specimen. That is, the surface of dry wood is usually subjected to compressive stress due to the subsequent drying shrinkage of the inner part of the wood [29]. Such stress acting on the surface of dry wood is stored in the wood itself [30]. Thus, it is difficult to detect, unlike the surface strain during the early drying stages [31,32]. Although detailed information, such as the effect of drying temperature on the degree of mechanical reduction, is still unknown, it is natural to consider that these unreleased residual stresses are responsible for the reduced compression properties of the dry wood. The second is that wood drying occasionally causes the collapse of wood cell walls under inappropriate conditions. Generally, drying low-density wood with high moisture content close to water saturation conditions at moderate to high temperatures is accompanied by collapse [2]. Thus, air-drying or kiln-drying at low temperatures below 45 °C or even lower should be adopted to avoid this phenomenon [2]. The Japanese cedar used in this study corresponds to tree species with low density and high moisture content, and all specimens were soaked in water before drying at each temperature. The collapse of wood cell walls may have occurred except in the specimens dried at 20 °C.

On the other hand, the strain energy results shown in Figure 3d never presented such apparent changes compared to other compressive properties. Figure 5 shows the typical result for the stress–strain curve. It was found that the yield point of most specimens was determined at strain more significant than 5%, and the strain energy shown in Figure 3d was mainly based on the mechanical behaviors of the elastic region. Then, two types of strain energy with different integrating ranges were compared, as shown in Figure 6. Each value of strain energy was divided by each strain level for comparison. It was found that strain energy based on the maximum strain showed a decreasing trend, while the strain energy evaluated up to 5% strain did not show significant changes. Higher drying temperatures might have changed the mechanical toughness of wood, especially in the plastic region. The wood dried at higher temperatures became more brittle in the direction perpendicular to the wood fiber.

3.2. Effects of Drying Temperature on Bending Stress

The bending test was performed to understand the effects of drying temperatures compared to partial compression properties. Figure 7 shows the results of the bending test. In contrast to the partial compression tests described above, no significant changes in mechanical properties were found in specimens dried at each temperature. Kuroda [12] presented that the 5% reduction in mechanical strength required at least 30 h of hygro-thermal treatment after the wood temperature reaches 100 °C. This report also estimated the treatment time of hygro-thermal condition in the timber with a cross-section of 132 mm and presented that it requires approximately 30 h for the average moisture content to fall below 25%. Likely, the wood dries more quickly on the surface areas of such timber and in the small specimens used in this study. This is one of the reasons why no significant changes were found in bending properties, which was determined by using small specimens with a cross-section of 5 mm, as shown in Figure 7.

Some previous studies indicated that the mechanical properties of softwood species, not only the bending properties but also shear strength, were reduced by the heating even in the range of 100 to 110 °C [11]. Thus, the fracture site in the bending test was classified to confirm the effects of drying temperature on the resistance to shear force. Figure 8 shows the results of the occurrence ratio of the fracture site in the bending test. It should be noted that most specimens dried at 100 °C fractured during the bending test, and the fractures often occurred under the loading point. This suggests that heating temperatures close to 100 °C reduce the toughness of wood and its resistance to shear forces and that wood drying should be performed under appropriate temperatures.

4. Conclusions

This study investigated the effects of drying temperatures on the mechanical properties of wood using Japanese cedar. This study’s main focus was on the partial compression properties, which have not been the subject of detailed investigations. The following significant findings were drawn based on the results, including four-point bending characteristics:

• Young’s modulus perpendicular to grain, yield stress, and compressive strength obtained by the partial compressive test were highest for wood dried at 20 °C. It was possible to explain that the reduction in mechanical properties found in the drying at 100 °C was due to the effects of excessive heating. On the other hand, the decreases in mechanical properties found in drying at lower temperatures are considered to result from other factors, such as occurrences of compressive residual stresses due to the progress of drying and the collapse of wood cell walls under inappropriate conditions.
• Comparisons of the strain energy of partial compression calculated at each strain level showed that the effects of drying temperature became apparent, especially in the plastic region during loading in the direction perpendicular to the wood fiber.
• Bending properties showed little influence of drying temperatures compared to the partial compressive properties. On the other hand, fractures of the wood dried at 100 °C were often found under the loading point, suggesting reduced resistance to shear forces.

This knowledge acquired in the present study might contribute to a detailed understanding of the relationship between drying temperature and mechanical properties of wood, particularly in the direction perpendicular to the grain, e.g., the effect on strength around the mechanical connection.