Mechanical performance: longitudinal and transverse direction
Figure 7 shows the longitudinal and transverse compression test results of the plywood with different thicknesses (9 and 12 mm) at different temperatures (25, − 20, − 65, − 110, and − 165 °C). At room temperature (25 °C) and low temperatures (− 20 and − 65 °C), most of the plywoods with 9 and 12 mm thicknesses exhibited an elastic behavior and fractured after reaching the ultimate stress. At cryogenic temperatures (− 110 and − 165 °C), most of the plywoods with 9 and 12 mm thicknesses exhibited a linear section and fractured with little plastic behavior. In summary, the plywood exhibited brittle characteristics in the longitudinal and transverse test directions at cryogenic temperatures (− 110 and − 165 °C).
Figure 8 shows the mean compressive strength and mean elasticity of the plywood specimens at room, low, and cryogenic temperatures based on Fig. 9 and Fig. 10. The calculation criterion for the mean compressive strength and elastic modulus in the longitudinal and transverse directions is as follows. The compressive strength was calculated from the ultimate strength of fracture before or near the yield point and from the strength of brittle fracture, as shown in Fig. 11 [25].
The experimental data were extracted with an error range of ± 10%. From the viewpoint of plate thickness, the compressive strength and elastic modulus of the plywood specimens with 9 and 12 mm thicknesses showed similar trends. With a decrease in temperature from room temperature to cryogenic temperatures, the compressive strength and elasticity increased. It has been reported that this phenomenon is highly influenced by water absorption and the ice crystals present in the wood cells, which absorb moisture at low temperatures. In addition, the adhesion strength increased, showing resistance to deformation at low temperatures [26,27,28].
Mechanical performance: vertical direction
Figure 12 shows the compression test results of the plywood specimens of varying thicknesses (9 and 12 mm) in the vertical fiber direction at different temperatures (25, − 20, − 65, − 110, and − 165 °C). Compared with the plywood with longitudinal and transverse fiber directions, the plywood with vertical fiber direction showed different characteristics. For the plywood with vertical fiber direction, the stress–strain curves show linear elastic and plastic regions at temperatures between 25 and − 110 °C. The yield strength increased with decreasing temperature. However, at − 165 °C, the yield point is hard to distinguish. In addition, the plywood broke immediately without plastic deformation. Thus, the plywood shows brittle characteristics at − 165 °C.
Figure 15 shows the average compressive strength and modulus of elasticity of the plywood specimens in the vertical direction at ambient (room), low, and cryogenic temperatures based on Fig. 13 and Fig. 14. The calculation criterion for the mean compressive strength and elastic modulus in the vertical direction is as follows. In the vertical direction, except at − 165 °C, the offset yield strength is the stress that corresponds to a point at the intersection of a stress–strain curve and a line that is parallel to the specified elastic modulus line. This line is horizontally offset by a predetermined amount. The value of the offset (expressed as a percentage of strain) is 0.2% as defined by ISO 604. The yield strength is similar for the 9- and 12-mm thickness specimens, as shown in Fig. 15a. The 9- and 12-mm thickness specimens showed similar tendencies without significant differences in elastic modulus values. The 12-mm thickness specimen had a slightly higher value than the 9-mm thickness specimen did as temperature decreased. The elasticity modulus of the 9-mm thickness specimen was higher than that of the 12-mm thickness specimen at ambient temperature; however, the difference in elastic modulus between 9- and 12-mm thickness specimens increased with decreasing temperature, as shown in Fig. 15b.
With a decrease in temperature, the stiffness increases because of ice crystal formation below 0 °C. The hardening caused by ice crystal formation increases the elastic modulus, thereby preventing deformation [26, 27]. This is also because of the tendency of the resin to increase in strength as temperature decreases [9]. The 12-mm thickness specimen has more resin and more layers than the 9-mm thickness specimen does. Thus, although the difference between the 9 and 12-mm thickness specimens was small and the tendency was similar, the elastic modulus of the 12-mm thickness specimen was higher than that of the 9-mm thickness specimen at low and cryogenic temperatures, because of an increase in resin strength and ice crystal formation at low temperature. Therefore, in the design approach to Mark-III, 12t plywood should be used near the primary barrier exposed to cryogenic environments, while 9t plywood should be used near the inner hull exposed to ambient temperature. That is, if the 9t and 12t plywoods are designed differently according to the temperatures of the environments to which they are exposed, the stiffness of the structure can be increased.
Macroscopic and microscopic observations
To investigate the effect of test temperature, fiber direction, and thickness on the fracture behavior of the plywood, macroscopic and microscopic analyses were conducted. In the present study, the fracture shapes in mainly the longitudinal and transverse directions were analyzed, because the fracture shapes in the vertical direction were almost the same regardless of temperature and thickness [10].
Figure 16 shows the macroscopic images of the tested plywood specimens. As can be seen, no noticeable macroscopic breakage occurred in the plywood specimens tested at temperatures between 25 and − 20 °C. At temperatures below − 65 °C, a well-marked macroscopic failure is observed with delamination and a transverse crack. Crack progression is clearly visible at cryogenic temperatures (− 110 and − 165 °C). Choi and Sankar reported the fracture toughness of a transverse crack in laminates at cryogenic temperatures and defined the damage progression in laminated composites such as micro-cracks, transverse cracks, delamination, and debonding of the facesheet [25]. The present study shows a similar trend of damage progression at cryogenic temperatures (− 110 and − 165 °C). It is thought that the fracture of the plywood at cryogenic temperatures is mainly due to the thermal contraction phenomenon, which occurred between the fibers with a decrease in temperature [26]. In addition, plywood is a material manufactured from thin layers of wood veneer that are glued to adjacent layers, unlike solid wood. Thus, the contraction of resin at the low temperature causes micro-cracks because of the difference in thermal contraction between resin and wood fiber at low temperature [28]. Moreover, PF resin is reported to have good temperature stability; however, for MUF resin, the extent of cracking is significant due to a reduction in temperature stability [28].
Following the macroscopic observation, microscopic observation was conducted by scanning electron microscopy (SEM) to confirm the crack growth characteristics of the tested samples. The identification of micro-cracks and prediction of cracks are crucial for sandwich-layered structures such as plywood. Figure 17 shows the SEM images of the tested samples. As can be seen in Fig. 17a, crack propagation occurs with a decrease in temperature from 25 to − 65 °C with the initiation of micro-cracks in the transverse direction [29]. As the temperature decreases to − 110 °C, the large thermal contraction causes transverse cracks and delamination (Fig. 17b). This leads to an interfacial crack in the adjacent layer at − 165 °C, as shown in Fig. 17c. It can be said that the phenomenon of micro-crack and interfacial crack formation is a major issue in LNG CCSs along with the risk of cryogenic liquid spillage that can cause the fracture of sandwich structures.
Ultimate strain analysis
The mechanical properties of plywood such as ductility and brittleness were confirmed from the stress–strain curves shown in Fig. 7. Based on this, the ultimate strain of plywood was determined, and is shown in Fig. 18. In the present study, the fracture characteristics of the plywood were determined from its mechanical properties in the longitudinal and transverse directions, as shown in Fig. 18 based on Fig. 7. With a decrease in temperature, a micro-crack occurred due to the thermal contraction effect. With the growth of micro-crack, fracture occurred with crack propagation. Therefore, the extent of crack growth can be determined from the ultimate fracture characteristics.
Figure 18 shows the temperature-dependent ultimate strain behaviors of the 9 and 12-mm thickness plywood specimens in longitudinal and transverse directions. As shown, the ultimate strain increases with a decrease in temperature; however, at − 110 °C, the ultimate strain remained the same or slightly decreased. This indicates that degradation started at a critical temperature of − 110 °C because the ultimate strain is the fracture strain caused by the brittle characteristics of plywood at − 110 °C. As shown in Fig. 17, the crack propagation trends are similar at − 110 °C and − 165 °C. This allowed us to determine the structural characteristics of the laminated wood from the critical temperature of − 110 °C for the mechanical properties.
Thermal expansion coefficient
To investigate the temperature-dependent thermal expansion coefficient of the plywood, TMA was performed in the longitudinal and transverse directions, which are the loading directions. Figure 19 shows the temperature-dependent thermal expansion coefficient of the plywood specimens in the longitudinal and transverse fiber directions. The thermal contraction in the transverse direction was higher than that in the longitudinal direction from 25 to − 110 °C. This is because a transverse crack occurs at the fiber layer with 90° fibers at ambient temperature (25 °C) and low temperatures (− 20 and − 65 °C), as shown in Fig. 17. The thermal contraction is higher in the transverse direction because it has more layers of 90° fibers than the longitudinal direction does. However, the thermal contraction in the transverse direction was similar to that in the longitudinal direction below − 110 °C. As shown in Fig. 16 and Fig. 17, interfacial cracks occur in 0° fibers and transverse cracks occur in 90° fibers at cryogenic temperatures (− 110 °C and − 165 °C). All the layers including 0° and 90° fibers have cracks regardless of the fiber direction. According to these phenomena, at − 110 °C, the fracture shape and thermal contraction are similar. Thus, in the design of LNG CCSs, the use of plywood in the longitudinal direction would be better from the viewpoint of structural safety since the thermal contraction is lower in the longitudinal direction than in the transverse direction.
In Fig. 19, it can be seen that the thermal expansion coefficient converges around a certain temperature. The convergence of the thermal expansion coefficient implies that the material does not shrink or expand anymore; however, breakage is more likely to occur due to external forces. The present study defines the convergence temperature of the curve as the critical temperature at which the transition from ductility to brittleness occurs. This critical temperature is abbreviated as “CT”. From Fig. 19, it is confirmed that the CT of the MUF resin plywood is approximately − 110 °C.
The CT is important because when a structural material is exposed to a cryogenic liquid, the material becomes brittle, which can be dangerous from the viewpoint of structural safety. In other words, the transition to brittleness causes mechanical degradation of plywood, which serves as a strong frame and distributes the local load. In addition, the fiber direction, which is most affected by the CT, should be considered an important design parameter for NO96 plywood box and the plywood wall plate of Mark-III because the thermal contraction is higher in the transverse direction than in the longitudinal direction at ambient and low temperatures. It is important to predict the types of fractures, such as ductile fracture and brittle fracture, of plywood depending on the temperature in the design of LNG CCSs to ensure structural stability. In this study, the CT was determined by both mechanical and thermal analyses, and was found to be − 110 °C for MUF resin plywood.
This implies that the MUF resin plywood requires extra consideration for impacts of over 79 MPa. However, MUF resin plywood can be applied to LNG CCSs because it can sufficiently withstand sloshing impact since its strength is higher than the required strength of 65 MPa at a cryogenic temperature, as specified by DNV GL [16, 30].