Analysis of the tensile and bending strengths of the joints of “Gigantochloa apus” bamboo composite laminated boards with epoxy resin matrix

2.1 Material characteristics

The smeared bamboo used in this study as depicted in Figure 1 was at least 3 years old with an average diameter of 150 mm. Apus bamboo belongs to the genus Gigantochloa which has a dense clump. The scientific name of apus bamboo is Gigantochloa apus. Apus bamboo is also known as tali bamboo, awi tali, or pring tali. In comparison to bamboo apus with twigs, bamboo apus without twigs has superior mechanical properties. In this study, the type of bamboo used as the base material for the composite was rope bamboo, also known as apus bamboo (Gigantochloa apus). Table 1 provides the detailed information on the mechanical characteristics of apus bamboo.

Figure 1

Apus bamboo (Gigantochloa apus).

Composites are made with bamboo fiber which has been arranged as an epoxy resin polymer reinforcement. Epoxy resins listed in Table 2 are suitable for coating ships, oil rigs, storage tanks, and water pipes [19]. Epoxy glue (2 components) is a multi-purpose glue that glues wooden planks to wooden boats. The advantages of epoxy glue are that it is effortless to use, the quality is unquestionable, has strong adhesion, and is seawater resistant. The disadvantages of epoxy glue are that the film tends to be more challenging, has poor elasticity, and has a relatively high price [20].

The influence of the many layers of laminae in the composition of the laminated board has a significant impact, according to the data obtained in previous studies [22]. The more the number of layers of the lamina, the more the pressing strength during the process of making the laminated board. In previous studies, one of the benefits of compressed air is that it minimizes the formation of air cavities caused by the gluing of the epoxy resin, for pressing during the process of making laminated boards that have maximum strength. In this test, the specimen with a compression pressure of 2 MPa has full strength [22].

2.2 Test specimen manufacturing

Apus bamboo laminated joint boards are made in stages. First, 3 year old, 150 mm-diameter bamboo apus from Yogyakarta, Indonesia, are felled. After cutting apus bamboo, the stem must be cut 30 cm before the branch. Bamboo stems are peeled and split into blades. Splitting bamboo yields 30 mm × 20 mm slats. Using a four-sided tool, bamboo slats are laminated. This process yields 1.2, 1.8, and 3 mm thicknesses. Figure 2 shows drying bamboo slats. This ensures that no water remains in the bamboo or bamboo fibers.

Figure 2

Apus bamboo slats.

Aligned bamboo slats are glued with epoxy resin to make apus bamboo laminate board. In this process, bamboo slats are arranged parallel to the direction and according to their thicknesses and glued on top of bamboo slats that have been glued and put together again with epoxy resin until they reach 10 mm. In terms of specific strength, the bamboo slats-prepared laminated board is superior to other methods [23]. Figure 3 shows how Apus bamboo is glued between layers to form a laminate board, then pressed to densify the layers. Figure 4 shows how more glue was used to reduce space risk on a laminated board. The laminated boards are connected using the following methods: butt joints, finger joints, scar joints, tongue and groove joints, and desk joints. After the joints dry and harden, laminated boards are cut to the sizes specified in tensile and bending tests. Then, the surface is smoothed by sanding.

Figure 3

Apus bamboo gluing process.

Figure 4

Laminate board pressing process.

2.3 Specimen variation and mass fraction

This study uses three variations based on the orientation of the angular arrangement of laminated bamboo, as shown in Table 3. The mass fraction of a material is the ratio of its mass to the overall mass of the mixture. In this study, the mass fraction of the material in each specimen is almost similar because each sample has the same number of layers as shown in Table 3.

In this study, the variants of the laminated board joints have five sorts of connections: butt joints, finger joints, scarf joints, tongue and groove joints, and desk joints. In Figure 5, the laminated board is connected right in the center of the test specimen. It is also carried out directly on the laminate junction during the testing procedure to assess the influence of the laminated board connection on the tensile and bending strengths.

Figure 5

Laminate board connection variations: (a) butt joint, (b) scarf joint, (c) finger joint, (d) tongue and groove joint, and (e) desk joint.

2.4 Mechanical testing

2.4.1 Tensile test

Tensile testing was carried out at the Laboratory of Ship Material and Strength at Diponegoro University using the Universal Testing Machine (UTM) Type WEW-1000B as shown in Figure 6. This study used the ASTM D-3039 standard with a specimen size of 250 mm × 25 mm × 10 mm [24]. Five specimens were tested in this test. Tensile tests are carried out to complete the information and basic design of the strength of the material. Based on the tensile test, the following properties can be obtained such as, tensile strength, strain, and modulus of elasticity.

Figure 6

(a) Specimen size based on ASTM D-3039, (b) tensile test specimen, and (c) Universal Testing Machine (UTM) WEW-1000B.

2.4.2 Three-point bending test

The bending test was carried out at the Laboratory of Ship Material and Strength at Diponegoro University using the WEW-1000B Universal Testing Machine (UTM). Four specimens were tested. Based on Figure 7, the test standard used in the parallel bending test is ASTM D-7264 [25], with a specimen size of 130 mm × 13 mm × 10 mm. From the bending test, the following properties are obtained:

(1) σ = 3 F L 2 b d 2 ,

where σ is the bending strength, F is the maximum load (N), L is the length of the support (mm), b is the width of the specimen (mm), and d is the thickness of the model (mm).

Figure 7

(a) Bending test specimen standards according to ASTM D-7264, (b) bending test specimens, and (c) specimen testing on the WEW-1000B UTM.

3.1 Result of tensile test

Tensile test using WEW-1000B UTM has been conducted to measure the maximum load capacity in each joint variations. Tensile strength, strain, and modulus of elasticity are the test results. The maximum load results obtained from the machine can be calculated using the maximum stress formula, where σ = P A 0 , so that the tensile strength of each specimen is obtained, as shown in Table 4.

The data in Table 4 and Figure 8 reveal that the specimens using desk joint type and 7 layers of bamboo produce the highest average tensile strength value of 118.64 MPa, which is 31% stronger than specimens using finger joint type and 3 layers of bamboo which has the lowest average tensile strength of 81.36 MPa.

Figure 8

Comparison of tensile stress of different joint types for (a) three layers, (b) five layers, and (c) seven layers.

The obtained results are consistent with previous research [30] in which the specimen with 7 layers of laminate has the highest tensile test results because the bonds between the fibers are more potent in the specimen with 7 layers and the bamboo slats used are thinner. The mass fraction between the epoxy resin and bamboo is smaller, that is 70:30%, which results in increasing the tensile strength. The area of the desk joint connection has the most expansive area resulting in increased bonding between the joints. In contrast, the specimen with the finger joint connection type with 3 layers is the weakest specimen due to the reduced mass fraction between the bamboo slats and the epoxy resin, which is 65:35%, resulting in reduced bonding. The number of air bubbles still left on the adhesive layer between bamboo joints.

From the test results, we can see that the influence of the type of connection and the number of layers affect the tensile strength of each specimen. Strain can be defined as the ratio of the increase in the object’s length to its initial size. Tensile strain occurs when the rod is pulled. From Table 5, we can see that each specimen subjected to a tensile test experiences an increase in length. The addition of the measurement itself can be used to find the tensile strain, where we can use the formula ε = Δ L L 0 100 % where Δ L is the increase in length, while L 0 is the length of the original specimen.

Data from Table 5 and Figure 9 show that the specimens with 7 layers have the highest average strain value of 1.08%, compared with the specimens with 3 and 5 layers, which experienced a decrease in strain.

Figure 9

Comparison of average strain of different layers of bamboo laminate.

Strain can occur because of the level of elasticity of a test material. The highest strain was obtained by the specimen with 7 layers, and this was because the mass fraction between bamboo and epoxy resin had the highest ratio. This strengthened the bond between the fibers, putting more strain on the laminated board.

If strain dictates flexibility, then the modulus of elasticity reveals a material’s stiffness. The higher the modulus of elasticity, the stiffer the material. After knowing the yield strength and strain values, the modulus of elasticity may be computed. Based on Table 6, it is clear that the specimen with 7 layers has the highest modules elasticity for all joint types. Figure 10 shows the modulus of elasticity distribution for specimens with different number of layers and joint types.

Figure 10

Comparison of the modulus of elasticity of different joint types for (a) 3 layers, (b) 5 layers, and (c) 7 layers.

The modulus of elasticity obtained from Table 6 shows that the desk joint connection with 7 layers has the highest strength value with an average of 2752.03 MPa. In comparison, the finger joint connection with 3 layers has the lowest strength value with an average of 1913.32 MPa. The increase in the modulus of elasticity for each type of connection is influenced by the number of layers in the laminate board arrangement. The more layers there are on the board, the modulus of elasticity increases and is directly proportional.

It can be seen from Figure 11 that there is a fracture in each tensile test specimen, wherein the fractures are located between the joints. Due to axial load, the fracture is initially started at the joint connection of the applied load on the testing specimen, in which the glue/adhesive layer breaks due to lower strength compared to bamboo material. The crack at adhesive layer surface then grows into delamination between the bamboo lamina. The desk joint seen in Figure 11e has the most substantial tensile strength due to its cross-sectional area​​, which is more significant than other types of connection. While the finger joint has the weakest tensile strength because the connection is quite complicated, and this type of connection allows air bubbles to be trapped between the joints of the boards.

Figure 11

Macrography of tensile test specimens: (a) butt joint, (b) scarf joint, (c) finger joint, (d) tongue and groove joint, and (e) desk joint.

3.2 Result of bending test

A bending test is carried out by bending or pressing a test material until it reaches its brittleness limit point. This research was conducted with a 3-point bending test (3-point bending). In this test, we can find the bending strength using the formula σ = 3 F L 2 b d 2 , where F is the maximum load, L is the length of support, b is the width of the specimen, and d is the model’s thickness.

In Table 7 and Figure 12, the results for the bending test show that specimens with a desk joint connection type and 7 layers of bamboo produce the highest average tensile strength value of 475.89 MPa, which is 17% stronger than the specimen with finger joint connection and 3 layers of bamboo and has the lowest average tensile strength of 395.28 MPa.

Figure 12

Comparison of bending strength of different joint types for (a) 3 layers, (b) 5 layers, and (c) 7 layers.

The results obtained follow previous research studies where the highest tensile test results are in the variation with 7 layers because in the specimen with 7 layers, the bonds between the fibers are more robust and the bamboo slats used are thinner. The mass fraction between the epoxy resin and bamboo is smaller at 70:30, resulting in increased tensile strength. The desk joint connection has the most expansive area resulting in increased bonding between the joints. In contrast, the specimen with the finger joint connection type with 3 layers is the weakest because the mass fraction between the bamboo slats and the epoxy resin decreases to 65:35, which results in reduced bonding. Between the fibers, there are a number of air bubbles remaining on the bamboo joints which are glued together using epoxy resin.

As shown in Figure 13, there are similar fractures phenomena in each bending test specimen, wherein the fractures are mainly located between joints. Due to bending load, fracture is initially started at joint connection at bottom part of the applied load on the testing specimen, during which the glue/adhesive layer breaks due to lower strength compared to bamboo material. Surface cracks are initiated at the lower surface of the specimen under tension. Surface crack has a similar pattern for each joint type variation. In all laminated bamboo composites, lamina delamination occurs due to the non-adherence of fibers between the laminae and the matrix material due to the weak adhesive layer. In addition, desk joints have the most substantial tensile strength because the cross-sectional area of the connection is more extensive than other types of joints. Finger joints have the weakest tensile strength because the connection is quite complicated, which allows air bubbles to be trapped between the joints board.

Figure 13

Macrography of bending test specimens: (a) butt joint, (b) scarf joint, (c) finger joint, (d) tongue and groove joint, and (e) desk joint.

In a previous study [22], testing was carried out on the technical analysis of apus bamboo, where apus bamboo was tested without using reinforcing fiber, commonly referred to as raw material. The results of these tests for tensile strength and bending strength can be seen in Tables 8 and 9.

As shown in Table 8, all specimens with different joint types experienced an increase in strength due to the addition of layers. The tensile strength of the laminated board connection specimens increased but could not match the tensile strength of previous studies. For samples with desk joint connections, the effect of the number of layers increases the tensile strength of each specimen.

Meanwhile the bending test in Table 9 shows that all specimens experienced a very significant decrease, for example, with different joint types, the tensile strength was 29–48% stronger than in previous studies. The type of connection for which the bending strength significantly increases is the laminate board with the desk joint connection type. In contrast, the laminated board with the finger joint connection produces the lowest flexural strength but still meets the required standards.

As per the regulations of the BKI for both tensile testing and bending testing, the test specimens that meet the test standards are the specimens with 7 layer bench connection, whose tensile strength is 109.76 MPa and bending strength is 151.51 MPa. In comparison, the specimen with the 3-layer connection and finger joint type has a value of 73.78 MPa for tensile strength and 125.85 MPa for bending strength. These results are consistent with the study of Manik et al., in which 7-layer bamboo composites with a greater epoxy mass matrix exhibited better mechanical qualities than 3 and 5 layers [26].