Correlation between lamina directions and the mechanical characteristics of laminated bamboo composite for ship structure

2.1 Material selection and characteristics

In this subsection, material selection, physical characteristics, and mechanical properties of each proposed material will be separately explained. The discussion will be presented in the following three subsections.

2.1.1 Apus bamboo laminate

Bamboo is a grass-root plant that grows significantly in Indonesia, mostly in rural areas. As a tropical natural resource, bamboo has many variations, is easy to obtain, grows fast, and is used in people’s daily lives as a sustainable source [9,17]. In Southeast Asia, bamboo is generally used as a material for building and various types of vegetable baskets. Other uses include making paper, musical instruments, handicrafts, and boat/shipbuilding [9]. In the shipbuilding industry, bamboo is employed as a structural material to replace wood on the fishing vessel. Bamboo belongs to the grass family and has a growth rate significantly higher than that of wood, and many bamboo species reach full height in less than 6 months and maturity in 3–5 years. The mechanical properties of bamboo depend on the species and can also be influenced by local climatic conditions [18].

The species of bamboo used as primary reinforcement material is Apus bamboo. This species was chosen because of the strong, flexible, and quite thick diameter of its culm. Apus bamboo is abundant, specifically in rural areas [19]. It has a sympodial clump and a diameter of 4–15 cm depending on soil fertility, is stiff and upright, and can grow to 22 m. It usually thrives on river banks and hilly slopes from lowlands to highlands (±1,300 m asl). Bamboo’s lamina of Apus bamboo was collected from Bondowoso Regency, East Java, Indonesia, and harvested at the age of 5 years. The bamboo used was 3 years old and had an average diameter of 150 mm. The selected part of the bamboo stem, 1 m to the base to 4 m, was used to make the lamina. A previous investigation [11] stated the example of mechanical properties of Apus bamboo in Salatiga Regency, Indonesia: air content 12–15%, specific gravity value of 0.59 g/cm³, bending strength of 502.3–1240.3 kgf/cm², modulus of elasticity (MOE) of 57.515–121.334 kgf/cm², and tensile strength of 1.231–2.859 kgf/cm² [11].

When laid flat, bamboo is thinner than a wood beam. Lamination with a special adhesive can increase the thickness of bamboo to match that of a wood beam. Bamboo’s initial spherical shape with a diameter that narrows toward the tip could be sliced lengthwise with a special tool to get straight blades or slats. The slats’ surfaces are planned and then bonded in various directions to achieve the desired width and thickness. These properly dimensioned bonded slats can be transformed into various composite lamina boards [20]. The manufacturing process of Apus bamboo lamina is illustrated in detail in Figure 1.

Figure 1

Apus bamboo lamina manufacturing process.

2.2 Material manufacturing

The manufacturing of laminated bamboo boards comprised several steps. In the first step, Apus bamboo was cut into bamboo strips with various blade widths and 1 mm thickness. Bamboo stalks were chopped at the height of 1 m above the ground. The portion of the stem was located at 1–4.5 m above the ground. The bamboo was cut crosswise (cross-cutting) according to the length of the section (40 cm). After the bamboo stalks were split into blades, the outer shell was removed to create bamboo slats. The splitting method was then carried out to generate a 400 mm × 20 mm bamboo blade. The preparation of the bamboo lamina is presented in Figure 2. The four-sided planning tool was used to produce the bamboo lamina blades with certain thicknesses. The bamboo laminas were then maintained by soaking them with a preservative solution containing 2.5% sodium tetraborate and dried after preservation in an oven until the water content reached 10%. The percentage of water content in the bamboo laminas was determined using a moisture meter. Depending on the weather, drying in an oven takes 4–6 h until the moisture content of the material is less than 13%. Dried bamboo slats were grouped by thickness, sanded to smooth the surface with a sandpaper machine, and utilized to manufacture laminated bamboo composites as reinforcing composites.

Figure 2

Lamina shape of Apus bamboo (Gigantochloa apus) [16].

As a matrix/adhesive substance, an epoxy resin polymer was employed. Hand layup techniques were used to construct seven bamboo laminas with different layer orientations. For the initial layer, epoxy glue was applied to uniformly smooth out the whole surface of the bamboo. For the second layer, the entire surface was covered with Waru fibers. The bamboo slats and Waru fibers that have been cut were aligned and formed into boards according to the size and arrangement of the laminated bamboo slats’ fibers. A cold press machine was used for pressing at a pressure of 1.5 MPa. When the press pressure had reached the desired level, the bolts on the mold’s iron clamp were tightened. Laminated bamboo planks that had been crushed were placed in the clamp for 24 h before being removed to enhance the adhesion between the layers. The 40 cm × 20 cm sized laminated bamboo composite boards with different layer orientations were cut with a laser cutting machine based on specimen dimension. The manufacturing process of the laminate bamboo board is illustrated in Figure 3.

Figure 3

Manufacture process of lamina bamboo board.

The layer configuration and mass fraction of laminated bamboo composites are presented in Table 2. The mass fraction of a substance (percentage by weight/wt%) within a mixture is the ratio of the mass of that substance (m _i) to the total mass of the mixture (m _to t). Table 2 shows that for three different layer configurations, the mass fraction between matrix and filler components was similar, that is, 50:30:20 for bamboo, Waru fiber, and epoxy resin.

Table 2

Comparison of the mass fraction of constituent material under different layer orientations

No	Layer configurations	Apus bamboo mass fraction (wt%)	Waru fiber mass fraction (wt%)	Epoxy resin mass fraction (wt%)
1		50	30	20
2		50	30	20
3		50	30	20

2.3 Mechanical testing

Several mechanical tests, including tensile, bending, and impact, were conducted to investigate the mechanical behavior of laminated bamboo composites under different layers. Specimen dimension and testing procedure of each experimental testing were explained in detail.

2.3.1 Tensile test specimen and procedure

The tensile strength of laminated bamboo composite was tested in accordance with ASTM D3039 [23]. As shown in Figure 4, the specimen size of 250 mm × 25 mm × 10 mm was prepared by water jet cutting using a Universal Testing Machine (UTM) type WE-1000B, Zhejiang, China, with a maximum capacity of 1,000 kN at the Materials and Construction Laboratory, Department of Naval Architecture, Diponegoro University, Indonesia. The bottom side of the test specimen was clamped on a testing machine, and loading was slowly increased up to a particular load until the test object broke. The tensile test parameters were tensile strength and MOE of specimens with different layers. The ultimate tensile strength and tensile stress were calculated at each required data point using the following equations, respectively:

(1) σ max = P max A o ,

(2) σ i = P i A ,

where σ _max is the ultimate tensile strength (MPa), P _max is the maximum load before failure (N), o _t is the tensile stress at ith data point (MPa), P _i is the load at ith data point (N), and A is the average cross-sectional area (mm²). Tensile strain from the indicated displacement at each required data point can be calculated using the following equation:

(3) ε i = δ i L g ,

where e is the tensile strain at ith data point, S _t is the extensometer displacement at ith data point (mm), and L _g is the extensometer gauge length (mm). MOE is a property of a material that tells how easy it can stretch and deform and is defined as the ratio of tensile stress (σ) to tensile strain.

Figure 4

(a) Tensile test specimen and loading configuration, (b) tensile test using UTM type WE-1000B, and (c) tensile specimen with 0°/90°.

2.3.2 Three-point bending test specimen and procedure

In addition to the impact test, bending testing was carried out to obtain information on the strength of the material using the three-point bending method in a UTM type WE-1000B, Zhejiang, China, with a maximum capacity of 1,000 kN conducted at the Materials and Construction Laboratory, Department of Naval Architecture, Diponegoro University, Indonesia. The test was conducted at a 2.0 mm/min rate of crosshead movement. The specimen sizes had a configuration of 130 mm × 13 mm × 10 mm as shown in Figure 5. The specimen dimensions and testing procedure were in accordance with ASTM D7264 [24]. The bending strength was taken as the average value for five specimens tested. The maximum stress at the outer surface occurring at midspan was evaluated as flexure, while the beam was simply supported at two points and loaded at the midpoint. Eq. (4) can be used to compute flexural strength at any point on the load–deflection curve.

(4) σ = 3 PL 2 b h 2 ,

where σ is the stress at the outer surface at midspan (MPa), P is the applied force (N), L is the support span (mm), b is the width of beam (mm), and h is the thickness of beam (mm). The maximum strain at the outer surface occurring at midspan can be computed using the following equation:

(5) ε = 6 δh L 2 ,

where ε is the maximum strain at the outer surface (mm/mm), S is the midspan deflection (mm), L is the support span (mm), and h is the thickness of the beam (mm).

Figure 5

(a) Bending test specimen and loading configuration, (b) bending specimen with 45°/−45°, and (c) bending test using UTM type WE-1000B.

Moreover, the flexural chord MOE (E _flex) is the ratio of stress range ( σ ) and corresponding strain range ( ε ).

2.3.3 Charpy impact test specimen and procedure

Charpy impact test was applied to assess the brittle performances of the laminated bamboo material when subjected to an impact load using the Charpy impact machine Model DB-300A, Dongguan Hongtuo Instrument Co., Ltd., Dongguan, China. This test determines the amount of energy absorbed by a standard notched specimen when it breaks under an impact load. The Charpy device is a dynamic three-point bending experiment using an experimental setup consisting of the specimen, anvils on which the specimen is maintained freely, and a pendulum with a defined mass coupled to a spinning arm pinned to the machine body. The pendulum descends in a circular path, striking the test specimen in the middle of the span and delivering kinetic energy. Total correction energy (E _TC ) was calculated using the following equation:

(6) E TC = ( E A − ( E B / 2 ) ) ( β / β max ) + ( E B / 2 ) ,

where E _TC is the total correction energy for the breaking energy of a specimen (J), E _B is the energy correction for windage of the pendulum (J), and E _A is the energy correction for windage of pendulum plus friction in dial (J). Impact resistance I _s can be calculated using the following equation:

(7) I s = ( E s − E TC ) / t ,

where I _s is the impact resistance of the specimen (J/m), E _s is the dial reading breaking energy for a specimen (J), and t is the width of specimen or width of notch (m).

In the impact test, the characteristic dimension size of the rectangular test specimen was 55 mm × 25 mm × 10 mm rectangular test object with a notch angle of 45° as presented in Figure 6. The impact test based on ASTM D256 [25] was conducted at the Materials and Construction Laboratory, Department of Naval Architecture, Diponegoro University, Indonesia. The impact energy of 150 Joule (little hammer), the impact speed of 5.2 m/s, and the pendulum angle of 150° were applied. The impact strength for each variation was taken as the average value from four test specimens.

Figure 6

(a) Charpy impact testing specimen, (b) specimen with different layer orientations, and (c) Charpy impact testing machine.

3.1 Tensile test under different layer configurations

Tensile strength/ultimate tensile strength is the maximum stress a composite can withstand when stretched before it fractures. Tensile strength is generally measured by performing a tensile test and measuring the strain and stress value changes. The highest point of the stress–strain curve is called the ultimate tensile strength. The strength value does not depend on the size of the material but the type of material. Table 3 shows the comparative tensile strength results under different layer orientations. The highest tensile strength at a value of 167.2 MPa was found in the laminated bamboo specimen with unidirectional (0°) layer orientation. Moreover, specimens with bidirectional (45°/−45°) and (0°/90°) layer orientations had a tensile strength of about 80.7 and 141.6 MPa, respectively. These values decreased approximately 141.6% for 0°/90° orientation and 51.7% for 45°/−45° orientations compared with specimens with 0° orientation.

Furthermore, Figure 7 shows that adding Waru fiber to a laminated bamboo composite improves tensile strength. Compared to the earlier discovery of the formation of laminated bamboo composites with a comparable laminate direction without employing Waru fiber conducted by Manik et al. [16], the tensile strength with the laminate direction of 0°, 0°/90°, and 45°/−45° increased by about 20.28, 4.88, and 6.18%, respectively. It was found that the addition of Waru fiber significantly influences the laminated bamboo composite. However, adding Waru fiber into a specimen with 45°/−45° layer orientation still has lower tensile strength compared to specimens with 0° and 0°/90°.

Figure 7

Comparison of tensile strength under different material combinations.

Compared with other material combinations, Supomo et al. [13] developed a bamboo-hybrid-slat laminate for a fishing vessel using a combination of Ori bamboo and Pterocarpus indicus (Angsana wood) under different angle variations: parallel (0°), transverse (45°), and transverse (90°). It can be found that tensile strength has a different value based on angle variations. The tensile strength on the parallel slat layer is 114.64, 80.07 MPa on the transverse slat layer (90°), and 53.81 MPa on the transverse slat layer (45°). Those values experience a lower tensile strength compared to the present study. These values were lower than those obtained in the present study, with decrease of approximately 31.4% for 0° layer orientation, 43.4 for 0°/90°, and 33.3% for 45°/−45°. Therefore, manufacturing laminated bamboo composites using compaction pressure at certain pressure can improve the strength of the material.

Compared with other material combinations using bamboo and meranti wood at different compositions given by Manik et al. [11] using unidirectional (0°) layer orientation, the tensile strength of the present study still had a higher value of approximately 41680.9%. Furthermore, the tensile strength of laminated bamboo composite using thick laminas such as 10 [14] and 6 mm [15] was compared with that from the present study. Analysis on the tensile strength of laminated bamboo composites showed that the highest tensile strength was found in the composites with thin bamboo reinforcement. Meanwhile, thick bamboo slats had a low tensile strength. Figure 7 shows that with the addition of Waru fiber layer into laminated bamboo composites in the present study, only samples with 0° and 0°/90° layer orientations fulfilled the BKI tensile strength standard (98 MPa) [26]. Although the addition of Waru fiber can increase the tensile strength by 45°/−45°, the value was still below the BKI tensile strength threshold.

Tables 4 and 5 list the comparison results for the tensile strain and MOE under different layer orientations. Figure 8 shows the highest strain in the sample with unidirectional (0°) laminate direction, followed by 0°/90° and 45°/−45° layer orientations. MOE is used to measure a material’s resistance to elastic deformation when a force is applied to the specimen. The MOE of a specimen was defined as the slope of the stress–strain curve in the elastic deformation region. Figure 8 shows that varying laminate orientations had different MOE values. Analyzed at 1.5 MPa compaction pressure, the highest MOE of 7.61 GPa was found in the specimen with 0° laminate direction. The MOE of a specimen with 0°/90° laminate direction was 11.8% lower than that of a specimen with 0° laminate direction. Moreover, the samples with 45°/−45° laminate direction experienced a lower trend near 22.5% than those with 0° laminate direction. Compared with the BKI standard for tensile MOE [26], only specimens with 0° and 0°/90° laminate direction had a tensile modulus above the given threshold.

Figure 8

Result of modulus elasticity and strain under different layer orientations.

Figure 9 depicts a macrophotograph of the surface fracture of the laminated bamboo composite under different layer orientations after tensile testing. Figure 9a shows the fracture of the specimen with the 0° laminate direction. The surface fracture of the specimens with 45°/−45 and 0°/90° laminate directions is shown in Figure 9b and c. In the specimen with the 0° laminate direction, the first phenomenon during tensile test was matrix fracture, followed by load transfer to bamboo laminas and subsequent bamboo lamina fracture. In the specimen with 0°/90°, the fracture of the matrix first occurred, followed by the delamination of the lamina and finally the fracture of the lamina in the 0° direction. In the specimen with +45°/−45° laminate orientation, matrix fracture first occurred, followed by delamination and lamina cleaving.

Figure 9

Fracture pattern of tensile strength under different layer orientations. (a) Unidirectional (0°), (b) bidirectional (45°/−45°), and (c) bidirectional (0°/90°).

3.2 Result of three-point bending test

Flexural testing was carried out to determine the ability of the test sample to withstand the maximum bending load until it fractures. Table 6 and Figure 10 show the comparative result of the three-point flexural strength under different layer orientations between the present study and previous investigation by Manik et al. [16]. The material selections, the manufacturing process of laminated bamboo composites, and testing specimens and procedures were similar to those in ref. [16]. The present results were compared with those of the previous study under the same compaction pressure at 1.5 MPa. A similar finding was obtained; that is, the highest bending strength of 273.4 MPa can be achieved for the specimen with 0° (on-axis laminate) layer orientation. The specimens with 45°/−45° and 0°/90° experienced a decreased trend at about 3.3 and 3.1%, respectively.

Figure 10

Results of bending strength under different material combinations.

Compared with the previous bending strength result by Manik et al. [16], the addition of a Waru fiber layer into the laminated bamboo composites in the present work led to an increasing trend in bending strength in all bamboo laminated layer orientations. The result showed that the addition of Waru fiber in 0° laminate direction caused an increase of about 3.17%. A similar trend was also found upon the addition of Waru fiber in 45°/−45° and 0°/90° laminate directions. Both materials experienced an increase of about 10.17 and 14.18%. Moreover, the bending strength in the present study (Apus bamboo and Waru fiber) was higher than that reported by Supomo et al. [13] in all layer orientations. In addition, the tensile strength of all specimens in the present research was higher than the threshold given by BKI (98 MPa) [26].

Table 7 and Figure 11 show the bending modulus and strain under different layer orientations. A similar result was also found: laminated bamboo composite with 0° layer orientation had the highest bending modulus compared with 45°/−45° and 0°/90° laminate directions. The bending modulus of bamboo composite with 45°/−45° and 0°/90° laminate directions decreased by about 28.8 and 17.6%, respectively, relative to that of the composite with 0° layer orientation. By contrast, the highest strain value was achieved by the specimen with 45°/−45° at 0.0208, followed by those with 0°/90° and 0° laminate directions. In terms of the BKI threshold, all prepared specimens with different layer orientations fulfilled the flexural modulus threshold at 6.86 GPa.

Figure 11

Results of flexural modulus and strain under different layer configurations.

Figure 12 shows the macrophotographs of the specimen after the bending test. Figure 12a reveals that the bending test of the specimen with the 0° laminate direction resulted in matrix fracture and subsequent laminate fracture. Figure 12b shows the fracture photographs of specimens with layer orientations of 45°/−45°. Fracture occurred at the bottom of the specimen due to lamina delamination. Failure progressed with delamination, and the lamina split due to shear stress. As shown in Figure 12c, the failure of the specimen with the lamina direction of 0°/90° indicated that the failure due to bending began with matrix fracture and lamina fracture at the bottom (outer) of the specimen, followed by the delamination of the lamina in the 90° direction.

Figure 12

Fracture pattern of the flexural test under different layer configurations. (a) Unidirectional (0°), (b) bidirectional (45°/−45°), and (c) bidirectional (0°/90°).

3.3 Result of Charpy impact resistance test

Impact strength was used to measure the material’s capability to withstand a suddenly applied load and was expressed in terms of energy. Impact testing aimed to determine the brittle nature of the test specimen against impact load. It requires energy to break the specimen with one hit using a hammer with a specific weight that is dropped by being released from a certain angle. The layer orientations strongly influenced the impact strength value in the laminated bamboo composite. Table 8 and Figure 13 show the comparison results of the impact strength of the bamboo laminate composite with different layer orientations. The specimen with 0° layer orientation showed the highest impact resistance of 340.7 J/m, and the specimen with 45°/−45° layer orientation had the lowest impact resistance of 149.3 J/m. In terms of the BKI impact resistance threshold, only the specimen with 45°/−45° layer orientation did not fulfill the minimum standard.

Figure 13

Results of impact strength under different layer orientations.

Figure 14 shows a macrophotography of the surface fracture of the laminated bamboo composite after impact testing for each fiber direction variation. In the laminated bamboo composites with a 0° lamina direction, the first phenomenon was matrix fracture, followed by load transfer to the bamboo lamina and fracture in the bamboo lamina as shown in Figure 14a. In the laminated bamboo composite with 45°/−45° lamina direction in Figure 14b, a fracture occurred in the matrix, followed by delamination and gap formation between the laminas. Figure 14c shows a fracture in the matrix occurred, followed by lamina delamination in the 90° direction and lamina fracture in the 0° direction.

Figure 14

Fracture pattern of impact test under different layer configurations. (a) Unidirectional (0°), (b) bidirectional (45°/−45°), and (c) bidirectional (0°/−90°).

3.4 Overall discussions

Results of several methods of mechanical testing were collected to investigate behavior distinction under three different laminate directions (unidirectional (0°), bidirectional (45°/−45°), and bidirectional (0°/−90°). It can be found that material with 0° lamina direction produces highest strength. The tensile, bending, and compressive properties will continue to decrease due to changes in the lamina orientation that increases from 0°.

Referring to the BKI requirements, laminated bamboo composites with a lamina direction of unidirectional (0°) and bidirectional (0°/90°) still meet the minimum limits of BKI for tensile strength, bending strength, tensile MOE, bending elasticity modulus, and impact strength, and only material with bidirectional (45°/−45°) does not fulfill the tensile strength, tensile modulus, and impact strength thresholds. The proposed material is categorized as Strength Class III based on BKI. It is feasible to be recommended for several structural locations, such as hull skin, frame, and the deck of the ship’s superstructure, except for the material for the keel of the boat.

The comparison of bamboo laminated composite materials that have been developed has higher tensile strength and bending strength than fiberglass material. Ruzuqy [27] studied the tensile strength of fiberglass material (chopped strand mat CSM 300 and 400) by varying the number of laminates from 4 to 8 layers resulting in tensile strength in the range of 59–87 MPa. The results of the strength of the fiberglass material have a strength equivalent to the cross-direction composite of 45°/−45°. However, the tensile strength is still far below the laminated bamboo composite material with the fiber direction of 0° and 0°/90°. The value of the bending strength between the developed and the fiberglass material needs to be reviewed. Marzuki et al. [28] conducted a series of fiberglass material strength tests using chopped strand mats and woven roving material for applications on fishing boats made of fiberglass resulting in bending strength in the range of 200–288 MPa. Setiawan and Supomo [29] showed that laminated fiberglass hull with multiaxial material has a tensile strength of 120–200 MPa and a bending strength of 200–300 MPa. The strength value of the fiberglass material has the same strength as the laminated bamboo composite material with the fiber direction in the direction of 0°. From density value, the laminated bamboo composite material has an average density of 0.84–0.88 g/cm³, while fiberglass composite materials for several types of fibers such as carbon, e-glass, and s-glass have a higher density between the range of 145–1.9 g/cm³ [30]. Therefore, from the aspect of material strength, the laminated bamboo composite material has a better strength-to-weight ratio than fiberglass composite material.

Supomo et al. [8] indicated that laminated bamboo has succeeded in becoming an alternative material with technological advantages, particularly compared to teak wood. They compared the qualities of laminated bamboo and found that bamboo is 60% more cost-effective than teak in use today and has a strength that is 1.5 times higher. Bamboo also has an advantage in terms of raw material accessibility because it can be harvested in 3 years as opposed to 25–30 years for wood. For laminated bamboo-based vessels, this is undoubtedly a production advantage as well as a financial benefit. Bamboo in construction offers advantages when exposed to water, especially salt water.