Evaluation of impact resistance and material properties of eco-friendly buoys with biodegradable plastic materials

2.1 Experimental method

The deformation behavior of plastic can be described based on the connection state of the molecular chain.

As the molecular chain entangled by the external load is released, a yield stress appears along with nonlinear elastic behavior. Afterward, a softening behavior is observed, and then, a hardening behavior emerges owing to the locally released molecular chain, which leads to rupture [5].

Generally, plastic materials exhibit different von Mises stresses, depending on the deformation mode in which pressure is applied, and pressure significantly affects the yield condition. Therefore, material data for short-axis, shear, compression, and biaxial tensile tests are required to obtain accurate mechanical properties of plastic materials.

However, in the case of the developed biodegradable material, the purpose of material test is to obtain physical properties for simulation, and it is limited to conduct various tests of the material. Therefore, this study obtained a stress-strain curve through a tensile test.

The tensile test was performed based on the ASTM D638 standard test method for the tension properties of plastic specifications. The test speed was 5 mm/min. The dimensions of the specimens are presented in detail in Figure 2 and Table 1 [6].

Figure 2:

Specimen dimensions for the tensile test

Table 1:

Specimen dimensions of biodegradable plastic material

The specimens used in the tests are shown in Figure 3. The specimen used in the test is composed of poly lactic acid (PLA) material. PLA is a representative plastic material composed of processed lactic acid obtained by fermenting and refining the glucose extracted from corn. It is an eco-friendly biodegradable material that naturally decomposes into water and carbon dioxide within a few months because of the action of microorganisms under certain conditions.

Figure 3:

Specimens of biodegradable plastic material

Ten specimens were used in the test, and the test results of the average elastic modulus and tensile stress of the 10 specimens were selected for the finite element analysis.

2.2 Experimental results

The setup arrangements for the tensile test and the images before and after the test are shown in Figure 4. The analyses of the fracture shape of biodegradable materials revealed that damage occurred immediately after the elastic range. This implied that a short plastic range was present before material fracture compared to similar materials.

Figure 4:

Tensile test for biodegradable plastic material

From the tensile test, the maximum tensile strength of the 10 specimens was estimated as 26.71 MPa; the minimum tensile strength was 23.69 MPa, and the average tensile strength was 24.99 MPa. Damaged specimens after the tensile test are shown in Figure 5, and the tensile strength and modulus of elasticity of the 10 specimens are listed in Table 2.

Figure 5:

Fracture section of biodegradable plastic material

Table 2:

Tensile test results for the biodegradable plastic material

The test results of PLA material showed that the tensile strength was similar to that of conventional PE and PP. However, the elongation at break of PLA was lower than that of PE and PP applied to conventional buoys. These findings imply that damage can occur if significant deformation arises when the material is used. Strength-deformation and strength-strain rate curves of the tensile test are shown in Figure 6. To further analyze the properties of this new material, we applied its physical properties to flotation and performed a numerical analysis.

Figure 6:

Material tensile test results (stress-strain curve)

3.1 Numerical method

Herein, the impact-resistance test was numerically analyzed using the finite element analysis software ANSYS LS-dyna. Impact-resistance analysis was conducted under the same boundary conditions as those of the eco-friendly buoy certification standard. Therefore, a transient analysis of the free fall of the eggplant pendulum was performed considering the nonlinear material for 1.0 s.

3.2 Modeling and meshing

Under the impact-resistance test conditions, a metal plate possessing a thickness of (2 ± 1) cm was fixed without movement. The buoy developed herein was eco-efficient, and it was visually examined for abnormalities, such as cracks, breaks, and tears, on the buoy surface by dropping the eggplant pendulum vertically from a height of 2000 mm at the center of the top surface. The eco-friendly buoy was in the form of a circular sphere with a radius of 119 mm and composed of PLA materials. Figure 7 shows the cross-section of the eco-friendly buoy, except that the thickness of the wired part in the figure has vacant shapes.

Figure 7:

Section shape of the eco-friendly buoy

The eggplant pendulum should have a height of 252 mm, an impact radius of 63 mm, a radius of 31.5 mm in the upper part, and a weight of (10 ± 0.1) kg, as shown in Figure 8. The drop direction and impact point were the spherical part of the bottom of the eggplant pendulum, and the impact point of the eco-friendly buoy was determined by selecting the thinnest curved part rather than the bonding part [7][8].

Figure 8:

Eggplant pendulum for the impact-resistance test

The model used in the analysis is shown in Figure 9. As mentioned previously, the model was composed of an eggplant pendulum and a biodegradable buoy. An eco-friendly buoy was a circular sphere with a diameter of 238 mm, and the eggplant pendulum was modeled with the same size as suggested in the eco-friendly buoy certification. The finite element type applied to the analysis was a 4-node finite strain shell, and the number of nodes and elements used in the finite element model for impact analysis were 10,212 and 10,042, respectively.

Figure 9:

FE Model for impact analysis

3.3 Material properties

The material tensile test results obtained in Figure 6 of the previous experiment were used as the boundary condition of the numerical analysis. As listed in Table 3, the material properties of the buoy and eggplant pendulum correspond to PLA and steel, respectively. The average values obtained from the experiments were used to determine the elastic modulus and tensile strength. Poisson's ratio followed the same conditions as existing plastics.

Table 3:

Material properties

3.4 Analysis conditions

The fixed boundary conditions were applied by fixing them on a metal plate as suggested in the specification, and the eggplant pendulum with a maximum weight of (10 ± 0.1) kg was modeled and freely dropped to the bouy.

To estimate the optimal thickness to ensure the impact-resistance performance of the newly developed material, analyses were conducted at 1 mm intervals from 1 to 3 mm, considering the injection manufacturing process.

The analysis was conducted for three cases: 0 (center), 25, and 50 mm. The suitability of biodegradable buoys was reviewed considering the diagonal collisions that may occur during the impact test using the eggplant pendulum.

Among the analysis cases, three analysis cases in the diagonal direction and the drop direction are illustrated in Figure 10. The analysis cases considering thickness and direction are summarized in Table 4, and the total number of analysis cases combined is 9.

Figure 10:

Load cases depending on free fall positions

Table 4:

Load cases depending on thickness and position

3.5 Analysis Results

The results of the simulation of collision with the additional buoy of the eggplant pendulum are shown in Figure 11. The eggplant pendulum collided with the buoy at approximately 0.7 s during free fall, and the collision ended at 1.0 s.

Figure 11:

Impact of the eggplant pendulum on buoy over time

A review of the impact shape revealed that owing to the structural characteristics of the eco-friendly buoy with an empty inside, the additional collision part of the eggplant pendulum was crushed and bent inward.

The thicknesses below the red line in the graph of Figure 12, which is the allowable stress value, can satisfy the required impact-resistance performance conditions. The thicknesses below the red line are 2t and 3t.

Figure 12:

Impact analysis results of buoy depending on thickness

The detailed results of the impact-resistance analysis are presented in Table 5 and Figure 12. Cases 2-1 and 3-1 had different thicknesses of 2t and 3t. However, 3t was high at a stress of 0.4 MPa. An interpretation error was observed owing to singularity. Because this was a significantly small difference and all values were below the allowable stress, it was defined as satisfying in the selection of the optimal thickness, and the results were reviewed.

Table 5:

Impact analysis results of buoy

Therefore, the impact-resistant performance was considered satisfactory because an allowable stress of 24.99 MPa or less was observed at a thickness of 2t or higher. When only the impact-resistance performance was considered in the eco-friendly buoy certification, 2t was considered the optimal thickness. Because the impact-resistant analysis showed a similar trend for all nine cases, only the figures of three cases based on positions as a representative result (Case 2) were attached.

The analysis results for the 2t-buoy are shown in Figures 13-15. The analysis of the buoy showed that the maximum stress occurs in a part where the metal plate and buoy are fixed across all load cases.

Figure 13:

Analysis result for maximum stress (Case 2-1)

Figure 14:

Analysis result for maximum stress (Case 2-2)

Figure 15:

Analysis result for maximum stress (Case 2-3)

Author Contributions

Conceptualization, J. S. Jo; Methodology, J. S. Jo; Software,J. S. Jo; Validation, J. S. Jo; Formal Analysis, J. S. Jo and S. J. Choi; Investigation, J. S. Jo; Resources, J. S. Jo; Data Curation, J. S. Jo; Writing—Original Draft Preparation, J. S. Jo; Writing—Review & Editing, J. S. Jo and J. S. Kim.