Abstract
Car body panels are one of the vehicle components that support the existence of the car interior and maintain the safety of passengers, especially in monocoque-type car bodies. Car panel materials, which generally use steel materials, have the disadvantage of being relatively heavy and having a short service life due to corrosion. To achieve higher energy efficiency in energy-efficient vehicles, honeycomb sandwich structures made of polymer composites, which are lighter in weight, can achieve a higher power-to-weight ratio. In this research, a polymer matrix composite material with a honeycomb sandwich structure was created and tested as an alternative material to replace steel for car body panels. Composite made from WR-200 Fiberglass as a reinforcing agent and SHCP 2668 CM-M resin as a bonding agent to determine the effect of the detailed honeycomb sizing, especially on the flexural strength and stiffness characteristics, have been used. Several specimens were made with variations in cell-pitch sizes of 20 and 40 mm, variations in the cell-height size of 10, 20 and 40 mm and variations in the thickness of the cell wall thickness obtained from the use of layers of Fiberglass of one, two and four layers. From the bending tests performed on all specimens, it was known that the highest flexural strength value have the specimens with a cell-pitch size of 20 mm, cell height of 10 mm and cell wall thickness of 4 layers of Fiberglass, namely 36.13 N mm−2. The specimen has the highest stiffness value with a cell pitch of 40 mm, cell height of 40 mm and cell wall thickness of 4 layers of Fiberglass, which is 338 N mm−1.
1 Introduction
Materials with a honeycomb sandwich structure have been widely used because they have specific strength and stiffness advantages. This property can be obtained because the material with a honeycomb sandwich structure is lightweight. Moreover, due to the stiffness behaviour, this material is widely used in the transportation sector [1, 2]. Forming a strong and rigid structure of the vehicle body is crucial to protect passengers in the event of a collision. The honeycomb sandwich structure can form a strong and rigid vehicle body structure and has crashworthiness properties, which increase vehicle safety [3]. The bending stiffness of the honeycomb sandwich structure is strongly determined by the stiffness of the face skin and the distance between the two face skins. However, the sandwich core must withstand all shear loads [4]. Besides the use of material, the shape and size of the core (honeycomb cell) also emphasize the mechanical properties of the honeycomb sandwich structure [5, 6]. Cell size is one of the critical parameters that would significantly influence the performance of honeycomb sandwich structures under bending loads [2]. One of the exciting parts of research on honeycomb sandwich structure was studying the effect of cell size on its mechanical properties.
Material selection to produce sandwich structure panels was also interesting to study. Polymer matrix-based composites have advantages in terms of lighter weight [7]. Then, using FRP (Fibre Reinforced Polymer) to produce materials with a sandwich structure is a good option. The lightweight properties possessed by FRP will produce a material with a sandwich structure with higher specific flexural rigidity. Another advantage of FRP is that it can be formed more efficiently to produce complex geometries and has good flexibility in assembly. Production process efficiency also impacts lower prices [8, 9].
Research on sandwich cores with various materials, namely fibre-reinforced composite and aluminium, shows that composite material always has a higher transverse shear modulus than aluminium properties for the same sandwich core density. This research also shows that the sandwich cores made from composite materials have lower weight, increased stiffness, and lower thermal distortion, making them compatible with face sheets [4]. The composite material makes the properties of the sandwich core easy to modify for specific needs [4]. Thus, applying a honeycomb sandwich structure with fibre-reinforced polymer material on a monocoque-type, high-energy-efficient vehicle chassis will be an excellent option for achieving a higher power-to-weight ratio. Previous research has investigated honeycomb sandwich panels with the exact cell pitch dimensions with two different cell heights. From the tests that have been carried out, it is known that the flexural strength of honeycomb sandwiches with different cell-height sizes but with the same cell pitch and the same material show relatively the same flexural strength [10, 11].
The flexural strength values of the two honeycomb sandwich specimens should be the same because both were made with composite materials with the exact composition of matrix and fibre fractions. Composites may have different flexural strength values if the fractional composition of the reinforcing agent and bonding agents' fractional composition are different. In this case, the thickness of the specimen does not matter because the flexural strength is specific. After all, it has been distributed over the cross-sectional area through calculations involving the moment of inertia.
Different from the value of flexural strength, cross-sectional dimensions significantly influence stiffness. The cross-section has a vital role in the magnitude of the stiffness figure. The same material can provide different stiffness values if the moment of inertia is different. As can be seen from the deflection equation for a simple beam, which is δ = −PL3/48EI, deflection (d) is inversely proportional to the moment of inertia. Since the deflections are inversely proportional to the stiffness, the stiffness is defined equivalently to the moment of inertia (I) and the modulus of elasticity (E). This was shown in previous studies, which showed that larger cell-height dimensions, which also means an increase in the moment of inertia value, provide higher stiffness [10, 11].
Although there has been extensive research on honeycomb sandwiches both structures [12–16] and materials [17, 18], studies on applying fibreglass composites to manufacture all honeycomb parts, including the core and skin, are still limited. Most honeycomb cores were made of aluminium, and the skin was made of polymer composite. Making honeycomb cores from fibreglass composite is certainly a challenge, considering that curing time is required for the resin to harden. This means the Fiberglass composite must remain in the mould until the resin hardens to achieve shape accuracy. Fibreglass composites have advantages in being widely available on the market, are cheap, and have a lighter weight than metal. In addition, no studies have shown a relationship between increasing the size of cell pitch, cell height, and thickness of cell wall and increasing flexural strength and stiffness using an experimental approach.
Therefore, it is important to find the relationship between the increase in flexural strength and stiffness values of honeycomb sandwiches made from Fiberglass composites as an effect of selecting the size of cell pitch, cell height and thickness of cell core. This information can be used to find a combination of flexural strength and stiffness of honeycomb sandwich structures that suits the needs of applications in the industry. In addition, this study also offers an alternative method for making honeycomb sandwiches from fibreglass composite materials that are easy and cheap but still provide consistent accuracy with honeycomb core shapes.
In this research, the mechanical characteristics of the honeycomb sandwich structure panels were studied, and variations in cell pitch, cell height, and cell wall thickness were observed. Cell pitch was varied with dimensions of 20 and 40 mm. Cell height varies with dimensions of 10 and 20 mm for cell pitches of 20 mm, while cell-pitch sizes of 40 mm vary by 20 and 40 mm. The honeycomb sandwich structure panel is made of composite materials, with Fiberglass as the reinforcing agent and resin as the bonding agent. The wall thickness is made with variations of 1, 2 and 4 layers of Fiberglass for all panel specimens of honeycomb sandwich. The honeycomb sandwich structure panel was tested by bending test primarily to determine its flexural strength, stiffness, and other physical and mechanical characteristics.
2 Methodology
2.1 Design of honeycomb sandwich panel
Specimens to be tested to determine their flexural strength and stiffness are made in panels. The honeycomb sandwich panel was structured of a layer of open cells as a core, which was covered on both surfaces with a thin layer of skin, as shown in Fig. 1a. Honeycomb sandwich panels are 500 mm long, approximately 200 mm wide, and vary in thickness depending on the height of the honeycomb cell core.
Honeycomb Sandwich: a) Specimen Panel [10], b) Cell-pitch, cell-size and cell height of honeycomb core [11], c) panel cross-section
Citation: International Review of Applied Sciences and Engineering 2025; 10.1556/1848.2025.00957
The honeycomb cell structure specifications which were made and tested in this study have detailed sizes: cell-pitch variations of 20 and 40 mm, cell-height variations of 10 mm, 20 and 40 mm and variations in cell wall thicknesses of one layer, two layers and four layers of Fiberglass. The definition of cell-pitch, cell size and cell height of honeycomb is shown in Fig. 1b. The core of the honeycomb sandwich is arranged constructively from corrugated sheets to form hexagonal cells. The corrugated sheets constructed into a honeycomb core are then covered with a layer of skin on the top and bottom. Previous studies also used the same technique [10], [11]. The materials used to make the honeycomb sandwich structural panels are Fiberglass WR200, which will be used as a reinforcing agent, and SHCP 2668 CM-M Resin, which will be used as a bonding agent. Fiberglass WR200 was chosen for reinforcing agents because it has thin sheets and is small-woven, making it easier to manufacture honeycomb cell cores. SHCP 2668 CM-M resin is a slightly viscous liquid good for fiberglass sheets because it can be well absorbed in fiberglass weaves. Mepoxe is used as a catalyst.
All these materials are available in chemical manufacturer stores. The core of this honeycomb sandwich structure was made with three variations of the core cell wall thickness obtained from using one layer, two layers and four layers of Fiberglass. Variations in the core cell wall thickness were made to determine the effect on flexural strength and stiffness. The top and bottom of the skin layers for all specimens were made with two layers of Fiberglass so that each specimen's flexural strength and stiffness could be compared equally, and the differences that occurred only came from differences in the thickness of the core cell walls.
2.2 Manufacture of honeycomb sandwich panel specimens
The core of honeycomb sandwich panels is composed of corrugated sheets made by a molding process. The mold for making a corrugated sheet consists of two parts, as shown in Fig. 2a. The process begins with resin preparation, mixing the resin with a catalyst of 3% of the resin weight. Corrugated sheets are formed by placing fiberglass in one mold, applying resin with a brush, and then pressing it with another. This mold pair is then pressed with the help of several C-clamps to ensure that the corrugated sheet obtained is formed accurately. The time required for resin solidification is approximately 50–60 min. The curing process occurs at room temperature of approximately 30 °C without additional heating. The corrugated sheet that has been made is seen in Fig. 2b. Then those corrugated sheets are arranged to get a honeycomb core, as seen in Fig. 2c. The arrangement of the corrugated sheets will form a honeycomb cell core with a hexagonal shape. The corrugated sheets that form the core of the honeycomb sandwich panel are then covered at the top and bottom with skin. The skin is made of 2 layers of Fiberglass and resin. The final form of the honeycomb sandwich panel is shown in Fig. 2d. All specimen manufacturing steps were done manually using the hand lay-up method and molds. For each condition, five specimens were prepared for testing.
The fabrication of honeycomb
Citation: International Review of Applied Sciences and Engineering 2025; 10.1556/1848.2025.00957
2.3 Bending test
Bending tests were conducted to determine the flexural strength and stiffness of honeycomb sandwich structural panels made from Fiberglass composite and SHCP 2668 CM-M resin. The specimen is placed on two supports with a distance between the supports of 400 mm. The load is applied at the midpoint of the span between the supports, as shown in Fig. 3a. This test follows the bending test standards for sandwich structures, ASTM C 393-00 [19]. The bending test machine will produce a graphic of the maximum load and deflection in the specimen panel. The bending test process is shown in Fig. 3b.
The bending test: (a) schematic, (b) experimental
Citation: International Review of Applied Sciences and Engineering 2025; 10.1556/1848.2025.00957
In this study, three parameters will be analyzed to determine the characteristics of a honeycomb based on dimensions and the number of layers, namely moment of inertia, flexural strength, and stiffness. According to Fig. 3a, the moment of inertia calculation is carried out by dividing this cross section into three sections:
- Section I: upper skin with equations:
- Section II: cross-section of core with equations:
- Section I: upper skin with equations:where:
Then the total moment of inertia is obtained by adding up the moments of inertia of the three section.
The flexural stiffness (K) of the specimen panel is calculated in the elastic region or the area where the deflection (
3 Result and discussion
3.1 Physical characteristic
Through dimensional measurements and weighing, several physical characteristics of honeycomb sandwich panels were obtained, such as panel weight, density, and weight fraction of Fiberglass and resin weight fraction. The density of honeycomb sandwich panels is obtained based on the weight and volume of the finished honeycomb sandwich panels. All weight measurements are carried out using a digital weighing scale with an accuracy of up to 0.1 g. However, in this study, the smallest value was only 1 g. The value of weight, volume and density (specific gravity) of each panel specimen of the honeycomb sandwich can be seen in Table 1, which has two variations of cell-pitch dimension, two variations of cell-height dimension and three variations of cell core wall thickness.
Calculation of honeycomb panel density
| Cell thickness | Average weight of panel specimen | Dimension (L × W × T) (mm) | Volume (m3) | Density | ||
| (gram) | St- Dev | (kg m−3) | St- Dev | |||
| Cell pitch 20 mm - Cell-height 10 mm (CP20-CH10) | ||||||
| 1 layer | 567 | 5.90 | 500 × 177 × 14 | 0.00124 | 457.63 | 4.70 |
| 2 layer | 734 | 6.14 | 500 × 188 × 14 | 0.00132 | 557.75 | 4.56 |
| 4 layer | 912 | 7.12 | 500 × 195 × 14 | 0.00137 | 668.13 | 4.99 |
| Cell pitch 20 mm - Cell-height 20 mm (CP20-CH20) | ||||||
| 1 layer | 730 | 5.79 | 500 × 177 × 24 | 0.00212 | 343.69 | 5.72 |
| 2 layer | 846 | 6.16 | 500 × 188 × 24 | 0.00226 | 375.00 | 4.81 |
| 4 layer | 1,012 | 6.10 | 500 × 194 × 24 | 0.00233 | 434.71 | 4.67 |
| Cell pitch 40 mm - Cell-height 20 mm (CP40-CH20) | ||||||
| 1 layer | 419 | 5.89 | 500 × 177 × 24 | 0.00212 | 197.27 | 4.24 |
| 2 layer | 548 | 7.00 | 500 × 188 × 24 | 0.00226 | 242.91 | 3.84 |
| 4 layer | 690 | 6.76 | 500 × 193 × 24 | 0.00232 | 297.93 | 5.46 |
| Cell pitch 40 mm - Cell-height 40 mm (CP40-CH40) | ||||||
| 1 layer | 647 | 6.44 | 500 × 177 × 44 | 0.00389 | 166.15 | 3.32 |
| 2 layer | 823 | 7.19 | 500 × 188 × 44 | 0.00396 | 207.83 | 4.25 |
| 4 layer | 1,077 | 6.06 | 500 × 193 × 44 | 0.00425 | 253.65 | 5.17 |
3.2 The weight of specimen panel
From the data in Table 1, it can be seen that for specimens with the same cell-pitch size, the weight of the honeycomb sandwich panel specimen is equivalent to the cell wall thickness and cell-height size. Specimens with the same cell pitch and height sizes and thicker cell core walls will increase the weight of the specimen. Furthermore, a larger cell height for the same cell pitch will make the specimen heavier. Thicker cell core walls and higher cell heights certainly require more material. It was evident that honeycomb sandwich panel specimens with thicker cell walls and higher cell cores would be heavier. If a comparison is made of the difference in cell-pitch size, the smaller specimen has a heavier weight. With relatively the exact size of the specimen panel, the smaller size of the cell pitch will form a more significant number of honeycomb core cells and produce a higher density of material structure so that the weight of the specimen panel will be greater. This comparison can be seen in the weight data from the panel specimens with the same cell height, 20 mm, but with different cell pitches, namely 20 and 40 mm. The data in Table 1 shows that for the specimen panel with the same cell height (20 mm), the specimen panel with a smaller cell pitch (20 mm) has a heavier weight.
Due to the effect of cell wall thickness on panel weight, the increase in weight of the panel specimen is not a direct function of the number of layers of Fiberglass used to make the honeycomb core cell thickness. For each cell pitch and cell-height size, the increase in cell wall thickness from 1 layer of Fiberglass to 2 layers of Fiberglass does not cause the panel weight to increase twice. Likewise, the increase in Fiberglass from 2 to 4 layers does not increase the weight of the panel to 2 times. This means that the increase in the number of layers of fiberglass does not have the effect of increasing the thickness of the cell core wall with the same multiplication.
3.3 Density
From the data of all honeycomb panel specimens that have been tested, specimens with larger cell pitches have a smaller density because larger cell pitches will result in a lower structural solidity. This can be seen in the graphic in Fig. 4, where the largest density is owned by the specimen panel with the cell-pitch size of 40 mm and cell-height of 40 mm, which is the largest cell-pitch and cell-height.
The value of density of honeycomb in different dimension and layers
Citation: International Review of Applied Sciences and Engineering 2025; 10.1556/1848.2025.00957
For panel specimens with the same cell pitch size, the cell height also has a fairly strong influence on density, where a higher cell height will produce a larger cavity in the core cell so that the density of the structure becomes lower. This makes the density of panel specimens with higher cell heights lower than those with lower cell heights.
The graph in Fig. 4 also shows that increasing cell-height has a more significant effect on panel specimens with smaller cell-pitch than those with larger cell-pitches. Two times higher cell height in the specimen panel with a cell pitch of 20 mm lowers the density to an average of 69%. In the specimen panel with a cell-pitch size of 40 mm, increasing the cell-height by 2 times only reduces the density to an average of 85%. The thickness of the cell core wall directly increases the specimen's weight and impacts its density, which also increases.
3.4 Fiberglass and resin fractions in composites
It is also necessary to observe the weight fraction of the composite's components to analyse the properties formed in the composite. The higher fractions in a composite will undoubtedly dominate the properties of the composite formed. From the weighing carried out, the weight fraction of Fiberglass and resin weight was obtained for each panel specimen, as seen in Table 2.
Weight fraction of fibre and resins in composites
| Cell thickness | Weight of panel specimen (gram) | Weight of fiberglass (gram) | Weight of fiberglass (%) | Weight of resin (gram) | Weight of resin (%) |
| Cell pitch 20 mm - Cell-height 10 mm (CP20-CH10) | |||||
| 1 layer | 567 | 222 | 39% | 345 | 61% |
| 2 layer | 734 | 337 | 46% | 397 | 54% |
| 4 layer | 912 | 562 | 62% | 350 | 38% |
| Cell pitch 20 mm - Cell-height 20 mm (CP20-CH20) | |||||
| 1 layer | 730 | 296 | 41% | 434 | 59% |
| 2 layer | 846 | 432 | 51% | 414 | 49% |
| 4 layer | 1,012 | 669 | 66% | 343 | 34% |
| Cell pitch 40 mm - Cell-height 20 mm (CP40-CH20) | |||||
| 1 layer | 419 | 223 | 53% | 196 | 47% |
| 2 layer | 548 | 332 | 61% | 216 | 39% |
| 4 layer | 690 | 496 | 72% | 194 | 28% |
| Cell pitch 40 mm - Cell-height 40 mm (CP40-CH40) | |||||
| 1 layer | 647 | 298 | 54% | 647 | 46% |
| 2 layer | 823 | 329 | 60% | 823 | 40% |
| 4 layer | 1,077 | 291 | 73% | 1,077 | 27% |
The use of fibre for more considerable cell heights requires more fibre, which is certainly followed by the need for more resin. The exciting part of the data in Table 2 is that for specimen panels with the same cell-height size, the specimen panels with smaller cell pitches use more fibre. This aligns with the above analysis regarding comparing weights in panels with the same cell-height size but different cell pitch sizes. Smaller cell pitches will form a more significant number of honeycomb core cells and produce a higher solidity of the material structure so that the weight of the specimen panel will also be higher. Then, the specimen panel with a smaller cell pitch will have a higher weight. Higher weights certainly require more material, both Fiberglass and binder resin.
Table 2 also shows a decrease in the percentage of resin used in specimens that use more layers of Fiberglass or thicker cell walls. The use of resin for a more significant number of layers of Fiberglass becomes more efficient, where a resin binding material that was used coincides with two layers of fibres at the joined surface of the fibre layers. The decrease in the percentage of bonding resin usage, which is equivalent to the increase in the use of Fiberglass layers, indicates that the increase in cell wall thickness is not equivalent to the increase in the number of Fiberglass layers with the same multiplication factor. This is in line with the above analysis regarding the addition of the number of layers of Fiberglass, which does not cause an increase in the thickness of the cell core wall by the same multiplication, so the increase in weight is also not multiplied by the same multiplier factor.
3.5 Moment of inertia
The value of the moment of inertia is determined by the internal structure that forms a material. For solid objects, the moment of inertia can be calculated directly based on the cross-section size. However, for materials with a honeycomb sandwich structure with cavities, the shape and dimensions of the structure within the material must be examined first. It is fascinating to know how the shape and dimensions of the honeycomb core structure also determine the value of the moment of inertia of the material with the honeycomb sandwich structure, including the size of the cell pitch and height, as well as the thickness of the core cell wall. From detailed measurements of the honeycomb sandwich panel specimens, the moment of inertia for each can be calculated as shown in Fig. 5. It can be seen in the figure that at the same cell-height size, the moment of inertia is not much affected by the cell-pitch size. Panel specimens with a smaller cell-pitch size and a more significant number of cells do not show a significantly higher moment of inertia value. For specimen panels with the same cell-height size (20 mm), specimens with a cell-pitch of 20 mm did not give much further increase in moment of inertia value; it was only about 5.5%, 9.8% and 12.4% compared to the specimen panel with cell-pitch 40 mm.
The value of moment inertia in different dimension and layers
Citation: International Review of Applied Sciences and Engineering 2025; 10.1556/1848.2025.00957
The increase in the moment of inertia value did not occur with an increase in the number of Fiberglass layers used to increase the thickness of the cell core wall in the same multiplication factor. The increase in the thickness of the cell core wall obtained from the use of the number of layers of Fiberglass, as much as 1, 2 and 4 layers, is not followed by an increase in the moment of inertia value with a multiplication factor that is the same as the number of layers of the Fiberglass. It is shown that the increase in the thickness of the cell core wall was also not proportional to the number of layers of Fiberglass used. This confirms the discussion of the problem of panel weight, in which the increase in the weight of the panel specimen was not a direct multiplication function of the number of Fiberglass layers used to obtain the thickness of the honeycomb core cell wall. This is also supported by the data in Table 2 regarding the use of resin, in which the use of resin also does not proceed in the same order as the use of Fiberglass layers. The use of resin decreased with an increasing amount of fiberglass layer. This means that the thickness of the honeycomb cell core wall does not increase in line with the multiplication of the number of layers of Fiberglass. Cell-height size has a significant influence on the moment of inertia value. For specimens with the same cell-pitch size, increasing cell height by two times can increase the moment of inertia value by 3.9–4.1 times for specimens with a cell-pitch of 40 mm. For 20 mm cell-pitch specimens, increasing the cell height by two times can increase the moment of inertia value by 3.5–3.8 times.
The most significant contribution to the increase in the moment of inertia value did not come from the cross-section of the vertical element of the honeycomb cell core, namely the thickness of the cell wall. Even the cell core walls, which are generally made thin on purpose, are not expected to increase the value of the moment of inertia significantly. The most considerable contributor to the value of the moment of inertia is the skin made as far as possible from the neutral line of the cross-section. The cell core functions more to widen the distance between the two skins rather than contributing to an increase in the cross-sectional area or the moment of inertia value. The small effect of the thickness of the core cell on the value of the moment of inertia led to the choice of thinner cell-core walls and higher cell heights. This is in line with the research results, which state that the moment of inertia is strongly influenced by the face-sheet planar stiffness and the distance between the two face sheets.
3.6 Flexural strength
The graph in Fig. 6 shows that the honeycomb sandwich panel specimens with the same cell pitch size show relatively the same flexural strength values. The flexural strength value of the panel specimen with a cell-pitch size of 20 mm shows a slight difference between a cell height of 10 mm and a cell height of 20 mm. Likewise, the flexural strength in specimens with a cell-pitch size of 40 mm also shows relatively the same value for cell heights of 20 mm and cell height of 40 mm. The graph in Fig. 6 shows that the honeycomb sandwich panel specimens with the same cell pitch size show relatively the same flexural strength values. The flexural strength value of the panel specimen with a cell-pitch size of 20 mm shows a slight difference between a cell height of 10 mm and a cell height of 20 mm. Likewise, the flexural strength in specimens with a cell-pitch size of 40 mm also shows relatively the same value for cell heights of 20 mm and cell height of 40 mm.
The value of flexural strength in different dimension and layers
Citation: International Review of Applied Sciences and Engineering 2025; 10.1556/1848.2025.00957
The similarity in flexural strength values for panel specimens with the same cell-pitch size shown by the results of this study is in line with previous studies, which showed test results where honeycomb sandwich panels with the same cell-pitch size had the same flexural strength values even though the cells-height were different [11]. This proves that the honeycomb sandwich panels for the same cell-pitch size are made with the same material. This is also supported by the data in Table 2, which shows that the composition of the Fiberglass and resin fractions for each number of layers of Fiberglass shows the same ratio. It is concluded that the composite material obtained from the hand lay-up method and the use of mold are the same. On the other hand, the hand lay-up and mold methods used to make honeycomb core cells were quite good regarding product consistency.
The graph in Fig. 6 also shows that the panel specimens with a cell-pitch size of 20 mm have a higher flexural strength value than those with a cell-pitch size of 40 mm. With a smaller cell-pith size, approximately half can increase the flexural strength between 2 and 3 times greater. These data indicate that the actual structural solidity of the material provides higher strength. This makes sense because a solid material with the same specimen size will undoubtedly have a higher strength.
When observing the moment of inertia value for each panel specimen, it can be seen that a higher value of the moment of inertia does not always affect higher flexural strength. Larger cross-sections and moments of inertia will increase the material's ability to withstand greater loads. However, suppose the ability to withstand loads increases with the same function as the increase in cross-sectional area and moment of inertia. In that case, the increase in flexural strength value will not be obtained. Flexural strength has a function that is equivalent to the ability to withstand loads, but it has an inverse function with cross-sectional area or moment of inertia. Therefore, the flexural strength will increase if the increase in load-bearing ability is faster or more progressive than the increase in cross-sectional area and moment of inertia.
3.7 Stiffness
The stiffness of the specimen panel is calculated in the elastic region or the area where the deflection increases linearly with the load. The results of the stiffness calculation for each specimen panel are shown in Fig. 7. The stiffness of the specimen panels can be seen as an effect of increasing the thickness of the core cell wall or increasing the number of layers of Fiberglass, as shown in the graph in Fig. 7. Moreover, these similarly occur for all cell-pitch sizes and all cell-height sizes. This trend was similar to the moment of inertia data shown in Fig. 5, where the increase in the value of the moment of inertia is equivalent to the increase in the thickness of the cell-core wall. This means that the increase in the value of the moment of inertia directly impacts the increase in stiffness.
The value of stiffness in different dimension and layers
Citation: International Review of Applied Sciences and Engineering 2025; 10.1556/1848.2025.00957
The same trend also occurs for cell pitch and cell-height sizes. The order of the moment of inertia values from the smallest, namely panel specimens with cell pitch 20 mm and cell height 10 mm, cell pitch 40 mm and cell height 20 mm, cell pitch 20 mm and cell height 20 mm, and the largest is cell pitch 40 mm and cell height 40 mm. This order is similar to the order of stiffness value in Fig. 3, starting from the smallest to the largest. So, it is true that the moment of inertia directly affects the stiffness value of the honeycomb sandwich panel specimen. The greater the value of the moment of inertia, the stiffness will also increase. This is also in line with the research results, which state that the bending stiffness is strongly influenced by the face-sheet stiffness and the span between the face sheets [4]. It also aligns with the research that examined several sandwich plates with different types of cores. It concluded that the best stiffness and high natural frequency is the sandwich plate with the higher honeycomb cores [20]. The natural frequency of a particular structure is related to its stiffness. The higher the structure's stiffness, the higher the natural frequency; otherwise, a higher natural frequency indicates higher stiffness. Research on the vibration characteristics of the honeycomb panels used in aircraft structures has also shown the same phenomenon, that higher stiffness was characteristic for the honeycomb, which has a higher core as indicated by a higher natural frequency [21].
Cell-pitch and cell-height dimension ratio indicate an effect on stiffness. Honeycomb, which has relatively the same cell-pitch and cell-height dimensions (CP40-CH40 & CP20-CH20), shows higher stiffness than honeycomb, which has a smaller cell height than its cell-pitch (CP40-CH20 & CP20-CH10). Higher stiffness is built up by the compact honeycomb core geometry, which can be defined as relatively the same dimensions of cell pitch and cell height. The same dimension of cell pitch and cell height makes the stiffness in the transverse direction equal to that in the longitudinal direction; thus, the structure is stiffer and not easily bent. A compact honeycomb core geometry that can build higher stiffness was found in a study on the effect of cell shape on the flexural strength of honeycomb structure [22].
3.8 Specific flexural strength and specific stiffness
The factor also considered when choosing a material based on mechanical properties is its weight. Solid materials almost certainly have high flexural strength and stiffness; however, solid materials have disadvantages in terms of heavyweight. The main reason that the material with honeycomb sandwich structure was developed is high strength and stiffness but is lightweight. Specific Flexural Strength and Specific Stiffness represent this aspect, dividing flexural strength and stiffness by their weight. The specific flexural strength value for each panel specimen is shown in Fig. 8. The panel specimen owns the highest specific flexural strength value with a cell-pitch size of 20 mm and cell height of 10 mm.
The value of specific flexural strength in different dimension and layers
Citation: International Review of Applied Sciences and Engineering 2025; 10.1556/1848.2025.00957
For specimen panels with the same cell height, specimen panels with thinner cell walls have a higher specific flexural strength number. Thinner cell-core walls will make the weight of the specimen panel lighter so that a greater specific flexural strength is obtained. This can be seen in the specimen panel with a smaller cell pitch of 20 mm, as shown in the graph in Fig. 8. In terms of the increase in the weight of all specimen panels, the increase in density due to the increase in cell-core thickness occurs more progressively in the sample panels with smaller cell pitch sizes than in the specimen panels with larger cell pitches. So, it is reasonable if the decrease in specific flexural strength is also very much seen in the specimen panels with smaller cell-pitch than those with larger cell-pitch.
For a panel of specimens with the same cell-pitch size, a higher cell height impacts a lower specific flexural value because the weight is heavier even though the density is lower while the flexural strength is relatively the same. Panel specimens with larger cell heights have lower densities, but more significant cell heights and lower densities do not make the value of specific flexural strength higher. This is because the specific flexural strength is determined more by its weight, not by its density. Furthermore, the specimen panel with a smaller cell height has a higher flexural strength value. In contrast, the smaller cell-height forms a tighter material structure and makes the material stronger to withstand loads. For panels with the same cell pitch size, where the flexural strength values are relatively the same, a smaller weight will result in a greater specific flexural strength. It can be achieved by a panel of specimens with a smaller cell height due to their lighter weight.
The specimen panel has the highest specific stiffness value with a cell-pitch size of 40 mm and cell height of 40 mm. Stiffness is strongly influenced by the moment of inertia, which is produced mainly by higher cell height. This also has a strong influence on the specific stiffness. Although panel specimens with a cell-pitch size of 40 mm and cell height of 40 mm have the highest weight, the stiffness increases progressively with higher cell height so that this panel specimen can have a higher specific stiffness than other panel specimens, as seen in Fig. 9.
The value of specific stiffness in different dimension and layers
Citation: International Review of Applied Sciences and Engineering 2025; 10.1556/1848.2025.00957
Cell height strongly influences stiffness by establishing the distance between the skins, significantly increasing the moment of inertia. This can be seen in panel specimens with the same cell-pitch size, where panels with higher cell heights will have higher specific stiffness. Even with a higher weight, a panel specimen with a higher cell height can still achieve a higher specific stiffness. Higher solidity of the material structure will cause the material to be heavy. Although it can reach higher strength, it is less able to achieve higher stiffness. This means that to get high strength, a material with a higher material solidity or a smaller cell pitch can be a better choice, but to get high stiffness, a large cell pitch and especially a higher cell height will be the better choice. Likewise, with the specific flexural strength and specific stiffness, the higher specific flexural strength is owned by the panel specimen with the smaller cell pitch and lower cell height. In contrast, the specific stiffness is characteristic for the panel specimen with the larger cell pitch and higher cell height.
4 Conclusions
This study shows that a lower honeycomb sandwich density can be obtained with a larger cell pitch, larger cell height and thinner cell walls. From the tests, it is known that the highest density is owned by the specimen with the cell-pitch size of 20 mm, cell-height size of 10 mm and the thickness of the cell core wall of 4 layers of Fiberglass, which was 668.13 kg mm−3. In comparison, the lower density is characteristic for a specimen with a cell-pitch size of 40 mm, a cell-height size of 40 mm and a cell core wall thickness of 1 layer of Fiberglass, which was 166.15 kg mm−3.
Regarding the ratio of the fiberglass fraction and the resin matrix fraction in composites, the study showed the same trend as previous studies, which showed that using more fibre layers to manufacture honeycomb cell core requires less weight fraction of resin matrix. Using one layer of glass fibre to manufacture a honeycomb cell core requires a resin with a weight fraction of 40%–55%. The more fiberglass (4 layers) used, the lower the use of resin, up to 27%–38% of the total weight of the specimen.
The value of the moment of inertia of the honeycomb sandwich panel mainly comes from the size of the cell height. The highest moment of inertia value is obtained by a specimen with a cell-pitch size of 40 mm, cell height of 40 mm and a cell wall thickness of 4 layers of Fiberglass, which is 440,080 mm4. Specimen panels possess higher flexural strength with a higher solidity of material structures. In this study, the highest flexural strength value was obtained by specimens with a cell-pitch size of 20 mm, cell height of 10 mm and a cell wall thickness of 4 layers of Fiberglass, which is 36.13 N mm−2. The specimen panel possesses the lowest flexural strength with a cell-pitch size of 40 mm, cell height of 20 mm and a cell wall thickness of 1 layer of Fiberglass, with a value of 8.32 N mm−2. It was also known that honeycomb sandwich panels with the same cell-pitch size tend to have the same flexural strength values even though the cell heights differed. Moreover, the highest stiffness value is obtained by a specimen panel with a cell pitch of 40 mm, cell height of 40 mm and a cell wall thickness of 4 layers of Fiberglass, with a stiffness value of 338 N mm−1.
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