Experimental Study on Low-Velocity Impact Performance of GFRP Trapezoidal Corrugated Sandwich Structures

: Glass ﬁber trapezoidal corrugated sandwich structures are composed of trapezoidal cores and glass ﬁber-reinforced polymer (GFRP) panels. A series of low-velocity impact tests were conducted to investigate the impact resistance performance, considering the effects of impact position, impactor shape, and impactor diameter on the damage mechanism of sandwich structures. When the impactor shape and impact energy remain constant, the maximum impact load at the node impact point is higher than at the base, while the displacement of impact at the base is signiﬁcantly higher than that at the node. Secondly, when the impactor diameter and energy of the impactor are the same, the hemispherical impactor requires less penetration energy to impact the sandwich structure compared to the ﬂat impactor. Comparing the shape of the impactor, it is found that the smaller the contact surface of the impactor, the more concentrated the stress, and the lower the required penetration energy. Finally, when the impactor shape and impact energy are the same, as the diameter of impactor increases, the damage expansion during impacting on the sandwich structure becomes more sufﬁcient, resulting in decreased impact displacement and smaller impact damage caused to the sandwich structure.


Introduction
Compared to traditional metal materials, advanced composite materials play an important role in many fields with high demand for lightweight materials, such as aerospace, vehicles, ships, constructions, and sports equipment, due to their light weight and high strength advantages [1].Therefore, composite sandwich structures have excellent mechanical properties, which are an important part of lightweight structures [2].Typically, the sandwich structure is composed of small-thickness but high-stiffness upper and lower panels, as well as lightweight porous cores.When a sandwich structure undergoes bending loads, the panels primarily provide strength and stiffness to bear tensile or compressive loads, and its main failure modes are fiber damage, matrix damage, and delamination.The core mainly bears the transverse shear force transmitted from the panels, supporting the panels and preventing local buckling [1,3,4].
Honeycomb core and foam core are the most widely used traditional sandwich structures at present.With the increasing development of fabrication technology, higher requirements are put forward for sandwich structures in many fields.The sandwich structures are not only required to satisfy the necessities of being weight-bearing and lightweight, but also need to have functional requirements such as heat insulation, noise reduction, energy absorption, vibration isolation, and embedded electronic devices.These multifunctional requirements also provide new ideas for the structure to further reduce weight [5,6].In sandwich structures featuring foam or honeycomb cores, once micro-cracks appear in the structures, water can easily flow into the core and is not easily discharged, which is likely to result in the detachment of the adhesive layer between the panels and core [7].However, the corrugated sandwich structures have an open cavity channel, which can ensure internal gas flow, avoid condensation and the accumulation of water vapor, and can add functional components inside the corrugated core to make the corrugated sandwich structure a structure-function integrated structure [8].Corrugated sandwich structures are lightweight and have high strength, impact resistance, shear resistance, and anisotropic structure.They have a certain deformation ability along the growth direction of the corrugation; in this direction, the corrugation core can deform with the deformation of the panel material [9].In some cases, the transverse shear effect can significantly affect the mechanical properties of the corrugated board, so it cannot be ignored.Garbowski et al. [10] proposed a method for calculating the transverse stiffness of the corrugated board based on plate torsion tests and correct formula analysis.This modification reduced the error of the results and enabled corrugated board structures to achieve higher strength.Radford et al. [11] conducted a comparison experiment of the dynamic response of sandwich beams with a corrugated core, foam core, and pyramid core under impact loads using high-speed photography, and found that the performances of the corrugated core and foam core sandwich beams were superior to that of the pyramid core sandwich beam.Rong et al. [12] studied the effect of a corrugated core geometric configuration on the low-velocity impact performance of sandwich structures by using the finite element method.It was found that the core's geometric shape played a crucial role in determining the energy absorption capacity under low-energy impact.In addition, compared with sinusoidal and arc-shaped corrugated sandwich structures, trapezoidal corrugated sandwich structures had better impact resistance.
Previous studies have used metals [13], plastics [14], ceramics [15], and thermosetting composite materials [16,17] as raw materials for preparing sandwich structures.The cores of these sandwich structures are usually formed by the pressing molding process.Over the years, the combination of carbon fiber composite materials and sandwich structures has opened up a new idea for lightweight materials' design and application.Meanwhile, the allcomposite sandwich structures have become a research hotspot, with new structural forms and preparation processes emerging.He et al. [18] fabricated a hybrid corrugated sandwich structure with carbon fiber-reinforced polymer (CFRP) panels and an aluminum alloy core.The impact resistance of the sandwich structure was studied by mainly changing the core thickness and comparing the failure mode and energy absorption characteristics under low-velocity impact experiments.The study revealed that there were significant increases in the slopes and peak values of impact load-time curves as the core thickness and impact energy increased.During the impact process, the duration of contact between the impactor and sandwich structure reduced as the core thickness increased and extended as the impact energy increased.Rejab et al. [19] performed a set of experimental and numerical studies on the compression behavior and failure mechanisms of corrugated sandwich panels made of aluminum alloy, glass fiber-reinforced plastic (GFRP), and carbon fiber-reinforced plastic (CFRP) while varying the number of unit cells and the thickness of the cell wall.It was found that the mechanical properties of the composite sandwich structures surpass those of the metal sandwich structure.Several studies have demonstrated that low-velocity impact experiments result in distinct damage modes and morphologies on sandwich structures, depending on the position of impact.Meanwhile, the shape and diameter of the impactor also play significant roles in the impact response and damage mechanism of the sandwich structure.For example, Boonkong et al. [20] investigated the features of impact response and energy absorption in a curved sandwich structure composed of an aluminum alloy.Their study focused on analyzing different impact parameters, such as the size of the impactor, the type of material used, and the angle of impact.The findings indicate a direct correlation between the penetration energy and elevated impact angles as well as larger impactor diameters.Liu et al. [21] examined how the shape of the impactor and its position impact the damage mechanism of sandwich structures, and revealed that impactors with a larger contact surface yielded a higher impact peak force and greater delamination between layers under identical impact energy.Moreover, when the energy of the impact was identical, the impact displacement of long-span impact was longer than that of short-span impact.
The research on the low-velocity impact performance of corrugated sandwich structures is currently mostly focused on all-metal structures and hybrid structures of carbon fiber-reinforced panels and metal cores.In addition, the research on the mechanical properties of corrugated sandwich structures mainly focuses on quasi-static compression.There is less research on the low-velocity impact performance of all-composite sandwich structures.CFRP is the fiber-reinforced polymer with the highest tensile strength at present, but its cost is too high to be popularized in practical engineering.However, GFRP has become the most widely used fiber-reinforced polymer in the market due to its high tensile strength, high elongation, low price, and mature production technology.In this paper, the all-glass fiber trapezoidal corrugated sandwich structure was fabricated by the vacuum hot-pressing process and die-pressing method.A series of low-velocity impact tests were carried out on the node and base positions of the sandwich structures with two different shapes of impactors and three different diameters of impactors.The corrugated sandwich structure is widely used in comprehensive protection systems in aerospace and marine engineering due to its superior shock absorption ability and bending resistance [22], and it has a wide range of applications in packaging, transportation, and other fields.Therefore, the impact resistance of a glass fiber trapezoidal corrugated sandwich structure based on the loaddisplacement curves and energy absorption characteristics of the sandwich structures is of great research significance, also providing practical reference value for future numerical simulation expansion research.

Material and Specimen Description
The glass fiber trapezoidal corrugated sandwich structure consists of the glass fiberreinforced panels and the trapezoidal corrugated core.The trapezoidal corrugated core and panel are composed of E-glass fiber satin fabric prepreg material (E-glass stain-weave fabric/CYCOM7701 epoxy composite, provided by Cytec Industries Inc., USA).Each layer of prepreg has a thickness of 0.25 mm.The mechanical properties are enumerated in Table 1.
The ply sequence of panels is [0 • /90 • ]s, and that of the trapezoidal core is [45 • /−45 • ]s. Figure 1 presents a schematic diagram of the trapezoidal sandwich structure.The crosssection design parameters of the core are shown in Figure 1a: the unit cell height is h = 20 mm, the core sawtooth step size is 2a = 24.78mm, the wall support angle is β = 60 • , the bonding platform width is d = 4 mm, and the material thickness is t = 1 mm.Table 2 presents the actual parameters of the trapezoidal core.The schematic diagram of the trapezoidal corrugated core and the sandwich structure specimen are shown in Figure 1b,c.The types of impactors used in this paper are illustrated in Figure 2.
trapezoidal corrugated core and the sandwich structure specimen are shown in Figure 1b,c.The types of impactors used in this paper are illustrated in Figure 2.

Preparation Process of the Specimens
The manufacturing process for the trapezoidal corrugated sandwich structure utilizing glass fiber-reinforced epoxy resin is shown in Figure 3.The trapezoidal core is prepared by die-pressing.The core preparation mold is composed of a split upper mold and an integral lower mold, as shown in Figure 4. First, preheat the entire mold in an oven to around 60 °C.After preheating, spray the release agent onto the surface of the mold to make it easier to separate the solidified core from the mold.Then, lay the glass fiberreinforced epoxy resin prepreg in the order of [45°/−45°]s and place it in the groove of the lower mold.Next, insert the upper mold sequentially into the groove of the lower mold and secure it in place using locating pins and clamping blocks.Finally, put it into the oven and monitor its temperature with the heat compensation instrument.trapezoidal corrugated core and the sandwich structure specimen are shown in Figure 1b,c.The types of impactors used in this paper are illustrated in Figure 2.

Preparation Process of the Specimens
The manufacturing process for the trapezoidal corrugated sandwich structure utilizing glass fiber-reinforced epoxy resin is shown in Figure 3.The trapezoidal core is prepared by die-pressing.The core preparation mold is composed of a split upper mold and an integral lower mold, as shown in Figure 4. First, preheat the entire mold in an oven to around 60 °C.After preheating, spray the release agent onto the surface of the mold to make it easier to separate the solidified core from the mold.Then, lay the glass fiberreinforced epoxy resin prepreg in the order of [45°/−45°]s and place it in the groove of the lower mold.Next, insert the upper mold sequentially into the groove of the lower mold and secure it in place using locating pins and clamping blocks.Finally, put it into the oven and monitor its temperature with the heat compensation instrument.

Preparation Process of the Specimens
The manufacturing process for the trapezoidal corrugated sandwich structure utilizing glass fiber-reinforced epoxy resin is shown in Figure 3.The trapezoidal core is prepared by die-pressing.The core preparation mold is composed of a split upper mold and an integral lower mold, as shown in Figure 4. First, preheat the entire mold in an oven to around 60 • C.After preheating, spray the release agent onto the surface of the mold to make it easier to separate the solidified core from the mold.Then, lay the glass fiber-reinforced epoxy resin prepreg in the order of [45 • /−45 • ]s and place it in the groove of the lower mold.Next, insert the upper mold sequentially into the groove of the lower mold and secure it in place using locating pins and clamping blocks.Finally, put it into the oven and monitor its temperature with the heat compensation instrument.
The composite panel adopts a vacuum hot-pressing molding process.First, lay the prepreg in the order of [0 • /90 • ]s.After the electric blanket, heated aluminum plate, isolation film, prepreg, gasket, thermocouple, equalizing plate, and insulation cotton are placed on the preparation platform, the vacuum bag is sealed with sealant and then vacuumed.The temperature and vacuum level inside the vacuum bag are regulated by the heat compensation instrument [23].Then, cure the panel according to the curve shown in Figure 5c.The temperature monitoring curve of the heat compensation instrument rises to 126 • C at a heating rate of 3 • C/min.After holding for one hour, the temperature drops to 50 • C at 2 • C/min.Then, the temperature drops naturally to room temperature to obtain the specimens of the sandwich structure.The composite panel adopts a vacuum hot-pressing molding process.First, lay the prepreg in the order of [0°/90°]s.After the electric blanket, heated aluminum plate, isolation film, prepreg, gasket, thermocouple, equalizing plate, and insulation cotton are placed on the preparation platform, the vacuum bag is sealed with sealant and then vacuumed.The temperature and vacuum level inside the vacuum bag are regulated by the heat compensation instrument [23].Then, cure the panel according to the curve shown in Figure 5c.The temperature monitoring curve of the heat compensation instrument rises to 126 °C at a heating rate of 3 °C/min.After holding for one hour, the temperature drops to 50 °C at 2 °C/min.Then, the temperature drops naturally to room temperature to obtain the specimens of the sandwich structure.The composite panel adopts a vacuum hot-pressing molding process.First prepreg in the order of [0°/90°]s.After the electric blanket, heated aluminum isolation film, prepreg, gasket, thermocouple, equalizing plate, and insulation co placed on the preparation platform, the vacuum bag is sealed with sealant a vacuumed.The temperature and vacuum level inside the vacuum bag are regu the heat compensation instrument [23].Then, cure the panel according to the curv in Figure 5c.The temperature monitoring curve of the heat compensation instrum to 126 °C at a heating rate of 3 °C/min.After holding for one hour, the temperatu to 50 °C at 2 °C/min.Then, the temperature drops naturally to room temperature t the specimens of the sandwich structure.The interface performance between the panels and core is the key to exerting the mechanical/multifunctional advantages of composite sandwich structures [24].First, the surface of the bonding area between the panels and core is polished using sandpaper to achieve a certain roughness.Then, wash and wipe the panels and cores to maintain a clean contact surface.Finally, an epoxy resin structural adhesive film is used to bond the panels and cores.Paste the cut adhesive film onto the contact surface between the core and panels, depending on the adhesive film's inherent adhesion before curing.The initially bonded sandwich structures are slightly compacted by using the pressure-equalizing plate, and then placed on a heating platform.According to the vacuum hot-pressing The interface performance between the panels and core is the key to exerting the mechanical/multifunctional advantages of composite sandwich structures [24].First, the surface of the bonding area between the panels and core is polished using sandpaper to achieve a certain roughness.Then, wash and wipe the panels and cores to maintain a clean contact surface.Finally, an epoxy resin structural adhesive film is used to bond the panels and cores.Paste the cut adhesive film onto the contact surface between the core and panels, depending on the adhesive film's inherent adhesion before curing.The initially bonded sandwich structures are slightly compacted by using the pressure-equalizing plate, and then placed on a heating platform.According to the vacuum hot-pressing process, perform the secondary curing.Figure 6 displays a concise schematic diagram of the vacuum hot-pressing process.After curing, the prepared sandwich structures were cut into the impact test specimens with the size of 120 mm × 120 mm.The interface performance between the panels and core is the key to exertin mechanical/multifunctional advantages of composite sandwich structures [24].Fir surface of the bonding area between the panels and core is polished using sandpa achieve a certain roughness.Then, wash and wipe the panels and cores to maintain a contact surface.Finally, an epoxy resin structural adhesive film is used to bond the p and cores.Paste the cut adhesive film onto the contact surface between the cor panels, depending on the adhesive film's inherent adhesion before curing.The in bonded sandwich structures are slightly compacted by using the pressure-equa plate, and then placed on a heating platform.According to the vacuum hot-pr process, perform the secondary curing.Figure 6 displays a concise schematic diagr the vacuum hot-pressing process.After curing, the prepared sandwich structures cut into the impact test specimens with the size of 120 mm × 120 mm.

Test Method
The low-velocity impact tests were performed using the Instron CEAST 9350 hammer testing machine, and the specimen was secured by a 76 mm pneumatic clam shown in Figure 7.The overall weight of the drop hammer consists of a force sen counterweight block, and an impactor, with a total mass of 10.48 kg.The load, tim impact velocity during the low-velocity impact test process were measured param and the initial height was automatically calculated and set by the software accord the set impact energy.Based on the results of the pre-test, the impact energy range hemispherical impactor with a diameter of 20 mm was set to 25-100 J, and the i energy range of the flat impactor with a diameter of 20 mm was set to 25-125 J.The i positions are node and base positions, respectively, as shown in Figure 8.

Test Method
The low-velocity impact tests were performed using the Instron CEAST 9350 drop hammer testing machine, and the specimen was secured by a 76 mm pneumatic clamp, as shown in Figure 7.The overall weight of the drop hammer consists of a force sensor, a counterweight block, and an impactor, with a total mass of 10.48 kg.The load, time, and impact velocity during the low-velocity impact test process were measured parameters, and the initial height was automatically calculated and set by the software according to the set impact energy.Based on the results of the pre-test, the impact energy range of the hemispherical impactor with a diameter of 20 mm was set to 25-100 J, and the impact energy range of the flat impactor with a diameter of 20 mm was set to 25-125 J.The impact positions are node and base positions, respectively, as shown in Figure 8.

Response of Node Impacts
Damage modes of the specimens resulting from the impact of a 20 mm diameter flat impactor at the node impact are illustrated in Figure 9.As can be observed, the failure modes of the sandwich structures are influenced by the impact energy.At an impact energy of 25 J, a circular impact mark similar to the diameter of the impactor was formed on the upper panel.From the damaged cross-section, it was found that the impact cracks were generated on the core wall surface under the in-plane compression load.Owing to the low impact energy, no damage occurred on the lower panel, and the impactor finally rebounded from the specimen due to insufficient impact energy.
At an impact energy level of 50 J, the upper panel sustained a tensile fracture to form a circular punching, and the damage expanded around the punching hole.The core mainly sustained buckling and brittle fracture.It is worth noting that no debonding occurred at the platform where the core was bonded to the upper panel, which proved that the core and the panel had a good bonding performance.The damage area surrounding the punching hole gradually expanded with increasing impact energy, leading to the delamination of the upper panel.At an impact energy level of 75 J, during the impact process, an impact plug was formed in the upper panel and the core underwent buckling, delamination, crushing, and even fracture.Due to the support of the core, the lower panel remained undamaged, as shown in Figure 9c.Damage modes of the specimens resulting from the impact of a 20 mm diameter flat impactor at the node impact are illustrated in Figure 9.As can be observed, the failure modes of the sandwich structures are influenced by the impact energy.At an impact energy of 25 J, a circular impact mark similar to the diameter of the impactor was formed on the upper panel.From the damaged cross-section, it was found that the impact cracks were generated on the core wall surface under the in-plane compression load.Owing to the low impact energy, no damage occurred on the lower panel, and the impactor finally rebounded from the specimen due to insufficient impact energy.When the impact energy was 100 J, the bonding between the core and the upper panel was broken, and the core in the impact area of the impactor completely failed.The formed impact plug with the upper panel was compacted and piled up on the lower panel, causing debonding of the core and the lower panel and the formation of small cracks on the lower panel, as shown in Figure 9d.At an impact energy level of 50 J, the upper panel sustained a tensile fracture to form a circular punching, and the damage expanded around the punching hole.The core mainly sustained buckling and brittle fracture.It is worth noting that no debonding occurred at the platform where the core was bonded to the upper panel, which proved that the core and the panel had a good bonding performance.The damage area surrounding the punching hole gradually expanded with increasing impact energy, leading to the delamination of the upper panel.At an impact energy level of 75 J, during the impact process, an impact plug was formed in the upper panel and the core underwent buckling, delamination, crushing, and even fracture.Due to the support of the core, the lower panel remained undamaged, as shown in Figure 9c.
When the impact energy was 100 J, the bonding between the core and the upper panel was broken, and the core in the impact area of the impactor completely failed.The formed impact plug with the upper panel was compacted and piled up on the lower panel, causing debonding of the core and the lower panel and the formation of small cracks on the lower panel, as shown in Figure 9d.
At an impact energy of 125 J, the sandwich panel was penetrated completely.This resulted in severe delamination and fiber pull-out of the upper panel.Because the tensile and compression damage were both generated and expanded inside and along the fiber direction [25], the damage morphology of tensile tearing on the lower panel evolved from small cracks to L-shaped tearing bands.Furthermore, the right side of the lower panel crack was concave downward, resulting in a more serious debonding phenomenon between the right core and the lower panel when compared to the left side.
The displacement of the impactor is defined as the distance the impactor travels from the moment it touches the specimen [26].The load-displacement curves and energy absorption-displacement curves of the sandwich panels at node impact are shown in Figure 10.When the sandwich panel is completely penetrated, a "double-peak" curve will be presented, whereby the distance between the two peaks indicates the extent of the core damage.At the initial stage, the load-displacement curves of each energy almost coincide and show a linear growth, suggesting that the sandwich panel is mainly under elastic deformation at the initial stage.At 25 J impact energy, the load-displacement curve displayed a "single-peak" pattern, wherein the curve sharply declined after reaching its peak value, attributed to insufficient impact energy.Additionally, an indentation was observed on the upper panel.At 75 J impact energy, the upper panel was punctured, and the tearing area continued to expand.At this time, core and upper panel debonding occurred, as well as severe buckling and crushing fracture.At an impact energy of 125 J, the sandwich panels were entirely penetrated.The load-displacement curve displayed a fluctuating decrease at a displacement of 26.69 mm, attributable to plastic deformation and friction of the lower panel.On the other hand, the calculation of energy absorption adopted the integration method of force and displacement.When the specimen was not penetrated, the sandwich panel absorbed all the impact energy.When the specimen was penetrated, the maximum absorption energy of the sandwich panel remained stable near the penetration threshold.From the energy absorption displacement curve in Figure 10b, the impact energy of each energy was completely absorbed at the node impact.

Response of Base Impacts
Figure 11 illustrates the damage modes of the specimens due to the flat impactor with a 20 mm diameter at base impact.At an impact energy of 25 J, the upper panel exhibited an irregularly shaped punching hole due to the plastic hinge formed through concave bending during the impact process.At an impact energy of 50 J, the upper panel displayed a tensile tear "channel" approximately 20 mm in length, resulting from the shear tensile effects of the impactor's right-angle edge and the extrusion of the upper panel impact plug.Furthermore, a severe crushing fracture occurred in the core wall, which became unstuck from the lower panel.For the 75 J impact energy, the upper panel exhibited a regular round hole measuring 20 mm in diameter.Notably, fiber pull-out and long cracks appeared on the lower panel when the core completely failed, leading to the formation of the core plug.
the sandwich panels were entirely penetrated.The load-displacement curve displayed a fluctuating decrease at a displacement of 26.69 mm, attributable to plastic deformation and friction of the lower panel.On the other hand, the calculation of energy absorption adopted the integration method of force and displacement.When the specimen was not penetrated, the sandwich panel absorbed all the impact energy.When the specimen was penetrated, the maximum absorption energy of the sandwich panel remained stable near the penetration threshold.From the energy absorption displacement curve in Figure 10b, the impact energy of each energy was completely absorbed at the node impact.

Response of Base Impacts
Figure 11 illustrates the damage modes of the specimens due to the flat impactor with a 20 mm diameter at base impact.At an impact energy of 25 J, the upper panel exhibited an irregularly shaped punching hole due to the plastic hinge formed through concave bending during the impact process.At an impact energy of 50 J, the upper panel displayed a tensile tear "channel" approximately 20 mm in length, resulting from the shear tensile effects of the impactor's right-angle edge and the extrusion of the upper panel impact plug.Furthermore, a severe crushing fracture occurred in the core wall, which became unstuck from the lower panel.For the 75 J impact energy, the upper panel exhibited a regular round hole measuring 20 mm in diameter.Notably, fiber pull-out and long cracks appeared on the lower panel when the core completely failed, leading to the formation of the core plug.
When the impact energy increased to 100 J, the damage forms of the upper panel and core remained similar to those observed at 75 J.However, due to the high energy input, the impact plug created by the upper panel and core was compressed onto the lower panel by the impactor after breaking, while the damage area of the lower panel expanded to the surrounding areas due to the loss of support from the core.When the impact energy was 125 J, the lower panel was penetrated, forming an approximately rectangular damage area, and the core and upper and lower panels both underwent debonding, as depicted in Figure 11e.Load-displacement curves and energy absorption-displacement curves of the five impact energies at base impact are presented in Figure 12.In order to eliminate environmental noise, we used percentile filtering to smooth the curves.Unlike for the node impact, the maximum load is presented at the second peak upon base impact.Damage to the upper panel is represented by the curve from the initial to the first peak.In the case of an impact energy of 125 J, the sandwich structure was completely penetrated, and the maximum energy absorption value of the When the impact energy increased to 100 J, the damage forms of the upper panel and core remained similar to those observed at 75 J.However, due to the high energy input, the impact plug created by the upper panel and core was compressed onto the lower panel by the impactor after breaking, while the damage area of the lower panel expanded to the surrounding areas due to the loss of support from the core.
When the impact energy was 125 J, the lower panel was penetrated, forming an approximately rectangular damage area, and the core and upper and lower panels both underwent debonding, as depicted in Figure 11e.Load-displacement curves and energy absorption-displacement curves of the five impact energies at base impact are presented in Figure 12.In order to eliminate environmental noise, we used percentile filtering to smooth the curves.Unlike for the node impact, the maximum load is presented at the second peak upon base impact.Damage to the upper panel is represented by the curve from the initial to the first peak.In the case of an impact energy of 125 J, the sandwich structure was completely penetrated, and the maximum energy absorption value of the sandwich structure was significantly less than the impact energy value, as shown by the purple curve in Figure 12.When the impact energy was 125 J, the lower panel was penetrated, forming an approximately rectangular damage area, and the core and upper and lower panels both underwent debonding, as depicted in Figure 11e.Load-displacement curves and energy absorption-displacement curves of the five impact energies at base impact are presented in Figure 12.In order to eliminate environmental noise, we used percentile filtering to smooth the curves.Unlike for the node impact, the maximum load is presented at the second peak upon base impact.Damage to the upper panel is represented by the curve from the initial to the first peak.In the case of an impact energy of 125 J, the sandwich structure was completely penetrated, and the maximum energy absorption value of the sandwich structure was significantly less than the impact energy value, as shown by the purple curve in Figure 12.The effects of impact position on the failure modes of trapezoidal corrugated sandwich structures were investigated by comparing damage modes at node impact and base impact.The results show that impact positions have a significant effect on damage evolution.When the impact energy is high, the damage evolution law of the upper and lower panels is similar at node impact and base impact.The upper panel formed a circular punching hole, while the lower panel evolved from small cracks to L-shaped tearing bands.At lower impact energies, such as 25 J, the damage degree of the upper panel at two specific positions is significantly different.Base impact led to significantly greater damage compared to node impact, as only the upper panel sustained shear failure before the impactor contacted the core.The experiments proved that impact positions have an obvious influence on the damage modes of the core.When an impactor hit the node position, the core primarily sustained in-plane impact loads that led to buckling, crushing, delamination, and fracture.In contrast, when the impactor hit the base position, the core walls mainly experienced tensile fracture and fiber pull-out under out-of-plane loads, resulting in the tearing of the bonding platform between the core and the lower panel.
The impact response curves of the flat impactor under varying impact energies are illustrated in Figure 13.When the sandwich panels were not penetrated, the sandwich panels absorbed all the impact energy at node and base positions.The highest fiber content points of sandwich structures are located in the bonding areas between the core and the panels, which have the highest impact resistance.Figure 13 indicates that the impact load at node impact increased rapidly and decreased sharply after peaking for the first time.The increase in impact load at base impact was more gradual compared to that at node impact, and after reaching its peak, the load continued to fluctuate for a distance before rapidly decreasing.At this stage, the main damage mode was the failure process of the upper panel, and the core wall had already been damaged but had not completely failed.panels absorbed all the impact energy at node and base positions.The highest fiber content points of sandwich structures are located in the bonding areas between the core and the panels, which have the highest impact resistance.Figure 13 indicates that the impact load at node impact increased rapidly and decreased sharply after peaking for the first time.The increase in impact load at base impact was more gradual compared to that at node impact, and after reaching its peak, the load continued to fluctuate for a distance before rapidly decreasing.At this stage, the main damage mode was the failure process of the upper panel, and the core wall had already been damaged but had not completely failed.The sandwich panel underwent penetration when subjected to an impact energy of 125 J. Figure 13e demonstrates a fluctuating decline in the load curves of both node and base impacts after achieving the second load peak.In addition, as shown in Figure 13, the energy absorption curves of node impact remain higher than those of base impact at any displacement.Table 3 outlines the maximum loads of node and base impacts corresponding to each impact energy, while Figure 14 presents the variation law of the maximum loads and maximum energy absorption of the sandwich structure at node and base impacts.The sandwich panel exhibited a considerably higher maximum impact load at the node impact location compared to base impact with the same level of impact energy.This was especially evident at low energy levels.An illustrative example is when the impact energy was 50 J, resulting in the node experiencing a maximum impact load that was 2.16 times higher than the maximum impact load experienced by the base.When the impact energy was 125 J, the energy absorption value during node impact was significantly greater than that of base impact.Therefore, the node position exhibited better impact resistance than the base position.This was especially evident at low energy levels.An illustrative example is when the impact energy was 50 J, resulting in the node experiencing a maximum impact load that was 2.16 times higher than the maximum impact load experienced by the base.When the impact energy was 125 J, the energy absorption value during node impact was significantly greater than that of base impact.Therefore, the node position exhibited better impact resistance than the base position.

The Effects of Impactor Shape
Damage modes of the specimen resulting from impact by a hemispherical impactor with a diameter of 20 mm at node impact are shown in Figure 15.When the impact energy was 25 J, the damage modes of the hemispherical impactor resembled those of the flat impactor, with an impact indentation forming on the upper panel and cracks occurring in the core wall.

The Effects of Impactor Shape
Damage modes of the specimen resulting from impact by a hemispherical impactor with a diameter of 20 mm at node impact are shown in Figure 15.When the impact energy was 25 J, the damage modes of the hemispherical impactor resembled those of the flat impactor, with an impact indentation forming on the upper panel and cracks occurring in the core wall.The damage morphology of the upper panel at an impact energy of 75 J appears to be similar to that observed at an impact energy of 50 J.The core sustained crushing fracture and delamination, while the lower panel produced small cracks.Upon being exposed to an impact energy of 100 J, the sandwich structure was penetrated, leading to the creation of a circular punching hole on the upper panel that measures around 20 mm in diameter.With the increase in impact energy, the core formed an impact plug after completely fracturing, with the impactor popping out and getting stuck at the crack opening of the lower panel.The fracture area was accompanied by more fiber pull-out and obvious delamination.The lower panel produced a cross-shaped crack and formed a pyramid-shaped protrusion.The damage morphology of the upper panel at an impact energy of 75 J appears to be similar to that observed at an impact energy of 50 J.The core sustained crushing fracture and delamination, while the lower panel produced small cracks.Upon being exposed to an impact energy of 100 J, the sandwich structure was penetrated, leading to the creation of a circular punching hole on the upper panel that measures around 20 mm in diameter.With the increase in impact energy, the core formed an impact plug after completely fracturing, with the impactor popping out and getting stuck at the crack opening of the lower panel.The fracture area was accompanied by more fiber pull-out and obvious delamination.The lower panel produced a cross-shaped crack and formed a pyramid-shaped protrusion.
Figure 16 illustrates the types of damage observed in the specimen following impact with a hemispherical impactor of 20 mm diameter at the base.At the energy level of 25 J, a four-corner star-shaped punching hole was formed on the upper panel.Owing to insufficient residual energy, the impactor was rebounded by the upper panel, so there was no obvious damage to the core and the lower panel.At the impact energy level of 50 J, the upper panel was penetrated and tensile tearing occurred, accompanied by fiber pull-out.Tensile tearing arose in the core due to the action of out-plane loads.At impact energy levels of 75 J and 100 J, the attachment point between the core and the lower panel failed completely, resulting in a cross-shaped crack with a conical protrusion formed by the lower panel.Upon comparing the damage modes of the flat and hemispherical impactors, it is found that the hemispherical impactor requires less energy to penetrate the sandwich panel compared to the flat impactor.This is because the stress of the hemispherical impactor is concentrated at the center point of the sphere during impact, which is more concentrated than that of the flat impactor.Therefore, the smaller the contact surface of the impactor, the more concentrated the stress, and the easier it is to penetrate the sandwich panel.Thereby, we simplified the flat impactor and the hemispherical impactor, both with a diameter of 20 mm, as F20 and H20, respectively.Under the impact energy of 25 J, 75 J, and 100 J, the load curves of F20 and H20 at the node and base impact are compared, as shown in Figure 17.When the impactor impacted at the node position, the load curves of F20 and H20 had the same trend, but the differences were the fluctuation of their load curves with displacement and the maximum load values.Upon impact, it became clear that the hemispherical impactor underwent greater displacement than the flat impactor, even though both were subjected to the same amount of energy.When the sandwich panel did not suffer any penetration, it absorbed the entire impact energy, resulting in the energy absorption curve reaching the same value as the impact energy.The variation curves of the maximum load and maximum energy absorption of F20 and H20 under various energy impacts are shown in Figure 18.It is evident that the flat Upon comparing the damage modes of the flat and hemispherical impactors, it is found that the hemispherical impactor requires less energy to penetrate the sandwich panel compared to the flat impactor.This is because the stress of the hemispherical impactor is concentrated at the center point of the sphere during impact, which is more concentrated than that of the flat impactor.Therefore, the smaller the contact surface of the impactor, the more concentrated the stress, and the easier it is to penetrate the sandwich panel.Thereby, we simplified the flat impactor and the hemispherical impactor, both with a diameter of 20 mm, as F20 and H20, respectively.Under the impact energy of 25 J, 75 J, and 100 J, the load curves of F20 and H20 at the node and base impact are compared, as shown in Figure 17.When the impactor impacted at the node position, the load curves of F20 and H20 had the same trend, but the differences were the fluctuation of their load curves with displacement and the maximum load values.Upon impact, it became clear that the hemispherical impactor underwent greater displacement than the flat impactor, even though both were subjected to the same amount of energy.When the sandwich panel did not suffer any penetration, it absorbed the entire impact energy, resulting in the energy absorption curve reaching the same value as the impact energy.The variation curves of the maximum load and maximum energy absorption of F20 and H20 under various energy impacts are shown in Figure 18.It is evident that the flat impactor exhibits a consistently greater maximum impact load than the hemispherical impactor at node impact, while the maximum impact load changes irregularly for flat and hemispherical impactors at base impact.In addition, the energy absorption curves of F20 and H20 were tightly fitted, and the impact energy was completely absorbed at node impact.However, when the impact energy at base impact was 100 J, the maximum energy absorption value of H20 was obviously smaller than F20.At that time, the hemispherical impactor had already penetrated the sandwich panel, but the flat impactor had not yet reached the penetration energy value, which proves that the penetration energy required for the hemispherical impactor was smaller than that of the flat impactor.

The Effects of Impactor Diameter
By analyzing the damage situations of F20 and H20 at node impact and base impact, it is concluded that when the impact energy is less than 75 J, the main damaged parts of the sandwich structure are the upper panel and core.As the impact energy exceeds 75 J, the sandwich structure is penetrated, resulting in local penetrating damage.Therefore, the damage modes of the sandwich structure are chosen at an impact energy of 75 J for the purpose of studying the performance of the sandwich structure under impact, affected by the impactor diameter.To conduct low-velocity impact tests on sandwich structures, hemispherical impactors with varying diameters were applied: 20 mm, 25 mm, and 38 mm.The impactors were simplified as H20, H25, and H38, and the impact test results were compared and analyzed.Figure 19 shows the damage modes of specimens with hemispherical impactors of specimens with H20, H25, and H38 at the node impact when an impact energy of 75 J was exerted.At node impact, the damage form of the upper panel of H25 was similar to that of H20.The upper panel sustained tensile tearing to form a cross-shaped damage morphology, but the buckling deformation and fracture of the core were not significant compared to H20.On the other hand, a shallow indentation was formed on the upper panel of H38 at node impact, causing impact cracks on the core wall and mild fracture along the cracks.The impact results in Figure 19 indicated that when three kinds of diameter impactors impact the sandwich panels at node impact, the lower panel generated damage only under H20 impact.

The Effects of Impactor Diameter
By analyzing the damage situations of F20 and H20 at node impact and base impact, it is concluded that when the impact energy is less than 75 J, the main damaged parts of the sandwich structure are the upper panel and core.As the impact energy exceeds 75 J, the sandwich structure is penetrated, resulting in local penetrating damage.Therefore, the damage modes of the sandwich structure are chosen at an impact energy of 75 J for the purpose of studying the performance of the sandwich structure under impact, affected by the impactor diameter.To conduct low-velocity impact tests on sandwich structures, hemispherical impactors with varying diameters were applied: 20 mm, 25 mm, and 38 mm.The impactors were simplified as H20, H25, and H38, and the impact test results were compared and analyzed.Figure 19 shows the damage modes of specimens with hemispherical impactors of specimens with H20, H25, and H38 at the node impact when an impact energy of 75 J was exerted.At node impact, the damage form of the upper panel of H25 was similar to that of H20.The upper panel sustained tensile tearing to form a cross-shaped damage morphology, but the buckling deformation and fracture of the core were not significant compared to H20.On the other hand, a shallow indentation was formed on the upper panel of H38 at node impact, causing impact cracks on the core wall and mild fracture along the cracks.The impact results in Figure 19 indicated that when three kinds of diameter impactors impact the sandwich panels at node impact, the lower panel generated damage only under H20 impact.
of H25 was similar to that of H20.The upper panel sustained tensile tearing to form a cross-shaped damage morphology, but the buckling deformation and fracture of the core were not significant compared to H20.On the other hand, a shallow indentation was formed on the upper panel of H38 at node impact, causing impact cracks on the core wall and mild fracture along the cracks.The impact results in Figure 19 indicated that when three kinds of diameter impactors impact the sandwich panels at node impact, the lower panel generated damage only under H20 impact.Figure 20 illustrates the damage modes of specimens with H20, H25, and H38 at base impact when the impact energy was 75 J.It can be observed that the damage morphology of the upper panel gradually became irregular as the impactor diameter increased.When the H20 impacted the base position, the bonding platform between the core and the lower panel was completely torn, and the core and the lower panel debonded, forming a cross-shaped crack and conical protrusion on the lower panel.The impact of H25 and H38 at the base position caused a significant collapse in the upper panel and resulted in the detachment of the core from the upper panel.The damage forms of the core with H25 at base impact were similar to that of H20, while the core was not significantly damaged with H38 at base impact.Therefore, through a comparison of the damage modes resulting from impact at the node and base positions using impactors of three different diameters, a notable variation can be observed in the damage sustained by the core with an increase in the diameter of the impactor at node impact.Conversely, a significant variation in damage is seen in the upper panel when impacted at the base position.Figure 20 illustrates the damage modes of specimens with H20, H25, and H38 at base impact when the impact energy was 75 J.It can be observed that the damage morphology of the upper panel gradually became irregular as the impactor diameter increased.When the H20 impacted the base position, the bonding platform between the core and the lower panel was completely torn, and the core and the lower panel debonded, forming a crossshaped crack and conical protrusion on the lower panel.The impact of H25 and H38 at the base position caused a significant collapse in the upper panel and resulted in the detachment of the core from the upper panel.The damage forms of the core with H25 at base impact were similar to that of H20, while the core was not significantly damaged with H38 at base impact.Therefore, through a comparison of the damage modes resulting from impact at the node and base positions using impactors of three different diameters, a notable variation can be observed in the damage sustained by the core with an increase in the diameter of the impactor at node impact.Conversely, a significant variation in damage is seen in the upper panel when impacted at the base position.The impact response performance of the sandwich structure is significantly impacted by the diameter of the impactor.Figure 21 displays the load variation curves and energy absorption associated with three different impactor diameters when impacted at both the node and base positions, with an applied energy of 75 J.Based on Figure 21, it is clear that The impact response performance of the sandwich structure is significantly impacted by the diameter of the impactor.Figure 21 displays the load variation curves and energy absorption associated with three different impactor diameters when impacted at both the node and base positions, with an applied energy of 75 J.Based on Figure 21, it is clear that the H20 had the longest impact displacement and the H38 had the shortest impact displacement.The analysis of damage modes resulting from the impact of three impactors of varying diameters on both the node and base positions of the sandwich panel revealed that the contact surface area between the impactor and the sandwich panel increased and the damage propagation became more extensive with an increase in the impactor diameter.This resulted in a smaller degree of damage to the sandwich panel.Therefore, for a given impact energy, an increase in impactor diameter led to a reduction in impactor displacement.

Conclusions
In this work, various low-velocity impact tests were performed to investigate the impact response and damage modes of glass fiber trapezoidal corrugated sandwich structures under lowvelocity impact by investigating the effect of impact position, impactor shape, and impactor diameter on the dynamic response characteristics and mechanisms of sandwich structures.The research shows the following: (1) Under scenarios of increased impact energy, principle failure modes of sandwich structures consist of three stages.At a low level of impact energy, the damage modes of sandwich structures are mainly tensile tearing in the upper panel.As impact energy increases, the core wall experiences buckling, crushing fracture, and delamination.When the impact energy is high, the lower panel sustains tensile tearing, and the sandwich structure is penetrated.(2) It is clear that the impact position can influence the damage mechanism of sandwich structures.Under the same impact energy, the maximum impact load generated at node impact significantly exceeds that of base impact, especially when the impact energy is relatively low.However, the impact displacement of the impactor at base impact is larger than that of the node impact.(3) By comparing damage modes with flat and hemispherical impactors, it is found that the penetrated energy required for the hemispherical impactor to impact the sandwich panel is less than that of the flat impactor.As a result, the smaller the contact surface of the impactor, the more concentrated the stress, and the easier it is

Conclusions
In this work, various low-velocity impact tests were performed to investigate the impact response and damage modes of glass fiber trapezoidal corrugated sandwich structures under low-velocity impact by investigating the effect of impact position, impactor shape, and impactor diameter on the dynamic response characteristics and mechanisms of sandwich structures.The research shows the following: (1) Under scenarios of increased impact energy, principle failure modes of sandwich structures consist of three stages.At a low level of impact energy, the damage modes of sandwich structures are mainly tensile tearing in the upper panel.As impact energy increases, the core wall experiences buckling, crushing fracture, and delamination.When the impact energy is high, the lower panel sustains tensile tearing, and the sandwich structure is penetrated.(2) It is clear that the impact position can influence the damage mechanism of sandwich structures.Under the same impact energy, the maximum impact load generated at node impact significantly exceeds that of base impact, especially when the impact energy is relatively low.However, the impact displacement of the impactor at base impact is larger than that of the node impact.(3) By comparing damage modes with flat and hemispherical impactors, it is found that the penetrated energy required for the hemispherical impactor to impact the sandwich panel is less than that of the flat impactor.As a result, the smaller the contact surface of the impactor, the more concentrated the stress, and the easier it is to penetrate the sandwich structures.(4) When the impact energy and impactor shape remain unchanged, the overall impact damage degree of the sandwich structure reduces with the rise in impactor diameter.
With the node position impacted by the impactor, the damage evolution of the core changes more significantly with impactor diameter.On the other hand, when the impactor impacts the base position, the damage to the upper panel changes more obviously as the impactor diameter increases.

Figure 2 .
Figure 2. Shape and diameter of impactor.

Figure 2 .
Figure 2. Shape and diameter of impactor.

Figure 2 .
Figure 2. Shape and diameter of impactor.

Figure 3 .
Figure 3. Schematic diagram of the procedure for preparing the sandwich panel.

Figure 4 .
Figure 4. Preparation of mold for trapezoidal core molding.

Figure 3 .Figure 3 .
Figure 3. Schematic diagram of the procedure for preparing the sandwich panel.

Figure 4 .
Figure 4. Preparation of mold for trapezoidal core molding.

Figure 5 .
Figure 5. Preparation equipment and curing curve of trapezoidal sandwich panel.

Figure 5 .
Figure 5. Preparation equipment and curing curve of trapezoidal sandwich panel.

FOR PEER REVIEW 7 of 20 Figure 7 .
Figure 7. Drop hammer testing system for low-velocity impact testing.

Figure 7 .
Figure 7. Drop hammer testing system for low-velocity impact testing.

Figure 7 .
Figure 7. Drop hammer testing system for low-velocity impact testing.

Figure 8 .
Figure 8. Impact position of trapezoidal sandwich structure.

Figure 8 .
Figure 8. Impact position of trapezoidal sandwich structure.

20 Figure 9 .
Figure 9. Damage modes of sandwich panel with flat impactor at node impact.

Figure 9 .
Figure 9. Damage modes of sandwich panel with flat impactor at node impact.

Figure 10 .
Figure 10.Load-displacement curves and energy absorption-displacement curves of sandwich panel with flat impactor at node impact.(a) Force-displacement, (b) energy absorptiondisplacement.

Figure 11 .
Figure 11.Damage modes of sandwich panel with flat impactor at base impact.

Figure 11 .
Figure 11.Damage modes of sandwich panel with flat impactor at base impact.

Figure 12 .
Figure 12.Load-displacement curves and energy absorption-displacement curves of sandwich panel with flat impactor at base impact.(a) Force-displacement, (b) energy absorption-displacement.

Figure 13 .
Figure 13.Load-displacement curve and energy absorption curve impact sandwich structure with flat impactor at node impact and base impact.

Figure 14 .
Figure 14.The variation law of the max.force and max.energy absorption of sandwich structure at node impact and base impact.

Figure 14 .
Figure 14.The variation law of the max.force and max.energy absorption of sandwich structure at node impact and base impact.

Figure 15 .
Figure 15.Damage modes of sandwich panel with hemispherical impactor at node impact.

Figure 15 .
Figure 15.Damage modes of sandwich panel with hemispherical impactor at node impact.

J 20 Figure 16 .
Figure 16.Damage modes of sandwich panel with hemispherical impactor at base impact.

Figure 16 .
Figure 16.Damage modes of sandwich panel with hemispherical impactor at base impact.

JFigure 17 .
Figure 17.Comparison of the impact load curves between F20 and H20 at node impact and impact.

Figure 17 .
Figure 17.Comparison of the impact load curves between F20 and H20 at node impact and base impact.

Figure 17 .Figure 18 .
Figure 17.Comparison of the impact load curves between F20 and H20 at node impact and base impact.

Figure 18 .
Figure 18.Comparison of max.force and max.energy absorption with F20 and H20 at node impact and base impact.

Figure 21 .
Figure 21.E = 75 J.Load-displacement curve and energy absorption curve of sandwich panel under impact from H20, H25, and H38 impactors at node and base positions.

Figure 21 .
Figure 21.E = 75 J.Load-displacement curve and energy absorption curve of sandwich panel under impact from H20, H25, and H38 impactors at node and base positions.

Table 1 .
Mechanical properties of composite material.

Table 2 .
Cross-section parameters of trapezoidal core.

Table 2 .
Cross-section parameters of trapezoidal core.

Table 3 .
Comparative analysis of the maximum impact load in sandwich panel.

Table 3 .
Comparative analysis of the maximum impact load in sandwich panel.