Study on the low-velocity impact properties of GF/PP thermoplastic composite sandwich structure and the influence of impact damage on vibration characteristics

Abstract

Thermoplastic composite sandwich structures are widely utilized in aerospace and other high-performance fields due to their advantages of lightweight construction, high strength, high flexural stiffness, excellent damage tolerance, and recyclability. The thermoplastic composite sandwich structure behaves ductilely under static load. However, under impact loading, it may exhibit brittleness or even lead to catastrophic failure. Therefore, in applications where impact considerations are paramount, the design of the sandwich structure must demonstrate good impact properties while also satisfying static and fatigue load requirements. Using a low-velocity drop-weight impact test, this paper investigates the impact response of sandwich structures with various panel configurations and different core thicknesses. Additionally, the influence of damage extent on the vibration characteristics of the sandwich structure under varying impact energies is examined with a vibrometer. The results indicate that the impact peak load of the sandwich structure increases with the panel thickness and core thickness, and the peak load for sandwiches with symmetrical configurations is greater than that for those with unidirectional configurations. The impact energy absorption capability of the sandwich structure is primarily influenced by its failure mode. When both the panel and core are damaged, the structure exhibits superior energy absorption properties. As impact energy increases, the vibration modal frequency of the sandwich structure decreases, while the damping ratio increases.

Keywords

GF/PP thermoplastic composites sandwich structure low-velocity impact properties vibration characteristics

Introduction

The sandwich structure is usually composed of two layers of panels and a lightweight core material in the middle. The panel is usually composed of high-strength and high-modulus materials such as metal and fiber-reinforced resin matrix composites, and the core material is generally composed of porous or lightweight materials such as foam and honeycomb. Ferri et al.¹ compared the damage resistance and energy absorption properties of graphite/bismaleimide composite laminates and sandwich plates through drop-weight impact and static indentation tests. It was found that sandwich plates absorb more energy than laminates and have less deflection when absorbing the same maximum energy. Among many different core materials, honeycomb core material is more common in the aerospace industry. Honeycomb sandwich structure is widely used in automotive, aerospace, and transportation engineering design because of its high specific energy absorption, high stiffness, and strength-to-weight ratio.^2–7 Because honeycomb core material is a porous solid material, it will not damage the stiffness and energy absorption characteristics of the structure while reducing the total weight of the structure.⁸ At the same time, compared with many other rigid foams, honeycomb can be better controlled in terms of mechanical properties because of its customizable porous thin-walled structure.⁹

Among different types of honeycomb core materials, Nomex honeycomb and aluminum honeycomb are widely used because of their low cost and low density. However, these two types of honeycombs are greatly affected by humidity factors, which will lead to a significant decrease in the mechanical properties of the structure, thereby reducing its service life.¹⁰ At present, many studies have focused on the sandwich structure of thermosetting composites, such as epoxy resin, because of its excellent mechanical and thermal properties, which are widely used in the matrix system of Fiber Reinforced Polymer (FRP) composites. However, the out-of-plane properties of thermosetting composites are relatively poor, especially under impact load. In addition, once the thermosetting composites are cured, they can neither be molded nor recycled.^11,12 Compared with thermosetting composites, thermoplastic composites have many advantages, including excellent impact resistance, post-formability, high chemical resistance, good shock absorption, and recyclability.^11,13

The dynamic impact response of the honeycomb sandwich structure is greatly affected by honeycomb size parameters and honeycomb materials. Cao et al.¹⁴ studied the impact mechanical properties of sandwich structures with cross-grid cores, and compared the energy absorption of the structure under different impact velocities. Foo et al.¹⁵ found that the low-velocity impact resistance of aluminum sandwich panels can be improved by higher aluminum foil density, larger aluminum foil thickness, or smaller unit size. Gunes and Arslan proved by numerical simulation that the stiffness and stability of the aluminum honeycomb sandwich structure increase with the decrease of the core size, but the core thickness has little effect on the impact response.¹⁶ Crupi et al.¹⁷ used 3D computed tomography to study the failure mode and damage of aluminum honeycomb sandwich panels under impact loading. It is found that the honeycomb collapse area with a smaller unit size is more concentrated. To improve the crashworthiness of the honeycomb sandwich structure, many scholars choose different kinds of composite materials with different specific strengths as the panel of the sandwich structure. Crupi et al.¹⁸ used glass fiber reinforced polymer (GFRP) panels to improve the energy absorption and bearing capacity of honeycomb sandwich structures. Liu et al.¹⁹ studied the impact response of Nomex honeycomb sandwich structure and fiber reinforced woven fabric composite panel under different impact energy, impactor mass, and tilt angle. Wu et al.²⁰ and He et al.²¹ compared the impact response of carbon fiber reinforced polymer (CFRP) sandwich panels with honeycomb cores of different parameters by experimental and numerical methods, respectively. Zhang et al.²² studied the drop-weight impact response of a Nomex honeycomb sandwich structure with a CFRP panel. The energy absorption mechanism of the sandwich structure is related to the failure mode of the structure. Various failure modes that may occur during the low-velocity impact of the sandwich structure include local indentation/cracking of the panel, buckling of the core material, debonding between the panel and the core material, extrusion failure of the lower panel, and full penetration of the sandwich structure.^14,23–26

Structural components in aerospace are subjected to various dynamic loads, and these dynamic loads can also produce excessive vibration. Therefore, it is necessary to understand the main response modes of the structure. Vibration is described as the time-dependent motion of particles around their equilibrium state. The dissipation of vibration energy with time and distance in solid media and structures is called damping. Damping occurs in places with friction, reduces motion, and disperses energy. The loss factor or ratio between the energy dissipated in each cycle and the energy still present in the system is called the damping capacity of each material.²⁷ The sandwich structure is usually composed of a high-strength panel and a lightweight core material. Compared with the mean structure, the sandwich structure has excellent density specificity, high bending stiffness and strength, as well as high impact resistance and vibration resistance.^28,29 Sheikhi et al.^27,30,31 studied the vibration behavior of sandwich structures and prepared sandwich structures composed of aramid fiber fabric reinforced polymer (AFRP ), carbon fiber reinforced polymer (CFRP ), glass fiber reinforced polymer (GFRP) panels, and aluminum panels with a fixed cork core. The impact vibration reduction and vibration test of the panels were carried out respectively. It was found that the panels based on CFRP showed the highest damping and stiffness characteristics. The shear thickening fluid (STF) and multifunctional shear thickening fluid (M-STF) with different rheological properties were added to the sandwich structure composed of GFRP and cork core, and the sandwich structure composed of polyvinyl chloride (PVC) and aluminum panel, respectively. The final research results show that the addition of intelligent materials such as STF and M-STF can greatly improve the vibration resistance of the structure.

Vaidya et al.³² studied the vibration response of composite sandwich plates composed of laminated plates and aluminum foam under free boundary conditions. Hao et al.³³ studied the bending-torsion coupling of functionally graded sandwich plates. Liu et al.³⁴ studied the nonlinear vibration of a functionally graded shell with porosity on an elastic substrate. Duc et al.³⁵ studied the dynamic response and impact of a composite honeycomb sandwich plate, and analyzed the influence of geometric characteristics on the natural frequency. Zhang et al.³⁶ studied the influence of elastic parameters and gradient classification on the natural frequency of functionally graded honeycomb sandwich panels, and studied the dynamic response of functionally graded honeycomb sandwich panels under low-velocity impact.

Although numerous studies have been conducted on the low-velocity impact behavior of sandwich structures, the impact response and post-impact vibration characteristics of thermoplastic composite honeycomb sandwich structures such as GF/PP composites have not been thoroughly investigated. Sandwich structures are widely used in load-bearing and functional applications due to their high strength, lightweight nature, and integrated functionalities. Thermoplastic composites like GF/PP exhibit excellent toughness, impact resistance, high damage tolerance, short molding cycles, and recyclability. GF/PP sandwich structures combine the advantages of both components: the high strength and stiffness of GF/PP face sheets and the toughness of PP core materials, forming a “rigid-flexible” hybrid structure. While sandwich structures typically demonstrate ductile behavior under static loads, they often exhibit brittleness under dynamic loads such as impact. Therefore, improving their impact resistance is critical for applications involving dynamic loading. This study employs low-velocity drop-weight impact tests to analyze the mechanical response and energy absorption of sandwich structures with different face sheet layup configurations and core thicknesses, revealing the influence of these parameters on impact performance. Since varying impact energies induce different damage modes that alter vibration characteristics (natural frequency, damping ratio, and modal shapes), vibration tests are conducted to evaluate the effects of impact-induced damage on dynamic behavior. A correlation is established among three key aspects: the impact performance of structural parameters, post-impact damage modes, and their influence on vibration characteristics. The results provide foundational data for optimizing the design of GF/PP sandwich structures in dynamic load applications.

The GF/PP sandwich structures in this study were fabricated using the non-isothermal molding process of high-temperature mold forming low-temperature workpiece proposed by Guo et al.³⁷ This method prevents excessive melting of the core material, thereby avoiding thickness reduction and mechanical property degradation that could lead to structural collapse. Furthermore, the process achieves bonding between face sheets and core solely through PP resin melting without introducing additional materials, eliminating potential interfacial factors that might compromise impact performance.

Test methods and test materials

Test Materials and Test Equipment

The test materials used in this paper include: Continuous glass fiber reinforced polypropylene (CGF/PP) unidirectional prepreg tapes, the reference number is SHMR10080517, which is produced by Shanghai Jieshijie New Material (Group) Co., Ltd. The mass content of glass fiber is 60%, the thickness of prepreg tape is 0.3 mm, and the density is 1.92 g/cm³. Table 1 lists the basic mechanical properties of CGF/PP. A polypropylene (PP) circular hole-like honeycomb panel was produced by Shixin Eurasia Plastic Hardware Co., Ltd, Zhongshan City, Guangdong Province. The circular hole diameter of the honeycomb panel was 8 mm, and the wall thickness of the honeycomb hole was 0.18 mm. Table 2 is the basic information of the test equipment.

Table 1.

Basic mechanical properties of CGF/PP unidirectional prepreg tape.

Tensile strength (MPa)	Tensile modulus (GPa)	Bending strength (MPa)	Bending modulus (GPa)
209.6	12.5	241.6	11.6

Table 2.

Test equipment information.

Equipment	Model	Manufacturers
Low-velocity drop-weight impact testing machine	CLC-A	Beijing Guance Jingdian Equipment Co., Ltd
Contact force sensor	208C05	National Instruments, USA
Modal data acquisition instrument	DH5922D	Jiangsu Donghua Testing Technology Co., Ltd.
Hammer	DH5922 L	Jiangsu Donghua Testing Technology Co., Ltd.
Acceleration sensor	3A102	Jiangsu Donghua Testing Technology Co., Ltd.
Contact force sensor	1A116 E	Jiangsu Donghua Testing Technology Co., Ltd.

Sample Preparation

Preparation of GF/PP thermoplastic composite sandwich structure by non-isothermal molding process of high-temperature mold forming low-temperature workpiece is shown in Figure 1.³⁸ The length of the sample is 150 mm, and the width is 100 mm. To facilitate the analysis, each sample uses a feature label. H indicates the thickness of the core material of the sandwich structure. For example, H10-0/90/0 indicates that the thickness of the core material is 10 mm, the panel is 3 layers of GF/PP prepreg, and the configuration is 0/90/0. Figure 2 is the impact specimen of the GF/PP sandwich structure prepared in this paper. Sandwich structures with different core thickness, different panel thickness, and different configurations were prepared, respectively, and then impact tests were carried out.

Figure 1.

High temperature mold forming low temperature workpiece non-isothermal molding process to prepare sandwich structure diagram.³⁸.

Figure 2.

GF/PP impact specimens.

Low-velocity Drop-Weight impact Test

The low-velocity drop-weight impact test machine was used to carry out the low-velocity impact test on the prepared samples. The test method was referred to as ASTM D7136/7136M《Standard Test Method for Measuring the Damage Resistance of a Fiber-Reinforced Polymer Matrix Composite to a Drop-Weight Impact Event》.³⁹ The low-velocity drop-weight impact test device used in this paper is shown in Figure 3. The test device consists of four parts: the double-column impactor guide device, the impactor and the force sensor, the impact load acquisition device, and the sample clamping device. There is a pneumatic valve on the upper part of the clamping device, which is automatically closed after the impact to prevent the drop-weight from re-impacting the test sample due to rebound. The total mass of the drop-weight is 1.28 kg. The impactor is a steel hemispherical impactor with a diameter of 16 mm. The specimen is clamped on four sides. The impact energy is adjusted by adjusting the distance between the drop-weight and the upper surface of the specimen. The formula (1) is the calculation formula of impact energy, which can be calculated according to the formula. Table 3 shows the drop-weight height and impact velocity corresponding to different energies.

E = m g h = \frac{1}{2} m v^{2}

(1)

In the formula, E is the impact energy, and the unit is J; m is the weight of the drop-weight, unit is kg; g is the acceleration of gravity, taking 9.81 m/s²; v is the speed of the drop-weight contact sample, the unit is m/s ;

Figure 3.

Drop-weight low-velocity impact test device (a) Drop-weight impact load acquisition device (b) Drop-weight impact overall device (c) impactor and sensor (d) Sample clamping device.

Table 3.

The drop-weight height and impact velocity corresponding to different energy.

Impact energy(J)	Falling weight height(m)	Impact velocity (m/s)
5	0.40	2.79
10	0.80	3.95
15	1.20	4.84
20	1.60	5.59

When the impactor connects to the sensor and contacts the sample, the load acquisition device displays the load during the impact process. The sampling frequency is 200 KHz, and 200,000 data points are collected per second. Finally, a continuous curve is output, and the load-time curve during the impact process is processed.

In the test, the load-time curve during the impact process is obtained by the sensor, and the velocity-time curve during the impact process can be obtained by integrating it.

v (t) = v_{i} + g t - \int_{0}^{t} \frac{F (t)}{m} d t

(2)

In the formula, v (t) is the velocity of the impactor at time t, in m/s; v is the initial impact velocity of drop-weight, and the unit is m/s; g is the acceleration of gravity, unit is m/s²; t is the action time, the unit is s; f (t) is the contact force recorded by the sensor at time t, and the unit is N; m is the mass of the impact drop-weight, in kg.

According to the velocity-time curve, the displacement-time curve can be calculated :

δ (t) = δ_{i} + v_{i} t + \frac{g t^{2}}{2} - \int_{0}^{t} [\int_{0}^{t} \frac{F (t)}{m} d t] d t

(3)

In the formula, δ (t) is the falling displacement of the impactor at time t, and the unit is m; δ_i is the relative displacement of the impactor at the initial time of t = 0, and the unit is m.

Finally, the energy-time curve of the sample absorbed during the impact process is obtained :

E (t) = \frac{m [v_{i}^{2} - v {(t)}^{2}]}{2} + m g δ (t)

(4)

In the formula, E (t) is the energy absorbed by the structure at time t, and the unit is J.

Finally, through step-by-step calculation, the load-time curve output by the load acquisition device is transformed into an energy-time curve.

The specific energy absorption of the structure is calculated according to the ratio of the total energy absorption to the mass of the structure :

S E A = \frac{E}{m}

(5)

In the formula, SEA is the specific energy absorption of the sandwich structure, the unit is J/g, E is the total energy absorbed by the structure, the unit is J, m is the mass of the sandwich structure, and the unit is g.

Modal Test of Force Method

Modal analysis is a common method in structural dynamics, which mainly includes experimental modal analysis and computational modal analysis. The modal test in this paper is to excite and respond to the test piece, and obtain the modal parameters of the structure through the identification and analysis of the vibration signal. Modal parameters include modal frequency, modal damping ratio, and modal shape, which are the natural vibration characteristics of the structure. Each mode has a fixed frequency, damping ratio, and vibration shape. Among them, the main research method of modal analysis is the frequency response function. By decoupling the matrix equation that expresses the vibration characteristics of the system, the modal parameters of each order are obtained, which provides a basis for studying the vibration characteristics of the structure. The acceleration frequency response function of the damped system can be expressed in the form of a complex exponent :

H_{A} (ω) = | H_{A} (ω) | {e^{i β}}^{(ω)}

(6)

In the formula, |H_A(ω)| and β(ω) are the amplitude and phase of the acceleration frequency response function, respectively. The expression is :

| H_{A} (ω) | = \frac{1}{m \sqrt{{[1 - {(ω_{0} / ω)}^{2}]}^{2} + {(2 ζ ω_{0} / ω)}^{2}}}

(7)

β (ω) = \tan^{- 1} \frac{2 ζ ω_{0} / ω}{1 - {(ω_{0} / ω)}^{2}}

(8)

In the formula, m represents the mass of the structure, ζ represents the structural damping ratio, and ω represents the frequency ;ω₀ represents the natural frequency of the structure.

In the process of structural system vibration, damping has the effect of dissipating energy, and its essence is to convert the dissipated energy into heat energy or other energy.⁴⁰ The free vibration attenuation method is a classical method for measuring damping in the time domain. When the system vibrates freely under the initial displacement, the amplitude will decay exponentially with time under the action of damping, and the damping ratio can be measured by calculating the corresponding amplitude. The expression of the free vibration of a single-degree-of-freedom damped system is :

x (t) = A e^{- ζ ω_{0} t} \cos (ω_{d} t + θ)

(9)

In the formula, A represents the initial amplitude of the system ; ω₀ represents the natural frequency of the structure ; ω_d represents the natural frequency of the damping system; ζ represents the structural damping ratio; θ represents the phase angle. To improve the accuracy of the damping ratio estimation, the amplitude of the interval n cycles can be used to calculate, and the damping logarithmic attenuation rate is defined as δ. The expression is :

δ = \frac{1}{n} \ln \frac{x_{1}}{x_{n + 1}} = \frac{2 π ζ}{\sqrt{1 - ζ^{2}}}

(10)

Figure 4 is the test device diagram of the modal test in this paper. The test device is mainly composed of a data acquisition system, signal collection software, a hammer with a contact force sensor, and an acceleration sensor. The signal collection software can collect the signal of the change of contact force and acceleration during the hammer striking.

Figure 4.

Modal test device diagram.

To effectively establish the relationship between damage severity and modal vibration characteristics of sandwich structures under varying impact energies while eliminating the influence of extraneous factors such as specimen dimensions and boundary conditions, this study maintained identical specimen dimensions (150 mm × 100 mm) for both vibration modal testing and impact testing. The sandwich structures were subjected to four distinct impact energy levels (5J, 10J, 15J, and 20J). Subsequent vibration characterization was performed to compare the dynamic responses between pristine (non-impacted) specimens and post-impact specimens across these energy levels. This controlled experimental approach ensures direct correlation analysis between impact-induced damage progression and corresponding modal parameter variations.

Figure 5 is the point diagram of the modal test. In this paper, the cantilever support method is used to fix the sandwich structure, and an approximate simulation of the free boundary condition is presented. The sandwich plate is fixed on the table surface by using the C-type clamp, and the acceleration sensor is fixed at the cantilever end, that is, the measuring point position 5, because the nodes near the support end are fixed, which is a plane constraint, so the measuring point is not set, as shown in Figure 5(a). The force hammer with a contact force sensor is used to hit the point of the sandwich structure point by point, and the acceleration signal at point 5 is obtained. The average value of each point is hit three times. The frequency response function is obtained by the dynamic signal test and analysis software. The PolyLSCF method is used to calculate the steady state of the collected data, and the modal frequency, modal damping ratio, and modal shape of the sandwich structure are obtained.

Figure 5.

Schematic diagram of modal test point (a ) Modal model in software ( b ) Modal model in test.

Results and discussion

Analysis of load-time curve of sandwich structure

Low-velocity impact response of sandwich structures with different configurations

In order to study the influence of the configurations of the sandwich structure panel on the overall bearing impact load of the structure, three kinds of panels with different thicknesses were prepared, respectively, and the thickness of the panel was changed by changing the number of layers of the GF/PP prepreg. The 0° unidirectional ply and the 0°、90° symmetrical ply were prepared for each panel thickness, and the sandwich structure impact samples were prepared with a core material of 10 mm thickness.

Table 4 lists the peak contact loads of sandwich structures with different face sheet layup configurations under four impact energy levels. Figure 6 presents the load-time curves of these configurations under varying impact energies. During low-velocity impact, the impact load primarily correlates with the structural stiffness of the sandwich configuration. As shown in Figure 6, significant fluctuations are observed in the load-time curve under 5J impact energy, indicating structural failure initiation during the initial loading phase. Progressive damage accumulation occurs with increasing indenter displacement, accompanied by continued load escalation. Notably, the 0/0 layup configuration exhibits an extended stress plateau in its load-time curve, demonstrating that the H10-0/0 structure sustains severe damage under 5J impact energy. It can be found that the load-time curves of the sandwich structure with two-layer panel thickness show an upward oscillation trend under the impact energy levels of 10J and 15J. It shows that the internal structure of the sandwich structure has begun to produce damage during the rising stage of the impact load of the sandwich structure during the impact process, which reduces the local stiffness of the structure. However, as the displacement of the impactor continues to increase, the stress is redistributed, and the impact load continues to rise.⁴¹ When the panel ply of the sandwich structure is 0/0, the impact response time is longer and the peak load is lower, because when the panel of the sandwich structure is only two layers, the overall stiffness and elastic modulus of the structure are lower, and the plastic deformation mainly occurs during the impact process, resulting in a more serious damage degree. Under the impact energy of 15J, the contact peak load of the sandwich structure of the 0/0 panel ply will be less than the contact peak load of the impact energy of 10J, because under the impact energy of 15J, the sandwich structure of the 0/0 panel ply has been broken, indicating that the structure at this time cannot bear the impact energy of 15J. In addition, the sandwich structure of the two-layer panel did not undergo a ' cliff-type ' decline after reaching the peak load, but a relatively gentle decline, because the deformation of the sample slowly recovered, and the elastic support of the impactor weakened, indicating that after the impact load reached the peak, the damage continued. Under the impact energy of 10J and 15J, the load-time curves of the sandwich structure of 0/90/0 and 0/90/0/90 are close to the sine type, indicating that the overall damage degree of the sandwich structure is low and the ability to bear the impact load is strong. The structure mainly undergoes elastic deformation during the impact process. Under the impact energy of 15J, the impact load-time curve of the sandwich structure with 0/0/0 and 0/0/0/0 panel ply will rise sharply first, and then fluctuate steadily. There is a platform area, and finally, a ' cliff-type ' decline occurs. It shows that after the impactor touches the sandwich structure, the core material also bears the load. The core material is compressed during the impact process, and there is a stress platform area and a densification phenomenon. Then, because the impactor rebounds rapidly, the curve’s ' cliff-type ' drops. Under 20J impact energy, the load-time curve of the 0/90/0/90 layup configuration no longer approximates a sinusoidal pattern. After reaching the peak load, a minor plateau phase emerges, followed by a gradual decline, indicating that the H10-0/90/0/90 sandwich structure undergoes plastic deformation at this energy level. For other layup configurations, the load plateau phase extends significantly, accompanied by severe structural damage, with some cases exhibiting indenter penetration through the sandwich structure.

Table 4.

The impact peak load of sandwich structures with different panel configurations.

Sandwich structure	5J contact peak load (N)	10J contact peak load (N)	15J contact peak load (N)	20J contact peak load (N)
H10-0/0	677.16	998.32	921.54	769.77
H10-0/90	1001.28	1188.26	1254.35	1041.79
H10-0/0/0	1030.22	1500.19	1619.31	1215.42
H10-0/90/0	1354.33	1578.48	1779.69	1811.56
H10-0/0/0/0	1041.79	1541.27	1819.77	1319.60
H10-0/90/0/90	1435.36	2116.13	2662.32	2482.94

Figure 6.

Impact load-time curves of different panel configurations.

Figure 7 shows the post-impact photos of sandwich structures with different panel configurations under 15J impact energy. According to the photos, when the impact energy is 15J, the sandwich structure of 0/0 panel has serious damage, and the core material and upper and lower panels of the structure are completely destroyed, indicating that the sandwich structure of 0/0 panel cannot withstand the impact energy of 15J, and has completely failed below the impact energy of 15J. Therefore, the peak contact load of the structure under 15J impact energy will be lower than the peak load of 10J impact energy. The sandwich structure with 0/90 face sheet layup exhibited micro-cracks around the indenter position and debonding damage at the structural edges. The 0/0 and 0/90 layup configurations induced distinct failure modes: The 0/0 layup aligned fibers unidirectionally, creating pronounced anisotropy. While demonstrating high strength and stiffness along the fiber direction, this configuration exhibited weak transverse properties, resulting in stress distribution heavily dependent on fiber orientation. Post-impact photographs revealed that damage primarily manifested as face sheet cracks. Notably, the 0/0 layup configuration displayed lower peak impact loads compared to the 0/90 configuration. For the 0/0/0 layup, cracks developed in both vertical and horizontal directions at the impact zone. In contrast, the 0/0/0/0 layup showed only fiber-aligned cracks. The 0/90/0 and 0/90/0/90 layups exhibited impact-induced denting with peripheral cracks, though the dent area in the 0/90/0 configuration significantly exceeded that of the 0/90/0/90 configuration. Increasing face sheet thickness through additional plies enhances structural stiffness, thereby improving impact resistance, reducing damage severity, and increasing peak load capacity. For 0° unidirectional layups, impact loads transfer predominantly along the fiber direction, generating localized stress concentrations near the impact site. This promotes penetrating cracks parallel or perpendicular to fiber orientation. Conversely, 0/90 symmetric layups distribute impact energy through cross-ply fiber networks, delaying localized damage. Consequently, such configurations develop finer web-like or radial crack patterns rather than catastrophic failures.

Figure 7.

Sandwich structure samples with different panel configurations after 15J energy impact.

Low-velocity impact response of sandwich structures with different core thickness

This section studies the low-velocity impact responses of sandwich structures with different core thicknesses under different impact energies. The panel uses three layers of GF/PP prepreg, all using a 0/90/0 configurations. The thickness of the core material of the sandwich structure is 5 mm, 10 mm, 15 mm, and 30 mm.

Table 5 shows the peak impact loads of the sandwich structures with four different core material thicknesses under different impact energies, and Figure 8 shows the load-time variation curves of the sandwich structures with four different core material thicknesses under different impact energies. Because in the process of low-velocity impact, the impact load is directly related to the stiffness of the sandwich structure. When the thickness of the core material of the sandwich structure increases, the stiffness of the structure will also increase.⁴² Therefore, as the thickness of the core material of the sandwich structure increases, the corresponding impact peak load will also increase. According to the load-time curve of Figure 8, at 5J impact energy, the load-time curves of sandwich structures with four core thicknesses approximate sinusoidal profiles, yet exhibit noticeable oscillations during both ascending and descending phases. This phenomenon indicates that all structures experienced initial damage at this energy level, though with minimal severity. The predominant elastic deformation during impact suggests the structures largely maintained their integrity under low-energy loading conditions. It can be seen that the impact response time of the sandwich structure with a core thickness of 5 mm is longer than that of the sandwich structure with other core thicknesses. After the impact load reaches the peak, the curve does not drop sharply, but there will be a relatively stable fluctuation platform area, because the section of the sandwich structure with a core thickness of 5 mm has a relatively small moment of inertia to the neutral axis. When the impactor contacts the panel of the structure and the upper panel is damaged, the lower panel will immediately bear the impact load together with the core material. Most of the impact load is borne by the panel of the sandwich structure, and the structure mainly undergoes plastic deformation during the impact process. The damage response time is relatively long, and the sandwich structure mainly undergoes progressive damage at this time. Therefore, after the load reaches the peak, there will be a certain degree of decline, and then begin to fluctuate. The load-time curves of the other three core thicknesses are similar to the sine curve, and the impact response time is relatively short. After reaching the peak value of the impact load, the impactor begins to rebound. In the rebound stage, the load-time curve drops steadily until the impactor is separated from the sample, and the load drops to 0, indicating that the overall damage degree of the sandwich structure is low. The core material of the sandwich structure also bears most of the load, and the sandwich structure mainly undergoes elastic deformation during the impact process. When the impact energy is 20J, the impact load-time curve of the sandwich structure with 5 mm and 10 mm core thickness will decrease sharply after reaching the peak value, and then begin to fluctuate steadily, and the response time is longer, indicating that the sandwich structure is seriously damaged and close to complete failure. After reaching the peak load, a platform area also appears in the sandwich structure with 15 mm core thickness; the impact load-time curve of the sandwich structure with 30 mm core thickness is still close to the sinusoidal shape. Indicating that under the impact energy of 20J, the sandwich structure with 5 mm and 10 mm core thickness has a serious damage failure. The sandwich structure with 15 mm core thickness has progressive damage and plastic deformation. The sandwich structure with 30 mm core thickness still has good resistance to impact load. The degree of damage during the impact process is small, and the structure is mainly elastic deformation.

Table 5.

The impact peak load of sandwich structures with different core thicknesses.

Sandwich structure	5J contact peak load (N)	10J contact peak load (N)	15J contact peak load (N)	20J contact peak load (N)
H5-0/90/0	1255.94	1261.58	1463.09	2083.58
H10-0/90/0	1354.33	1578.48	1779.69	1811.56
H15-0/90/0	1412.21	2131.19	2577.80	2043.07
H30-0/90/0	1516.38	2351.03	3081.33	2662.35

Figure 8.

Impact load-time curves of different core material thicknesses.

At 5J impact energy, all four core thickness configurations exhibit similar load-time curve profiles resembling sinusoidal patterns with pronounced oscillatory behavior. For the 5 mm core thickness configuration under impact energies exceeding 10J, the load-time curve maintains a prolonged plateau phase after peak load attainment rather than immediate decline, followed by gradual load reduction accompanied by significant structural damage. At 20J impact energy, both 10 mm and 15 mm core thickness configurations demonstrate load plateau formation before slow decay, indicating plastic deformation failure. These observations reveal that thinner-core sandwich structures fail earlier with increasing impact energy due to their panel-dominated stiffness characteristics. The reduced global bending stiffness in thin-core configurations promotes large deformations under equivalent impact energies, where localized plastic deformation becomes the primary failure mechanism. This initiates progressive damage propagation through the outward expansion of plastic zones. Conversely, thicker-core configurations demonstrate substantially enhanced global stiffness, enabling predominant elastic bending deformation under impact. The elevated structural stiffness effectively suppresses localized deformation by converting impact energy into global bending moments, allowing energy dissipation through elastic stress redistribution before reaching material yield limits. However, when impact energy exceeds structural capacity, core shear fracture failure occurs, manifested by characteristic load plateau phases followed by gradual load reduction in the load-time curves.

Figure 9 shows the peak load comparison histogram of the sandwich structure prepared in this paper under different impact energies. It can be seen that under the same impact energy, the impact peak load of the sandwich structure will increase with the increase of core thickness and panel thickness. Under the same impact energy, the same panel thickness and core thickness, the peak load of the sandwich structure with 0° and 90° alternating ply is higher than that of the 0° unidirectional ply.

Figure 9.

The peak load of different sandwich structures under different impact energies. Note : A is H10-0/0, B is H10-0/90, C is H10-0/0/0, D is H10-0/90/0, E is H10-0/0/0/0, F is H10-0/90/0/90, G is H5-0/90/0, H is H10-0/90/0, I is H15-0/90/0, J is H30-0/90/0.

Energy Absorption Properties Analysis of the sandwich Structure

Energy absorption properties of sandwich structures with different panel configurations

The energy absorption-time curve is mainly divided into two stages. The first stage is the downward movement of the impactor, and the impact energy is gradually transformed into the internal energy of the sandwich structure, corresponding to the rapid rise stage of the energy absorption-time curve. The second stage is the process of impactor rebound. The elastic potential energy stored in the sandwich structure is converted into the kinetic energy of the impactor, and the impactor rebounds upward, corresponding to the gentle decline stage of the energy absorption-time curve. In the early stage of impact, the sandwich structure sample absorbs energy through deformation under a small impact load, so the slope of the curve is low. With the continuous downward movement of the impactor, the contact area of the impactor increases, so that the impact load increases rapidly, the deformation displacement of the sandwich structure will increase, and even the sandwich structure will exhibit damage behavior, so the slope of the energy absorption-time curve will be relatively high. Finally, because the impact load decreases, the impact displacement reaches the maximum value, the energy absorption-time curve gradually stabilizes, and the slope of the curve decreases to 0.

Table 6 lists the energy absorption and specific energy absorption of sandwich structures with different configurations. Figure 10 is the energy absorption-time curve of sandwich structures with different configurations under different impact energies. At 5J impact energy, the H10-0/0 sandwich structure exhibited the highest energy absorption due to its severe damage and prolonged response duration under this loading condition. Conversely, the H10-0/90/0/90 configuration demonstrated the lowest energy absorption, as a greater proportion of the impact energy was converted into indenter rebound kinetic energy through panel deformation. The energy absorption capacity increased with face sheet thickness, with 0/90 symmetric layup configurations showing lower absorption values than their 0° unidirectional counterparts. None of the structures experienced complete failure, resulting in an inverse relationship between peak impact load and energy absorption – configurations with higher peak loads dissipated less energy through structural damage, instead transferring more energy to the indenter rebound. Under the impact energy of 10J, because the thickness of the sandwich structure panel of the 0/0 is small, the upper panel is seriously damaged during the impact process, and the impact displacement is large, resulting in the honeycomb core of the sandwich structure being crushed by the impactor, so the energy absorption is higher than the 0/0/0 panel and the 0/0/0/0 panel sandwich structure. Under the impact energy of 10J, the energy absorption value of 0/90 and 0/90/0 panel ply is smaller than that of 0/0 and 0/0/0 panel ply. Because under the impact energy of 10J, the sandwich structure of the panel symmetrical ply can better bear the impact load during the impact process, the degree of damage is low, and the ability of damage to synergistically absorb energy is low. When the movement displacement of the impactor reaches the maximum, the elastic potential energy stored in the structure will be converted into the kinetic energy of the impactor rebound, so the final structure absorbs less energy. According to Figure 10, it can be seen that under the impact energy of 15J, the highest point of the energy-time curve of the sandwich structure with 0/0 panel ply did not reach 15J, and there was no stage with a slope of 0 and a curve decline. It can be seen from Figure 7 that when the impact energy is 15J, the H10-0/0 sandwich structure has been destroyed, indicating that the sandwich structure at this time cannot withstand the energy impact of 15J and has been excessively damaged. However, the energy absorption values of the sandwich structures with 0/0/0 and 0/90/0 panels are not much different, even close to 15J, indicating that the thickness and ply of the panel can make the impact energy disperse to a larger area during the impact process. The damage degree of the structure is larger, and better energy dispersion and stability can be obtained, resulting in better energy absorption performance. When the impact energy is 20J, the energy absorption curve of the H10-0/0 sandwich structure does not show a downward and stable stage due to complete failure, so the energy absorption value is also the smallest. The sandwich structure of H-0/90 has the largest energy absorption value because the damage degree of the sandwich structure of H-0/90 is serious, the core material receives the impact and undergoes compression deformation, and the panel also has a large area of pits and cracks. The synergistic energy absorption of multiple damage modes makes the structure have good energy absorption performance.

Table 6.

Energy absorption and specific energy absorption of sandwich structures with different ply modes.

Layering method	Energy absorption(J)				Specific energy absorption (J/g)
Layering method	5J	10J	15J	20J	5J	10J	15J	20J
0/0	4.77	8.95	10.08	16.29	0.119	0.225	0.256	0.402
0/90	4.34	8.16	11.773	19.56	0.107	0.204	0.298	0.485
0/0/0	4.44	8.25	14.56	19.44	0.085	0.157	0.275	0.371
0/90/0	3.88	7.59	14.51	18.15	0.070	0.142	0.271	0.338
0/0/0/0	3.91	6.73	13.68	17.02	0.058	0.100	0.208	0.257
0/90/0/90	3.73	7.79	9.01	17.16	0.056	0.118	0.137	0.393

Figure 10.

Energy absorption-time curves of sandwich structures with different panel configurations.

As shown in Figure 7, sandwich structures with 0° unidirectional face sheet layups primarily exhibit large penetrating cracks in the panels, oriented both parallel and perpendicular to the fiber direction. This failure mechanism involves fiber breakage and longitudinal tearing of prepreg materials during impact. Progressive crack propagation under loading creates continuous damage paths through synergistic interactions between transverse fiber fracture and longitudinal prepreg delamination, resulting in enhanced energy absorption performance. In contrast, the 0/90/0/90 layup configuration demonstrates reduced energy absorption. Post-impact analysis reveals localized denting at the impact zone with peripheral microcracks. The high-stiffness cross-ply layup effectively constrains panel deformation and damage propagation, redirecting impact forces to the core material. Consequently, the core experiences predominantly brittle compressive failure rather than plastic deformation, failing to establish effective energy dissipation pathways. While 0/90 symmetric layups mitigate bending-tension coupling effects and reduce stress concentration to delay delamination growth, excessive symmetry concentrates impact energy in localized regions. This phenomenon restricts global energy absorption capacity and reduces energy absorption efficiency by limiting the development of distributed damage mechanisms.

According to Tables 6, it can be seen that within the impact range that the sandwich structure can withstand, as the thickness of the sandwich structure panel increases, the energy absorption of the sandwich structure will decrease. Because when the panel thickness of the sandwich structure increases, the overall stiffness of the structure will also increase, and the structure’s ability to resist deformation will be stronger. At the same time, the increase in the thickness of the panel makes it more difficult for the impactor to penetrate the panel of the structure, so that the core of the structure is squeezed to withstand the impact load. Although the increase in panel thickness will increase the impact peak load of the structure, the thin-walled structure of the honeycomb core material does not undergo a densification stage, which will reduce the energy absorption capacity of the structure. The panel thickness of the sandwich structure increases, and the mass of the structure will also increase, so the specific energy absorption of the sandwich structure will also decrease with the increase in the thickness of the structural panel.

Energy absorption properties of sandwich structures with different core thickness

Table 7 lists the energy absorption and specific energy absorption of sandwich structures with different core thicknesses under different impact energies. Figure 11 is the energy absorption-time curve of sandwich structures with different core thicknesses under different impact energies. According to Table 7, at 5J impact energy, the energy absorption values increase with core thickness due to the minimal structural damage and predominant elastic deformation under this low-energy loading. Thicker cores provide greater compressible deformation range, enabling enhanced energy absorption through elastic strain mechanisms. All four core thickness configurations exhibit energy absorption-time curves with distinct phases: ascending, descending, and stabilization periods. Under 10J impact energy, the 10 mm core thickness configuration demonstrates superior energy absorption compared to other thickness variants. The sandwich structure with a core thickness of 10 mm has a longer impact response time and a larger impact displacement during the impact process. The core material in the impact process belongs to progressive crushing, so the damage absorbs more energy. When the impact energy is 15J, the energy absorption-time curve of the H10-0/90/0 sandwich structure does not show a significant downward trend after reaching the peak, indicating that after the end of the impact process, the structure has been plastically damaged. There is no energy converted into kinetic energy for the rebound of the impactor, and the impact energy is absorbed by the sandwich structure. Under the impact energy of 15J, the energy absorption of the sandwich structure with 15 mm core thickness and 30 mm core thickness is not much different, because when the core thickness increases, the overall stiffness of the structure increases, and the displacement of the impactor decreases during the impact process. The structure undergoes elastic deformation, and the stored potential energy will be converted into the kinetic energy rebound of the impactor. It can be seen in the figure that the energy absorption-time curve will decrease significantly after reaching the peak, and the slope of the descending curve is significantly greater than that of the core thickness of 5 mm and 10 mm. At 20J impact energy, the energy absorption of the sandwich structure exhibits a decreasing trend with increasing core thickness. This phenomenon occurs because the increase in core thickness enhances structural stiffness, resulting in reduced damage severity under high-energy impact conditions. Consequently, stiffer configurations absorb less energy through damage mechanisms due to their limited plastic deformation and suppressed failure progression.

Table 7.

Energy absorption and specific energy absorption of sandwich structures with different core thickness.

Core thickness	Energy absorption(J)				Specific energy absorption (J/g)
Core thickness	5J	10 J	15J	20 J	5J	10 J	15J	20 J
5	3.66	6.50	13.72	18.39	0.076	0.152	0.321	0.393
10	3.88	7.59	14.51	18.15	0.070	0.142	0.271	0.338
15	3.96	7.16	10.91	18.10	0.069	0.125	0.192	0.322
30	4.20	7.61	10.40	17.60	0.056	0.108	0.143	0.242

Figure 11.

Energy absorption-time curves of sandwich structures with different core thicknesses.

Figure 12 shows the energy absorption and specific energy absorption of the sandwich structure under different impact energies. Combined with the previous analysis, it can be found that the influence of panel thickness, core thickness, and panel configurations on the energy absorption and specific energy absorption of the structure is not monotonous. The thick panel can resist the direct penetration of the impactor, so that more energy can be absorbed through the compression and shear of the core material and the bending deformation of the panel. However, when the panel is too thick, the impact energy will be rebounded by the elastic deformation of the panel; that is, the elastic deformation of the structure occurs during the impact, and the impact energy is converted into the kinetic energy of the rebound of the punch. The core material cannot participate in the crushing energy absorption, so the energy absorption of the structure is reduced. When the thickness of the sandwich structure increases, the compressible volume increases, and more energy is absorbed through plastic deformation, buckling, or crushing. At the same time, the thicker core material can delay the local buckling of the panel, allow greater overall bending deformation, and improve energy absorption efficiency. However, if the core material is too thick, the stiffness of the sandwich structure will be significantly improved, which will lead to the change of the failure mode of the structure during the impact process, which will lead to the local buckling of the structure or the delamination of the interface, rather than the progressive crushing, resulting in the reduction of the energy absorption capacity of the structure. When the layup of the structure is completely composed of 0° unidirectional layup, the structure has high axial stiffness and strength, but it is prone to brittle fracture of the fiber, and the energy absorption is reduced. The 0/90 symmetrical ply can reduce the bending-tensile coupling effect caused by impact, reduce the local stress concentration, delay the delamination expansion, and improve the energy absorption capacity of the structure through multi-mode damage synergistic energy absorption. However, excessive symmetry will lead to the impact energy concentrated in the local area, so that the energy absorption efficiency is limited.

Figure 12.

The energy absorption and specific energy absorption histogram of the sandwich structure under different impact energies (a) Energy absorption ( b ) Specific energy absorption.

The study reveals that the energy absorption of sandwich structures during impact does not exhibit a monotonic relationship with variations in peak impact load. The peak impact load is predominantly governed by the structure’s local stiffness. Enhancing the thickness of both face sheets and core materials, along with optimizing the face sheet layup configurations, effectively increases the peak impact load of sandwich structures. Energy absorption capacity is primarily determined by the failure mechanisms: structures experiencing severe damage progression with diversified failure modes demonstrate superior energy absorption through coordinated damage evolution during impact events.

Effect of impact on vibration Characteristics of sandwich Structure

The GF / PP sandwich plate is impacted with different impact energies, and the picture of the sandwich plate after impact is shown in Figure 13. It can be seen from the figure that as the impact energy increases, the area and depth of the pits in the sandwich structure will also increase. There are cracks near the pits of the panel. The larger the impact energy is, the more obvious the cracks of the sandwich structure panel are. The pit generated by the impact of the impactor is approximated as a circle, and the diameter and depth of the pit are measured. The diameter and depth of the pit produced by the sandwich structure under different impact energies are shown in Table 8.

Figure 13.

Sandwich structure after different impact energies.

Table 8.

Pit size and mode of sandwich plate under different impact energies.

Impact energy(J)	Crater diameter (mm)	Crater depth (mm)	First-order frequency (Hz)	Second-order frequency (Hz)	First-order damping ratio (%)	Second-order damping ratio (%)
0	\	\	98.377	242.631	2.968	3.766
5	6.24	0.09	97.139	197.487	3.641	5.202
10	8.42	0.16	96.879	178.748	3.830	8.302
15	10.61	0.34	87.617	169.629	4.055	8.678
30	13.86	0.46	60.988	163.343	5.519	11.914

Figure 14 presents C-scan damage images of sandwich structures after impacts at different energy levels. Impact-induced pits appear as blue regions in the C-scan images. At 5J impact energy, a near-circular blue zone is observed without significant peripheral damage. Under 10J impact energy, narrow, elongated, irregular features emerge around the pit periphery, indicating microcrack formation adjacent to the primary damage zone. When the impact energy increases to 15J, both the blue area and associated cracks exhibit substantial expansion in size and penetration depth. At 20J impact energy, dramatic increases in pit dimensions and crack propagation are evident, with deep, extensive cracks radiating from the impact site, confirming severe structural degradation. These observations demonstrate that increasing impact energy not only amplifies pit dimensions (area and depth) but also accelerates crack propagation, thereby exacerbating structural damage severity.

Figure 14.

C-scan images of the sandwich structure under different impact energies.

The vibration test of the sandwich plate with different impact energies is carried out by the force method. Table 8 lists the modal frequency and damping ratio of the sandwich plate with different impact energies. According to the table, it can be seen that the natural frequency of the sandwich plate decreases with the increase of the impact energy. Because the increase of the impact energy leads to the internal damage of the structure, the structural panel has pits, and the core material will also be compressed, thus weakening the overall stiffness of the structure. The natural frequency is positively correlated with the stiffness, and a decrease in stiffness directly causes a decrease in structural frequency. The higher-order modes are more sensitive to the change of local stiffness, and the local damage caused by the impact further weakens the local stiffness of the structure. Therefore, with the increase of impact energy, the second-order mode of the structure decreases more than the first-order mode. The damping ratio of the structure increases with the increase of the impact energy, because the internal damage of the structure caused by the impact (crack propagation of the panel, compression deformation of the core material, etc.) and its synergistic effect increase the energy dissipation path of the structure. Because the vibration modes involved in the higher-order modes are more complex, the local damage after the impact of the sandwich structure will aggravate the local energy dissipation, so the increase of the second-order damping ratio of the structure is more significant.

The impact damage of the sandwich structure affects the local stiffness of the structure and then affects the natural frequency of the structure. Combined with Figures 13 and 14, it can be seen that under the impact energy of 5J, the panel of the structure has local small cracks, which will reduce the local stiffness and lead to a decrease in the modal frequency of the structure. The friction between the cracks will increase the path of energy dissipation, and the structural damping ratio will increase. When the impact energy is 10J, the structure has obvious pits, the damage area is larger, and more serious cracks appear around the pits. The cracks and pits of the panel will lead to a decrease in the interface inertia moment of the structural panel, and the modal frequency will continue to decrease. As the degree of damage increases, the structural damping ratio continues to increase. Under the impact energy of 15J, the pit depth and area of the structure are larger, and the core material also receives obvious extrusion deformation. The interlayer results show delamination damage, which destroys the continuity of the structure. At the same time, the stratification produces viscous dissipation, and the damping ratio of the structure will also increase. At the impact energy of 20J, in addition to the large area of pits in the impact position, there are also penetrating cracks around the pits, and the core material is seriously compressed or even crushed. Therefore, the large-scale stratification of the structure and the crushing of the core material will greatly reduce the equivalent stiffness of the structure and reduce the modal frequency. However, due to the serious damage, the energy dissipation path increases, and the structural damping ratio will increase.

In the low-velocity impact test of this paper, the impact energy is relatively small, and the impactor will not penetrate the sandwich structure, but there will be pits in the sandwich structure. As the impact energy increases, the diameter and depth of the pits will increase accordingly. At the same time, the local stiffness of the structure decreases, but the whole structure remains intact, resulting in a small decrease in the modal frequency of the structure. At this time, the energy dissipation mode of the structure is mainly viscoelastic deformation and interface friction, so the damping ratio of the structure will increase.

Figure 15 is the modal shape diagram of the sandwich plate under different impact energies. The basic vibration modes of the sandwich plate with different impact energies are basically the same. The first-order modal shape corresponds to the bending deformation of the structure, and the second-order modal shape corresponds to the torsion deformation of the structure. According to the schematic diagram of the modal shape, it can be seen that when the sandwich plate is fixed by the cantilever support method, the displacement of the cantilever end of the first-order modal shape of the structure is the largest, and the two-point displacement of the cantilever end of the second-order modal shape is the largest.

Figure 15.

Modal shape diagram of sandwich plate with different impact energies.

Comparing the modal shapes of sandwich plates with different impact energies, it is found that the modal shapes of sandwich plates are less sensitive to impact damage, and there is no obvious difference. This is because the damage caused by different impact energies to the sandwich structure is mostly local non-penetrating damage, and the damage location generally occurs in the high-stress area in the center of the sample. A small range of damage is equivalent to uniform stiffness attenuation in the overall vibration, rather than introducing new vibration mode nodes or anti-nodes. In addition, the boundary conditions of the cantilever support suppress the disturbance of the local damage to the overall modal shape of the structure through geometric constraints. The displacement of the fixed end of the sandwich structure approaches 0, and the first-order bending modal energy is mainly concentrated at the free end. The damage area is generally located at the center of the structure, so the contribution of damage to modal strain energy is limited, resulting in a small change in the overall vibration mode of the structure.

According to the analysis of the influence of different impact energies on the vibration characteristics of the sandwich structure, it is found that when the structure is impacted at a lower energy, the failure mode of the structure is mainly the micro-cracks of the panel. At this time, the stiffness of the structure is slightly reduced, and the damping ratio will be slightly increased. When the impact energy is high, the damage to the structure extends to the core material of the structure. In addition to the damage to the panel, it is also accompanied by the compression collapse of the core material of the structure. The synergistic effect of various damages further reduces the stiffness, and the damping ratio of the structure will increase nonlinearly. When the impact energy of the sandwich structure increases, the stiffness degradation and energy dissipation capacity of the structure increase, which shows that the vibration frequency of the structure decreases and the damping increases, reflecting the progressive damage process of the structure from matrix damage to delamination failure.

Conclusion

In this paper, the low-velocity drop-weight impact test of the GF/PP sandwich structure is carried out. By analyzing the contact load response and energy absorption properties of the structure during the impact process of different impact energies, the influence of the configurations of the sandwich structure panel and the thickness of the core material on the impact properties of the structure is studied, and the damage degree of the structure is analyzed. Combined with the vibration modal analysis, the effect of damage modes on the vibration characteristics of the sandwich structure after impact is studied.

(1) Under the same impact energy, the impact peak load of the sandwich structure will increase with the increase of core thickness and panel thickness. Under the same impact energy, the same panel thickness, and core material thickness, the peak load of the sandwich structure with symmetrical panel configurations will be higher than that of the sandwich structure with unidirectional configurations.

(2) The energy absorption of the sandwich structure mainly depends on the failure mode of the structure, and the energy absorption will not change monotonously with the change of the thickness of the core material or the configurations of the panel. When pits appear on the panel after impact, the core material is crushed and densified, and the whole structure undergoes plastic deformation and progressive damage failure. The structure absorbs energy through multiple damage modes. At this time, the sandwich structure has good energy absorption properties.

(3) The influence of impact on the vibration characteristics of the sandwich structure depends on the damage mechanism of the sandwich structure in the process of impact. In the process of low-velocity impact, with the increase of impact energy, the local stiffness of the sandwich structure will decrease, so the modal frequency of the sandwich structure will decrease slightly, the damping ratio will increase, and the modal shape will be less sensitive to impact damage.

Footnotes

CRediT authorship contribution statement

Da Guo:Writing – original draft、Conceptualization、Methodology、Formal analysis、 Data curation. Yong Li:whole scheme design、Methodology、Writing – review & editing、Funding acquisition. Chen Liu:Methodology、Validation. Haoran Chen:Validation、Writing – review & editing. Xue Dong:C-scan test data analysis. Xinyuan Zhu:Preparation of samples for C-scan test.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: Our work is supported by the funding of “Special Funds of 2023 Jiangsu Provincial Science and Technology Plan”, and the funding number is ZAG23009.

ORCID iDs

Da Guo

Yong Li

Data Availability Statement

All data generated or analyzed during this study are included in this published article.*

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