Investigating the Behavior of Glass Fiber/Polyester Layered Composites Under Low-Velocity Impact

Abstract

This study investigates the behavior of glass fiber/polyester layered composites under low-velocity impact conditions, focusing on optimizing layer configurations and fiber types to enhance impact resistance. Composites were fabricated using a combination of E-glass mat and woven glass fibers with orthophthalic polyester resin, employing the hand lay-up method. Impact tests were conducted using a drop hammer device. Key variables included the number of layers and fiber types, with impacts performed using flathead and hemispherical impactors. The results showed that increasing the number of layers significantly enhanced energy absorption and peak contact force. Specifically, five-layer samples exhibited a 351% increase in energy absorption and a 212% increase in peak contact force compared to two-layer samples. Mat fibers outperformed woven fibers, with mat-only samples absorbing 21.8% more energy and showing 5.8% higher peak forces. Additionally, flathead impactors generated 101% higher peak forces and caused more severe damage than hemispherical impactors. Observed failure modes included fiber breakage, matrix cracking, and delamination, all influenced by impact parameters and material characteristics. These findings provide valuable guidance for designing composite materials with improved resistance to impact, with potential applications in the aerospace, marine, and automotive industries.

1. Introduction

Parts made with composite materials have extensive applications in various industries, including aerospace, marine, and automotive industries. The high strength-to-weight ratio, corrosion resistance, long fatigue life, and the ability to produce complex parts have increased the use of components made with composite materials [1,2]. Cantwell and Yule [2] investigated the effect of loading rate on composites made with needle-punched fibers and epoxy resin. They observed that with an increase in the loading rate, the failure stress and elastic energy stored in the composite increased. Habibi et al. [3] examined the effect of short needle-punched fibers on the mechanical properties and damage behavior of the composite. They concluded that using short fibers in the composite reduces crack growth at the fiber–matrix interface. Gnanavelbabu et al. [4] investigated the strength of glass/polyester composites using fibers with different angles under varying fiber volume percentages. According to their results, a combination of short and long fibers performed better than all other fiber types. Daouache et al. [5] studied the effect of low-velocity impact on carbon/epoxy/AL2024-T3 hybrid laminates and proposed a novel stacking sequence aimed at enhancing the stiffness and elasticity of composite structures through multi-material integration. Their work focused on key parameters such as impact velocity, impact sequence, composite thickness, and energy levels using the HASHIN failure criterion. Tie contacts were modeled between plies to simulate realistic interactions within the laminate. The results highlighted that impact energy and laminate thickness significantly influence energy absorption and damage distribution across the composite, matrix, and fibers. This study provides valuable insights into improving the impact resistance of hybrid aerospace structures.

Rabouh et al. [6] studied the effect of impactor head shape on damage in T700/Epoxy laminates under low-velocity impact. Using a 3D finite element model and a LARC_VUMAT subroutine based on the LARC05 damage model, they compared hemispherical and flat impactors. The results, validated with experimental data, showed that head shape significantly affects damage patterns. The proposed model effectively predicted key failure modes such as fiber kinking and matrix cracking. Fatima et al. [7] studied the effect of adhesive amount at the interface between the skin and core in carbon fiber/balsa wood sandwich structures. Their results indicated that increasing the amount of additional adhesive at this interface, without requiring a complex manufacturing process or increasing density, improves the structure′s performance. Wang et al. [8] tested the behavior of fiberglass composites reinforced with shape memory alloy fibers under low-velocity impact loading. Their results showed that the use of shape memory alloy wires enhances the impact resistance of the fiberglass composite and improves the damaged area under impact loading. Bombimba et al. [9] subjected samples made with glass fibers and tri-block copolymers to low-velocity impact tests. Their research results indicated an increase in absorbed energy, especially at higher energy levels, with the addition of lead nanoparticles.

Karsun et al. [10] experimentally and numerically investigated the effect of impactor shape on the behavior of sandwich panels under low-velocity impact loading. They used a drop weight test device for impact loading. They found that in the flathead impactor, more and faster damage occurs, and the displacement increases with a reduction in the impactor′s contact area. Additionally, they reported brittle failure for the flathead impactor and homogeneous plastic deformation for the hemispherical and pointed impactors. Mahesh et al. [11] numerically studied the behavior of jute/epoxy composite plates under low-velocity impact. They used three shapes of impactors—flathead, spherical, and pointed—for penetrative impact loading. They found that the number of layers in the composite plates is an important parameter in energy absorption, and reducing the impactor’s contact area with the composite surface increases energy absorption. Yao et al. [12] examined the behavior of metal fiber composites under low-velocity impact both numerically and experimentally. They showed that the resistance of composite plates depends on the shape of the impactor, impact energy, and the distribution of metal layers. They reported regular penetration accompanied by shear failure in the flathead impactor and petal-like deformation in the spherical impactor.

Zaniker et al. [13] investigated the energy absorption and failure modes of glass fiber-reinforced composite plates with a PVC foam core. They used a pendulum impact tester with repeated impacts to study the energy absorption. Their results showed that the penetration and energy absorption levels are the same for layered composite and sandwich panels, and the presence of foam, due to its low impact strength, does not significantly affect the energy absorption. Liao et al. [14] conducted a similar study on damage accumulation in carbon fiber-reinforced composite plates under repeated low-velocity impact loading. They imparted 15 joules of energy to the sample with each impact until penetration occurred. Their results showed that fiber failure is the main reason for energy absorption, and at complete penetration, the impact energy is equal to the absorbed energy. Sao et al. [15] studied the behavior of three-dimensional carbon fiber-reinforced composite plates under low-velocity impact loading. Their loading was performed at three impact energy levels: 30, 65, and 100 joules. Their results showed that samples subjected to 65 joules of impact energy had higher impact properties compared to those subjected to 30 and 100 joules of impact energy. Additionally, they reported that as the impact energy increased from 30 joules to 100 joules, the corresponding damage volume increased from 6.886 cubic millimeters to 477.133 cubic millimeters.

Due to the widespread use of needle-punched fibers in the production of composite components across various industries, this study explores how the layering arrangement of needle-punched fibers—and their combination with woven fibers—affects the behavior of composite materials under low-velocity impact loading. Optimizing layer configuration offers a cost-effective way to enhance the impact resistance of composites. Accordingly, this research investigates the effects of both the number of layers and their specific arrangements to maximize energy absorption. Additionally, it examines how the panels respond to impacts from flathead and hemispherical impactors and identifies the dominant damage mechanisms that contribute most to increased energy absorption and impact strength.

2. Materials and Methods

2.1. Sample Preparation

First, the glass fibers were layered using the hand lay-up method, and then a metal roller was used to eliminate trapped bubbles. The samples were layered using a combination of E-glass mat and woven glass fibers, where the mat fibers had specific weights of 225 and 450 g per square meter, and the woven fibers had a specific weight of 400 g per square meter, as well as orthophthalic polyester resin, according to the arrangement in

Table 1. Layout of the layers along with the weight of the samples and the type of impactor.

Specimen	Specimens’ Thickness (mm)	Lay-up	Mass (g)	Type of Impactor
F450-2	1.6 ± 0.1	[450/450]	20.4 ± 2	Flat
F450-3	2.6 ± 0.1	[450/450/450]	33.9 ± 3
F225-2	1.4 ± 0.1	[225/450]	17.99 ± 2
F225-3	2.4 ± 0.1	[225/450/450]	33.15 ± 3
F225-4	2.9 ± 0.1	[225/450/450/450]	41.2 ± 4
F225-5	3.4 ± 0.1	[225/450/450/450/450]	48.5 ± 4
F225-5-plain	3 ± 0.1	[225/450/450/400(Plain)/450]	41.05 ± 4
H450-2	1.6 ± 0.1	[450/450]	20.45 ± 2	Hemispherical
H450-3	2.6 ± 0.1	[450/450/450]	33.8 ± 3
H225-2	1.4 ± 0.1	[225/450]	18.1 ± 1
H225-3	2.4 ± 0.1	[225/450/450]	33.2 ± 2
H225-4	2.9 ± 0.1	[225/450/450/450]	41.28 ± 5
H225-5	3.4 ± 0.1	[225/450/450/450/450]	48.8 ± 5
H225-5-plain	3 ± 0.1	[225/450/450/400(Plain)/450]	41.15 ± 4

. To fully cure the resin, the panels were placed in an oven at 70 degrees Celsius for 3 h after being removed from the mold. Test samples were cut from the mentioned panels using a water jet. The use of a water jet eliminates or minimizes any damage to the edges of the samples during the cutting process. The dimensions of the samples are 100 × 100 mm, with thicknesses varying between approximately 3.5 mm and 9 mm depending on the number and type of layers. Figure 1 shows an image of the test samples after fabrication and cutting.

2.2. Test Device

A drop hammer device was used to conduct the low-velocity impact test. In this device, a 7 kg impactor provides the necessary energy for applying penetrating force to the composite panels by free-falling from a height of 50 cm. The force–time graph is directly obtained from the piezoelectric force sensor. The displacement–time graph is obtained by double integrating the results of the acceleration–time graph, which is directly recorded from the accelerometer sensor on the device. A schematic representation of the test device and the sensors used is shown in Figure 2. The total energy applied in the impact test for all samples is the same and is calculated to be approximately 35 J using Equation (1). The impact test conditions were kept constant for all samples and assessments were conducted at room temperature.

$$E=mgh$$

(1)

2.3. Characterization

In this article, to investigate and analyze the behavior of composite panels under low-velocity impact loading, important indices such as energy absorption ($E_a$), maximum contact force, specific energy absorption (SEA), and damaged area were used to compare the samples. The amount of energy absorbed is equal to the area under the force–displacement curve, which is calculated by integrating this curve according to Equation (2).

$$E_a={\int }_0^{{\delta }_{max}}f\left(x\right)dx$$

(2)

In this equation, $E_a$ is the energy absorption, and ${\delta }_{max}$ is the maximum penetration depth of the projectile in the composite panels. Since the mass of composite panels is always a consideration for designers, the amount of energy absorbed relative to the mass of the sample becomes important and provides a suitable parameter for comparing the behavior of samples. The specific energy absorption (SEA) is calculated using Equation (3).

$$SEA={\displaystyle \frac{E_a}{m}}$$

(3)

In this equation, m is the mass of the composite panels with the same surface area, where the arrangement and type of layers cause mass variations in the panels. Another important parameter is the maximum contact force ($F_{peak}$). This value represents the peak force applied to the panels during impact loading before serious damage occurs; in other words, it is the maximum force that the composite panels can withstand.

The area of the damage zone in composites under impact loading is a criterion for analyzing damage tolerance and is used to compare the samples with each other and analyze the behavior of the composite panels. In this article, AutoCAD V.2024 software was used to calculate the area of the damage and delamination zones. All impact tests were performed in triplicate for each configuration, and the results presented in

Table 2. Low-velocity impact test results.

Specimen	SEA (J/g)	Energy Absorption (J)	Peak Force (N)
F450-2	54.99 ± 2	1121 ± 20	3746 ± 112
F450-3	50.26 ± 1.3	1704 ± 18	5017 ± 170
F225-2	43.56 ± 1.5	7830 ± 50	2730 ± 40
F225-3	44.79 ± 0.9	1484 ± 15	4890 ± 92
F225-4	58.35 ± 0.9	2404 ± 34	6096 ± 153
F225-5	72.85 ± 1	3533 ± 38	8509 ± 230
F225-5-plain	70.67 ± 0.8	2901 ± 30	8044 ± 224
H450-2	29.4 ± 1.2	6010 ± 22	1270 ± 59
H450-3	40.73 ± 1.2	1376 ± 16	2508 ± 58
H225-2	33.44 ± 2.1	6050 ± 62	1460 ± 37
H225-3	41.24 ± 0.8	1369 ± 11	2254 ± 58
H225-4	48.31 ± 1.5	1994 ± 29	3302 ± 87
H225-5	57.52 ± 1.1	2807 ± 33	4223 ± 96
H225-5-plain	57.01 ± 1.6	2346 ± 27	4064 ± 90

represent the average values. Standard deviations are also reported to indicate measurement repeatability.

3. Results and Discussion

Low-velocity impact tests were conducted on all samples using two types of impactors: flathead and hemispherical. The results of energy absorption, peak contact force, and SEA for all samples under low-velocity impact tests are reported in Table 2. Subsequently, the study on the fracture surface and the effect of important parameters on the impact properties of the panels is introduced.

3.1. Failure Modes

Several modes of failure may occur during low-velocity impact loading in composites. These failures include fiber breakage, fiber–matrix debonding, fiber bridging, fiber pull-out from the matrix, matrix cracking, and delamination between layers. The failure modes are highly dependent on impact parameters such as the shape of the impactor, impact velocity, composite material properties, and layer arrangement. It should be noted that the damage modes discussed in this section are based on macroscopic observations, as the fiber size and damage features were sufficiently visible without microscopic analysis.

The observed damage and fracture surface of the composite plate sample are shown in Figure 3. Fiber breakage, delamination, and splitting in the panels can be identified as the main mechanisms in energy absorption and strength enhancement under low-velocity impact loading. In some samples, matrix cracking was also observed.

As seen from the top surface in Figure 4, in the flathead impactor, besides matrix cracking, fiber tearing and fiber pull-out from the matrix are the predominant modes, which are rarely seen in the hemispherical impactor. In fact, the composite surface under the hemispherical impact results in overall tearing of the panels and petal-like formation.

3.2. The Effect of Impactor Shape on Low-Velocity Impact Behavior

As previously mentioned, this study used two types of impactors, as shown in Figure 5. In the force–displacement graph of Figure 6a, the effect of the impactor shape on the sample with mat fibers is shown. The peak force in the flathead impactor for sample F255-5 is twice as much as that of the hemispherical impactor (H255-5). It is also observed that in the flathead impactor, the peak force occurs at a displacement of 7.5 mm, whereas in the hemispherical impactor, it occurs at a displacement of 6 mm. In the sample with one layer of woven fibers, as shown in Figure 6b, a similar behavior is observed. Samples subjected to impact loading with the flathead impactor (F255-5-plain) exhibit higher peak force, and this initial peak force occurs at a greater displacement. This behavior can be attributed to the difference in contact mechanics between the flathead and hemispherical impactors. However, as shown in Figure 7, the overall contact time is shorter in the flathead impactor due to its full surface contact at the onset of impact.

Similar findings have been reported by Sahu and Ansari [16]. It is observed that the damage initiation in the flathead impactor starts with matrix cracking and then progresses with fiber breakage. However, in the force–displacement graphs of the samples impacted by the hemispherical impactor, it is seen that the force level increases with a lesser slope and then gradually decreases. This behavior is explained by the gradual engagement of the hemispherical impactor’s surface during the impact.

A view of the impact surface of the samples after the impact test is shown in Figure 8. In this figure, it can be seen that the flathead impactor, by applying shear stress, results in matrix failure and sharp notches at the damage site. However, the hemispherical impactor creates a curved surface at the damage initiation site, taking the shape of the impactor. On the rear surface of the sample, as shown in Figure 9, it can be seen that the flathead impactor causes a circular plug, indicating high shear stress in this area. On the other hand, the sample impacted by the hemispherical impactor exhibits a petal-like phenomenon with four tears visible.

The effect of the impactor shape on the force–displacement graph for samples with three layers of mat fibers is shown in Figure 10a,b, respectively, for samples with layering [450/450/225] and [450/450/450]. The results show that in the sample with three layers of 450 g mat fibers (450-3), the peak force for the flathead impactor is 100% higher than that of the hemispherical impactor. Conversely, with a reduction of 0.2 mm in the thickness of the panel achieved by replacing a 450 g layer with a 225 g layer in the composite sample, it is observed that the peak force in the flathead impactor is 117% higher than that of the hemispherical impactor. A higher peak force indicates the composite panel’s ability to withstand impact force before severe damage and failure occur [17].

In Figure 11a,b, the force–displacement graphs for two samples with different layer counts of 2 and 4 layers are shown. Overall, the results show that using the flathead impactor increases the peak force level. For samples with 2, 3, 4, and 5 layers, an increase of 87%, 117%, 84%, and 101% in peak force is observed, respectively. The bar chart in Figure 12 compares the peak force values for all samples.

Also, in Figure 13, it can be seen that the delamination between layers and the damaged surface area in the sample with the flathead impactor is greater than in the sample with the hemispherical impactor. The results in

Table 3. Damaged area of samples after impact test.

Specimen	H225-5-Plain	H225-5	H225-4	H225-3	H225-2	H450-3	H450-2	F225-5-Plain	F225-5	F225-4	F225-3	F225-2	F450-3	F450-2
$$\begin{gathered} \mathrm{Damaged} \mathrm{area} \\ {(mm}^2) \end{gathered}$$	525	538	486	443	331	412	354	474	940	496	455	588	972	732

, considering the damaged surface areas, confirm this phenomenon. The large damaged area caused by the flathead impactor results in an energy absorption increase of up to 86.5% for the 450-2 sample and a minimum increase of 8.6% in the 225-3 sample. It can be said that the larger contact area at the start of the impact leads to greater damage, higher contact force, and reduced contact duration. This finding is also reported in the work of Liu et al. [17].

3.3. The Effect of Using Woven Fibers on Low-Velocity Impact Behavior

To examine the influence of incorporating woven glass fibers, specimens labeled F225-5-plain were fabricated and tested. As can be seen in Table 2 and Figure 14 and Figure 15, the impact performance of these samples was compared with that of the F225-5 specimens, which consisted entirely of mat fibers. The peak impact force in both types of impactors for samples H225-5 and F225-5 is 3.9% and 5.8% higher compared to H225-5-plain and F225-5-plain with one layer of woven fibers, respectively. The same figure shows that the inclusion of woven fibers (plain) in samples H225-5-plain and F225-5-plain resulted in a reduction in displacement at peak force by approximately 12.2% and 6.9%, respectively, compared with samples H225-5 and F225-5. The energy absorption and specific energy absorption (SEA) results for all samples under low-velocity impact loading with flathead and hemispherical impactors are compared in Figure 16 and Figure 17. The results show that replacing one layer of mat fibers with woven fibers in the flathead and hemispherical impactors reduces energy absorption by 21.8% and 19.6%, respectively. However, the specific energy absorption (SEA) in both samples is approximately the same. Using one layer of woven fibers instead of mat fibers reduces the mass of the sample by about 15.6%, which is the reason for the increase in SEA in the sample containing one layer of woven fibers.

In fact, it can be said that the presence of woven fibers as a layer in composite panels reduces fracture toughness and peak force under low-velocity impact loading. Therefore, if only energy absorption is important in a structure subjected to impact, mat fibers can be used entirely. If lower toughness is desired by the designer, a combination of mat and woven fibers can be used. It can also be concluded that when woven fibers are used in a sample under impact, the energy absorption is less than when equivalent mat fibers are used. One of the reasons for this occurrence is the fiber orientation in all directions in samples made with mat fibers, which increases the damaged area. Short fibers with random orientation help prevent crack growth in the matrix material of the composite [3].

To more accurately examine the damaged area around the impact region, the damaged area was calculated using AutoCAD software. Table 3 shows the extent of impact damage for various samples. As can be seen from the values in Table 2 and the images in Figure 18, the amount of damage in the H225-5 and F225-5 samples, which are entirely made of mat fibers, is greater than in other samples under any type of impactor. The greatest delamination in the H225-5-plain and F225-5-plain samples occurred at the interface between the woven and mat fibers. Additionally, the damaged area in the H225-5-plain and F225-5-plain samples is more uniformly distributed compared to the H225-5 and F225-5 samples. This indicates the effectiveness of woven fibers in better distributing the impact force in all directions.

3.4. Effect of the Layer Number on Low-Velocity Impact Behavior

To examine the effect of the number of layers on energy absorption, samples with varying numbers of layers were subjected to low-velocity impact loading. As shown in the force–displacement graphs in Figure 19a,b, the force level increases with the increase in the number of layers. The increase in energy absorption with both spherical and flat impactors is noticeable with the increase in the number of layers. Additionally, in the samples F225-2, F225-3, F225-4, and F225-5, the peak impact force and the strength of the sample under impact increase with the number of layers. In the force–displacement graphs in Figure 19a, it is seen that in samples with the flathead impactor, the peak force occurs at a greater displacement. In other words, with the onset of damage due to the flathead impactor, matrix cracking initially occurs, followed by fiber breakage. However, as seen in Figure 19b, in the samples H225-2, H225-3, H225-4, and H225-5, the force level increases suddenly and then gradually decreases. In the samples impacted by the hemispherical impactor, the force reduction occurs gradually and at a greater displacement compared to the samples impacted by the flathead impactor (Figure 19b). The sample F450-3, due to having one more layer of mat fiber, has a 33.9% higher peak force than the sample F450-2. Additionally, the sample H450-3 has a 97.4% higher peak force compared to H450-2. It can be concluded that the difference in maximum force in samples impacted by the flathead impactor is less than this parameter in samples impacted by the hemispherical impactor (Figure 20a,b).

While the results in Figure 19 and Figure 20 confirm that increasing the number of layers enhances the peak force and energy absorption under both flathead and hemispherical impactors, this behavior is expected due to the increased material mass. A more insightful comparison is achieved by examining normalized parameters such as specific energy absorption (SEA) and energy absorbed per unit thickness.

As shown in Table 2, for samples impacted by the flathead impactor, the SEA increases from 43.56 J/g (F225-2) to 72.85 J/g (F225-5), showing a total increase of about 67% as the number of layers increases from two to five. Similarly, for the hemispherical impactor, SEA increases from 33.44 J/g (H225-2) to 57.52 J/g (H225-5), representing a rise of 72%. However, the rate of SEA improvement slows down beyond four layers, particularly for the flathead impactor, where SEA reaches 58.35 J/g at four layers (F225-4) and only slightly increases to 72.85 J/g at five layers (F225-5). This trend indicates a saturation point in energy efficiency relative to weight.

Moreover, when comparing the same layered configurations under different impactors, the flathead impactor consistently results in higher SEA values. For example, the five-layer flathead sample (F225-5) shows 72.85 J/g, while its hemispherical counterpart (H225-5) reaches only 57.52 J/g, a 26.7% difference. This highlights the significant role of impactor geometry and contact surface area in energy dissipation mechanisms of these composites.

4. Conclusions

This study provides a detailed analysis of the behavior of glass fiber/polyester layered composites under low-velocity impact conditions, with an emphasis on optimizing layer arrangements and fiber types to improve impact resistance. The results show that increasing the number of layers significantly enhances both energy absorption and peak contact force. Specifically, five-layer samples impacted with a flathead impactor exhibited a 351% increase in energy absorption and a 212% increase in peak contact force compared to two-layer samples under the same conditions. Mat fibers outperformed woven fibers, with mat-only samples absorbing 21.8% more energy and achieving 5.8% higher peak force. The geometry of the impactor was also critical: flathead impactors generated 101% higher peak forces and caused more severe damage than hemispherical ones. Observed failure modes included fiber breakage, matrix cracking, and delamination, all significantly affected by impact parameters and material properties.

Specific energy absorption was a key performance metric. Mat fiber samples showed a 3.0% higher SEA than those made with woven fibers, highlighting the importance of balancing energy absorption with weight efficiency for targeted applications. The SEA analysis also revealed that while energy efficiency improves with additional layers, the rate of gain plateaus beyond four layers.

These findings provide practical guidance for designing composite materials with high impact resistance, particularly for applications in aerospace, marine, and automotive industries. Future research should investigate other fiber and resin systems, a wider range of impact velocities, and the influence of environmental conditions to further advance composite performance.