1. Introduction
Fiber-reinforced composite polymers are used in almost all types of advanced engineering structure. They combine glass or carbon reinforcing fibers with a matrix material such as epoxy, phenolic, or polyester. Composite materials are complex, mainly due to the degree of anisotropy induced by the reinforcing fibers. Thermosetting polymers consist of chain molecules that chemically bond or cross-link with the others when heated together. Composite materials have a high resistance to internal and external corrosion. They are light with a very smooth inside for higher throughputs. Manufacturing a composite structure starts by incorporating a large number of fibers in a thin layer of the matrix to form a ply. The required load in a fiber-reinforced composite structure is obtained by stacking a plurality of layers in a specified order and then grouping them to form a ply. Various layers in a ply can contain fibers in different directions. It is also possible to combine different types of fibers (for example, glass and carbon) to form a hybrid laminate.
A low-velocity impact by foreign objects is a significant concern for composite laminate as this can cause damage to the interior of materials, which significantly reduces the strength of the composite component and may not be easily detected. The complexity of such an impact on composite laminate is due to the different failure modes that occur in composites compared with metals. The selection of optimal parameters for composite plates that give a high resistance to low-velocity impact loads should consider various factors related to the material properties, as well as the manner of manufacturing the composite product. To obtain the desired impact resistance, it is essential to know the correlation between these parameters and the energy absorbed by the composite plate. The development process of this correlation is not an easy task because unknown process parameters are non-linear. Knowing the parameters affecting the impact resistance and the degree to which these parameters affect are the most significant problems of the composite laminate industry.
Drop-weight impact testing is the standard test procedure used to study the impact of resistance and the behavior of composite laminates. Drop-weight testing also tends to be the preferred method when performing low-velocity impact testing. American Society for Testing and Materials (ASTM) Test Method D7136/D7136M [
1] is the governing international standard used to study the impact testing on a rectangular plate. This test technique determines the damage resistance of multidirectional polymer matrix composite laminated plates exposed to a drop-weight impact event. The standard test utilizes constant impact energy normalized by specimen thickness. The properties obtained using this test method can guide researchers concerning the anticipated damage resistance of composite structures of similar material, thickness, stacking sequence, and so forth. To compare samples quantitatively, several equations may be used, which can be found in ASTM D7136.
The total amount of energy introduced to a composite specimen and the energy absorbed by the composite specimen through the impact event are essential parameters to assess the impact response of the composite structures. The introduction of new fiber materials is a promising method for strengthening interfacial bonding between the matrix and fibers in hybrid composite laminates. This alteration of the material has been used to enhance the impact resistance of polymer composite materials. A considerable improvement in the impact resistance was achieved by using hybrid composites. The formation of delamination generally relates to matrix cracking. Generally, in any impact situation, matrix cracking occurs first, followed by delamination [
2]. Many useful techniques have been successfully devised to improve delamination resistance in the past three decades, namely three-dimensional (3D)-weaving, stitching, braiding, embroidery, Z-pin anchoring, fiber hybridization, toughening the matrix resin, and interleaving with tough polymer, short fibers or micro-scale particles. These methods enhanced the interlaminar properties but at the cost of in-plane mechanical properties [
3]. Multiwall carbon nanotube-reinforced carbon-fiber laminates have better energy absorption capacity as compared to neat carbon-reinforced fiber laminate [
4]. On the other hand, glass-fiber composites exhibited evident delamination between the plies, matrix transverse cracks within plies, and significant fiber damage at relatively low impact energies [
5].
Several low-velocity impact tests considering target size, projectile diameter, and test temperature were carried out by many authors to determine the response of four different combinations of hybrid laminates to low-velocity impact loading using an instrumented impact testing machine [
6,
7,
8,
9]. Impact resistance is proportional to the thickness of the composite panel, and it was not affected by the geometry of the plate.
New types of fiber materials and different staking were considered [
10]. Fiber metal laminate exhibits outstanding impact absorption capacity under various energy levels, where its energy resistance is lower than standard woven fabrics [
11]. There is a recent increase in the use of ecofriendly, natural fibers as reinforcement for the fabrication of lightweight with an increasing trend in research publications and activity in the area of basalt fibers. Natural fiber composite has the potential to be widely applied in the alternative to fiberglass composites in sustainable energy impact-absorption structures [
12,
13]. Deposition of micro- and nano-fillers, such as aluminum powder, colloidal silica, and silicon carbide powder, in glass fiber-reinforced epoxy composites can enhance the impact resistance and impact energy absorption of the hybrid composite laminates [
14,
15]. Microencapsulated epoxy and healing agents can be incorporated into a glass fiber-reinforced epoxy matrix to produce a polymer composite capable of self-healing with excellent mechanical strength [
16,
17,
18,
19].
Sensitivity analysis is one of the approaches that can be used to ascertain the degree of influence of various mechanical and material parameters on the impact performance of the composite laminated plates. Many researchers have tried optimization of the impact performance of the composite plates using the design of experiment (DOE) and artificial neutral network (ANN) model with Finite Element Modelling (FEM) techniques. Based on several studies using ANN models to find the optimal laminate combination, the low-velocity impact resistance of fiber-reinforced polymer composite plates depends more significantly on the thickness and the stacking sequence and the effect of the elastic moduli of the fibers. At the same time, the matrix has less effect than the strength of the fiber and matrix materials of the composite [
20,
21,
22,
23].
Analysis of absorbed energy and velocities during impact testing of composites may not be all that is needed for characterization. For damage mechanism characterization and type of failure identification, post-impact analysis is required to be carried out for the damaged sample [
24]. Several techniques were tried and tested and have been proven to provide useful results to characterize the damaged areas in a polymer composite resulting from a low-velocity impact. Visual inspection can be used to analyze the impact tested samples for specific damage types that include dent/depression, cracking/ splitting, fiber failure, and delamination. Correlation between detected volumes and absorbed energy using optical measurement is an excellent tool to estimate impact effects [
25]. Nowadays, plenty of Non-Destructible Testing (NDT) techniques are investigated for composite inspection [
26,
27,
28,
29,
30,
31]. Infrared thermography has shown great potential and advantages, which has greater inspection speed, higher resolution and sensitivity, and detectability of inner defect due to heat conduction.
Finite element modeling and simulations are commonly used as well to evaluate the impact resistance of single- and multi-layer fibers [
32]. The finite element (FE) model, in conjunction with the material model, is capable of capturing the behavior of composite for multi-layer and staking configurations under low-velocity impact [
33,
34].
Others have comprehensively studied the behavior of low-velocity impact on the composites in the literature. However, the current work aims to investigate the impact response of the angle-ply laminated plates using different fibers (carbon and glass). A combination of two types of fibers was also examined. Several types of stacking sequence and resin were considered. Absorbed energy-time curves were presented to understand the behavior of the low impact velocity loading. A flowchart of the entire procedure, experimental and modeling, is given in
Figure 1.
4. Finite Element Modeling
The composite plates were modeled using 3-D shell elements as an area without thickness. The thicknesses and orientations were given as the composite layup data using ANSYS ACP (Canonsburg, PA, USA). The striker was modeled as a 3-D rigid body. Frictionless contact between striker and plate was considered. The amount of damage was calculated as the loss in the kinetic energy of the striker. The FE model is shown in
Figure 11.
In this work, the fixed composite plate was subjected to impact loading represented by the dropped object at a velocity of 2.06 m/s. The impactor (12.7 mm in diameter) was modeled as a rigid hemispherical body. The mechanical properties of the glass and carbon fiber-reinforced composites are listed in
Table 7. The initial velocity and mass of the striker were set depending on the energy level considered. The impactor was constrained to movement within 5 degrees of freedom (x and y translations and 3 rotations) and was allowed to move only in the -y-direction. A sufficient density of 7860 kg/m
3 was assigned to the rigid impactor.
The Probabilistic Design System (PDS) module of the commercial finite element software ANSYS was used for the Monte Carlo simulation. A total of 1000 analysis loops are run to obtain the output parameters as a function of the set of random input variables. The 19 plates were manufactured and then tested based on previous work using the Monte Carlo method for random variables to evaluate the effect of variability in the governing parameters for the outcome of the experiment. We selected the von Mises equivalent stress as the outcome of the numerical experiment for the new proposed plates design. To determine the mesh size of elements in finite element modeling, a convergence test was conducted on several cases of the models where
Table 8 shows the convergence case for 20 J impact testing. To determine the size of elements in finite element modeling, a convergence test was conducted on several cases of the models where
Table 8 shows the case for 20 J impact testing. The analysis showed that, the optimum mesh size was when we selected the axial edge sizing to be 100 divisions with a bias factor of 2.
The simulation was accomplished through a concept known as birth and death of elements in ANSYS. To achieve the “element death” effect, the ANSYS program does not actually remove “killed” elements. Instead, it deactivates them by multiplying their stiffness by a severe reduction factor (ESTIF). This factor is set to 1.0 × 10−6 by default. An element’s strain is also set to zero as soon as that element is killed. In like manner, when elements are “born”, they are not actually added to the model; they are simply reactivated. When an element is reactivated, its stiffness, mass, element loads, etc. return to their full original values.
Contact between the impactor and the whole laminate composite was simulated using the automatic-surface-to-surface penalty-based contact algorithm to accommodate impact initiation and progress. A contact criterion based on 0.01 mm of the normal distance between the contact surfaces was adopted for the simulation. The loads and boundary conditions are shown in
Figure 12. The finite element model is generated in ANSYS ACP for angle-ply laminate having different stacking sequences.
Table 9 lists a comparison of the absorbed energy response, while
Figure 13 shows the absorbed energy-time history of the fiberglass plate G4 and Carbon-fiber plate C1 for the case of 20 J obtained by the experiment and by the FE model. The FE results are very close to the experimental values for the final absorbed energy, with an error of less than 9%. This means that the test results validate the FE model. Hereafter, the FE model can be used to perform a parametric analysis; this indicates that the developed FE model can reasonably predict the actual behavior of any composite plate under low-velocity impact loading. The difference in the starting energy time is due to the placement of the impactor close to the plate during modeling to optimize the computational time, whereas in the experiments the impactor falls from a height based on the required energy, which is 20 J in this case. The effect of gravity was considered on the calibration cases for C1 and G1 plates. This is done by including the gravity as an initial condition for the system then by suppressing this feature during analysis. It was found that it is acceptable to ignore gravity as the impact process was performed in a fraction of a second. Including the gravity affects the absorbed energy steady-state line as the bouncing of the impactor decreases with time due to the gravity, and hence the slight inclination in the steady-state line of the absorbed energy.
6. Conclusions
In this study, the low-velocity impact behavior of laminated carbon-, glass-, and mixed-fiber plates with different types of resin was investigated experimentally and numerically. The results presented in the current study give an insight into the effects of the considered parameters on the impact-resistance performance. Several types of stacking sequence at constant impact energy are considered in order to investigate the behavior of composite structures. The main conclusions of the study are:
The amount of energy absorbed (impact performance) varies significantly for the variations in the thickness of a single layer, number of layers, and stacking sequence.
The experimental test data showed an increased energy absorption for the composite plates made with phenolic resin.
The carbon-fiber/epoxy composite plate has better impact resistance compared to the glass-fiber/epoxy composite plate due to the higher measured absorbed energies of the carbon-fiber/epoxy.
Visual inspections showed a large extent of damage was observed on the polyester and epoxy resin plates when they are compared with the same plates made with phenolic resin.
The effect of the carbon-fiber plies location for the mixed plates was not exceptionally pronounced.
The stacking sequence with [90/0/45/−45]s was better than [60/45/−45/−60]s in terms of impact resistance, as concluded from the simulated cases.
The results that were captured from the current work provide a motivation to further examine the samples in the future in order to determine the damage morphology under scanning electron microscopy. Moreover, the results from this study help researchers in designing composite laminated plates with better impact resistance. The results allow for a more methodical approach in selecting the parameters to vary in order to achieve better impact performance of composite laminates against low-velocity impact loadings.