**1. Introduction**

Energy absorbing panels are installed wherever there is a risk of damage to important machine components, human life, or health risk. Nowadays, energy-absorbing covers are used in many fields, such as energy, automotive, rail, aviation, mining, and maritime industries [1–3]. The most demanding branch of industry in which we deal with energy-absorbing shields is the military industry [4]. In the case of the military application, energy absorbing panels depending on this purpose must meet the relevant requirements set out in the standards [5,6]. Depending on the application, the designed covers must meet a number of different requirements. The important criterion is of course the degree of energy absorption. However, an increasingly important aspect of the newly designed solutions is to minimize their weight [7,8]. Therefore, the demand for solutions using composite materials is growing [9]. Polymer composites in addition to high strength are also characterized by a low weight and high

value of specific strength [4,10]. Depending on the application, composite shields can be subjected to the impacts of stones, hail, birds, bullets, or explosive debris. Therefore, resistance to impact loads perpendicular to the plane of laminate reinforcement layers is particularly important. Composite energy absorbing panels in the military application are used in various types of solutions. They are used both in personal protective equipment in the form of bulletproof vests or helmets [11–13], as well as in special purpose vehicles [9,12,14,15]. In the case of combining a polymer composite material with a ceramic material [7,11], the ceramic layer is responsible for defragmenting the striker and changing its trajectory, as a result of which a large part of the impact energy is absorbed. The layer made of fiber composite is designed to capture the remains of the striker, the defragmented ceramic layer, and the striker's inhibition. In the case where a multilayer panel with fiber reinforcement is used individually, it must stop the striker and absorb all of the impact energy. Impact loads cause exceeding the strength of the polymer matrix in the form of shear and bending. A delamination process is initiated between successive layers of reinforcing fabrics [16]. This is also a disadvantage of fibrous composites, consisting of the loss of connection between adjacent layers of reinforcing material, which leads to decreasing of the composite strength. During the continuous operation of composite products, this is particularly important because, in combination with fatigue strength [17], it leads to a significant weakening of the material. Delamination in the multilayered composites could be located using, for example, the wave propagation method [18,19]. The material susceptibility for delamination depends on many factors. Some of them could be mentioned, for example, the material used as an reinforcement, its properties, or bond quality between the fiber and the matrix [20]. Material delamination occurs because the energy threshold initiating this process is much lower than the energy threshold causing destruction of the composite reinforcement fibers [21]. In other words, delamination occurs because the strength of the matrix material is much less than the strength of the reinforcing material. Therefore, the largest part of the impact energy is absorbed as a result of fiber destruction [16,22].

The process of destroying the energy absorbing panel made of laminate can be divided into two phases. In the first phase, a high-speed striker hits the panel, causing fiber shear and matrix cracking. Primary yarns carry the highest loads. Secondary yarns are much less stressed. In the second phase, as the successive penetration of reinforcement layers progresses, the smaller and smaller energy of the striker is distributed over an ever larger surface, and finally it becomes insufficient to break the fibers in subsequent reinforcing layers [13,16]. The remains of the impact energy initiates the matrix delamination process. The reinforcement fibers in deeper layers are stretched under the influence of the impact energy concentration [12,16,23]. The microscopic image of damage in the composite with the hybrid reinforcement that was used in the conducted research is shown in Figure 1. The presented sample was hit by the hemispherical striker. The jagged fragments of the fibers at the side opposite to the impact side are the result of the cutting process. In Figure 1, it can be seen that the aramid fibers are deformed, broken, and sheared as a result of the striker impact. Around the impact area, accumulation of the damaged reinforcing layers could also be observed. The carbon fibers located between the aramid fibers owing to the high Young's modulus were deformed elastically and rebounded the striker.

The total energy at impact is the sum of the remaining kinetic energy of the striker and the energy absorbed by the composite panel. The total energy during impact could be divided at the energy of the moving striker, energy absorbed by shear plugging, energy absorbed by deformation of secondary yarns, energy absorbed by tensile failure of primary yarns, energy absorbed by delamination, energy absorbed by matrix cracking, and energy absorbed by friction [22].

Considering energy absorption, the most commonly used reinforced material is aramid fibers [16]. These fibers can absorb a large amount of the impact energy before their breaking. Furthermore, aramid fibers are more elastic than, for example, carbon fibers [24]. They are characterized by high rigidity and orientation, and they are connected to each other by strong, dense hydrogen bonds [25]. They have a good strength to density ratio (specific strength). Strength expressed in this way for Kevlar® fibers is greater; that is, five times greater than steel [16]. The tensile curves of aramid fibers, similar to glass and graphite fibers, are approximately linear to break. Aramid fibers have a density

43% lower than that of glass fibers. Because of that, aramid fibers are particularly attractive for the production of many composites [25]. Energy-absorbing panels built using aramid fibers provide excellent protection against pistol bullets, revolver bullets, or fragmenting debris. Aramid fibers are one of the most important materials used for the production of energy absorbing panels [16]. However, in the application of energy absorbing panels, stiffness is also very important and should be enough high to prevent too much deformation after impact. To minimize that deformation employment of carbon fibers between aramid fibers, reinforcement layers could be a solution. However, carbon fibers have some disadvantages. The first of them is the brittle damage [26] after impact, which can be dangerous for nearby people. It is worth mentioning that the brittle damage in some cases can be desirable, especially during modelling of the separation process [27–30]. In this case, the process should be designed in such a way that the equivalent stress should exceed the allowable stress that appears in the direct cutting zone. Otherwise, the material being removed would not be separated. The proper combination of different reinforcing materials allows the composite to take advantage of each of the applied fibers [16,31,32]. Undoubted advantages of the carbon fiber reinforced polymer composites are their low density, good static and fatigue strength, high modulus of elasticity, resistance to abrasion, and corrosion resistance [25]. Because they consist almost exclusively of graphite, they are non-melting and chemically resistant. The heat resistance of carbon fibers is unique and outperforms any known materials in this respect, except graphite. The high values of Young's modulus mean that this fiber is often used in hybrid composites to increase the stiffness of the structure [1,20].

**Figure 1.** Microscopic image of damage formed in a multilayered composite with hybrid reinforcement after impact of the hemispherical striker.

Naik et al. [33] showed that the proper configuration of reinforcing layers in a glass–carbon epoxy composite could increase post-impact compressive force. It was also shown that the damage area in a hybrid composites depends on the reinforcing material configuration. Studies presented in [34] also show that the hybridization in glass–carbon epoxy composites provide greatly enhanced damage tolerance of these structures. The experimental results show that the hybrid composites can absorb more energy in the impact event compared with non-hybrid composites [35]. Aramid fibers are often used in the case of the personal protective equipment, such as helmets. In order to increase the amount of information about the examined object, experimental and numerical research is carried out [36–38]. In another paper [39], the impact resistance of composite panels reinforced with aramid fibers and the matrix made from various thermoplastic materials were compared. The authors of [40] compared the

puncture resistance of the composites reinforced by 2D and 3D aramid fabrics. A further paper [41] presents the analysis of puncture resistance of aramid laminates on styrene–butadiene–styrene and epoxy resin matrix. The authors of [42] present the influence of the introduction of nanoclay into the resin on the increase of the maximum impact load of aramid fiber composite. Owing to the popularity of aramid fibers, research on composites with hybrid reinforcement (where aramid fibers were one of the used reinforcing materials) was also conducted. Research focused on a hybrid composite with reinforcement made of aramid and basalt fibers [43] shows that the use of both types of reinforcing materials in the proper configuration can increase the composite energy absorption during impact. The comparison of the hybrid composites with reinforced made of aramid and carbon fibers, based on the DI parameter (defined as the ratio between the damage propagation energy and the damage initiation energy), presented in the work of [35], showed that the highest value of this parameter was achieved for the composite in which reinforcing layers made of aramid fibers were used alternately with carbon fiber reinforcement layers. It was also noticed that the adjacent aramid fibers can also play a role in bridging the broken carbon fibers, which could improve the toughness of hybrid composites [35]. Studies on hybrid reinforcement made of aramid and carbon fibers in a sandwich structure [44] showed that the use of hybrid reinforcement increases the energy absorption of the composite during low speed impact. The authors of [44] obtained the highest values of absorbed energy in the case of a combination of three reinforcing layers of carbon fibers for one reinforcing layer of aramid fibers, and vice versa. However, the reduction of compressive strength after the impact was much smaller in the case where three reinforcing layers of aramid fibers for one reinforcing layer of carbon fibers was used. The damage area formed in the composite and its impact energy absorption during low velocity impact depend on the striker geometry [45]. Experimental and numerical research [45,46] showed that the influence of the striker geometry was changed with the impact velocity and the thickness of the composite.

The authors of that paper decided to assesses the influence of the striker geometry on the damage formed in the epoxy composite with hybrid reinforcement (made from aramid and carbon fabrics) after low velocity impact. Di fferences in the formed damage depending on the striker geometry were described. The authors of the present work decided to perform experimental and numerical research. The numerical calculations were performed in order to increase the amount of information about damage caused in the composite panel after impact of the strikers with di fferent geometry; in particular, information about delamination between reinforcing layers. In Section 2, the authors present materials used as a reinforcement and as a matrix. The producing method of the composite and the adopted research methodology are also presented. The results of experimental research, which was carried out using four strikers with di fferent geometries, are presented and discussed on the basis of microscopic images. In Section 3, the authors describe the process of preparation of the physical model. A methodology of modeling of the multilayered composite is presented. The damages caused in the composite plate as well as the phenomenon of delamination were presented and discussed. The authors present the values of the rebounded strikers' kinetic energy and the relationship between the amount of damage in the reinforcing layers and the occurring diameter of the delamination area. The experimental and numerical results were compared. The di fferences in obtained results were discussed. Composite panel wear was considered in the local response.

## **2. Experimental Research**
