1. Introduction
Fiber metal laminates (FMLs) are composite materials made by laying up reinforcing fibers and metal sheets alternately and preparing them under certain temperature and pressure. As it combines the excellent mechanical properties of fiber and metal materials, it has the advantages of high specific strength, high specific stiffness, resistance to impact damage, high temperature and corrosion resistance, etc., and is widely used in aerospace, electronic information, and the military industry, and other fields [
1,
2,
3,
4,
5]. FMLs are not susceptible to chemical corrosion, and their mechanical properties do not change under a certain pressure. The mechanical properties of FMLs at specific temperature are related to the metal used, and degradation of the mechanical properties of FMLs occurs when the temperature exceeds the threshold value that the metal can withstand.
It is often unavoidable for composite components to be damaged by low-velocity impacts in the process of its use, for example, when a car is hit by another vehicle while driving, when an airplane is hit by a flock of birds during flight, or when a ship is hit by a floating object while sailing, and so on [
6,
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11]. The occurrence of such events will cause structural damage to FMLs, thereby reducing their service life and greatly weakening their load-carrying capacity. Therefore, it is particularly important to investigate the mechanical behavior of FMLs under low-velocity impact.
In recent years, many researchers have investigated the mechanical behavior of FMLs under impact loading by means of experiments and finite element simulations. Jaroslaw et al. [
12] compared the mechanical behavior of aluminum-based glass fiber laminates and aluminum-based carbon fiber laminates under low-velocity impacts in terms of damage area and damage depth. Research showed that the important damage mode of FMLs is interfacial debonding. Aluminum-based carbon fiber plywood has a higher tendency to perforate, and the two have completely different energy absorption modes. Yao et al. [
13] investigated the mechanical behavior and damage mechanism of aluminum-based composites under low-velocity impact using different shapes of punches, and the results showed that the damage mode of FMLs under the impact of conical punches was mainly petaling crack, and the damage mode under the impact of flat-shaped punches was mainly smooth perforation. The experimental results were verified by establishing a finite element model, and it was found that the impact load–time curves and damage morphology of the two could be better coincided. Lee et al. [
14] found that FMLs fabricated using autoclave molding have higher strength and stiffness, better impact resistance, and lighter mass compared to conventional FMLs through drop-weight impact tests. They also investigated the crack extension and damage modes of FMLs with different ply sequences, and the results showed that the direction of crack extension was affected by fiber orientation. Some scholars have investigated the mechanical behavior of FMLs under multiple impacts. Pai et al. [
15] incorporated aramid—epoxy, ultra-high molecular weight polyethylene—epoxy, and paperboard—epoxy as interlayers, and observed and analyzed the deformation distributions and damage modes of FMLs under primary and secondary impacts. It was concluded that the FMLs incorporating the paperboard layer had smaller deformations compared to those without the paperboard layer. Moreover, it was found that the addition of high acoustic impedance metal panels could enhance the impact resistance of FMLs to a certain extent. The established numerical model can predict the deformation profile of the backplate of FMLs more accurately.
Hybrid composites are composite materials made by combining two or more fibers in the same matrix, in which the fibers can be selected from natural fibers, artificial fibers, or synthetic fibers, and various fibers can be arranged in different layup sequences to optimize the performance of the composites. Compared with single-fiber-reinforced composites, hybrid composites have the advantages of reducing weight, lowering production costs, improving strength and stiffness, and improving impact resistance and fatigue resistance, which are widely used in aerospace, construction industry, health care, and other fields [
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Several scholars have successfully used hybridization to enhance the impact resistance of homogeneous laminates. For research on hybrid composites, at the beginning, most of the literature mentioned the combination of carbon/aramid and carbon/polyethylene to enhance the impact resistance of the laminate. As the environmental requirements became more stringent, the research on basalt fibers gradually increased. Sarasini et al. [
21,
22] investigated the effect of basalt hybridization on the impact properties of aramid fiber laminates and carbon fiber/epoxy laminates and performed four-point bending tests after impact. The results showed that the hybrid laminates have superior energy absorption and flexural behavior and better damage tolerance than the homogeneous laminates. With the development of hybrid composites, some scholars began to combine hybrid fibers with FMLs. Megeri et al. [
23] investigated the mechanical properties of glass/carbon fiber-reinforced aluminum alloy laminates under low-velocity impacts and found that hybrid fiber-reinforced aluminum alloy laminates have better impact resistance and smaller central deflection compared to pure glass-fiber-reinforced aluminum alloy laminates. Kazemi et al. [
24] hybridized carbon and UHMWPE fibers and investigated the mechanical properties of fiber-reinforced titanium alloy laminates under low-velocity impact. It was found that FMLs with added UHMWPE fibers had better toughness and ductility and performed better at LVI compared to pure carbon fiber reinforced titanium laminates. Hussain et al. [
25] used the matrix and hybrid fibers as variables to investigate the low-velocity impact behavior of hybrid fiber-reinforced aluminum alloy laminates. They found that FMLs have better toughness and higher impact resistance when using Polyvinyl Butyral as the matrix, while epoxy-based FMLs have poorer impact resistance. A comparison of different types of hybrid reinforcements reveals that aramid/jute hybrid fibers have higher deformation resistance and show better overall LVI performance compared to carbon/jute hybrid fibers.
At present, many scholars usually choose an aluminum alloy as the metal layer when studying the mechanical properties of FMLs, while research on magnesium alloy, especially on hybrid fiber-reinforced magnesium alloy laminates is less common. In this paper, the mechanical properties of pure carbon-fiber-reinforced magnesium alloy laminates and carbon and glass hybrid fiber-reinforced magnesium alloy laminates under low-velocity impact are investigated through experiments, theoretical models, and numerical simulations. Firstly, the displacement–load curves and energy–time curves of FMLs were obtained through experiments, and then a finite element model is established based on Abaqus-VUMAT subroutine to observe and analyze the damage modes and damage morphology of the metal layer, the fiber layer, and the interlayer, and the experimental results were compared with those of the finite element analysis. The above study applied hybrid fibers to FMLs and investigated their impact properties and damage mechanisms, filling a gap in the field. The established finite element model can better predict the mechanical properties and damage patterns of FMLs, which contributes to the application and development of FMLs in practical engineering.
5. Conclusions
Compared with pure carbon-fiber-reinforced magnesium alloy laminates, carbon/glass hybrid fiber-reinforced magnesium alloy laminates have improved peak load and energy absorption by 5.1–8.5% and 3.7–5.7%, respectively, although the stiffness is slightly reduced. The use of hybrid fibers leads to better the impact resistance of FMLs.
The FEM results of FMLs were compared with the experimental results. It was found that the simulated values were slightly higher than the experimental values, and the differences between the peak load and energy absorption were 2.0–6.5% and 0.7–4.6%, respectively. The load–displacement curves and energy–time curves of the two are in good agreement, which indicates that the established finite element model can predict the experimental results better.
The damage morphology and damage modes of FMLs under low-velocity impact were observed and analyzed. When the incident energy is low, the damage mode of FMLs is petaling crack, and when the incident energy is increased, the damage mode of FMLs changes to circumferential crack.
At incident energies of 40 J, 60 J, and 80 J, the fiber damage area and debonding area of different fiber-reinforced magnesium alloy laminates become larger with the increase in incident energy. Compared with carbon-fiber-reinforced magnesium alloy laminates, hybrid fiber-reinforced magnesium alloy laminates have smaller damage area and better impact resistance and load-bearing capacity. An observation of the damage morphology of the fiber layer and the interlayer of FMLs reveals that the damage of the upper fiber layers is more serious compared to the lower fiber layers. Compared with the interlayers between the fiber layers, more serious debonding occurs between the fiber layers and the metal layers.