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Vol. 8. Issue 5.
Pages 4620-4630 (September - October 2019)
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Vol. 8. Issue 5.
Pages 4620-4630 (September - October 2019)
Original Article
DOI: 10.1016/j.jmrt.2019.08.006
Open Access
Experimental research on damage characteristics of CFRP/aluminum foam sandwich structure subjected to high velocity impact
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Enling Tang
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tangenling@126.com

Corresponding author.
, Xiaoqi Zhang, Yafei Han
School of Equipment Engineering, Shenyang Ligong University, Shenyang 110159, China
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Tables (3)
Table 1. Kinetic energy loss of projectile at different initial velocities.
Table 2. Kinetic energy loss of projectile at the different impact velocities.
Table 3. Kinetic energy loss of projectiles with different material.
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Abstract

In view of the wide demands of sandwich structure with porous material as core layer in many important fields such as protection engineering, aerospace, automobile manufacturing and ship, etc. In order to reveal the energy absorption and damage characteristics of high velocity impact aluminum foam core and CFRP/aluminum foam core structure, the quasi-static loading system, one-stage light gas gun loading system, the impact pressure testing system and the high-speed camera acquisition system were used to perform the quasi-static compression experiments of aluminum foam and the projectiles with different materials impacting on the aluminum foam core and CFRP/aluminum foam sandwich structure at the speeds of 100–300 m/s. The influences of impact velocity and projectile’ density on the damage characteristics of CFRP/aluminum foam sandwich structure and the attenuation effect of shock wave in sandwich structure have been studied. The experimental results showed that the curve of stress and strain has three typical zones similar to porous materials: elastic zone, yield platform zone and compaction zone under quasi-static compression loading. Due to its brittle and low tensile strength of aluminum foam material, the single aluminum foam plate has poor ballistic performance under the high-speed impact of projectiles with different materials. Therefore, closed cell aluminum foam needs to be combined with high strength panels to form sandwich structure to improve its ballistic performance.

Keywords:
High velocity impact
Aluminum foam core
CFRP/aluminum foam sandwich structure
Energy dissipation
Damage characteristics
Full Text
1Introduction

Aluminum foam Sandwich structures with porous materials as core layers are widely applied in many important fields such as protection engineering, aerospace, automobile manufacturing and ships [1]. The application of sandwich structures with porous materials as core layers is widen, which may be subjected to impact loads such as flying debris and projectiles during service, and the impact performance of sandwich structures has attracted wide interest of plenty of scholars. Typical sandwich structures consist of front, rear panels and lightweight core. They have many advantages, such as a high stiffness, a high strength, a high energy dissipation, a good vibration reduction and stress wave attenuation. In the past, metal plates were mostly used as panels in sandwich structures. With the development of material science, more and more carbon fiber reinforced composites replaced lightweight metal as panels of sandwich structures. The core layer material is mostly polymer or metal foam, aluminum honeycomb and other dot matrix structures. With the development of new lightweight porous material, the closed cell aluminum foam has opened up an exciting new gate. Carbon fiber reinforced composites have been studied at the early stage at abroad. In 1993, Lee et al. [2] studied the dynamic penetration of graphite/epoxy composite laminates impacted by blunt-nosed projectiles. The ballistic limit of the target was obtained through experiments, and the dynamic penetration process of graphite/epoxy composite laminates was simulated. In 1995, Hitchen et al. [3] have carried out plenty of experimental studies on impact damage behavior of carbon fiber/epoxy resin reinforced composites under different laying orders. By analyzing the impact-time response and the energy absorption of the target, it is found that the stacking sequence has a significant influence on the energy absorption induced by delamination. The energy consumed by delamination increases linearly with the increasing of the total delamination area. In 2005, Breen et al. [4] studied the effect of impact velocity on CFRP reinforced laminates under low velocity impact by using finite element method. In 2006, Hwang et al. [5] carried out experimental study on the anti-penetration and fracture properties of carbon fiber reinforced composite laminates by using one-stage light gas gun as loading tool. In 2008, Wicklein et al. [6] performed the research on the ultra-high speed impact on CFRP performance, deducing and validating the material model of high dynamic performances of CFRP under high speed impact. In 2012, Francesconi et al. [7] carried out the CFRP experiment of hypervelocity impact with velocities ranging from 2 to 5 km/s, and programming the prediction model of the ultimate penetration velocity of target. By studying the high-speed shadow image of debris cloud, the basic characteristics of impact damage and its evolution were qualitatively analyzed. In 2016, Yashiro et al. [8] carried out experimental study on the damage and propagation mechanism in CFRP panel under high-speed impact conditions. It was found that the main factors that the kinetic energy consumption of projectiles are the breakage and stratification of fibers below the impact point. In 2007, Crupi et al. [9] carried out three point bending tests on two types of aluminum foam sandwich panels under quasi-static and dynamic conditions. The results showed that the collapse mode of sandwich panels depended on the distance of support span and the characteristics of sandwich panels themselves. In 2010, Dong et al. [10] used MTS universal testing machine to perform indentation experiments on foam aluminum sandwich structures under quasi-static loading. The influences of the thickness for the panel and core layer, adhesive layer, surface and boundary conditions on the mechanical properties and energy absorptive ability of the target were analyzed. In 2007, Zhang et al. [11] built a quasi static indentation testing system for sandwich structures, and conducting a quasi-static indentation test of sandwich structures. In 2017, Wang et al. [12] studied the indentation response of glass fiber/closed cell aluminum foam sandwich structure by combining theory with experiments, and obtaining the failure mode of sandwich structures. In 2011, Ahmed et al. [13] conducted the test of CFRP/foam aluminum sandwich structure by using drop hammer, and discussing the energy absorptive characteristics of sandwich structure under different panel materials and densities of aluminum foam. In 2011, Mohan et al. [14] carried out low-speed impact tests about three different panels (aluminum alloy, stainless steel, CFRP) and closed cell aluminum foam sandwich structure. In 2015, Mohd et al. [15] utilized Instron Dynatup 9250 HV impact testing machine to impact four types of targets (CFRP, aluminum foam, aluminum/aluminum foam sandwich structure, CFRP/foam aluminum sandwich structure), and the impact velocity was 6.7 m/s until the specimen was destroyed completely. In 2017, Colak et al. [16] studied the vibration response of CFRP/foam aluminum sandwich structure by experiments and finite element method, and measuring the natural frequency and damping coefficient of CFRP/foam aluminum sandwich structure by drop hammer. In 2013, Luo et al. [17] used ABAQUS software to study the dynamic behavior and energy absorptive characteristics of CFRP/foam aluminum sandwich structure under low impact velocity. The influences of different laying angles and relative density of core on the deformation, failure, contact force time curve, maximum contact force and energy absorption performances of sandwich structure were discussed. In 2018, Xiao et al. [18] simulated the explosion load of aluminum foam by one-stage light gas gun loading. The dynamic behavior of CFRP/foam aluminum sandwich structure was studied, and the influences of impact energy and relative density of foam aluminum on dynamic behavior were investigated, and the failure mode of the target was obtained. In order to reveal the energy absorption and damage characteristics of high velocity impact aluminum foam and CFRP/aluminum foam sandwich structure, respectively, the quasi-static loading system, one-stage light gas gun loading system, the impact pressure testing system and the high-speed camera acquisition system were used to perform the quasi-static compression experiments of aluminum foam and the projectile impacting on the aluminum foam core and CFRP/aluminum foam sandwich structure at the impact velocities of 100–300 m/s. The influences of impact velocity and projectile’ density on the damage characteristics of CFRP/aluminum foam sandwich structure and the attenuation effect of shock wave in sandwich structure were given.

2Experiment

The experiments have been performed on one-stage light gas gun at Intense Dynamic Loading Research Center, Shenyang Ligong University, China. The projectile used in experiment is a cylindrical flat-nosed projectile with a diameter of 15.3 mm and a ratio of length to diameter of 1:1. The sealing ability of the barrel is improved by adding a rubber ring to the projectile, and the wear of the barrel is reduced at the same time. The target is a typical aluminum foam core or sandwich structure. The panel material is CFRP, and the core material is closed cell aluminum foam. The geometric dimension of the sandwich structure is 120 × 120 × 26 mm3, the thickness of front and rear panels are 3 mm, and the orthogonal ply of [0/90] is adopted. The planar geometric dimensions of the core layer with a thickness of 20 mm is the same as that of CFRP, which the density of foam aluminum is 0.47 and a relative density of 0.17. However, the porous size distribution of aluminum foam is between 4–8 mm. The experimental system consists of a shock pressure measurement system and a high-speed camera acquisition system.

2.1Experimental loading system

A series of impact experiments have been performed about sandwich structure with a cylindrical flat-nosed projectile and the velocity ranges from 100 to 300 m/s. Fig. 1 lists a schematic diagram of the experimental loading system.

Fig. 1.

Schematic diagram of the experimental loading system.

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2.2Measurement system of impact pressure

In order to determine the attenuation effect of the shock wave in foam aluminum, two sets of PVDF piezoelectric sensors are installed between the panels and the core layer, and electrodes of the sensor are drawn out by carving a groove on the aluminum foam. When the projectile impacts on the sandwich structure and adhesion by epoxy resin, the shock wave first passes through the front panel and then spreads to the aluminum foam core layer. By analyzing the pressure changes measured by two group of PVDF piezoelectric sensors, it can reflect the attenuation effect of the shock wave in the core layer. Fig. 2 is the sensor layout diagram for impact pressure measurement of sandwich structure.

Fig. 2.

Sensor layout diagram for impact pressure measurement of sandwich structure.

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PVDF (Polyvinylidene Fluoride) piezoelectric film sensor was used to obtain the impact pressure between the panels and the core in the sandwich structure. Two copper electrodes were overlapped at both ends of the PVDF film, and packaged with insulating materials. Finally, Separate Hopkinson Pressure Bar (SHPB) was used to calibrate the dynamic piezoelectric coefficient. PVDF piezoelectric sensor testing circuit in current mode is used to measure the impact pressure. Fig. 3 is a testing circuit diagram of current mode.

Fig. 3.

Testing circuit diagram of current mode.

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In Fig. 3, discharge circuit of the current mode consists of a resistor R connected in parallel at both ends of the PVDF. When the PVDF is subjected to impact pressure, the charge quantity Q is obtained through the circuit formed by the external resistance R. The voltage signals U(t) at both ends of the resistor R are collected by an oscilloscope, then the impact pressure σ(t) of the PVDF film is obtained by integral operation.

In formula (1), K is the dynamic piezoelectric coefficient of the PVDF piezoelectric sensor and its units of pC/N, R is the sampling resistance and its unit of Ω, A is the effective bearing area of PVDF and its unit of mm2. Formula (1) is programmed by Matlab software, and the time history curve of impact pressure can be obtained.

2.3High speed camera acquisition system

Continuous images of impact process were acquired by using pco.dimax HS4 high-speed camera, produced by PCO company in Germany, and the high-speed camera can clearly record the whole physical process of projectile impacting target. By reading the pictures of the critical moments in the process of impact by combining with TEMA software, the instantaneous velocity before impact, flight attitude, the action time between projectile and target, the final velocity after impact, the impact plug and the dispersion of fragments can be obtained.

3Experimental results and analysis3.1Quasi-static compression and high velocity impact damage characteristics of aluminum foam3.1.1Quasi-static compression properties of aluminum foam

In order to obtain the data of plastic hardening of closed cell aluminum foam, a quasi-static compression experiment of closed cell aluminum foam was carried out by electronic universal testing machine. In order to reduce the damage to aluminum foam caused by mechanical processing, the aluminum foam was cut by wire cutting machine. The geometric size of the specimens is Ø60 × 30 mm2, the density and the relative density are is 0.47 and 0.17, respectively, and the aperture sizes are distributed between 4 and 8 mm. The quasi-static compression is performed at a loading rate of 0.5 mm/min. Fig. 4 is a quasi-static compression device. Fig. 5 shows the stress/strain curves obtained from quasi-static compression experiment.

Fig. 4.

Quasi-static compression device.

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Fig. 5.

Stress/strain curves of closed cell aluminum foam in compression experiment.

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As can be seen from Fig. 5, stress and strain curve has three typical zones similar to porous materials: elastic zone, yield platform zone and compaction zone. At the initial stage of compression, the cell wall of aluminum foam is elastically deformed, and the corresponding curvilinear strain is 1–6%. With the compression lasting, a wider range of stress platform (strain is in the range of 6–35%) appears on the curve. At the same time, the foam aluminum begins to bend at the weak point of the cell wall. With the increase of pressure, the cell wall of aluminum foam begins to bend, fold and collapse from top to bottom, meanwhile, due to the size of the cell body and the heterogeneity of cell wall, the yield platform of aluminum foam appears. The stress amplitude fluctuates in the region, but the stress varies little. When strain reaches 35%, the stress starts to rise suddenly. At this moment, the cell pores of aluminum foam begin to be compacted then enter the compaction and densification stage. Due to the gradual failure of aluminum foam, the stress and strain curve does not exhibit unloading phenomena at the yield platform, but their stress values are constantly increasing, which means that the ability of aluminum foam resisting deformation is increasing with the increase of densification degree. The stress of the yield in platform area is taken as the platform stress of the aluminum foam, and the platform stress of the aluminum foam is determined about 7.85 MPa.

3.1.2Damage characteristics of steel projectile impacting on aluminum foam core at high velocity

For the impact resistance of CFRP/aluminum foam sandwich structure, the core layer has a very important influence on the energy absorptive characteristics. Therefore, a cylindrical flat-nosed steel bullet with diameter of 15.3 mm hits the closed cell aluminum foam experiment was conducted by one-stage light gas gun loading. The initial and residual velocities of the projectile are measured by high-speed camera acquisition system. Fig. 6 shows the damage morphologies of aluminum foam at the impact velocity of 200 m/s.

Fig. 6.

Damage morphologies of aluminum foam at impact velocity of 200 m/s.

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It can be seen from Fig. 6 (a) that there is a shear failure at the cell wall of the aluminum foam at the surface of the projectile, and the size of perforation is almost the same as that of the projectile. The damage occurs mainly within the local area around the impact point, and there is almost no damage in the position far away from the impact point and the boundary of the fixed constraint. It can be seen from Fig. 6 (b) that the perforation of the aluminum foam at the rear surface is slightly larger than the cross-sectional area of projectile, which is the interaction of spares wave due to the brittleness and low tensile strength of the aluminum foam. The cell walls of the rear surface stretch and fracture under the action of the reflected tensile wave. At the same time, due to the uneven distribution of cell pores and the thickness of the aluminum foam, there will be a larger local tear around the bullet holes. Fig. 7 shows the damage photos collected by high speed camera at the different key moments during the process of impact.

Fig. 7.

Typical failure process of aluminum foam impacted by projectile at the impact speed of 205 m/s.

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In Fig. 7, it can be seen that the initial moment is just the contact moment between aluminum foam and projectile, and the impact process begins. The cell walls of the aluminum foam around the projectile area are bent, cut and torn, and the cell pores begin to collapse gradually. At the moment of 143 µs, the aluminum foam fragments begin to appear on the rear surface of aluminum foam. The aluminum foam broke up to form a large number of fragments. After the moment of 286 µs, the projectile pierces through the aluminum foam completely, and a large number of aluminum foam fragments scatter around. At the same time, some aluminum foam fragments of the front part of the projectile are flying at the same speed with the projectile. At this time, the residual velocity of the projectile is 178 m/s. Fig. 8 list the aluminum foam fragments collected in the chamber.

Fig. 8.

Aluminum foam fragments.

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From Figs. 7 and 8, one can see that the aluminum foam breaks and crumbling at the weak wall, and the diameter of fragments is distributed between 4–8 mm. As can be seen from Fig. 9, the projectile hardly undergoes deformation and failure after impact. Table 1 shows the loss of kinetic energy of projectiles at different initial velocities.

Fig. 9.

The projectile after impact.

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Table 1.

Kinetic energy loss of projectile at different initial velocities.

Experimental codes  Projectile’ s material  Initial velocity/m.s−1  Residual velocity/m.s−1  Kinetic energy loss ΔE/J  Kinetic energy loss rate % 
Steel  191  174  66.7  17 
Steel  200  178  89.3  20.8 
Steel  205  185  83.8  18.6 

As can be seen from Table 1, when the initial velocities of the projectile are 191 m/s, 200 m/s and 205 m/s, the residual velocities of the projectile are174 m/s, 178 m/s and 185 m/s, and the kinetic energy loss rates of the projectile are 17%, 20.8% and 18.6%, respectively. Under three kinds of initial velocities, the maximum kinetic energy loss rate of the projectile is only 20.8%, which indicates that the aluminum foam has poor anti-impact capacity due to its brittleness and low tensile strength of aluminum foam. Therefore, closed cell aluminum foam needs to combine with high strength panels to improve its anti-shock performance.

3.2Damage characteristics of CFRP/aluminum foam sandwich structure caused by high-speed impact3.2.1Damage analysis of CFRP/aluminum foam sandwich structure

The projectile is a steel cylindrical flat-nosed projectile with a diameter of 15.3 mm and a ratio of length to diameter of 1:1. The impact velocities of the projectile are from 100 to 300 m/s. The geometric dimension of sandwich structure is 120 × 120 × 26 mm3, and the thickness of panels and core layer are 3 mm and 20 mm, respectively. Fig. 10 is the damage morphologies of each components of CFRP/aluminum foam sandwich structure at the impact velocity of 296 m/s.

Fig. 10.

Damage morphologies of each components of CFRP/aluminum foam sandwich structure at the impact velocity of 296 m/s.

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As can be seen from Fig. 10, the sandwich structure is pierced through completely at the impact velocity of 296 m/s, and the existence of the core layer changes the failure mode of the CFRP panel. Due to the supporting effect of aluminum foam, the front CFRP panel suffers from shear failure, which produces a circular bullet hole corresponding to the diameter of the projectile, and cutting down a round CFRP plug. A crack extension area is generated near the bullet hole of the front surface of the projectile. The crack zone is square with the edge length about 19 mm due to the interlayer of the [0/90] fiber layer, crack arrest boundary is obvious. At the rear surface of the front panel, due to the unique pore structure of the aluminum foam, a small area of fibrous layer crack occurs at the corresponding hole around the bullet hole. An elliptical bullet hole is formed on the aluminum foam core, and there is an obvious opening lip on the bullet hole. The aluminum foam around the bullet hole is compacted, and the bullet hole on the rear surface is tapered. Due to the inhomogeneity of the aluminum foam, the aluminum foam will be stripped at the weak periphery of the hole when the projectile and the front panel plug work together, and the projectile will deflect during the process of impact. The perforation diameter of aluminum foam is larger than that of the bullet hole and the shape is irregular. Severe delamination damage occurs on the rear panel, resulting in a large area of crushing damage on the rear surface of rear panel, with a damage area of about 1230 mm2; and a "cross" petal-shaped crack appears on the rear surface of rear panel, and the fibers are ruptured by tension and pulled out. The damage area was much larger than the diameter of the bullet. The longest tear of CFRP fiber layer is 97 mm, and the damage area is about 3104 mm2 on the rear surface of rear panel. Fig. 11 shows the damage morphologies of each components of CFRP/aluminum foam sandwich structure at the impact velocity of 100 m/s.

Fig. 11.

Damage morphologies of CFRP/aluminum foam sandwich structure at the impact velocity of 100 m/s.

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It can be seen from Fig. 11 that the sandwich structure has not been pierced through, and the projectile stays between the front panel and the aluminum foam finally. A cylindrical crater with a depth of 9 mm are formed between the aluminum foam and the front panel, and the diameter of the crater is the same as that of the projectile. The CFRP slug sheared under the front panel stays at the bottom of the crater, and some cell walls of the aluminum foam on the rear surface collapse and break. There is no visible damage to the rear panel. The difference between the damage morphology of CFRP/aluminum foam sandwich structure under different initial velocity is the damage scope of the rear panel. Fig. 12 shows the damage states on the rear surface of rear panel at different initial velocities.

Fig. 12.

Damage states on the rear surface of rear panel at the different impact velocities.

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As can be seen from Fig. 12, when the projectile impacts sandwich structure at a speed of 100 m/s, the sandwich structure is not pierced through and the rear panel is not damaged visibly. With the increase of impact velocity, the damage area of the rear panel is approximately rectangular, and the front CFRP panel cracks inward with the impact point as the center. The carbon fibers are pulled out and broken during the impact, and the epoxy resin matrix material cracks. When the initial velocity of the projectile is 172 m/s, the geometric dimensions and damage area of the front surface in the rear panel are about 34 mm × 33 mm and 1122 mm2, respectively; when the initial velocity of the projectile is 224 m/s, the geometric dimensions and damage area of the front surface in the rear panel are about 42 mm × 26 mm and 1092 mm2, respectively; when the initial velocity of the projectile is 296 m/s, the geometric dimensions and damage area of the front surface in the rear panel is about 41 mm × 30 mm and 1230 mm2, respectively. Fig. 13 shows the damage states of the rear surface in rear panel under the different impact velocities.

Fig. 13.

Damage of the rear surface in rear panel at different initial velocities.

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As can be seen from Fig. 13, when the projectile pierces through the sandwich structure, the damage region of the rear surface in the rear panel is much larger than the diameter of the projectile, which the rear panel produces a typical "cross" shaped petal cracking area. The rear surface of the rear panel cracks outward with the impact point as the center, and the cracking length of the carbon fiber layer varies greatly in the horizontal and vertical directions. When the projectile impacts at the speed of 172 m/s, the damage area of the rear surface in the rear panel is about 1800 mm2; when the projectile impacts at the speed of 224 m/s, the damage area of the rear surface in the rear panel is about 2496 mm2; when the projectile impacts at the speed of 296 m/s, the damage area of the rear surface in the rear panel is about 3104 mm2. Fig. 14 shows the relationship between the impact velocity and the damage area of the rear panel.

Fig. 14.

The relationship between the impact velocity and the damage area of rear panel.

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As can be seen from Fig. 14, when the sandwich structure is penetrated, the damage area of the front surface in the rear panel is basically equal, and the damage area of the rear surface in the rear panel increases approximately linearly with the increase of the impact velocity.

3.2.2Influences of different parameters on damage characteristics of CFRP/aluminum foam sandwich structure3.2.2.1Influence of impact velocity on damage characteristics of CFRP/aluminum foam sandwich structure

The experiments of flat-nosed steel projectile impacting on CFRP/aluminum foam sandwich structure have been performed at the speeds of 100–300 m/s, and the influence of impact velocity on the damage characteristics of CFRP/foam sandwich structure is determined. Table 2 shows the kinetic energy loss of projectile at the different impact velocities.

Table 2.

Kinetic energy loss of projectile at the different impact velocities.

Experimental codes  Projectile’smaterial  Initial velocity/m.s−1  Residual velocity/m.s−1  Kinetic energy loss ΔE/J  Error/%  Perforation status of sandwich structure 
Steel  296  123  779  82.7  Perforation 
Steel  224  98  436  80.8  Perforation 
Steel  213  81  417  85.5  Perforation 
Steel  172  75  258  80.9  Perforation 
Steel  100  108  100  No perforation 

In Table 2, it can be seen that when the impact velocity of the projectile is 100 m/s, the sandwich structure is not pierced through, and the kinetic energy of the projectile is consumed completely by the sandwich structure. Within the impact velocity ranges of 172–300 m/s, the sandwich structure is completely pierced through, and the kinetic energy loss rate of the projectile reaches about 80%, which shows that the anti-impact performance of the sandwich structure is improved significantly compared with the single aluminum foam.

3.2.2.2Influence of projectile density on damage characteristics of CFRP/aluminum foam sandwich structure

Two kinds of projectiles with different materials are used to perform impact tests on CFRP/aluminum foam sandwich structure. The projectiles are cylindrical flat-nosed bullets with diameter of 15.3 mm and the ratio of length to diameter is 1:1, the masses of steel and aluminum pellets are 21.5 g and 7.5 g, respectively. Table 3 shows the kinetic energy loss of the projectile.

Table 3.

Kinetic energy loss of projectiles with different material.

Experimental codes  Projectile’s material  Initial velocity/m.s−1  Initial kinetic energy Ei/J  Kinetic energy consumed ΔE/J  Error/%  Perforation status of sandwich structure 
steel  172  318  258  80.9  Perforation 
Aluminum  292  320  320  100  No perforation 

It can be seen from Table 3 that the CFRP/aluminum foam sandwich structure has different damage under the same impact energy levels and different densities, and the kinetic energy loss of aluminum projectile is 62 J, which is higher than that of steel projectile. Under the steel projectile impact, the sandwich structure is pierced through completely, and the kinetic energy loss of the projectile is 80.9%. However, the sandwich structure is not pierced through when the aluminum projectile impacts, and the kinetic energy of the projectile is completely consumed by the sandwich structure. This shows that the density of the projectile has an obvious effect on the anti-shock performance of the sandwich structure. When the impact energy is the same, the anti-shock performance of the sandwich structure is better for the low density projectile.

3.2.3Typical physical process of steel projectile impacting on CFRP/aluminum foam sandwich structure

The whole physical process of projectile impacting on CFRP/aluminum foam sandwich structure can be recorded by high-speed camera, and the initial and residual velocities of projectile impact composite target can be measured. Fig. 15 is the penetration failure process of a CFRP/aluminum foam sandwich structure at the impact velocity of 172 m/s.

Fig. 15.

Penetration failure process of CFRP/aluminum foam sandwich structure impacted by projectile at the speed of 172 m/s.

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As can be seen from Fig. 15, at the moment of t = 0 µs, the target is justly shot by projectile, with the continuous penetration of the projectile, the front panel will be sheared and plugged, and the cell wall of the foam will collapse. At the moment of t = 143 µs, the projectile will enter the target completely. At the moment of t = 286 µs, the rear panel will appear bulging, the local surface cracks and the tensile fracture of the fiber will appear on the rear surface, with the gradual increase of deformation; at the moment of t = 572 µs, the rear panel will be enlarged. On the rear surface, a large number of fibers broke down, and perforating with "cross" petals. At the same time, the deformation area continues to expand outward along the surface of the panel, and a small amount of fragments spatter outward. At the moment of 743 µs, the projectile pierces through the sandwich structure completely, and the rear panel suffers from serious matrix cracking, fiber fracture and delamination. The cracked filaments fly forward at a certain speed, and the projectile deflects after passing through the sandwich board. When the moment is t = 2000 µs, it can be clearly seen that the CFRP slices sheared flies above the syncline, and the residual velocity of the projectile is 75 m/s.

3.2.4Attenuation effect of shock wave in sandwich structure

In order to reveal the attenuation effect of shock wave in CFRP/aluminum foam sandwich structure, PVDF piezoelectric sensors are placed at two measuring points between the panels and the core layer, and the impact experiments at different speeds were carried out by using one-stage light gas gun loading system. When the projectile hits the CFRP/foam aluminum sandwich structure, the projectile first contacts the front panel, and the shock wave propagates through the front panel to the foam aluminum. The PVDF I sensor is used to record the impact pressure time history of the interface between the front panel and the foam aluminum, and PVDF II sensor is used to measure the impact pressure time history of the contact interface between the foam aluminum and the rear panel. The attenuation effect of core layer on shock wave is analyzed by compared the impact pressures of two measuring positions. Fig. 16 is the original piezoelectric signals and shock pressure time history curves of PVDF sensors under the impact velocity of 296 m/s.

Fig. 16.

Time history curves of PVDF piezoelectric signals and impact pressure at the impact velocity of 296 m/s.

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As can be seen from Fig. 16, the peak value of impact pressure at measuring point 1 is 55 MPa, the peak value of impact pressure at measuring point 2 is 25.8 MPa, and the difference of peak value between two measuring points is 29.2 MPa. The peak value of impact pressure attenuated by 53.1% through the action of core layer, which shows that core layer has obvious attenuation effect on shock wave. As can be seen from Fig. 16 (b), time history curve of the stress duration of sensor 1 is very short due to the front panel sheared quickly at the high impact velocity. Then the PVDF piezoelectric film at the measuring point 1 will be sheared because of the penetration of the projectile, therefore the curve of the pressure peak at the measuring point 1 decreases rapidly and lasts for a short time. The impact pressure attenuation of CFRP/foam aluminum sandwich structure can be explained as: the interface effect between the front and rear panels and the core layer, the deformation of the panels and core layer, and the unloading wave. In addition, there are a large number of irregular holes in the aluminum foam, which forms many aluminum substrate/air interfaces. When the shock wave propagates in the aluminum foam, there will be a series of interfaces. Due to the complex reflection, transmission and diffraction processes, the interaction of shock waves at the cell wall interface makes the signal received by the PVDF piezoelectric sensor 2 greatly weaken. Therefore, the existence of the foam aluminum core has a strong attenuation effect on the shock wave. Fig. 17 is the original piezoelectric signals and the impact pressure time history of the PVDF piezoelectric sensor when the impact velocity of the projectile is 100 m/s.

Fig. 17.

Time history curves of PVDF piezoelectric signals and impact pressure at the impact velocity of 100 m/s.

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As can be seen from Fig. 17, the pressure peak at the measuring point 1 is 38.4 MPa, and the pressure peak at the measuring point 2 is 21.8 MPa, and the peak value of the impact pressure decreases by 43.2% after the interaction of the aluminum foam core. The peak attenuation of impact pressure at impact velocities of 100 m/s, 172 m/s and 296 m/s are 43.2%, 48.2% and 53.1%, respectively. Fig. 18 is the relationship between the impact velocity and the peak value of impact pressure in CFRP/aluminum foam sandwich structure.

Fig. 18.

Relationship between impact velocity and impact pressure peak in CFRP/aluminum foam sandwich structure.

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It can be seen from Fig. 18 that the impact pressure peaks at the measuring points 1 and 2 increase with the increasing of the impact velocity of the projectile. In other words, with the increasing of the impact velocity of the projectile, the difference of the peak values of the impact pressure at the two measuring points will increase, which indicates that the aluminum foam core has a good attenuation effect on strong shock wave.

4Conclusion

The energy absorption and damage characteristics of high velocity impact aluminum foam and CFRP/aluminum foam sandwich structure are important index for engineering application. In the paper, the quasi-static loading system, one-stage light gas gun loading system, the impact pressure testing system as well as the high-speed camera acquisition system are established and used for performing the quasi-static compression and dynamic loading experiments. The influences of impact velocity and projectile’ density on the damage characteristics of CFRP/aluminum foam sandwich structure and the attenuation effect of sandwich structure on shock wave. The following conclusions can be drawn:

  • 1)

    In the elastic region, the strain of aluminum foam is in the range of 1–6%, the yield plateau area appears when the strain is 6–35%, and the plateau stress is about 7.85 MPa. When the strain reaches 35%, the compacted dense zone appears in the quasi-static compression experiment.

  • 2)

    When the sandwich structure is penetrated, the damage areas of the front and rear surfaces of rear panel are basically equal, and the damage areas of the rear surface of the rear panel increases approximately linearly with the increasing of the impact velocities.

  • 3)

    When the projectile’ velocity is 100 m/s, the sandwich structure is not penetrated, and the kinetic energy of the projectile is completely consumed by the sandwich structure. When the impact velocities are in the range of 172–300 m/s, the sandwich structure are completely pierced through, and the kinetic energy loss rate of the projectile reaches about 80%.

  • 4)

    The density of the projectile has an obvious effect on the anti-impact performance of the sandwich structure. When the impact energy is the same, the anti-shock performance of the sandwich structure is better for the low density projectile. With the increasing of impact velocity, the differences between the peak value of impact pressure of aluminum foam will increase, which indicates that the aluminum foam core has a good attenuation effect on strong shock wave.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgments

The authors would like to acknowledge National Natural Science Foundation of china (Grant No. 11472178), Open Foundation of Hypervelocity Impact Research Center of CARDC (Grant No. 20190201) and Open Project of State Key Laboratory of Explosion Science and technology in Beijing Institute of Technology (Grant No. KFJJ18-04M) to provide fund for conducting experiments.

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Journal of Materials Research and Technology

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