Comparative experiments were conducted with two different structures to study the mechanism of aluminum foam sandwich attenuating blast shock wave. The sandwich structure is composed of “steel–aluminum foam–steel,” and the mild steel structure is composed of “steel–steel.” In the experiment, the polyvinylidene fluoride transducers were used to directly test the load of stress wave between different interfaces of sandwich and mild steel structures. The strain of back sheet was simultaneously measured using high-precision strain gauge. The accuracy of the test results was verified by Henrych’s formula. Experimental results show that the wave attenuation rate on the mild steel structure is only 11.3%, whereas the wave attenuation rate on the sandwich structure can exceed 90%. The interface effect is clearly a more crucial factor in the wave attenuation. The peak value of back sheet strain in the mild steel structure is much higher than the sandwich structure. The apparent overall “X” crushing band is produced in the aluminum foam core, and scanning electron microscope (SEM) observation clearly shows the collapse of the cell wall. Experiments on the sandwich structure with different aluminum foam densities indicate that increasing the relative density results in increased attenuation capability of the aluminum foam and decreased attenuation capability of the sandwich structure. Experiments on the sandwich structure with different aluminum foam thickness indicate that increasing the thickness results in increased attenuation capability of the aluminum foam and the sandwich structure.
Aluminum foam (ALF) is porous material that consists of thousands of random three-dimensional polyhedral pores embedded in a continuous aluminum or aluminum alloy matrix. ALF is divided into closed-cell and open-cell structures depending on the properties of the internal pores. Compared with other composite materials, ALF has many excellent advantages such as light weight and high strength. It is widely used in aerospace and military applications, such as fuselage of space shuttle, armor of military tanks, and naval warship. In addition, the excellent thermal insulation and sound absorption properties of ALF make it popular in civil engineering [
A typical sandwich structure is composed of two thin metallic face plates and a thick porous metal core in the middle. The face plates can be steel plates, aluminum plates, or fiber metal laminates, which mainly bear shock loads and bending moments. The core material can be metallic honeycomb and ALF, which mainly bear transverse shear loads [
The theoretical research on the stress wave attenuation effect of sandwich with ALF core mostly focuses on the establishment of a simplified one-dimensional model. Based on the R-P-P-L mode of ALF, Tan et al. [
Few people have reported the experimental test results of the stress wave attenuation during the sandwich structure with ALF under blast loading. Goel et al. [
The polyvinylidene fluoride (PVDF) transducers are used to test the load of interface stress in the sandwich and mild steel structures. The dynamic sensitivity coefficient of the PVDF transducers should generally be calibrated before use in the explosive experiments. The Split Hopkinson Bar (SHPB), which could provide dynamic loading, is perfectly suited for dynamic calibration of PVDF sensors. The relationship between charge amount and pressure is linear for the PVDF sensors. PVDF transducers connect with the appropriate resistance, and the amount of charge can be determined by measuring the voltage of the resistor. The relationship between the stress
In the present work, ALPORAS closed-cell ALF was manufactured in liquid state using the blowing agent procedure with a chemical composition Al-1.5Ca-1.5Ti. The relative density of ALF is defined as the density of ALF and the matrix metal density. Two typical images of the experimental sample are shown in Figure
Images of aluminum foam with different relative densities.
After detonation, the explosives produced high temperature, pressure detonation gas products, and shock waves, which could cause serious damage to the surrounding structures. A 500-gram cylindrical TNT explosive was selected in each experiment. It was suspended at the center of the mild steel or sandwich structure at a distance of 320 mm. As the detonation distance was almost 10 times the diameter of the TNT explosive, the front sheet of the structure could be deduced to be affected only by blast shock wave. The PVDF transducer was installed in the interfaces between different materials, as shown in Figure
Experimental measurements of stress wave and strain: (a) mild steel structure, (b) aluminum foam sandwich structure, and (c) experimental devices.
Three groups of rectangular strain rosettes were set on the surface of the back sheet, as shown in Figure
The typical signal recorded by the PVDF sensor in the mild steel structure under blast loading is shown in Figure
Measurement signal in the mild steel structure: (a) voltage signal; (b) stress signal.
Figure
Measurement signal in the sandwich structure: (a) voltage signal; (b) stress signal.
The stress wave attenuation in the sandwich structure with ALF core can be divided into two steps. The first step is the multiple transmission and reflection of the stress wave between the interface of steel and the ALF, which is defined as interface effect. The second step is the effect of the porous characteristics of the ALF materials on stress wave attenuation. The attenuation progress of stress wave at the interface can be simplified into a one-dimensional elastic–plastic problem. Owing to the different impedance materials in the sandwich structure, the intensity value of reflected waves and transmitted wave intensity can be expressed as follows:
The equivalent proportional distance method is generally used to express the peak overpressure of the shock wave. The equivalent proportional distance is defined as shown in (
Henrych and Major [
The regular reflected pressure of the air shock wave in the rigid wall is shown in (
When the mass of TNT explosives is 500 g, the detonation height is 320 mm,
The strain time history curve of back sheet shown in Figure
Strain time history curves of (a) mild steel structure and (b) sandwich structure.
Figure
The peak value of the back sheet strain is shown in Table
Peak value of strain in mild steel and sandwich structure.
Structures | | | | | | |
---|---|---|---|---|---|---|
Mild steel | 2513 | 2284 | 1540 | 993 | 943 | 1490 |
Sandwich | 672 | 634 | 507 | 411 | 408 | 596 |
Figure
(a) Destruction image of aluminum foam under blast loading, (b)–(e) SEM images in different regions.
The cover plate and the front sheet show large plastic deformation and the apparent overall “X” crushing band produced in the ALF core caused by the free edge effect from the sides. The deformation of ALF can be divided into four regions. In Region I, the unaffected zone, an elastic deformation occurred in the cell wall. In Region II, an apparent plastic deformation occurred in the cell wall. In Region III, fragmentation occurred in the cell wall, indicating that the ALF was destroyed. In Region IV, the cell wall was collapsed.
Actually, multiple reflections occurred in the interface between the cell wall and air as a result of the porous structure of aluminum foam. The wave propagating became more and more dispersed due to the multiply wave interaction in the aluminum foam. So the pressure perpendicular to the aluminum foam core was attenuated since the dispersive effect. In addition, the plastic deformation and cracks growth of the cell wall are obvious in Figure
Some experiments were conducted to study the effect of the relative density of ALF on stress wave attenuation. The relative density of ALF ranges from 11.6% to 20.6%. Stress waves between different interfaces (
Stress wave in the sandwich structure with different ALF relative densities.
Relative density | | | | | | |
---|---|---|---|---|---|---|
0.116 | 29.32 | 5.37 | 1.13 | 0.817 | 0.145 | 0.961 |
0.125 | 29.73 | 5.92 | 1.45 | 0.801 | 0.150 | 0.951 |
0.143 | 29.34 | 6.61 | 1.69 | 0.775 | 0.167 | 0.942 |
0.154 | 29.64 | 7.36 | 1.88 | 0.752 | 0.185 | 0.937 |
0.162 | 29.78 | 7.96 | 2.05 | 0.733 | 0.198 | 0.931 |
0.206 | 29.64 | 8.58 | 2.41 | 0.711 | 0.208 | 0.919 |
Although the relative density of ALF is different, the value of
Influence of ALF relative density.
Figure
Some experiments were also conducted to study the effect of the thickness of ALF on stress wave attenuation. The thicknesses of ALF ranges from 38 mm to 53 mm, while the relative densities of ALF are all in the scope of 19.8%–20.6%. Stress waves between different interfaces (
Stress wave in the sandwich structure with different ALF thicknesses.
Thickness (mm) | | | | | | |
---|---|---|---|---|---|---|
38 | 30.26 | 8.72 | 3.73 | 0.712 | 0.165 | 0.877 |
43 | 29.54 | 8.61 | 2.96 | 0.709 | 0.191 | 0.900 |
48 | 29.64 | 8.58 | 2.41 | 0.711 | 0.208 | 0.919 |
53 | 29.63 | 8.60 | 2.33 | 0.710 | 0.212 | 0.921 |
58 | 29.65 | 8.53 | 2.20 | 0.712 | 0.213 | 0.926 |
Figure
Influence of ALF thickness.
Some experiments were conducted in this study to investigate the attenuation of the stress wave in the sandwich structure with ALF under blast loading. The PVDF transducers with high frequency response were used to test the stress waves between different interfaces, whereas the high-precision strain gauges were selected to measure the strain of the back sheet. The experimental results show that the ALF sandwich structure is extremely helpful in reducing the peak load of the stress wave compared with the mild steel structure without ALF core. The attenuation coefficient could exceed 90%, whereas the attenuation coefficient of the mild steel structure is only 11.3%. By measuring the strain of back sheet in different structures, the strains in the sandwich structure are found to be much less than the mild steel plate. By measuring the stress between the front sheet and the ALF, the interface effect is determined to be the main factor of the attenuation of the stress wave in the sandwich structure. By measuring the stress of sandwich structure with different ALF relative densities, increasing the relative density is found to increase the attenuation capability of the ALF but decrease the attenuation capability of the sandwich structure. Experiments on the sandwich structure with different aluminum foam thickness indicate that increasing the thickness results in increased wave attenuation capability of the aluminum foam and the sandwich structure. The apparent overall “X” crushing band was produced in the aluminum foam under blast loading, and the phenomenon could be considered as an important reason for the stress wave attenuation.
The authors declare that they have no conflicts of interest.