Military Technical College, Kobry Elkobbah, Cairo, EgyptReceived 8 November 2014; revised 10 February 2015; accepted 1 April 2015 Available online 11 July 2015

Multi-layer protective armour for underwater shock wave mitigation
A hmed HAWASS, Hosam MOSTAFA, Ahmed ELBEIH*

Military Technical College, Kobry Elkobbah, Cairo, Egypt
Received 8 November 2014; revised 10 February 2015; accepted 1 April 2015 Available online 11 July 2015

Abstract

The effect of underwater shock wave on different target plates has been studied. An underwater shock wave generator (shock tube) was used to study the interactions between water and different constructed targets which act as shock wave mitigation. Target plates, composed of sandwich of two aluminum sheets with rubber and foam in between, were prepared and studied. For comparison, the target plates composed of triple aluminum sheets were tested. The study includes the testing of the selected plates in water under the effect of different peak pressures and the analysis of the results.

The strain gauge data and displacement sensors results showed that the multi-layer plates have higher level of underwater shock wave mitigation than the triple aluminum plates with strain and deflection of nearly 50%.

Copyright©2015, China Ordnance Society. Production and hosting by Elsevier B.V. All rights reserved.

Keywords:Shock simulator; Mitigation; Aluminum; Foam; Rubber

E-mail addresses: elbeih.czech@gmail.com, elbeih.a@gmail.com (A. ELBEIH).

Peer review under responsibility of China Ordnance Society.

http://dx.doi.org/10.1016/j.dt.2015.04.006

2214-9147/Copyright©2015, China Ordnance Society. Production and hosting by Elsevier B.V. All rights reserved.

1. Historical background and introduction

In the modern battles, there is an aim to build a new naval body structures composed of light and strong composite materials [1]. This naval vessel must be able to withstand the damage produced by underwater explosion (UNDEX) Understanding that the interaction between the composite material and the applied load simulate the underwater explosion is an interesting study. Low rates of the stress applied to the composite material were deeply studied in many studies unlike the higher loading caused by the underwater explosion [1].

Since 1968, the different terrorist attack cases have been occurred [2]. To protect the naval vessel from these threats, the structure has to be supported by shock wave mitigation methods. Regarding to the good mechanical characteristics of the composite materials, they have varieties of applications, including military and defense applications. The understanding of blast response and resistance of these materials is very important to design a new material and decrease the effect of this attack. Many academic researchers have studied the response of different plates in the form of sandwich structure under the effect of different applied blast stresses [3-5]. Many studies have investigated the homogenous composite materials subjecting to different loads [6-10]. The addition of polymeric materials to different structures was studied to enhance the blast wave resistance [11]. The polymeric materials were used to decrease the weight of naval vessels and increase the protection level of their bodies.

The dynamic response of metallic lattice sandwich plates under impulsive loading applied by ballistic pendulum system was studied [12]. Liu et al. studied the effect of blast loading on metallic sandwich-walled hollow cylinders with graded aluminum foam cores [13]. Sandwich tubes under internal explosive loading were investigated experimentally and the deformation of sandwich tubes occurred sequentially from the inner tube to the outer one was discussed [14]. Short duration of pressure pulses resulting from underwater explosions was represented by Riccardo et al. [15]. The energy dissipation and deformation occurred to sandwich structures subjected to underwater blast loading was investigated [16].

Several typical phenomena can be observed by explosion of charge under water. These phenomena are the shock wave, gas bubble, cavitation, etc. Park [17] indicated that the explosive charge is converted to gaseous products at high temperature of 3000°C and produce shock wave pressure of approximate 500 MPa. The main product of the explosion conversion is gas at high temperature and pressure [18,19]. The reaction advance from the c-j plane to complete explosion reaction was presented in Ref. [20]. The resulted gaseous products form spherical gas bubbles producing initial shock wave followed by a further series of bubble oscillations until arriving to the surface or any target. After underwater explosive conversion, the generated shock wave propagates spherically at a speed which is faster than sound speed at first and then decreases to the similar value [21]. The formed gases provide rapid rise for the pulsed waves which are difficult for controlling and require a suitable safety arrangement. Underwater shock generator can be easily controlled and safer during test. Many researches studied and proved that the underwater shock wave generator can produce a pressure wave profile similar to the shock wave profile resulting from free underwater explosion tests [22-26].

Deshpande et al. designed an underwater shock wave generator filled with water to study the effect of different pressure pulses on several plates under water, reducing the time and cost of underwater explosions test [27]. Guan et al. studied another novel technique based on transmission tube and explosion method [28]. Using steel projectile as a source of impact energy, a laboratory underwater shock wave generator was designed to produce underwater shock pressure. The underwater shock wave generator can be controlled by changing the mass and the velocity of the impact projectile [29]. Another apparatus made from water-filled fiber composite tube and 1.5 kg striker was used to test different fiber plates [30].

The aim of this research is to investigate the mitigation of shock wave occurred at multilayer sheets made of light material such as rubber and polyethylene foam subjected to an underwater pressure wave.

2. Material specification

Multi-layer sandwich panels were fabricated. The outer sides of the panels were aluminum plates and the core was composed of two sheets of foam and one sheet of rubber, as shown in Fig. 1. Each aluminum 204 alloy has a composition of (93Al, 4.15Cu, 0.1 Cr, 0.5 Mn, 0.5 Si, 0.25 Zn, 1.5 Mg wt %), and the 8 mm thick and 0.98 kg weight multi-layer panel has a 330×330 mm face area. For comparison, an examined aluminum jackets made from three aluminum plates was prepared, which has the same face area mentioned above, and is 4.5 mm in thickness and 1.13 kg in weight. The constructions of the different plates are shown in Fig. 1 and the further details are provided in Table 1.

The material properties of aluminum 204 are listed in Table 2. The characteristics of EPDM rubber (based on ethylenepropylene diene monomer) are listed in Table 3. The rubber hardness test was carried out by using Shore A which is used for soft elastomers and its value varies between 10 and 90.

Fig. 1. Construction of the examined target.

Table 1Specification of target sheets.

Table 2Material properties of aluminum 204.

Table 3Specification of EPDM rubber sheet.

Cross-linked low density polyethylene (LDPE) foam, named VOLARA type A, produced by Sckisui Voltek,LLL, was selected as a shock wave mitigation material for the desired object of this research. LDPE have wide application in naval industry as it have many desirable properties including water resistance, chemical resistance, energy absorbance, buoyancy and cushioning. The specification of the foam used in the test is listed in Table 4. All test results of LDPE foamare related to standard test methods for flexible material ASTM D3575.

Table 4Specification of LDPE foam sheet.

3. Experimental testing

An underwater shock generator in the form of cylindrical tube was used to produce a small scale laboratory blast wave. In this research, the underwater shock generator consists of shock tube made of mild steel with 5 mm in thickness. It is 1 m in length and 20.7 cm in diameter, and placed over a steel plate. A hammer with a pendulum arm is fixed at the end of the shock tube in a vertical position to generate the impact energy needed for the creation of underwater shock wave. Moving of the hammer at different angles causes the increase of the impact energy. A 25 mm thick steel piston is placed inside the shock tube at the end part facing to the hammer. The examined target is fixed at the front part of shock tube. The positions of strain gauge and sensor are shown in Figs. 2 and 3.

The hammer has a mass of 21.3 kg. The impact of the hammer on the end part of the tube causes the piston to produce pressure in the water. The target is exposed to an impulse of a planer wave which propagates along the length of the tube as a result of the impact pressure imparted by the hammer. The generated pressure pulse is measured by the pressure sensor which is placed on the top of the underwater shock tube.

Kistler type 211B series pressure sensors (range: 700 kPa, sensitivity: 8 mV/kPa), produced by Inter Technology Co., Canada, was used to measure an exponentially decaying pressure history. The pressure sensor recorded the first signal which is the first shock pressure and the second signal which is a reflected pressure.

Fig. 2. The underwater shock generator.

Fig. 3. Construction of underwater shock generator.

Fig. 4. Sensors used for underwater testing.

The examined targets were subjected to similar pulsed pressure wave during different applied tests while the resulted reflected pressure depends on the material properties of the examined target. N11-FA-5-120-11 strain gauge, produced byShow a Measuring Instruments Co., Japan, was used to measure the strain generated by the pressure wave. S13FLP12A displacement sensor, produced by Alther bv, Netherlands, was used to record the displacement of the examined target. The three sensors used in the experiments are shown in Fig. 4.

Table 5Effect of the impact angles of hammer on the pressure wave characteristics.

4. Results and discussion

The results are divided into two main category: the measuring result of the pressure due to hammer impact and the measuring result of the targets' resistance to the underwater pressure wave, which are characterized by three main parameters, strain measurements, displacement measurements and visual examinations. Target visual examination containsthe depth of the resulted deformation produced as a result of different pressure waves after the different shots. Two examined groups were tested. The first group contains three different shots (peak pressure lies between 70 and 700 kPa) which permit the use of strain gauges, pressure anddisplacement sensors, and the second group contains one shot (peak pressure is more than 700 kPa) which imparts material damage and only allows strain gauge measurements. The angle between the impact hammer and the moving piston can be adjusted according to impact velocity and energy required. The adjustment can be produced by changing the distance between the hammer head and the piston. The detailed output characteristics are presented in Table 5.

Fig. 5. Pressure profile of shock tube.

Fig. 6. Measuring result of strain gauge.

Fig. 7. Measuring results of displacement sensors.

4.1. Measuring results of pressure sensor

A typical pressure profile obtained by the pressure sensor for different shots at different impact angles is shown in Fig. 5. For impact energy of 3.12 J (10°impact angle of the hammer), the incident shock pressure was nearly 5.6×104Pa, and the reflected pressure wave has higher value than the incident one (6.2×104Pa), as shown in Fig. 5(a). This result might be due to the combination of the incident and reflected waves to form a high value of shock pressure.

In case of impact energies of 12.4 J and 27.6 J, the incident pressures have maximum values of 2.5×105Pa and 5.5×105Pa, respectively, as shown in Fig. 5(b) and (c). The results show that the incident shock pressure increases as the impact energy of the hammer increases.

Fig. 8. Deformation of aluminum plate caused by underwater shock wave.

4.2. Fluid response (strain measurement)

As a result of the impact energy of the hammer, underwater pressure wave is formed and travels through the water from the end part of the shock tube to the front part until reaching the sheet of the target. Local cavitation and bubbles are formed on the boundary between water and target. The incident pressure wave impacts on the target and is reflected back into the water. The corresponding measurements of the compressive core strain histories are presented in Fig. 6.

The two test target plates had similar trend but different strain values were obtained for each particular impact energy. The multi-layer plate showed a great reduction in strain values at all the shot examined. The strain deformation of the multilayer target is nearly half the deformation value of the aluminum target in the case of 10°impact angle of the hammer. These results give indication that the rubber and foam sheets have the ability to absorb the incident shock wave energy and make the multi-layer plate more reliable against the underwater shock wave.

4.3. Deflection of center point

The displacement of sandwich plate was determined by the displacement sensors which present the displacement-time history for the triple-layer aluminum plate and multilayer sandwich materials. The results show that the deflections occurred in the multilayer sandwich material was less than the values recorded in the case of using the triple aluminum plates for particular shock pressure values. Reduction in the measured deflections is presented and explained in Fig. 7.

4.4. Visual examination

After four shots by the impact hammer, the degree of deformation of the triple-layer aluminum plate are 2.06 mm for the first aluminum plate face, 1.2 mm for the middle plate and 1.4 mm for the outer plate. In the case of multi-layer plate, no deformation occurred for the aluminum plate which firstly subjected to the underwater shock wave, and the degree of deformation of the outer aluminum plate is 2.24 mm. These results show that there is a significant absorption of the incident shock pressure and no reflection occurs. These results indicate that it is better to use polymerized material as internal layers of targets to obtain better mitigation in shock wave. Photos of the target sheets are shown in Fig. 8.

5. Conclusions

Underwater shock wave generator had been used successfully to investigate and compare the interaction between shock wave and aluminum target made of triple aluminum plates and multilayer plate with rubber and polyethylene foam. Pressure sensors were used to record the pressure wave produced by impact of a hammer. The measurements of strain gauge and displacement sensors proved that the multilayer plate has better mitigate shock wave compared to the aluminum target. Also the mass of multilayer plate is less than that of aluminum target. From this study, it is recommended to continue studying the possibility of replacing the ordinary plates of naval vehicle by multilayer plates.

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* Corresponding author.