Experimental Study on the Damage Characteristics of Polymer Slabs Subjected to Air Contact and Close-In Explosions

School of Water Conservancy Engineering, Zhengzhou University, Zhengzhou 450001, China National Local Joint Engineering Laboratory of Major Infrastructure Testing and Rehabilitation Technology, Zhengzhou 450001, China Hubei Key Laboratory of Construction and Management in Hydropower Engineering, China /ree Gorges University, Yichang 443002, China Faculty IV Electrical Engineering and Computer Science, Technical University Berlin, Berlin 13353, Germany


Introduction
Polyurethane is mainly composed of isocyanate and polyol polymerization, which has a series of advantages [1], such as rapid and adjustable reaction, high expansion rate, impermeability and waterproofness, and no environmental pollution. Hence, it has outstanding effect on solving specific complex engineering disaster problems, such as improving the properties of soft soil and increasing its strength [2]. Polyurethane has been widely used in high-speed railway unballasted track [3], underground pipeline [4,5], dam, cutoff wall [6], tunnel, highway [7], airport runway [8], and other infrastructure seepage rescue and rapid repair.
Researchers have carried out relevant investigations on the static properties of polymer, such as compressive strength and shear properties. In addition, they have also carried out research on its properties of high expansion and diffusion, strong permeability resistance, rapid curing, durability, and environmental protection [9]. For example, Shi et al. [10] explored the effect of temperature on the compressive strength and volume change of polyurethane grouting materials through uniaxial compression test; Li et al. [1] carried out uniaxial compression test on polyurethane grouting materials to study the influence of density and size of polyurethane samples on compressive strength. e direct shear behavior of polyurethane bentonite interface under different normal stress, water content, and density was studied [11]; Beverte et al. [12] tested the shear strength and modulus of polyurethane material using a selfmade axial clamping tensile tester; Fang et al. [13] explored the interface between polyurethane material and concrete using a scanning electron microscope and a series of direct shear tests. Niedziela et al. [14] calculated and analyzed the uneven expansion of polyurethane materials; Hao et al. [15] studied the diffusion process of polyurethane slurry in cracks through numerical analysis and experiments; Wang et al. [16] explored the permeability resistance of polyurethane grouting materials through experiments; Chen et al. [17] studied the durability of porous polyurethane mixture. However, the above research is mainly carried out on the static mechanical properties of polyurethane grouting materials, and there are few studies focusing on the dynamic mechanical properties, especially the damage characteristics under explosive impact load.
Field explosion test is widely used in the study of dynamic mechanical properties of concrete slabs, steel plates, bridges, and other structures. For example, Wang et al. [18] studied the damage characteristics and damage modes of unidirectional square reinforced concrete slabs under different TNT initiating charges through four groups of field explosion tests; Zhao et al. [19] explored the damage characteristics of concrete slabs under air and underwater contact explosion through field explosion tests and considered the boundary conditions, reinforcement, charge shape, and charge quality. e dynamic response of reinforced concrete slabs subjected to 500 g TNT contact explosion was studied by Dua and Braimah [20]. Gao et al. [21] designed and manufactured one-way sea-sand seawater concrete slabs reinforced with basalt fiber reinforced plastic (BFRP) bars and explored the damage mode and antiexplosion characteristics of one-way sea-sand seawater concrete slabs reinforced with basalt fiber reinforced plastic (BFRP) bars under air close-in explosion through field explosion test. Li et al. [22] explored the failure mode and antiknock performance of steel mesh reinforced concrete slab under contact explosion through field explosion test. Chen et al. [23] investigated the failure mode, damage characteristics, and antiknock performance of autoclaved aerated concrete (AAC) slab strengthened with polyurea coating under close-in explosion through field explosion test; Yang et al. [24] explored the damage mode of reinforced concrete slab under air and underwater contact explosion through field explosion test. Zhuang et al. [25] studied the dynamic response and damage mode of reinforced concrete cylinder under underwater explosion through field explosion test. Yang et al. [26] proposed an EDZ rock mass properties estimation method based on the generalized Hoek-Brown failure criterion and variable disturbance factor, which is used to quickly estimate the rock mass properties of EDZ change when the blasting method is used to excavate the slope in the rock mass. Furthermore, Yang et al. [27] monitored the vibration data at the tunnel blasting excavation site, analyzed and obtained the vibration characteristics of the tunnel surface and surrounding rock, and verified the site survey results using a three-dimensional dynamic finite element model. It is worth noting that the above field explosion tests are mainly carried out for concrete materials, while the field explosion tests for polymer slabs made of polyurethane materials are relatively less.
In this research, the air contact and close-in explosion tests of square polymer slabs were carried out, and the damage characteristics and dynamic response of polymer slabs were explored. e shock wave experimental data of the upper and lower sides of the slab could provide experimental basis for the antiknock performance research of polymer slab.

Specimen Preparation.
In this paper, six polymer slabs with the same density, mass, volume, and shape were made by using self-made injection mould. Nonaqueous reactive two-component foaming polyurethane was used as polymer material. Isocyanate and polyol were injected into the steel model with a grouting gun at a mass ratio of 1 : 1, and gelation reaction and foaming reaction occurred [13]. e density of polymer slab is 0.2 g/cm and the density is controlled by grouting quality. Polyurethane grouting material with this density is widely used in engineering applications such as dyke seepage prevention, so we choose 0.2 g/cm as the density of our polymer slabs. e length and width of the polymer slab are 50 cm and the thickness is 6 cm. During the foaming process, the swelling force of polyurethane material develops rapidly within 35 seconds after pouring, and the swelling force increases exponentially with the increase of foam density. e foam gradually solidified in 10 min, and the strength reached a stable level after 1 hour. e samples were placed at 20°C and 40% relative humidity for 24 hours and then demoulded and taken out to make the polymer slab. e polymer slab samples have the typical characteristics of plastic materials [1,11]. In addition, Li et al. [1] studied the mechanical properties of polyurethane grouting materials under quasistatic conditions and gave the static mechanical parameters of polyurethane grouting materials of different densities under uniaxial compression tests, such as stress-strain curve, Young's modulus, secant modulus, and yield strength. And an important conclusion is obtained: the mechanical properties of low-density polyurethane (≤0.3 g/cm ) are hardly affected by geometric dimensions. Figure 1 shows the specimen preparation process.

Equipment.
During the test, the Blast-PRO impact tester was used to monitor the air shock wave generated by the explosion load, as shown in Figure 2(a). e vibration acceleration of the polymer slab in X, Y, and Z directions was measured with the Blast-Cloud blasting vibration tester, as shown in Figure 2(b).

Explosion Load.
e rock emulsion explosive with density of 1.05 g/cm , detonation velocity of 4.2-5.0 km/s, and briskness of 12 mm was the used test, as shown in Figure 3(a). In order to avoid mass loss during weighing and explosive placement, thin plastic bags were used to wrap the explosives. It should be noted that the shape of the explosive is not an absolute cube or cuboid.
is is because rock emulsion explosives have good fluidity, and the weight of the explosive test charge is very small (only 15-50 g), and it is difficult to ensure that the explosive charge has a regular shape. However, we used plastic film to wrap the explosives so that the shape of the explosives used in each group of tests was basically spherical, which greatly reduced the impact of the shape of the explosives. e accuracy of the electronic balance used for weighing is 0.01 g. e charge was initiated with a nonelectric detonator, as shown in Figure 3(b).

Experiment Device.
In order to reduce the impact of the reflection of shock wave on the ground on the test results, a special steel frame was used to assemble a test device with a height of 1.5 m, which was placed on the flat ground (see Figure 4). e polymer slab was fixed on the welded iron bracket, and steel slabs were inserted between the two boundaries of the polymer slab and the iron bracket, which were fixed with special bolts in advance, to provide approximate fixed boundary conditions for the specimen. e welded iron support was fixed on the erected steel frame. Four air shock wave sensors were arranged on both sides of the upper and lower surfaces of the polymer slab in order, and two probes were arranged on each side, respectively, 80 cm and 100 cm away from the explosion facing surface of the polymer slab, which were numbered as # 1, # 2, # 3, and # 4 from top to bottom. e air shock wave sensor used in our test has good sensitivity and test accuracy, and a slight tilt will not affect its receiving shock wave signal. In addition, we will check whether the sensor is tightly fixed before each test, so as to prevent it from generating a large tilt that may affect the measurement of the test data. e sensor of the blasting vibration meter was fixed in the polymer slab with bolts. In the case of close-in explosion, the charge was suspended above the center of the slab, and the distance between the explosion centers was 20 cm. e detonator for initiation was placed in the center of the charge. Figure 5 is a simplified diagram of the experiment device.

Experiment
Grouping. Six identical polymer slabs were labeled A0, B0, B1, B2, B3, and B4, respectively. Among them, A0 and B0 are compared to analyze the influence of the burst distance on the failure mode and damage characteristics. From B0 to B4, the explosive charge weight has been increasing, to explore the influence of the explosive charge weight on the failure characteristics of polymer slab

e Influence of Standoff Distance.
e relative positions of explosive and polymer slab in air contact and close-in explosion are given in Figure 6. e test results of polymer slab A0 are shown in Figure 7. It can be seen from the figure that, under the action of contact explosion, the polymer slab is mainly characterized by local punching, collapse, and spalling on bottom surface.
It can be seen from Figure 7 that, after the test, the central area of the polymer slab A0 is penetrated; a hole with a diameter of about 15 cm is formed on the top surface of the slab, and a damage area with a diameter of about 20 cm is formed on the center of bottom surface. Besides, the polymer in the damage area peels off. e reason is that, under the action of contact explosion, the high-pressure shock wave generated by explosion directly acts on the top surface of the polymer slab, and the generated compressive stress is higher than the dynamic compressive strength of the polymer. erefore, firstly, a crater is formed on the top surface. After the shock wave pressure propagates to the interior of the polymer slab, it propagates to the bottom in the form of compression wave, and the strong tensile wave is reflected on the bottom surface of the slab. It causes the bottom surface collapse and spalling damage, so that the middle part of the polymer slab results forms a hole. Figure 8 shows the test results of specimen B0 subjected to close-in explosion. Compared with the specimen A0, the standoff distance changes from 0 cm to 20 cm, and other conditions remain unchanged.
It can be seen from Figure 8 that the damage of polymer slab B0 under close-in explosion is more serious than that under air contact explosion. One of the nonfixed edges of the polymer slab is completely broken, the maximum length of the damage area of the top surface is about 40 cm, and the polymer spalling damage is serious on the bottom surface. e maximum length of the damage area of the bottom surface is about 30 cm, and there are two shallow pits formed by the polymer spalling in the top surface. e reason might be that when the explosive explodes in the air, it can strongly compress the air to form a shock wave. After the shock wave reaches the top surface of the slab, the resulting compressive stress is higher than the dynamic compressive strength of the polymer slab, resulting in the damage on the top surface. en, the compression wave is transmitted into the slab through the air; as it propagates to the bottom surface of the slab, it reflected to a strong tensile wave. e tensile stress produced is higher than the dynamic tensile strength of the polymer, which eventually leads to the collapse of the bottom surface and the spalling of some polymers into pits.
erefore, under the action of close-in explosion, the polymer slab is damaged as a whole, and the damage degree of specimen B0 is more serious than that of specimen A0. Figure 9 shows the time history curves of shock wave pressure measured in contact explosion test. During the air contact explosion, the time history curve of shock wave pressure is divided into positive pressure zone and negative pressure zone (with the standard atmospheric pressure as the boundary).
According to the time history curve of shock wave pressure in Figure 9(a), when t � 0.47 ms after explosion, the air shock wave propagates to the #1 measuring point. And the pressure reaches the peak value instantly; its overpressure peak value is 0.133 MPa. en it decays exponentially to the standard atmospheric pressure, and then it enters the negative pressure area, and the decay rate becomes slower and slower. As shown in Figure 9(b), the air shock wave reaches the symmetrically set #4 measuring point at t � 0.58 ms, and the peak overpressure is only 0.053 MPa. From the comparison of #4 and #1 measuring points, the peak overpressure at #1 measuring point is 2.51 times of that at #4. is might cause the damage and breakage of polymer slab, and most of shock wave energy is consumed quantity. Figure 10 is the time history curve of shock wave pressure measured at the same distance from the upper and lower sides of the polymer board B0.
It can be seen from Figure 10 that, under the action of close-in explosion, the peak overpressure measured using #4 sensor is 0.1 MPa, which is far lower than 0.296 MPa measured using #1 sensor, and the latter is 2.96 times of the former.
is shows that the polymer slab has a good weakening effect on the propagation of blasting shock wave in air.
Comparing Figures 9 and 10, under contact explosion, the difference between #1 and #4 peaks is 0.0793 MPa, while for close-in explosion, the difference is 0.2019 MPa, which is 2.55 times of the former. erefore, compared with the contact explosion with the same amount of explosive, the weakening effect of the polymer slab on the air shock wave is stronger in the close-in explosion. Figure 11 shows the vibration acceleration curves of A0 and B0 in X, Y, and Z directions. e maximum acceleration values of A0 in X, Y, and Z directions are 8.6 g, 9.8 g, and 9.9 g, respectively. ese values of B0 are basically the same, which are 7.2 g, 9.1 g, and 10.4 g, respectively.
rough the explosive tests of A0 and B0, under same conditions, the damage degree of polymer slab under the action of close-in explosion is more serious than that of contact explosion. erefore, more attention should be paid to the damage mode and dynamic response of polymer slab under the action of close-in explosion.

e Influence of the Explosive Charge Weight.
In order to explore the influence of the explosive charge weight on the damage performance of polymer slabs under the action of close-in explosion, explosive charge weights of 15 g, 25 g, 30 g, and 50 g are carried out on specimens B1, B2, B3, and B4, respectively. e explosive distance is controlled as 20 cm, and other test conditions are the same. e damage results are shown in Figures 12∼15. Figure 12, the damage degree of specimen B1 is lighter than that of specimen B0, due to the explosive quality which decreases to 15 g. e polymer in the center of the bottom surface peels off to form a shallow pit with a diameter of 2 cm, and there is an obvious longitudinal crack in the middle of the bottom surface.

As shown in
As the explosive quality increases to 25 g, the damage degree of the polymer slab (specimen B2) is more serious than that of specimens B0 and B1, as shown in Figure 13. It is observed that the polymer slab breaks along the two fixed edges, resulting in two fracture surfaces approximately parallel to the fixed edge.
is might because, under the downward explosive impact, serious shear action occurs at the contact part between the two fixed edges and the steel frame, and the shear stress produced is far greater than the dynamic shear strength of the polymer slab. erefore, the polymer slab breaks, and the fracture surface is parallel to the two fixed boundaries of the slab. In addition, spalling occurred on the bottom surface due to the tensile shock wave action.

Advances in Materials Science and Engineering
If the charge weight is still increased to 30 g, the damage degree of specimen B3 continues to increase, as shown in Figure 14. Most of the middle part of the specimen B3 has been basically broken, and the uncracked part is divided into three large pieces. Two broken pieces along the fixed boundary and a small piece are connected with the right boundary crack. Moreover, there is a longitudinal fracture surface about 30 cm long between them.
Under the impact with explosive charge equal to 50 g, the damage degree of B4 is very serious. As given in Figure 15, almost all the polymer slabs are destroyed to small pieces except for the left and right fixed boundary parts.
As shown in Figures 16∼18, typical time history curves of air shock wave pressure measured using #1 and #4 sensors in three groups of tests with the explosive charge weights of 15 g, 30 g, and 50 g are given. e wave forms of the three  Advances in Materials Science and Engineering groups of air shock waves are similar, which can be divided into positive pressure zone and negative pressure zone (with the standard atmospheric pressure as the boundary). As shown in Figure 19, the peak pressure values measured using #1, #2, #3, and #4 sensors under different charge mass are given.
For the explosive quality of 15 g, 30 g, and 50 g, the peak pressures obtained in #1 sensor are 0.0925 MPa, 0.4426 MPa, and 0.7183 MPa, respectively. e peak pressure at #1 measuring point increased rapidly as the increase of explosive quality. However, for the peak pressure in #4 sensor, they is only 0.0407 MPa, 0.1503 MPa, and 0.1987 MPa. e peak pressures obtained in #4 measuring point are small and increase slowly compared to that in #1 measuring point. is might be because the polymer slab has a certain weakening effect on the propagation of shock wave under close-in explosion.
In order to describe the damage results of the polymer slab specifically and conveniently, the residual areas of the polymer slabs after explosion field tests with different charge weight are given in Figure 20.
When the explosive charge weight is 15 g, specimen B1 is basically intact, while as the explosive charge weight increases to 30 g, the residual area of specimen B3 is only about half of the original area. If the explosive charge weight continues to increase to more than 30 g, the residual area of the polymer slab is smaller, which indicates that the specimen has been basically destroyed.    Figure 17: Time history curves of shock wave pressure in slab B3. Air shock wave measured using (a) sensor #1 and (b) sensor #4.

Conclusions
In this study, the failure modes of polymer slabs under the impact of air contact and close-in explosions were studied by field explosion tests. e effects of standoff distance and charge weight were studied. e propagation characteristics of the shock wave were also compared and analyzed. e following conclusions can be drawn: (1) Under the same explosive charge weight, the damage degree of polymer slab under close-in explosion is more serious than that with contact explosion. For contact explosion, the polymer slab is mainly damaged locally, and penetrating damage is formed in the center of the slab. Under the action of close-in explosion, the polymer slab is mainly damaged as a whole, resulting in serious fracture. (2) e damage degree of the polymer slab becomes larger as the explosive quality increases. e failure mode of the specimen changes from cracking failure to penetrating failure and then to overall failure. (3) e polymer slab has a certain weakening effect on the shock wave generated by the contact and close-in explosions. Under close-in explosion, as the explosive charge weight increases, the weakening effect of the polymer slab is enhanced within a certain range.

Data Availability
e data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest
e authors declare that they have no conflicts of interest.