Meng Gu ,Xio-dong Ling ,An-feng Yu ,* ,Guo-xin Chen ,Ho-zhe Wng ,Hn-xing Wng
a State Key Laboratory of Safety and Control for Chemicals, SINOPEC Research Institute of Safety Engineering Co., Ltd., Qingdao, 266104, China
b College of Mechanical and Electronic Engineering, China University of Petroleum (East China), Qingdao, 266580, China
Keywords: Polyurea Fiber-reinforced cement board Gas explosion Failure criterion Glass transition
ABSTRACT Five types of polyurea elastomers were synthesized by changing the isocyanate component and the mechanical properties of polyurea materials were measured.Fiber-reinforced cement boards (FRCB)strengthened by polyurea with different formulations were processed,and a series of experiments were carried out on the specimens with gas explosion devices.The results showed that the conventional mechanical properties of different types of polyureas had their own advantages.Based on the gas explosion overpressure criterion,the blast resistances of reinforced plates were quantitatively evaluated,and the best polyurea was selected to guide the formulation design.The three typical failure modes of polyurea-reinforced FRCBs were flexural,shear,and flexural-shear failure.Dynamic thermodynamics and shock wave spectral analysis revealed that the polyurea did not undergo a glass transition in the gas explosion tests but retained its elastic properties,allowing it to effectively wrap the fragments formed by the brittle substrates.
Polyurea is a specific chemically cross-linked elastomeric copolymer formed by a rapid chemical reaction between a shortchain amine and an isocyanate curative.Its molecular microstructure is composed of hard segments and soft segments/phases[1,2].Compared with other elastomers,polyurea has the advantages of a high tensile strength,high ductility,and superb impact resistance[3].Furthermore,the properties of polyurea can be further improved by adjusting the content and type of isocyanate and amino compounds.
Most existing buildings have not been designed for blast loading.In external explosion accidents,most casualties are not directly caused by explosion shock waves but secondary injuries caused by the collapse and high-speed debris of a wall [4].When polyurea is sprayed on the back surface of a wall,it can effectively improve the explosion resistance performance of the wall,prevent wall disintegration,and catch lethal fragments,thus reducing the possibility of secondary damage[5-8].Chen et al.[9]evaluated the blast-resistant ability of polyurea strengthened autoclaved aerated concrete(AAC)plates through close-in explosion experiments.The results showed that the explosion resistance of the top polyureacoated AAC plate was weaker than those of the bottom-layer and double-sided polyurea-coated AAC plates.Shi et al.[10]conducted contact blasting tests on polyurea-reinforced concrete slabs.It was found that polyurea could cause the concrete blast fragments to scatter intensively along the rupture area,reducing the size of the threatened area caused by the blast fragment scattering.
In the processes of production,transportation,and storage and the use of flammable gases,once a gas explosion accident occurs,it can cause significant structural damage and casualties [11].However,the above research mainly focused on the performances of components strengthened by polyurea under high-strength explosive loading,and the related research under gas explosions is limited in the published literature.A gas explosion load has a small amplitude and long positive pressure duration [12,13].The mechanical properties and failure modes of the components are different from those of high-strength explosives [14,15].Mirzababaie Mostofi et al.[16]carried out experiments on aluminum plates with and without polyurea coatings using a gas detonation forming(GDF) apparatus.The results showed that the polyurea coating significantly improved the resistance of aluminum plates by increasing the tangent modulus,and using the polyurea coating as the backing layer was useful for reducing deformation of the structure.The premixed pressure method was used to control the concentration of reaction gas,but the time-history curve of the actual explosion overpressure was not recorded.For a polyureacoated composite steel plate,the steel layer rather than polyurea played a key role in dissipating kinetic energy,regardless of the structural configuration and blast intensity [17].However,the strength of ordinary building walls is far lower than that of metal plates,and they are among the most vulnerable components under an explosion load.Experimental studies of polyurea-strengthened brittle materials under gas explosions have rarely been reported.Therefore,it is necessary to study the dynamic responses and failure mechanisms of such composite structures.
In this study,five types of polyurea elastomers were synthesized by changing the isocyanate component and the mechanical properties of polyurea materials were measured.A series of gas explosion experiments were carried out on fiber-reinforced cement boards (FRCBs) sprayed with different polyurea formulations,and the overpressure time-history curves during the explosions were measured.Based on the overpressure criterion,the influences of the coating thickness,relative position,and formula type on the explosion resistance performance of the reinforced board were investigated,and a quantitative evaluation method of the polyurea coating was proposed.Time-temperature superposition(TTS)was used to establish the relationship between the glass transition of polyurea and the frequency of the dynamic loading using dynamic mechanical analysis data.Combined with gas explosion tests,the mechanism through which polyurea improves the blast resistance of a cement board was analyzed.
2.1. Materials
The main raw materials used to prepare different polyurea formulations are presented in Table 1.The raw materials in the table are commercially available products that were used without any further purification.
Table 1 Main experimental raw materials.
2.2. Polyurea processing
2.2.1.Component A
Polytetrahydrofuran ether glycol(PTMEG)and diol with special structure were inserted into a reactor with nitrogen protection,heated to 95-100 ℃ under stirring,and dehydrated for 1-1.5 h under a vacuum of -0.1 MPa.After cooling to below 40 ℃,diphenylmethane diisocyanate (MDI-100) was added and reacted at 80 ℃ for 2 h.After filtration,the product was labeled 89#.The preparation process of other types of semi-prepolymer A was the same as above,and only the polyol used for end capping changed.The raw materials and mass parts of component A are listed in Table 2.
Table 2 Formula composition of component A.
2.2.2.Component B
According to the composition in Table 3,each component was weighed and inserted into the storage tank of a high-speed disperser successively,stirred at room temperature for 60 min,and then filtered to obtain component B.
Table 3 Composition of component B.
A Graco HXP-3 spray machine was employed to mix different formulations of component A and component B according to a 1:1 vol ratio and spray a uniform 2-mm-thick polyurea layer on a polytetrafluoroethylene sheet.The sheet was then peeled off to evaluate the mechanical properties.
3.1. Quasi-static testing
Quasi-static tensile and tear tests of five different polyurea specimens were carried out using the MTS servo-hydraulic machine(Fig.1(a)).Tensile and tear samples were prepared according to GB/T 528-2009 and GB/T 529-2008 respectively,as shown in Figs.1(b) and (c).In tensile and tear tests,the test speed was 500 mm/min.Each test was repeated at least three times and conducted at room temperature.
3.2. Gas explosion tests
3.2.1.Test boards
The processing steps of polyurea-reinforced concrete board include cement casting and curing,brush primer,spraying operation and curing after spraying.The cast-in-place concrete slab generally needs 28 days of curing time,and the use of prefabricated lightweight FRCB as the substrate of polyurea can avoid concrete curing time and shorten the test period.After the polyureareinforced FRCB was manufactured,it needed to be cured indoors at room temperature for 10 days before gas explosion test.The nominal size of the FRCB was 350×350×18 mm3.According to the manufacturer,the bending strength was 16.1 MPa,and the density was 1.36 g/cm3.The coating performance was strongly influenced by its ability to adhere properly to the substrate material[18].The FRCBs were cleaned to remove excessive cement slurry and crumbly particles,and then the samples were brushed with coating primer.OSDN450 (Ningbo Ousheng Marine Anti-corrosion New Material Technology Co.,LTD.) was used as the primer for this study,which was a two-component reactive material.Primer A component was mainly composed of modified resin,additives and diluents,primer B component was modified isocyanate polymer.The two components of primer were evenly mixed according to the weight ratio of A: B=4:1,and brushed on the surface of FRCB.Polyurea was then sprayed on the primer after primer solidified for 3 h.The polyurea-reinforced FRCBs are shown in Fig.2.A total of 18 specimens of four categories (unstrengthened,impact-face strengthened,rear-face strengthened and double-face strengthened) were prepared.
Fig.2.Fiber-reinforced concrete board (FRCB) strengthened with polyuria: (a) Spraying operation;(b) Curing completed.
3.2.2.Gas explosive device
The gas explosion device can generate the required gas explosion shock wave and apply an explosive load to the test specimen.The schematic diagram of the device is shown in Fig.3.The gas explosion device was composed of a gas distribution system,pipeline system,ignition system,and data measurement system.The gas distribution system pumped combustible gas and air into the pipe after premixing.During the pumping process,the oxygen concentration in the mixed gas was monitored by an oxygen content analyzer in real time,and the gas flowmeter was adjusted dynamically to ensure the gas mixing uniformity.The inner diameter of the pipe system was 300 mm,and the length was 12 m.A specimen square clamp was welded at the end of the pipe,and the transition section between the clamp and pipe was reinforced with stiffeners.Eight M20 bolts were used to fix the specimen between the two 20-mm-thick steel frames of the clamp.The area exposed to the gas explosion load was 300 × 300 mm2.
Fig.3.Schematic diagram of gas explosion device.
After the specimen installation and gas distribution were completed,the spark plug of the ignition system detonated the gas mixture at the closed end of the pipe.The data measurement system consisted of three piezoelectric pressure sensors,a synchronous trigger,and a high-speed camera.The distances between P1,P2,P3,and the test piece were 1.3,0.8,and 0.1 m,respectively.The measuring range of P3 sensor was±5 MPa,and that of P2 and P1 was ±1 MPa.Upon ignition,the trigger instrument synchronously triggered the pressure sensor and high-speed camera to record the pressure and deformation process of the specimen.The gas explosive device is shown in Fig.4.
Fig.4.Photo of gas explosion device.
3.2.3.Testing scheme
Based on the coating thickness,relative position,and formulation type,including the uncoated control group,a total of 18 groups(Table 4)of tests were carried out to study the explosion resistances of the polyurea-strengthened FRCBs.The coating thickness was measured by ultrasonic thickness measurement instrument (PosiTector 200) with the measurement accuracy of ±20 μm.The first two digits of the specimen number in Table 4 represent the formulation type,the third digit represents the coating thickness of the impact face,and the fourth digit represents the coating thickness of the rear face.In test 2,it was found that propane explosion could not destroy the specimen and the plastic deformation of specimen was small,so it was difficult to conduct comparative analysis between tests.Therefore,ethylene with higher explosion pressure was used in other tests.
Table 4 Testing scheme.
3.3. Dynamic thermomechanical analysis
Gas explosion tests show that the ultimate failure pressure of polyurea 86# was the highest under the same spraying mode.Therefore,polyurea 86# was selected for dynamic thermomechanical analysis.A TA dynamic thermomechanical analyzer(DMA 850) single cantilever bending mode was used in the test.Four samples were used in the test,with nominal dimensions of 30×6×2 mm3(length×width×height).The temperature sweep frequencies were 1,5,10,and 15 Hz,and the test temperature increased at a rate of 2 ℃/min.The temperature ranged from-50to 150 ℃,which was expected to cover the glass transition temperature(Tg)and most of the transition zone between the glass and rubber states.
4.1. Basic mechanical properties
The basic mechanical properties and comparison of polyurea are listed in Table 5.Compared with the existing research results,the polyurea elongation in this study had no obvious advantage,but the tensile strength and tear strength were significantly improved.
Table 5 Basic mechanical properties and comparison of polyuria.
4.2. Explosion overpressure
Based on the energy release and explosion distance,a gas explosive loading process is divided into a shock wave and a pressure wave [20].In this paper,due to the reflection of the explosion overpressure and the confines of the test parts,a more complex pressure loading curve was produced.The typical overpressure curve is shown in Fig.5.The first digit of the legend PX-X represents the pressure sensors at different positions,and the second digit represents the serial number of the test.
Fig.5.Typical pressure time-history curve.
In test 2,the explosion resistance ability of the specimen was greater than the explosive loading.As shown in Fig.6,the specimen experienced plastic deformation,but it was intact overall.When the pressure reached the peak,it gradually decreased to atmospheric pressure due to the consumption of combustible gas and the air tightness of the device.The overpressure curve resembled an isosceles triangle,and the positive time of the shock wave could reach 1446.6 ms.In contrast,the explosion resistance of the specimen in test 4 was lower than the test loading,and the reinforcement plate acted like a pressure-relief plate.After the reinforcement plate was broken,the pressure in the device decreased rapidly to a negative pressure,and the positive time(127.8 ms)of the shock wave was far less than that of test 2.All test explosion overpressures,positive pressure times,and impulses are shown in Table 6.As shown in Table 6,the peak pressure was basically consistent with the cracking time of the coating,which indicated that the peak pressure was the failure pressure of the coating.
Fig.6.Specimen after test 2.
Table 6 Test results.
The failure modes of the FRCB strengthened by polyurea were closely related to the performance of the polyurea coating.The typical failure modes were flexural,shear,and flexural-shear failure.Only when the shear strength of the polyurea was relatively weak,shear and flexural-shear failure of the strengthened plate were observed.The failure modes of each specimen are listed in Table 6,and the schematic diagram of three typical failure modes is shown in Fig.7.
Fig.7.Schematic diagram of failure modes: (a) Flexural failure;(b) Shear failure;(c)Flexural-shear failure.
Fig.8.Pressure time-histories in test 1.
4.3. Damage modes of specimens
4.3.1.Unstrengthened
The pressure time-histories and sample failure process in test 1 are shown in Figs.8 and 9,respectively.Whent=665.4 ms,the first visible crack appeared in the FRCB.At this time,the peak reflection pressure was 279.72 kPa.An “X”-type through-crack developed,and the board was divided into four main fragments,showing a typical two-way bending response mode[14].The main fragments were broken into smaller fragments under a shock wave and accelerated to be thrown outward.High-speed fragments are the main cause of casualties in explosion accidents.There was still a considerable amount of residual gas burning and pressure reflection superposition in the pipeline,so the secondary pressure peak appeared at 682.2 ms.Fig.9 shows that the board had been completely destroyed at this time,and the secondary pressure peak could not represent the failure pressure of the specimen.
Fig.9.Failure process of unstrengthened specimen in test 1.
Fig.10.Pressure time-histories in test 6.
4.3.2.Impact face
The pressure time-histories and sample failure process in test 6 are shown in Figs.10 and 11,respectively.As shown in Fig.11,whent=692.8 ms,the first visible crack appeared near the center point of the impact-face-strengthened FRCB,at which time the pressure was 314.21 kPa,which was greater than the pressure of 279.72 kPa for the crack formation of the unstrengthened specimen.This was different from the metal plate damage being aggravated by spraying polyurea on the blast face [21-23],which indicated that the coating on the blast face of the FRCB improved the explosion resistance of the substrate.When polyurea was only sprayed on the blast face,even if the primer and FRCB adhered firmly,there was still a risk of FRCB self-destruction and detachment from the coating,forming high-speed debris,which could not effectively reduce the risk of additional personal injury.
Fig.11.Failure process of specimen in test 6.
Fig.12.Pressure time-histories in tests 7 and 8.
Fig.13.Pressure time-histories in tests 16 and 17.
4.3.3.Rear face
There were three main failure modes in the rear-facestrengthened specimen: flexural failure,shear failure,and flexural-shear failure,and the typical processes are shown in Fig.14-16,respectively.Shear failure and flexural-shear failure modes were only observed in the polyurea 89# with the lowest tear strength.
Pressure time histories and the process of typical flexural failure are shown in Figs.12 and 14,respectively.The pressure reached the peak of 983.62 kPa at 735.0 ms in test 8.The FRCB substrate was damaged,and the debris did not splash due to the coating effect of the polyurea.The first visible crack of the rear face polyurea occurred after 4.2 ms and then extended radially to four corners to form five major fragments.Att=745.2 ms,the main fragments were further damaged along the edge of the steel frame as indicated by the dotted line in Fig.14,and the fragments finally flew out.
Fig.14.Flexural failure process of specimen in test 8.
According to Fig.13,the pressure reached a peak of 983.62 kPa at 735.0 ms in test 17.Fig.15 shows that polyurea also prevented the splashing of the FRCB debris.As indicated by the red dotted line in the figure,the first visible crack appeared at the bottom edge of the rear face polyurea 3.2 ms later,and then the crack appeared at the upper edge.The crack developed along the edge,and specimen did not split into major fragments and flew out as a whole.
Fig.15.Shear failure process of specimen in test 17.
According to Fig.13,the pressure reached the peak value of 551.73 kPa at 695.4 ms in test 16.Fig.16 shows that the flexural failure and edge shear failure occurred almost at the same time,forming four main cracks(red dotted line).The crack development divided the specimen into three main fragments.The number of main fragments of the flexural shear failure was greater than that of the shear failure and less than that of the flexural failure.
Fig.16.Flexural-shear failure process of specimen in test 16.
For the metal plate sprayed with polyurea on the rear face,the energy dissipation mainly occurred through tensile deformation,debonding,and spalling of the coating.Fragments of rear-face strengthened specimens are shown in Fig.17.It can be seen from Fig.17 that there was no debonding between polyurea and FRCB,and the separation between them was caused by the destruction of FRCB.Therefore,the strength of the FRCB was lower than the adhesion strength of the coating.During the failure process,FRCB failed first,and the broken FRCB moved with the polyurea coating,and the kinetic energy transferred to the structure by the explosive loading was mainly dissipated through the deformation and failure of the coating.
Fig.17.Fragments of rear-face strengthened specimens.
Fig.18.Pressure time-histories in test 9.
4.3.4.Double face
In the double-sided reinforcement tests,there were two main failure modes: flexural failure and shear failure.The shear failure mode was also observed only in the polyurea 89# tests.The pressure time-history curves of polyurea 86# and 89# are shown in Figs.18 and 19,respectively.The peak pressures of the 86#and 89#plates with 3-mm coatings on both sides were 1059.78 kPa and 814.69 kPa,respectively,which were higher than that of the plate with the 6-mm coating the rear surface.
4.4. Overpressure criterion
The structural response to blast loading is significantly influenced by the value of ωtd.Three loadings regimes are categorized as follows [24]:
ωtd＜0.4: impulsive loading;
ωtd＞40: quasi-static loading;
0.4＜ωtd＜40: dynamic loading;where ω is the natural circular frequency of vibration of the structure,andtdis positive pressure time.ω is defined as follows:
whereTis the natural period of vibration of the structure,Kis the structural stiffness,andMis the structural mass.
The polyurea-strengthened FRCB was simplified as a singledegree-of-freedom system.For the same length and width bearing a uniform load,the stiffness of the plate in the elastic stage is 810EIa/l2[25],and the stiffness of the elastic-plastic stage is 252EIa/l2[25].In the process of an explosion,the reinforced plate gradually enters the plastic stage from the elastic stage,so the average stiffness is 531EIa/l2[25],where E is the elastic modulus of the structure,andIais the moment of inertia of the interface.E is calculated as follows:
whereEpis the elastic modulus of polyurea,EFis the elastic modulus of the FRCB,hpis the thickness of polyurea layer,andhFis the thickness of FRCB.The moment of inertia of the rectangular section (Ia) is defined as follows:
wherebis the width of the section.
Eqs.(2)and(3)are substituted into Eq.(1)to obtain ω.In the gas explosion tests,the positive pressure action time was 112.3-231.2 ms,and the polyurea thickness was 3-6 mm.The temperature during the field gas explosion test was about 5 ℃.The modulus of elasticity of the polyurea was 1.6 GPa,as obtained from the DMA tests.The density of polyurea was 1.04 g/cm3measured by HTY-120SL density tester.The elastic modulus and density of FRCB were obtained from the information provided by the manufacturer.The relevant parameters of polyurea and FRCB are listed in Table 7.According to the above calculation,it was concluded that ωtd=173-401 ＞40,so the loading type was quasi-static loading.The damage of the specimens was mainly related to the explosion overpressure,which was also consistent with the phenomenon observed during the tests.
Table 7 Wave impedance comparison of materials.
The peak overpressures of the tests are shown in Fig.20,where the red line is the overpressure value of the unreinforced FRCB.Fig.20 shows that the peak overpressure of polyurea 86# with 3-mm coatings on the impact surface and rear surface were 496.54 kPa and 962.46 kPa,respectively.Therefore,the reinforcement effect of the polyurea on the impact surface was lower than that on the rear face.For the polyurea with a low explosion resistance,increasing the thickness was more effective.When the coating thickness increased from 3 to 6 mm,the peak overpressure of polyurea 86# increases from 962.46 to 983.62 kPa,and the peak overpressure of polyurea 89#increased from 551.73 to 799.72 kPa.The effect of double side reinforcement is better than that of rear surface reinforcement with the same thickness.Under the same spraying mode,polyurea 86# has the highest peak pressure,so polyurea 86# is the best polyurea formulation.propagates from one medium A to another medium B with different wave impedances,there will be a reflected wave and a transmitted wave.The stress amplitude of the transmitted wave(σT)and the reflected wave（σI）can be calculated by the following formulas [26]:
Fig.20.Comparison of peak overpressures.
where ρA,ρBare the density of the medium,andcA,cBis the wave velocity of the medium.
The product of longitudinal wave velocity and material density is called the wave impedance of the medium.The longitudinal wave velocity is calculated as follows [26]:
4.5. Wave impedance
Once a shock wave hit the top surface of a tested plate,compressive stress wavefronts were produced,thus propagating along the through-thickness direction.When the incident wave whereEAorBis the elastic modulus of the medium.
The elastic modulus and density of concrete were determined by the GB 50010-2010 standard,and the elastic modulus and density of steel were obtained from Ref.[30].The wave impedance parameters of polyurea and common coating substrates are listed in Table 7.The wave impedances of the polyurea,FRCB,and concrete basically matched.Spraying polyurea on the impact face could still improve the blast resistance of the brittle base material reinforcement system,which was consistent with the results of gas explosion tests.There was a wave impedance mismatch in the polyureacoated steel plate.When the explosion shock wave passed from the coating with a lower wave impedance to the steel plate with a higher wave impedance,the increase in the transmitted shock wave amplitude aggravated the damage of the steel plate[21-23].Wave impedance could be one of the main reasons for the difference in the reinforcement effects.
4.6. TTS and shock wave spectrum
The variations of the storage (E’) and loss modulus (E’’) of the polyurea 86# at various frequencies as a function of temperature are presented in Fig.21.At the same test frequency,the E’ curve value of polyurea 86#was higher than that of Iqbal et al.and the E’’curve value was similar to Iqbal et al.[18],indicating that polyurea 86# had a high energy storage capacity.The peak value of E’’appeared at about-4.5 ℃ for polyurea 86#,and the corresponding temperature was the glass transition temperature,which also indicated that polyurea was in an elastic rubber state under ambient conditions.As the frequency of the dynamic loading wasincreased,both the loss and storage modulus curves shifted to higher temperatures[27-30].
Fig.21.Variation of storage and loss moduli of polyurea 86#.
Fig.22.Storage modulus TTS curves.
For most polymers,increasing the temperature,and decreasing speed of testing(frequency)have the same effect on its mechanical properties[29].This relationship permits evaluation of mechanical properties over a wide frequency range by conducting tests at relatively narrow frequencies but at various temperatures,which is called Time-Temperature Superposition (TTS) method [18].Based on the DMA data,the classical TTS method was applied to develop the frequency-domain master curves,which spanned 10-15to 1020Hz.The master curves obtained according to the changes of the storage and loss modulus at different reference temperatures are shown in Figs.22 and 23.Fast Fourier Transform(FFT)was applied to the pressure curves of the gas explosions.The spectrum analysis results of the polyurea 86#tests are shown in Fig.24,and the upper limit of peak area frequency was 210 Hz.Fig.23 shows that under gas explosion test temperature condition(5 ℃),the glass transition frequency needs to reach 8636 Hz,which is far greater than the upper limit of the frequency in the peak area.The “vitrification”process of polyurea under explosive loads is considered to be one of the most important mechanisms related to the excellent explosion resistance of polyurea[31,32].In contrast toTNT explosion tests,the polyurea in the gas explosion tests did not undergo a vitrification process,but retained its elastic properties,which could effectively wrap the fragments formed by the failure of the brittle substrate.Shock-wave-induced hard-domain ordering and crystallization[33-35],hydrogen bond breaking and recombination [36],and shock wave capture and neutralization[37,38]seem to be the main modes of the micro-energy absorption mechanism of polyurea under explosive loading.
Fig.23.Loss modulus TTS curves.
Fig.24.Gas shock wave spectrum.
In this paper,five kinds of polyurea elastomers with different formulations were synthesized by varying the isocyanate components and the mechanical properties of polyureas were measured,The dynamic response and explosion resistance performances of the polyurea-strengthened FRCB under explosive loading were analyzed using a gas explosion device.The main conclusions are summarized as follows:
(1) The unstrengthened FRCB experienced typical two-way flexural failure.The failure modes of the polyureareinforced FRCBs were closely related to the mechanical properties of the polyurea and spraying face,and the three typical failure modes were flexural,shear,and flexural-shear failure.
(2) The wave impedances of the polyurea,FRCB,and concrete basically matched.Spraying polyurea on the impact face could improve the blast resistance ability of the brittle base material reinforcement system.
(3) For the rear surface and double-sided reinforced specimens,the polyurea coating moved with the substrate plate in the failure process,and the energy dissipation was mainly completed through the deformation and failure of the coating.The polyurea played a key role in the kinetic energy dissipation.
(4) Dynamic thermodynamics and shock wave spectrum analysis revealed that the polyurea did not undergo glass transitions in the gas explosion tests but retained elastic properties.Thus,it could effectively wrap around the fragments formed by the failure of the brittle substrate.
(5) The mechanical properties of polyurea could be adjusted by changing the isocyanate composition,but each formula had its own advantages.The dynamic response of the polyureastrengthened FRCB mainly depended on the peak overpressure.Based on the failure criterion of the overpressure,the blast resistance ability of the reinforced plate could be quantitatively evaluated,and the best polyurea formula could be determined.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This project was funded by National Natural Science Foundation of China (No.12002392).