Mechanical behavior and failure mechanism of polyurea nanocomposites under quasi-static and dynamic compressive loading

Qing Liu ,Peng-wn Chen ,Yn-song Guo ,Jin-jun Su ,Lu Hn ,Ali Ar ,Jin-fei Yun

a State Key Laboratory of Explosion Science and Technology,Beijing Institute of Technology,Beijing 100081,China

b Xi’an Modern Chemistry Research Institute,Xi’an 710065,China

Keywords:Polyurea nanocomposites Mechanical properties Strain rate Micro-CT Failure

ABSTRACT Polyurea is an elastomeric material that can be applied to enhance the protection ability of structures under blast and impact loading.In order to study the compressive mechanical properties of SiC/polyurea nanocomposites under quasi-static and dynamic loading,a universal testing machine and split Hopkinson pressure bar(SHPB)apparatus were used respectively.The stress-strain curves were obtained on polyurea and its composites at strain rates of 0.001-8000 s-1.The results of the experiment suggested that increase in the strain rates led to the rise of the flow stress,compressive strength,strain rate sensitivity and strain energy.This indicates that all of the presented materials were dependent on strain rate.Moreover,these mechanical characters were enhanced by incorporating a small amount of SiC into polyurea matrix.The relation between yield stress and strain rates were established using the power law functions.Finally,in order to investigate the fracture surfaces and inside information of failed specimens,scanning electron microscopy(SEM)and micro X-ray computed tomography(micro-CT)were used respectively.Multiple voids,crazes,micro-cracks and cracking were observed in fracture surfaces.On the other hand,the cracking propagation was found in the micro-CT slice images.It is essential to understand the deformation and failure mechanisms in all the polyurea materials.

1.Introduction

With the development of polymers,a number of polymeric materials have been widely used to improve the protective performance against blast and ballistic impacts[1-6].This can be attributed to polymeric materials which have exhibit excellent properties such as high specific strength,light weight and low cost.Polyurea is an elastomeric polymeric material that is implemented in a form by chemical reaction between amine component and isocyanate component[7-10],which can improve the survivability of structures under blast and intensive impact loading conditions[11].For protective structures,polyurea is extensively used as a coating in the outer or inner layer of composite structures[12-16]such as vehicles,ships and buildings[17].Therefore,it is essential to investigate the mechanical properties and failure mechanism of polyurea materials at various strain rates.

The compressive mechanical properties of polyurea have been investigated by some researchers at a limited range of strain rates[17-22].Yi et al.[18]and Sarva et al.[19]have studied the compressive mechanical behavior of polyurea using split Hopkinson pressure bar(SHPB),indicating rate dependency,and they discovered the transition from rubbery-regime behavior at low rates to leathery-regime behavior at the high rates.Shim and Mohr[20]used a modified SHPB system to study the strain rate sensitivity of polyurea.Wang et al.[17]studied the compressive mechanical properties of polyurea at a wide range of strain rates(0.001-7000 s-1).It was found that flow stress strongly depended on the strain rate.Guo et al.[21]studied the compressive mechanical behavior of two polyurea materials at strain rate ranging from 0.001 s-1to 12 000 s-1,the results suggested that the stressstrain curves of polyurea was sensitive to strain rate.In addition,Zhang et al.[22]have studied the stress-strain behaviors of two polyaspartic esters(PAE)polyureas at the strain rate ranging from 0.001 s-1to 15 000 s-1.However,only a limited amount of research have been reported on the failure and damage mechanism of polyurea under dynamic loading.The combination of mechanical properties and failure behaviors can have an insight into polyurea at high strain rates,and play an important role on the application of polyurea in protection field.

Recently,nano-reinforced composites materials have attracted many researchers for improving the mechanical performance[23].Therefore,for enhancing the properties of polyurea material,some researchers[24-27]added the nanoparticles to the polyurea matrix in order to improve its mechanical properties.In the literatures,some nanofiller materials including multiwalled carbon nanotubes,nanoclay and graphene were incorporated into polyurea matrix,and the mechanical behavior of these composites were investigated under low strain rate loading.However,a very limited number of studies reported on the dynamic mechanical properties of polyurea nanocomposites at high strain rates.Therefore,considering that the application of polyurea nanocomposites at high impact strain rates,it is necessary to investigate the mechanical behavior and failure mechanism of various types of polyurea materials under dynamic loading.

In the study,the compressive mechanical properties of polyurea and its nanocomposites including silicon carbide(SiC)particles were investigated using an electronic universal testing machine and a SHPB[28,29]under quasi-static and dynamic loading.The deformation behavior and strain rate effect of polyurea nanocomposites with SiC contents of 0.7 wt%,1.5 wt%and 2.5 wt%were systematically analyzed at lower and higher strain rates.Moreover,the fracture surface of compression specimens at higher strain rate were analyzed by scanning electron microscopy(SEM).Micro X-ray computed tomography(micro-CT)scanning can be used to obtain the insider information of these damage specimens[30,31].Finally,the failure mechanism of the compression specimens at higher strain rate were analyzed and discussed.

2.Materials and specimens

The material used for pure polyurea in this study was supplied by Qingdao SanheSheng Polymer Technology Co.,Ltd.Pure polyurea was composed of isocyanate component and amine component.The nanofillers of SiC with dimension of average dimension of 60 nm in size were provided from Hefei Kaier Nanotechnology Co.,Ltd.To incorporate nanoparticles of SiC into the amine component of polyurea,in-situ polymerization method was applied to fabricate polyurea nanocomposites.In order to fabricate the materials,measured amount of isocyanate was added to reactor at a stirring speed of 80-120 rpm.Polyetherpolyol was also added to the reactor when the temperature increased by 40°C,then the mixture was maintained in container at the N2atmosphere at 80°C for about 3 h.In addition,SiC nanocomposites were weighed and dried at 80°C.For dispersion of nanoparticles in the matrix,measured amount of SiC nanofillers was gradually added into the amine component solution while rotating the mixer at 1200 rpm for 1 h.The diethyltoluenediamine exists in amine component mixture as a chain extender.At last,both the amine component with SiC nanofillers dispersion and the isocyanate component at the volume ratio of 1:1 were transferred to the polyurea spraying machine.The mixture component was sprayed on the template to solidify the polyurea composites.The same procedure was carried out to fabricate the other SiC/polyurea nanocomposites.Three different composites with SiC(0.7 wt%,1.5 wt%and 2.5 wt%)were fabricated and investigated in this study,namely,PU/0.7 SiC,PU/1.5 SiC and PU/2.5 SiC nanocomposites,respectively.The symbol of PU in this work represents polyurea.

PU and PU/SiC nanocomposites were produced in the pattern of thin plate with the thickness of about 2 mm and 5 mm.The cylinder specimens with a diameter of 8 mm and height of about 5 mm were machined from the polyurea sheet with the thickness of about 5 mm.These specimens are used for quasi-static compression test.In addition,the cylinder specimens with a diameter of 8 mm and height of about 2 mm were fabricated from the 2 mm polyurea thin sheet for dynamic compression tests.

3.Experiments

3.1.Quasi-static compression test

The quasi-static uniaxial compression test was carried out on 4104 MTS universal testing machine at room temperature(23°C),as shown in Fig.1.The loading rates of the test were set to 0.3 mm/min and 3 mm/min,which corresponded to the quasi-static engineering strain rates of 0.001 s-1and 0.01 s-1,respectively.The maximum engineering strain of compressed specimen was about 0.7.Before each test,a small pre-pressure was applied to both the top and the base of the specimen to ensure that the seal face was in close contact with an indenter.The engineering stress-strain curves of the process of compression were calculated from the pressure and displacement data obtained from the pressure sensor and the displacement sensor respectively.At last,the true stress-strain was calculated on the base of engineering stress-strain.

Fig.1.Quasi-static compression testing setup.

3.2.High strain rate compression test

The dynamic compression experiments were carried out by using a SHPB at room temperature,Fig.2 shows the schematic setup of the SHPB.Tests were carried out under the different stain rates(3000 s-1,5100-1s and 8000 s-1).As shown in Fig.2,the apparatus consisted of 3 bars(striker,incident,and transmitted bars).Both the incident bar and transmitter bar were made of aluminum alloy(elastic modulus E=75 GPa and density ρ=2700 kg/m3)with a diameter of 15 mm and a length of 130 cm the aluminum alloy was selected to reduce the impedance mismatch between the specimen and the bars.The striker bar was also made of aluminum alloy with length of 40 cm and diameter of 15 mm.The contact surfaces of the bars and the sample were sufficiently lubricated by molybdenum disulfide to reduce friction and specimen barreling.A copper pulse shaper(thickness of 0.5 mm and diameter of 3.5 mm)was located between the striker and the incident bars in order to improve the dynamic stress equilibrium and reduce the high frequency oscillation parts of the incident stress pulses[19,21].The copper discs with a diameter of 3.5 mm and thickness of about 0.5 mm,which was pasted at the center of incident bar using lubricating grease.The strain gauges were mounted on the middle of the incident bar and transmitter bar to measure the voltage signals.Then,the voltage signals were converted into strain signals and recorded on a computer by using Wheatstone bridge and data acquisition instrument,respectively.Fig.3 shows the typical strain gauge data of polyurea from SHPB set up.

Fig.2.The schematic of the SHPB apparatus.

Fig.3.Strain gauge data for polyurea material at various high strain rates.

Based on the one-dimensional stress wave and uniformity assumption,the engineering stress,the engineering strain and strain rate can be calculated as follows[32].

Here,E is the elastic modulus of aluminum alloy,C0is the wave velocity in the bars,S0is the cross sectional area of the bars,Lsis the length of the specimen,Ssis the initial cross sectional area of the specimen,εRis the reflected strain,εTis transmitted strain,andis the strain rate.

3.3.Micro X-ray computed tomography scanning

Micro X-ray computed tomography scanning can be used to investigate the microstructural of materials.In this study,a YXLON FF20 micro-CT was utilized to scan four specimens with failure.The parameter of source voltage was selected as 40 kV,the source current was selected as 60μA,and the resolution was adjusted as 14μm.During the scanning,a platform was used to hold the specimen.Every specimen rotated 360°to be scanned one time,and the process took 30 min to complete.

3.4.Scanning electron microscopy analysis

The fracture morphologies of compressive specimens were observed using a Quanta 600FEG scanning electron microscopy,the fracture surfaces of pure polyurea and its composites were treated by gold spraying in vacuum before observation.

4.Results and discussion

The quasi-static and dynamic experiments were conducted on specimens at various strain rates.After the quasi-static compression tests,the deformation of the specimens basically return to the shape before the tests.So only the deformation images of dynamic compressive test were shown as in Fig.4.These specimens(from left to right)are PU/0.7 SiC,PU/1.5 SiC and PU/2.5 SiC,respectively.It was found out that the recovered specimens at strain rates of 3000 s-1and 5100 s-1kept their original shape and no obvious failure occurred at these strain rates.Nevertheless,at strain rate of 8000 s-1,failure is occurred in the all specimens.

Fig.4.Photograph of specimens after dynamic compression test.

4.1.The stress-strain properties

The stress-strain curves can directly reflect the mechanical characteristic of specimens tested.Fig.5 presents the true stressstrain curves of PU and PU/SiC nanocomposites for the quasistatic test and dynamic test.As observed in Fig.5,the stress-strain curves for the all specimens follow the same pattern in each of the same strain rate.However,at high strain rate,the stressstrain curves of the specimens show a different trend from those curves of low strain rates.In Fig.5(a)and Fig.5(b),three stages of true stress-strain curves were observed at strain rate of 0.001 s-1and 0.01 s-1,namely the initial linear elastic stage,the high elastic stage and the strain harden stage.Similar finding was also reported by the previous study in the literatures[17,21].However,at high strain rates in Fig.5(c),(d),Fig.5(e),the slope of initial linear elastic part becomes increasingly large.Then,non-linear transition of stress-strain curves represents the yield of the materials.After that,at strain rates of 3000 s-1and 5100 s-1,the stress increases with the increasing of strain until maximum stain.It indicates that stain hardening take places before maximum stress level occurs.In the end,the final drop of stress-strain curves appear in Fig.5(c)and Fig.5(d),which is the result of the specimen loading rather than specimen failure.Generally speaking,low impact energy can not induce specimen failure.For most polymeric materials,it could be found that the rise of temperature in materials can induce thermal softening in the process of deformation at higher strain rates loading[33,34].At the given strain rate of 8000 s-1,a part of impact energy on the four materials under high strain rate was transformed to heat energy,resulting in thermal softening and the decrease of stress.At last,these specimens showed failure at this strain rate.

Fig.5.The true stress-strain curves for compression tests at various strain rates of(a)0.001 s-1;(b)0.01 s-1;(c)～3000 s-1;(d)-5100 s-1,and(e)-8000 s-1.

From Fig.5(a)to Fig.5(e),the true stress of all specimens show a rising trend while the strain rate is increasing.Meanwhile,the stress value corresponding to the same deformation appears to be much more prominent at high strain rates.Meanwhile,the stressstrain curves shape of PU are similar to those of polyurea composites.It can be seen that the addition of nanoparticles has a significant influence on the mechanical behaviors at various strain rates,indicating that the flow stress and maximum stress of composites are obviously changed in compare to that of pure polyurea.

4.2.Strain rate sensitivity

For describing the strain rate sensitivity(SRS)of polymeric materials,the SRS index is used as a references to estimate the sensitivity.There are several methods for obtaining the SRS index.For instance,the power-law relation describes the relationship between stress and strain rate for polymer materials.The strain rate sensitivity can be approximately estimated as the slope of the flow stress versus the logarithm of the strain rate,using the following equation[35,36]:

whereσ1andσ2are the flow stress at the same true strain under different strain rates,the reference true strain is 0.5,andare the strain rate,and＞.

The strain rate sensitivity index of specimens are presented in Table 1.At a fixed strain value,strain rate sensitivity rises by the increase strain rate range for all polyurea materials.In addition,the increase of strain rate sensitivity is more pronounced at higher strain rates.At quasi-static region,the lower sensitivity index indicates that the PU and nanocomposites are not sensitive at low strain rate.However,at the stain rate region from 3000 s-1to 5100 s-1,all polyurea nanocomposites show a higher strain rate sensitivity as compared to that of PU.On the other hand,at stain rate region from 5100 s-1to 8000 s-1,only PU/0.7 SiC nanocomposites display higher sensitivity than that of PU,the sensitivity index of the other two nanocomposites are lower as compared to pure polyurea.In other word,the addition of higher content nanoparticles influences the strain rate sensitivity.Overall,PU/0.7 SiC has recorded the highest level of strain rate sensitivity at high strain rate region.

Table 1The strain rate sensitivity of the four materials at various region of strain rates.

4.3.Flow stress,yield stress and compressive strength analysis

Fig.6 shows the effect of SiC contents on the flow stress and compressive strength at different strain rates.It is observed that the flow stress and compressive strength increase as the strain rates go up.There is no obvious difference under quasi-static compressive loading.However,compared to the pure polyurea,the nanocomposites exhibit prominent improvement in SHPB experiments.Particularly,the composite with 0.7 wt% SiC presents the highest flow stress and compressive strength under dynamic experiments using SHPB loading.The increase of flow stress for PU/0.7 SiC material at strain rates of 3000 s-1,5100 s-1and 8000 s-1are 15.4%,15.4%and 12.8%as compared to pure polyurea,respectively.On the other hand,in comparison to pure polyurea,the compressive strength of the same composites increased by 15.1%,6.61% and 12.1% when the strain rates were at 3000 s-1,5100 s-1and 8000 s-1,respectively.This indicates that a small amount of nanofiller material can reinforce the mechanical properties.This reason may be a smaller amount nanoparticles dispersed uniformly in the matrix and improve the interfacial strength of the specimen[23],leading to a certain reinforcement effect.

For obtaining the yield stress of polyurea materials,tangent method[37]was used to estimate the yield stress value at various strain rates.The relationship between yield stress and strain rate can be described by a power-law function for different materials.For instance,Fan et al.have made use of a power-law relation[38,39]for polymeric materials at high strain rate loading.To predict the relation between yield stress and strain rate for polyurea materials at low and high strain rates,the yield stress and strain rate relation is plotted in Fig.7,and the function of the yield stress can be expressed as

where A is a material constant;B is a material parameter that is a function of temperature,strain and material microstructure;C,D are the constants dependent on the nanoparticles content;mpis the mass fraction of nanoparticles,and m is the strain rate sensitivity index.The function considers the effect of the nanoparticles content on the yield stress.As shown in Fig.7,the model results agree well with the experimental values.From all the strain rates,the yield stress of all materials shows insensitivity to strain rate at low strain rates,but increasing sharply with the enhancement of strain rates from 3000 s-1to 8000 s-1.For the yield stress,the fitting parameters of the 4 materials have been shown in Table 2.

Through the fitting results,it can be seen that the parameter A of polyurea nanocomposites(PU/0.7 SiC and PU/1.5 SiC)is higher than that of pure polyurea,which is due to the improved mechanical properties of nanocomposites under quasi-static loading.In addition,the parameter B of all the nanocomposites is higher in comparison to the pure polyurea,which can be attribute to the improved stiffness of nanocomposites.Furthermore,parameter m of PU/0.7 SiC is also higher than that of pure polyurea.It indicates that the faster increasing of yield stress with the increase of strain rate.However,parameter m of the other nanocomposites is lower than that of pure polyurea.This is due to a large amount of nanoparticles can affect the strain rate sensitivity under dynamic loading.

4.4.Strain energy characteristic

To characterize the energy absorption capacity of pure polyurea and its nanocomposite materials,the strain energy of all materials can be calculated by measuring the area under the stress strain curves up to a certain strain[35,40].In this study,in order to determine the strain for calculating,the value of strain was selected to be 0-0.5 as the range of integration at different strain rates.Fig.8 shows the strain energy results of the four materials under low strain rates and high strain rates.As observed in Fig.8,the strain energy increases as strain rate goes up.In addition,the strain energy of high strain rates is significantly larger than quasi-static tested.However,there is no prominent difference between strain rate at 0.001 s-1and strain rate at 0.01 s-1,which is mainly due to the fact that the stress-strain curves have not significant difference below the strain of 0.5.The stress-strain curves are obviously different when the strain values exceed 0.5 at low stain rates.Compared to the pure polyurea,the strain energy of PU/0.7 SiC appear to be the highest at dynamic loading.It indicates that appropriate amount of SiC nanoparticles can improve the strain energy of polyurea.Thus,PU/0.7 SiC material has an obvious advantage in absorbing energy which is generated during dynamic impact loading.

Fig.6.Effect of SiC on mechanical properties of the four materials at various strain rates:(a)flow stress at strain of 0.5;(b)compressive strength.

Fig.7.The relationship between yield stress and strain rates of the four polyurea materials:(a)PU;(b)PU/0.7 SiC;(c)PU/1.5 SiC,and(d)PU/2.5 SiC.

Table 2Model parameters of polyurea and its composites materials in power-law function.

4.5.Failure behavior under high strain rate loading

For a better understanding of the fracture process and the failure mechanisms,the fractured surfaces of failed specimens were examined using a SEM.The SEM images are shown in Fig.9,which could depict the damage mechanisms of specimens under dynamic compressive loading at strain rate of 8000 s-1.The fracture is rough and presents numerous voids(Fig.9(a)and Fig.9(b)).From Fig.9(b),it can be seen that one crack is observed at the edge of void,the other crack propagate towards one direction.Pure polyurea molecular chain consists of soft segments and hard segments.As shown in Fig.9(c),the hard segments of polyurea inhomogeneous distribute in soft segments.The micro-crakes in the dotted line area among hard segments are initiated and propagated,resulting in multiple cracks which connect to form a whole.In this case,the distribution of hard segments has a significant influence on the initiation and propagation of cracks.In Fig.9(d),numerous crazes are present in the dotted line area.The arrow pointed part displays rough and bumpy qualities.A small amount of debris is peeled off from the fracture surface,which is result of the formation of crazes and cracks during high strain rate impact.Similar phenomenon have been reported in the literature.As pointed by the arrow in Fig.9(e),the direction of crack propagation is changed,which is attributed crack deflection induced by nanoparticles.An obvious crack can be seen surrounding the void,and numerous crazes are observable in the radial direction(Fig.9(f)).In general,crazes can grow and develop into micro-cracks,and the micro-cracks will grow and propagate to damage the specimen with more impact energy.Crack through the three voids along the direction of vertical compression(Fig.9(g)).It indicates that the distance of voids at short range is easy to provide a path of crack propagation,leading to a serious damage.The fracture surface that is complex has numerous cracks,and also has some ditch pattern formed in the direction of compression loading(Fig.9(h)).This sample is the most seriously damaged.In addition,as shown in Fig.9(i),a prominent crack is formed along the void,which due to the local agglomeration of multiple particles addition leading to the weak interfacial interaction.

Fig.8.Relationship between strain energy and the content of SiC nanoparticles.

Fig.9.SEM images of fracture surfaces of specimens at strain rate of 8000 s-1:(a & b & c)PU;(d & e)PU/0.7 SiC;(f & g)PU/1.5 SiC;(h & i)PU/2.5 SiC.

From these SEM observations,we can observe the craze,crack initiation,propagation and fracture through these specimens.The deformation and fracture of polyurea materials can be attributed to craze initiation and growth into cracks.When polyurea composites with small amount fillers,nanoparticles have good dispersion properties,the formation of craze and the deflection of crack can enhance the absorption of fracture energy.The existence of a large number of craze can prevent the propagation of crack,reduce the generation of crack and improve the toughness of composites.However,when the amount of nanoparticles is large,the cohesiveness of particles is enhanced,the possibility of uniform dispersion is reduced,and the stress concentration effect is increased which result in a large number of cracks.

On the other hand,a micro-CT scanner was utilized to obtain micro-CT images for polyurea and its composites.Since nanoparticles are usually much smaller in size,the nanofillers of composites can not be detected in CT images.However,the cracks and voids of failure specimens can be observed by using these CT images,as shown in Fig.10,numerous small black dots suggest the existence of voids in the specimens.Fig.10 depicts the surface CT images of four cylinder specimens.With micro-CT images,we can observe neat polyurea sample with perfectly fracture and a crack appeared in the damage zone(Fig.10(a)).Compared to Fig.10(a),polyurea composites show only a few cracks and do not have a fracture as seen in the specimens shown in Fig.10(b)and Fig.10(c).This might be due to the existence of numerous cracks which absorb more impact energy.This indicates that higher number of cracks on the materials can limit the fracture of samples.Meanwhile,as observed in Fig.10(c),a crack caused deflection in the process of propagation,reducing the extent of damage in the specimen.There exist a lot of cracks that are presented in Fig.10(d).The specimen(see Fig.10(d))appear to have serious damage,but is not completely broken.

Fig.10.Micro-CT surface images of polyurea and its composites under dynamic compressive loading at strain rate of 8000 s-1:(a)PU;(b)PU/0.7 SiC;(c)PU/1.5 SiC,and(d)PU/2.5 SiC.

These indicate that polyurea composites with a small amount of nanoparticles can reduce the extent of fracture,and a large amount of nanoparticles can easily to cause a large area of damage.These phenomena of micro-CT images are consistent with Fig.9.In addition,according to the relationship of energy absorption,polyurea composites can absorb more energy than pure polyurea,reducing the extent of the damage in the composites.

Fig.11 illustrates crack progression(from left to right)under the compressive loading(axial direction)of four specimens at the strain rate of 8000 s-1.From the three slices images in Fig.11(a),a crack appears in red frame all of the slices images,which shows that crack propagates along the axial direction in the sample,and forms the axial crack through the specimen.An internal crack in specimen propagates toward two direction,leading to a long and wide crack in the SiC-filled sample,and forming new damage area near the edge as shown in Fig.11(b).Compared to the crack in Fig.11(a),the crack propagation of specimen(Fig.11(c))appeared to be similar properties in the axial direction.Due to the serious damage of the specimen with the largest amount of SiC-filled,the red frame can not be appeared in Fig.11(d).It can see from Fig.11(d)that multiple cracks appear in the specimen along the axial direction.As the cracks expand,some fragmentation occurs on the surface of the specimen,resulting in local fracture.

5.Conclusion

In this research,the quasi-static and dynamic compressive testing methods have been carried out using the electronic universal testing machine and SHPB apparatus on pure polyurea and polyurea/SiC nanocomposites with three different contents of nanofillers including 0.7 wt%,1.5 wt%and 2.5 wt%at various strain rates.The main conclusions of the study can be drawn as follows:

At various strain rates,the true stress-strain curves obviously appeared to be nonlinear,and the profiles of the stress-strain curves of nanocomposites are similar with those of pure polyurea.The mechanical behavior of all the tested specimens suggested strong dependency on strain rate.The level of flow stress,compressive strength and strain energy went up by the increase of strain rate.Increasing the strain rate resulted in the elevation of the strain rate sensitivity in all of the polyurea specimens.The relationship between yield stress and the strain rate was expressed in the four materials according to the power law function.

In comparison to pure polyurea,the addition of nanoparticles influences on the compression properties.Under static loading,the nanocomposites with the content 1.5 wt%fillers affected greatly the compressive mechanical properties.However,under dynamic loading,the mechanical behaviors of nanocomposites with the additional amount of SiC(0.7 wt%)was observed to be more active compared to other nanocomposites.The reason may be that more cracks were formed on inside of the specimens with the increased content of particles under a high stain rate which leads to the decrease of mechanical properties.

The fracture surface of all specimens at the strain rate of 8000 s-1were observed by using SEM.It noticeable that voids,crazes,micro-cracks and cracking were shown in microstructure images.These features will be helpful for gaining better insight into the dynamic response and failure mechanisms.The main factors which result in the failure of polyurea materials are existence of microcrakes among hard segments,the formation of craze,the deflection of crack,crack propagation and the cohesiveness of nano particles.Moreover,propagation of cracks in the failed specimens can be revealed from the micro-CT slice images.These phenomena of micro-CT images are consistent with microstructures images.

Fig.11.Micro-CT slice images of damage specimens at strain rate of 8000 s-1 in axial direction:(a)PU;(b)PU/0.7 SiC;(c)PU/1.5 SiC,and(d)PU/2.5 SiC.

Declaration of competing interest

None.

Acknowledgments

The authors would like to thank the State Administration of Science,Technology and Industry for National Defense of China(Grant No.WDZCKYXM20190503)and Xi’an Modern Chemistry Institute for the financial support.Beijing Institute of Technology and Northwestern Polytechnical University are thanked for providing experimental equipments for this work.