Experimental and numerical study of concrete targets under high rate of loading

Experimental and Numerical Study of Concrete Targets Under High Rate of Loading Procedia Engineering 173 ( 2017 ) 130 – 137 1877 7058 © 2017 The Authors Published by Elsevier Ltd This is an open acces[.]

Procedia Engineering 173 ( 2017 ) 130 – 137

1877-7058 © 2017 The Authors Published by Elsevier Ltd This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the organizing committee of Implast 2016

doi: 10.1016/j.proeng.2016.12.049

ScienceDirect

The 11th International symposium on plasticity and impact mechanics, IMPLAST 2016

Experimental and numerical study of concrete targets under

high rate of loading

Abhishek Rajput* and M A Iqbal

Department of Civil Engineering, Indian Institute of Technology Roorkee, Roorkee, 247667, India

Abstract

In order to examine the ballistic resistance of plain concrete and reinforced concrete targets the perforation tests conducted in the laboratory, on square concrete targets size (450 mm x 450 mm x 80 mm) of plain concrete and reinforced concrete and to validate the experimentally obtained results Numerical simulations were carried out in Abaqus/Explicit finite element code The

unconfined target compressive strength of concrete was 48 MPa A grid of steel bars having 8mm diameter has been incorporated

in reinforced concrete plates The plates were subjected to normal impact of 0.5 kg ogive nosed hard steel cylindrical projectile having diameter of shaft 19mm The projectiles were accelerated by the laboratory pneumatic gun to velocities range between 53m/s to 220m/s impact and residual velocities were measured with the help of Phantom-V411 high speed digital camera system Ballistic limit of plain concrete and reinforced concrete targets had been obtained in the experiments as well as in the Numerical simulations in Finite element code Abaqus\explicit Also calculated spalling and scabbing volume of plain and reinforced concrete targets after perforation experiments, the reinforcement in the concrete has been found to be effective in minimizing the scabbing and spalling of material The ballistic limit of reinforced concrete target was experimentally found 16.9% higher than plain concrete target, However Numerical simulations predicted the ballistic limit of plain concrete target within 8% and that of reinforced concrete target within 3% deviation in comparison to experimentally obtained ballistic limits Peer-review under responsibility of the organizing committee of IMPLAST 2016

Keywords: Ballistic Impact; Spalling; Scabbing; Calibre Radius Head (CRH); Rigid Projectile

1 INTRODUCTION

Concrete structures have been widely used in construction of civilian structures and important structures such as bunkers, nuclear power plants, buildings, brides, dams, tunnels etc for those structures important criteria is to with

* Corresponding author Tel.: +918430397812

E-mail address: hiitsme.abhishek@gmail.com

© 2017 The Authors Published by Elsevier Ltd This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the organizing committee of Implast 2016

impact velocities and also the launch acceleration in the gun bore and the deceleration during the perforation event was recorded with an acceleration transducer Several perforation tests using concrete targets of different thicknesses were conducted with a nominal striking velocity of 400 m/s and the residual velocity against kinetic energy consumed versus the target thickness was analyzed, the perforation limit was also obtained [7] Li et al [8] studied the effect of hard missile impact on concrete plates and proposed analytical formulae to measure the ballistic resistance of the target In this study perforation experiments and numerical simulations have been done on plain and reinforced concrete of low compressive strength of 48 MPa with hard steel projectile of 0.5 kg and 19mm diameter The projectiles were accelerated by the laboratory pneumatic gun to velocities range between 124 m/s to 180 m/s impact and residual velocities were measured by the high speed digital camera system Ballistic resistance of plain concrete and reinforced concrete at different thicknesses had been find out in the experiments Damage pattern of concrete also discussed

2 CONCRETE TARGETS

In the earlier studies several researchers have studied with high strength concrete and also normal strength concrete but they have found that there is very minor effect in performance of the concrete targets with respect to their compressive strengths The target materials used were M40 concrete grade see Table-1, Different thicknesses

of 80mm and 100mm of M40 grade concrete had been casted, and 6 cube of dimension (150*150*150) mm also casted The samples for the compressive strength tests were allowed to cure for at least 28 days After curing of concrete cubes had been tested on compression testing machine which gives unconfined average compressive strength of 48 MPa

Table 1 Constituents of concrete

Cement (kg/m 3 ) Water (kg/m 3 ) Aggregate (10mm) (kg/m 3 ) Sand (kg/m 3 ) admixture

The square concrete plates of span 450 mm x 450 mm were taken and concrete plates thickness were considered 80mm geometry of concrete plate shown in fig-1a The experiments were conducted on a pneumatic gun 1 kg projectile as shown in Fig-1b, was launched up to an incidence velocity 200 m/s The length of the barrel was considered 18 m to enable adequate acceleration of the projectile for obtaining the required velocity see Fig-2 The angle of incidence was considered normal to the target

Fig 1 Geometry of concrete plate and hard steel projectile (All dimensions in mm)

3 Experimental setup

A 20mm diameter and 18m long barrel was used to launch the projectile to hit the target plates at normal incidence The plain and reinforced concrete plates were hit at normally at incidence velocities 53-200 m/s The incidence velocity of the projectile was varied by the pneumatic pressure gun attached with 18 m long seamless steel barrel see Fig 2 however, impact and residual velocities were measured with the help of phantom high speed video camera system In the experiment three non-flickering LED lights were used to ensure the visibility of perforation

phenomenon

Fig.2- Experimental setup of pneumatic gun

diameter 19 mm and total length 225 mm, Fig 1(c).The projectile was also assigned surface to surface contact with reinforcement in reinforced concrete and the reinforcement and prestressing strands in prestressed concrete target using kinematic contact option assuming negligible friction The edges of the target were fixed by constraining all the degree of freedom The central core of the target which constitutes the contact region of diameter 20 mm was meshed with 1 mm x 1 mm x 1 mm C3D8R elements, Fig 3 The outer region of the targets was also meshed with C3D8R elements of size 5 mm x 5 mm x 3 mm

The constitutive modelling for simulating the ballistic penetration of concrete was carried out using Holmquist-Johnson-Cook (HJC) elasto-viscoplastic material model and the material parameters for concrete having compressive strength 48 MPa has been used [9] The material behavior of reinforcing steel bars were incorporated

in the ballistic impact simulations using the Johnson-Cook elasto-viscoplastic material model [10-11]

Fig 3 Finite element model of concrete target

5 Ballistic impact

The projectile of 0.5 kilogram at the different striking velocity were impacted into the plain and reinforced concrete plates and residual velocity of projectile was measured using Phantom high speed digital camera system after perforation of target The ballistic resistance of the 80 mm thick plain and reinforced concrete plate has obtained Further the numerical simulations have been carried out to reproduce the experiments in the finite element code Abaqus/Explicit to validate the results The damage induced in the plain and reinforced concrete targets during experimentation has been quantified by measuring the volume of material removed from the front and rear target surface The equivalent diameter of the spalled and scabbed area was calculated as indicated in Fig 4 in four different directions and the mean diameter has been obtained Table-2 shows the variation in the spalling and scabbing diameters as a function of incidence velocity of projectile for 80 mm target thickness It has been found that equivalent scabbing diameter increases with decrease in impact velocity until ballistic limit It may therefore be concluded that the global deformation has decreased with increase in projectile velocity The spalling of material however is not influenced considerably by the incidence velocity

3 mm C3D4 element

1 mm C3D8R element

3 mm C3D8R element

5 mm C3D8R

Fig 4 Equivalaent diameter measurment of affected area after perforation

4

4 3 2

D

The volume of spalling and scabbing has also been calculated as a function of incidence velocity and the variation thus obtained and variation has been plotted in Fig.6a-6b for Plain and reinforced concrete target respectively The volume of material removed due to scabbing has been found to increase almost linearly with decrease in incidence velocity for both type of concrete The volume of spalling however remained almost constants for both types of concretes in the considered velocity regime For a given incidence velocity, the volume of scabbing has been found to be higher in the smaller thickness For a given thickness however, the volume of both scabbing and spalling of material has been found to be comparatively lesser in reinforced concrete target compared to plain concrete target This is due to the fact that the pressure wave propagation in reinforced concrete was very slow due

to partial confinement of concrete hence the cracks were very small On the other hand, in case of plain concrete cracks wide and long cracks were observed Equivalent diameter at front and rear face of the impacted specimen have been measured in four different directions and find out the mean of that four diameters in in four directions which is termed as equivalent diameter at front and rear face It is found that equivalent diameter was increases with decrease of the impact velocity till ballistic limit however at low velocity higher global deformation had been found when compare to higher velocity for the same specimen The failure modes of the impacted concrete target have been found in good agreement with the simulated results see Fig 5

Fig 5 Typical 80 mm thick reinforced concrete target after projectile impact (a) Experiment (b) Numerical

simulation

Fig 6 Front and rear surface eroded material of (a) Plain concrete (b) Reinforced concrete

Table 2 Equivqlent diameter of 80 mm thick Plain concrete (PCC) and reinforced concrete (RCC) target after Impact

Target

Thickne

ss

(mm)

Specimen

No – impact

velocity (m/s)

Front Surface Rear Surface D1

(mm)

D2 (mm) D3 (mm) D4 (mm) Equivalent Diameter (mm)

D1 (mm)

D2 (mm) D3 (mm)

D4 (mm)

Equivalent Diameter (mm)

82.4 65.57 64.8 78.85 72.90 161.8 159.5 175.7 141.0 159.5

62.4 71.1 60.75 61.42 63.91 170.1 124.8 152.2 167.2 153.6

60 52.93 64.8 63.08 60.20 182.6 135.0 167.0 188.6 168.3

6 Result and discussion

Projectile impact experiment and numerical simulations had been carried out successfully on the 80mm thickness

of plain and reinforced concrete plates, in the experiments impact and residual velocities have been measured with the help of high speed phantom video camera system Impact and residual velocities of projectile impact experiments are shown in table 3 Results of impact and residual velocities for plain and reinforced concrete plates have been plotted in fig.7a and 7b Equivalent diameter was increases with decrease of the impact velocity till ballistic limit however at low velocity higher global deformation had been found when compare to higher velocity for the same specimen spalling of the impacted concrete plate were almost equal at different velocities however scabbing was increase with decrease of the velocities till ballistic limit of the specimen The ballistic limit of reinforced concrete target was experimentally found 16.9% higher than plain concrete target, However Numerical simulations predicted the ballistic limit of plain concrete target within 8% and that of reinforced concrete target within 3% deviation in comparison to experimentally obtained ballistic limits

Table 3 Incidence and residual projectile velocities (m/s) for different concretes

Plate thickness

(mm)

80

Experimental Results Numerical Results Experimental Results Numerical Results Ini Vel

(m/s)

Fin Vel

(m/s)

Ini Vel

(m/s)

Fin Vel

(m/s)

Ini Vel

(m/s)

Fin Vel

(m/s)

Ini Vel

(m/s)

Fin Vel

(m/s)

Fig.7 Experimental (actual) and numerical simulations (predicted) Impact velocity vs residual velocity a) Plain, b) Reinforced concrete targets

7 CONCLUSION

Conducting ballistic performances on plain concrete and reinforced concrete plates taking 48 MPa as unconfined compressive strength of the target specimen for both plain concrete plates and reinforced concrete plates of dimensions 450mm x 450mm x 80 mm for both plain concrete and reinforced concrete plates and studied the outcomes for both plain concrete and reinforced concrete plates from the experiments it is found that The ballistic limit of reinforced concrete target was found 16.9% higher than plain concrete target

Also Residual velocities for reinforced concrete plates are decreased when compared to plain concrete plates Due to the incorporation of reinforcement in the concrete plates there is very huge decrease in the scabbing however when striking velocity of the projectile is low then scabbing in the rear face of the concrete plate is high for plain concrete plates and as well as reinforced concrete plates.it is also found that pressure wave propagation is very slow due to confinement of concrete so cracks were very small while in case of plain concrete cracks were wide and long Spalling is almost same for plain concrete plates and reinforced concrete plates Scabbing were increase with decrease of the velocities till ballistic limit of the specimen Global deformation was higher at lower velocities while

at higher velocities local deformation was found Numerical simulations predicted the ballistic limit of plain concrete target within 8% and that of reinforced concrete target within 3% deviation in comparison to experimentally obtained ballistic limits

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