Study mass loss at microscopic scale for a projectile penetration into concrete

Accepted Manuscript Study mass loss at microscopic scale for a projectile penetration into concrete L. Guo , Y. He , X.F. Zhang , C.X. Pang , L. Qiao , Z.W. Guan PII:

S0734-743X(14)00108-0

DOI:

10.1016/j.ijimpeng.2014.05.001

Reference:

IE 2343

To appear in:

International Journal of Impact Engineering

Received Date: 5 November 2013 Revised Date:

10 March 2014

Accepted Date: 7 May 2014

Please cite this article as: Guo L, He Y, Zhang XF, Pang CX, Qiao L, Guan ZW, Study mass loss at microscopic scale for a projectile penetration into concrete, International Journal of Impact Engineering (2014), doi: 10.1016/j.ijimpeng.2014.05.001. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

Study mass loss at microscopic scale for a projectile penetration into concrete L. Guoa, Y. Hea,*, X.F. Zhanga, C.X. Panga, L. Qiaob, Z.W. Guanc a

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School of Mechanical Engineering, Nanjing University of Science & Technology, Nanjing 210094, P.R. China b Beijing Institute of Space Long March Vehicle, Beijing 100076, P.R. China c School of Engineering, University of Liverpool, Brownlow Street, Liverpool L69 3GQ, United Kingdom

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Abstract: Mass loss of the kinetic energy (KE) projectile results in a dramatic drop of DOP (Depth of Penetration) due to the change of nose shape. This is often observed in

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the high-velocity penetration into concrete, which significantly influences the penetration efficiency. In order to investigate the intrinsic mechanisms of mass loss, experiments were conducted on 30CrMnSiNi2A kinetic energy projectiles with an ogival nose penetrating concrete targets in the normal direction at a striking velocity

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of 843 m/s and 1400 m/s, respectively. Microstructural features of various sections, obtained from different locations of the residual projectiles recovered from concrete

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targets, were systematically characterized by using optical microscope, energy dispersive X-ray (EDX) detector and microhardness tester. Based on the experimental

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observations and analysis, evolution mechanisms of the microstructures induced by plastic deformation and high temperature during penetration were proposed. It involves formation of the mixed zone (MZ), the refined zone (RZ) and the original zone (OZ). Experimental evidences and analyses of the surface formation suggest that the thermal softening, material flow and eventual mass loss associated with high strains and high strain rates at high temperature are the main mechanisms of material *

Corresponding author Tel.:+86 025 84315790

E-mail address: [email protected] 1

ACCEPTED MANUSCRIPT failure during projectile penetration. Keywords: deep penetration; mass loss; penetrator; thermoplastic 1. Introduction

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Low-velocity penetration (<1000 m/s) into concrete with kinetic energy (KE) projectile has been studied systematically [1-5] by assuming the projectile as a rigid body. However, with increasing impact velocity (1000 m/s
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loss of the projectile becomes more and more significant during the penetration

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process. Phenomena, such as nose abrasion, bending or breaking of the projectile resulting in the trajectory deviation as well as dramatic drop of DOP (Depth of Penetration), are observed in many experimental results [6,7]. The assumption of a rigid projectile penetration through concrete targets is inappropriate in the case of

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high velocity penetration. It is an indisputable conclusion that the unstable projectile structure induced by mass loss will significantly affect the performance of terminal lethality. Therefore, ensuring the integrity of projectile at high velocity impact

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becomes an imperative issue in the international research community. For further

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development of the earth penetration weapon (EPW), a clear cognition of the underlying mechanisms is required to the mass loss of a projectile during high velocity penetration.

With increasing the research interest in high velocity impact, a series of

experimental studies as well as theoretical exploration have been carried out to reveal the mechanisms of the mass loss during penetration. Through the experimental work, Forrestal et al. [6,10] and Frew et al. [8,9] found that mass abrasion mainly occurred

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ACCEPTED MANUSCRIPT at the exterior surface of projectiles’ nose. This was based on the six groups of penetration experiments with different CRH (caliber radius head) projectiles and uniaxial compressive strength of the concrete targets. The mass loss is up to 7% when

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the striking velocity exceeds 1200 m/s. Furthermore, Silling and Forrestal et al. [11] indicated a liner relationship between the mass abrasion and the initial kinetic energy of a projectile below the velocity of 1000m/s based on the experimental data from

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Sandia National Laboratory (SNL). In consideration of the yield strength of

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projectiles, He et al. [12] and Yang et al. [13] investigated the penetration ability, stability and deformation of projectiles in high velocity impact. The results obtained from extensive experimental work showed that the mass loss of the projectile is closely related to the initial impact velocity, the dynamic mechanical property of

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penetrator, the category of the aggregate casted in concrete and other random factors. Basic phenomena during high velocity penetration were observed in those experiments, which support the hypotheses for theoretical research.

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In the case of theoretical analysis, assuming that the rate of mass loss is

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proportional to the normal stress and the sliding velocity between projectile and concrete, Beissal et al. [14, 15] developed an axisymmetrical mass abrasion model for carrying out the 3D finite element analysis. Jones et al. [16] proposed an abrasion model based on the assumption that the mass loss is caused by the heat produced by the sliding friction between projectile and concrete. Chen et al. [17] identified the hardness of aggregates having a significant influence on the mass loss after examining the experiment data from Forrestal et al. [6]. In combination of Jones’ model [16] and

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ACCEPTED MANUSCRIPT Silling’s model [11], He et al. [18] derived a model with seven main influential variables, i.e., the initial impact velocity, initial nose shape, melting heat, shank diameter of projectile, density and strength of target as well as aggregate hardness of

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target, which is the most successful one to predict the mass abrasion of a projectile. The abrasion effect for the projectile nose has a significant influence on penetration. There are several abrasion models developed to modify the variation of projectile

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nose, such as Davis et al. [19], He et al. [20], Wen et al. [21] Yang et al. [22] and Qian

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et al. [23]. Zhao et al. [24,25] and He et al. [26,27] analyzed the penetration processes considering mass loss based on the assumption that the nose maintained arc during the penetration with the minimum CRH equal to 0.5. Klepaczko et al. [28] gave a definition on the universal parameters of the rate of wear and the rate sensitivity of

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wear. Mu et al. [29] and Zhang et al. [30] conducted relevant experiments with the impact velocity up to 1600 m/s and proposed an engineering model to determine transition velocity into the semi-hydrodynamic penetration regime. Good consistency

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has been obtained between the existing theoretical calculations and experimental

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results, to some extent. However, due to the limitations of theoretical assumptions, most predictions on the new situation derived from the theoretical models do not provide satisfactory calculations. The existing method to predict mass loss by the simple assumption of material melting or curve fitting of the cited experimental data is inappropriate. Therefore, a clear understanding of the mechanisms of mass loss is essential for the establishment of reasonable theoretical assumptions and further analysis.

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ACCEPTED MANUSCRIPT Involving with high temperature, high pressure and high strain-rate induced by high velocity, it is difficult to obtain or record some real-time data experimentally to support the theoretical analysis in the instantaneous penetrating process. Therefore,

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an efficient approach to analyze mass loss in microscopic level is imperative to further improve the penetration ability of EPW. The objective of this work is therefore to reveal the underlying mechanisms of mass loss of a projectile at a high striking

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velocity. Experiments were conducted on 30CrMnSiNi2A kinetic energy projectiles

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with an ogival nose shape penetrating into concrete targets in the normal direction with a striking velocity range of 800~1400 m/s. Specimens drawn from the residual projectiles were analyzed by optical microscope and scanning electron microscope with energy dispersive X-ray (EDX) detector to examine the metallographic structural

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evolution in detail along the surface depth affected. In addition, microhardness tests were undertaken to further obtain the gradient variation of hardness from the outer to the inner surfaces of the projectile. The mechanisms of mass loss were proposed and

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the dominant factors were identified.

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2. Penetration experiments 2.1. Experimental setup

A 105mm caliber smoothbore gun was used to launch a 60 mm diameter projectile

to penetrate into concrete with a velocity in a range of 800~1400 m/s by the sub-caliber launching way. A reasonable mass charge was calculated according to the required striking velocity. A time-measuring system, containing a multi-channel time recorder and several aluminum foils used to provide signals for the time recorder, was

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ACCEPTED MANUSCRIPT placed at the flight trajectory of the projectile to obtain the striking velocity. In addition, two high-speed cameras were set orthogonally to record and measure the flight attitude, e.g. pitch and yaw angles of the projectile. All units were aligned and

experiment layout is showed in Fig. 1.

Fig. 1. Plan sketch of experiment layout.

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2.2. Projectiles and targets

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fixed to make sure that the projectile could hit the target normal to its surface. The

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The CRH=3 ogive-nose projectiles, with a length of 300mm and diameter of 60 mm, were machined using 30CrMnSiNi2A steel. The hollow structure was used to reduce launch weight in order to satisfy the impact velocity with the consideration of structural safety during launching and penetrating processes, as shown in Fig. 2. The

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concrete targets with limestone aggregate were casted in steel tubes of 1800 mm in diameter with the wall thickness of 10 mm, which were cured in standard conditions

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for 28 days. Three 150×150×150 mm cubes used for compressive strength tests were cored from the cured concrete inside the steel tube. Tests indicated that the concrete

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samples had a nominal unconfined compressive strength in a range of 40~45 MPa and a density of 2400 kg/m3. Fig. 2. A projectile used in the experiment (Length 300 mm, diameter 60 mm).

2.3. Penetration results Eight experiments were conducted with relevant data recorded. In order to investigate mechanisms of the mass loss during penetration, two projectiles related to the minimum and the maximum velocities were recovered for further analysis. Fig. 3 6

ACCEPTED MANUSCRIPT shows the images of the ogive-nose projectiles tested at a velocity of 853 m/s and 1401 m/s, respectively. It is clear that the projectile subjected to the velocity of 853 m/s shows a negligible abrasion (Fig. 3a) and however, the projectile at the velocity of

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1404 m/s must have experienced a grim penetration process featured with the nose shape significantly changed (Fig. 3b). Table 1 summarizes the experimental results of

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these two projectiles.

Fig. 3. The photographs of the projectiles recovered, (a) Projectile at 853 m/s, (b) Projectile at 1401 m/s.

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Table 1. Penetration results of projectiles related to the minimum and the maximum velocities

3. Microscopic analysis of the recovered projectiles

The high-strength alloy steel 30CrMnSiNi2A is widely used for manufacturing

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load-supporting structure, such as aircraft landing gear due to its excellent comprehensive properties. The other chemical compositions in the alloy steel are listed in Table 2, while its original metallographic structure is displayed in Fig. 4. It is

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found that the main composition of tempered sorbite structure has high dislocation

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density and thin bunches due to the refined effect by element Ni. As a result, the tensile strength σb of the projectile with high strength martensite can reach up to 1800 MPa.

Table 2. Other chemical compositions in the steel investigated (wt%)

Fig. 4. Metallograph of 30CrMnSiNi2A steel.

Most abrasion erosion of a projectile occurs on its surface during penetration, as expected. Obviously, materials on the surface at different positions of the projectile 7

ACCEPTED MANUSCRIPT experience different abrasion. By comparing the microstructure erosion in different locations of the recovered projectiles, mechanisms of the mass loss process are proposed to support the theoretical research. In order to support the proposed

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mechanisms, samples with the size of 5mm×5mm×10mm were acquired from three typical locations, i.e. the front, middle and back of these two projectiles by using a wire cut electrical discharge machine, as shown in Fig. 5. The original external

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surfaces of the samples were maintained, while some steps such as grinding and

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polishing were undertaken to the others surfaces. The ground coupons were ultrasonically cleaned in ethanol for 15 minutes.

Fig. 5. Sketch of the sampling locations.

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3.1. Metallographic observation

Fig. 6. Cross-sectional morphology of the samples from the projectile subjected to a velocity of 853 m/s, (a) the nose, (b) the shank, (c) the tail, (d) detail view of typical cross-sectional microstructure

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morphology

Fig. 7. Cross-sectional morphology of the samples from the projectile subjected to a velocity of

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1401 m/s, (a) the nose, (b) the shank, (c) the tail.

Fig. 6 and Fig. 7 show metallographic images of the cross-sections of the two

recovered projectiles at three typical locations, obtained from optical microscope along the depth from the surface. Clearly, microstructure morphology of the surface differs from that in the substrate. Evidences of the phase transformation can be seen in the surface layer, in which the layer pattern could have gone through a refinement process. 8

ACCEPTED MANUSCRIPT The transient heat flux generated by intensive sliding friction between projectile and concrete would likely have an effect on the constitutive behaviour of the material. Many physical mechanics phenomena, such as softening, melting and phase

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transformation, occur on the nose of a projectile during the penetration process. Fig. 6d shows a typical cross-sectional microstructure morphology of the surface from a partial close-up of Fig. 6a. From the metallographic structure, it can be observed that

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the surface layer may be subdivided into three sections along the depth from the

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surface, i.e. the mixed zone (MZ), the refined zone (RZ) and the original zone (OZ). In addition, the heat affected zone (HAZ) is composed of the MZ and the RZ. Each region has its own characteristics, for example, the tempered sorbite that has excellent mechanical properties forms the main composition of the OZ. Also, there are

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minus changes between the projectile states before and after the penetration. It can be inferred that the phase transformation as well as grain size refinement only occur at the surface layer of the projectile during penetration. The distribution of the refined

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grain becomes more dispersive from the OZ to the RZ due to the high temperature

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and pressure caused by the contact interaction between projectile and target. Tempered sorbite is gradually decomposed, while extremely tiny ε-carbide and ferrite are obtained in that environment. Finally, the RZ is filled with ferrite covered with ε-carbide, and the corresponding hardness has been declined due to the decrease of supersaturation of ferrite structure which will be confirmed by the microhardness tests. There exists a temperature gradient along the depth from surface. Therefore, microstructure

in

the

MZ

endured

tough and harsh conditions

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with

phase

ACCEPTED MANUSCRIPT transformation, melting of material, resulting in filling with the melted projectile material and concrete particles. The MZ is finally formed due to a rapid cooling rate, as the white materials shown in Fig. 6d.

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Penetration along with rapid thermal change history can lead to differences in grain size, phase composition and microstructures. By comparing Fig. 6 and Fig. 7, it is noted that the surface layer thickness affected is not uniform from place to place,

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indicating the different plastic deformations occurred in different locations due to

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different initial penetration conditions. The MZ, RZ and OZ with clear boundaries exist in the nose surface of the projectile subjected to a velocity of 853 m/s (Fig. 6d), whilist the MZ and RZ at the shank and tail become blurred. On the contrary, projectile with a velocity of 1401 m/s has clear boundaries at the shank and tail, but

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not at the nose. Further observation indicates that micro-cracks only appear at the nose of the projectile with the velocity of 1401m/s. It may be reasonable to assume that the high pressure and temperature are the main cause of the projectile mass loss.

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Plastic deformations of the projectile material generated by the transient effect

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between projectile and concrete were converted to heat mostly, which make the thin layer of the exterior surface of the projectile thermally softened or even melted. Cracks were formed with the shear stress upon the softening layer materials with a thickness of 10~30µm. The material with cracks was erased away by the aggregate during the penetration, finally resulted in the mass loss of the projectile. In addition, the nonhomogeneity of the concrete media likely leads to the asymmetry of stress distribution and even results in material broken away from the projectile or trajectory

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ACCEPTED MANUSCRIPT deviation with increasing the impact velocity. The cross-sectional microstructure morphology shown in Fig. 7(b) presents a circular feature caused by the rapid friction on the exterior surface of the penetrator which has surface defects. Obviously, the

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affected area is enlarged and centered on the location of the defects due to the influence of the transient heat flux. 3.2. Microstructure evolution with EDX method

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The optical observation clearly demonstrates the microstructure developed in the

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surface layer of 30CrMnSiNi2A after penetration. In order to understand the microstructural evolution process during the penetration, a systematic investigation on the composition in the surface layer is carried out.

A JSM-5610LV made by Japan Electron Optics Laboratory (JEOL) Energy

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Dispersive X-ray Detector (EDX) was employed to characterize the detailed compositions of the surface layers of the recovered projectiles. Specimens from the two recovered projectiles were placed on a plate of aluminum. Fig. 8 shows the

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images obtained from the EDX. The line on the left of Fig. 8 indicates the scanning

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path of the spatial-resolution energy dispersive X-ray spectroscopy (EDS), while the letter “S” represents the starting position of the line-scanning. With consideration of elements Al, C, Fe and Si, further analysis along the straight scanning line is shown on the right of Fig. 8. The line scanning initiates from the outside region, crosses the MZ and the RZ of the specimen, and finally enters the OZ. It is convinced that the onset boundary of the MZ or the boundary of the specimen can be determined precisely when the counts of Al are decreased to a low level, while

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ACCEPTED MANUSCRIPT the other boundary locations are identified when a steep increase is turned up in the counts of Fe. The MZ is marked out in Fig. 8 and the thickness values are listed in Table 3. It was found that the counts of Si increased between the marked two lines

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indicating the scope of the MZ, which further enhances the standpoint that the MZ was filled with melt projectile material and concrete particle. The back-scattered electron imaging (BEI) photographs from scanning electron microscope (SEM), as

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shown in Fig. 9, give a confirmation that there existed a layer, i.e. the MZ, with a poor

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electrical conductivity on the outside surface of the projectile with the velocity of 853 m/s. As to the RZ and the OZ, the contents maintain the same since there is no impurity introduced, which is understandable.

The heat caused by the friction and the plastic deformations reaches the melting

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point of the projectile, which is one of the mass loss sources during penetration. However, by comparing with the ratio of the counts of Fe to Si in the MZ, it was found that there are higher counts of Fe on the shank than that on the nose and tail of

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the projectile. This may be due to the flow of the molten mixture with high Fe content

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from the nose to the shank and weaken to the tail, regardless of which projectile. Besides, the outer part of the MZ is mainly comprised of concrete due to the direct contact to the target, with a relatively large thickness of the MZ. Also, as shown in Fig. 8, the percentage of Fe rises when approaching to the inner region of the MZ. Here, an interesting phenomenon is observed from Table 3. The thickness of the MZ is significantly reduced from the nose to the tail of the recovered projectile with a velocity of 853 m/s, whereas that related to the velocity of 1401 m/s is almost the

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ACCEPTED MANUSCRIPT opposite, i.e. the thickest MZ exists on the tail. This indicates that the projectile with a striking velocity of 1401 m/s must have been undergone a different penetration process characterized with the higher contact stresses and friction, which result in a

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higher pressure and higher temperature rise, than that of projectile with a striking velocity of 853 m/s. The surface layer material of the projectile may encounter thermo-softening, especially at the nose region, leading to the reduction of yield

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strength. Furthermore, the hardness of aggregate added to concrete plays an important

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role in this process, as discussed by Chen [17]. Apparently, the distribution of aggregate in the asymmetrical plain concrete enhances the local stress on the surface of the projectile which leads to the generation of the MZ on the surface of projectile. Mixed with concrete and metal, a certain shear stress is required to strip the mixed

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layer away from the projectile. It is likely that the temperature and pressure generated at a striking velocity of 853 m/s may not reach the critical values resulting in failure and material flow along the contact surface between projectile and target. However, at

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a striking velocity of 1401 m/s, the clear loss of materials shown in Fig. 3 covered

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from the nose to the shank of the projectile. Based on the discussion above, it is reasonable to conclude that material softening sheared from the projectile is the main mechanism that contributes to the projectile mass loss and therefore, increasing the yield strength at high temperature for a projectile surface layer is an effective approach to combat the mass loss. (a) the nose of the projectile tested with a velocity of 853m/s (b) the shank of the projectile tested with a velocity of 853m/s

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ACCEPTED MANUSCRIPT (c) the tail of the projectile tested with a velocity of 853m/s (d) the nose of the projectile tested with a velocity of 1401m/s

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(e) the shank of the projectile tested with a velocity of 1401m/s

(f) the tail of the projectile tested with a velocity of 1401m/s Fig.8. Results of EDX

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Table 3. Thickness of the MZ covering on the residual projectile

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Fig. 9. SEM images of a sample from the nose of projectile with a velocity of 853 m/s by back-scattered electron imaging (BEI)

3.3. Microhardness tests

Although it is difficult to obtain the real-time measurements on the change of

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physical parameters of a projectile during penetration, many methods are proposed to work out the mechanisms of mass abrasion in the transitory penetration process. This is usually through observing the changes in microstructures of the surface materials of

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a projectile before and after penetration. Jones et al. [31] and Montgomery [32]

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indicated that the phase transformation due to temperature rise on the thin exterior exceeding the austenitic transformation temperature has a remarkable influence on the properties of materials. Previous studies [33] showed that the heat affected zone (HAZ) with a high hardness is formed by a grain refining process due to a rapid cooling rate. However, the detailed investigation on the hardness of the HAZ is limited up to date. Mechanical properties of a metallic material will vary according to grain size, phase

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ACCEPTED MANUSCRIPT composition and microstructures. In order to support the microstructure classification and evolution proposed, microhardness tests were conducted with a HVS-1000Z digital Vickers microhardness tester operated at the load of 0.5 kg for a dwell period

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of 10 s using Vickers diamond pyramid indenter at room temperature. The indented impression of Vickers was approximately a square-based pyramid. Fig. 10 shows relationships between the microhardness and the depth from exterior surface for

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different sampling locations of the two projectiles studied. The result values obtained

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were transformed to Rockwell C by measuring the length of the two diagonals using a calibrated micrometer attached to the eyepiece of microscope.

The average Rockwell hardness of the original projectile structure was marked out as a value of 49, as shown in Fig. 10. Based on the experimental data, approximate

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quadratic function relationships between the surface microhardness and the distance to the surface were fitted. According to the definition proposed in Section 3.1, the HRC in the RZ, filling with a low saturation of ferrite and ε-carbide, is lower than that

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in the OZ. In general, the region of the RZ can be determined at the intersection point

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of the polynomial fitting curve and the original HRC line. Here, the MZ is marked out in Fig. 10 according to Table 3. As mentioned before, the HRC of the MZ is lower than that of others under the

same velocity and location, which is caused by mixing with concrete particles. Next to the MZ, the microhardness in the RZ is somewhat lower than that in the OZ due to filling with a low saturation of ferrite and ε-carbide. However, the hardness in the OZ does not revert to the average level immediately. With the effect of temperature and

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ACCEPTED MANUSCRIPT pressure gradients along the depth, the mechanical properties of grains at the edge of the OZ next to the RZ are enhanced after the low temperature tempering process, and tempered martensite with high HRC is obtained [7]. Comparing Figs. 10(a), 10(c) and

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10(e) with each other, results indicate that the fitted hardness curve of the nose has higher fluctuation than that of others. Gradually, this hardness fluctuation influences the material at the shank and the tail with the increase of impact velocity. Meanwhile,

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the HAZ is generated from the nose to the shank, and finally expands to the tail,

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which proves the point that the HAZ is the result of material thermal failure. As to the tail of the projectile (Fig. 10(e) and 10(f)), the partitions of the RZ by curve fitting method is not accurate for the less influenced region due to low temperature rising. The trends of the microhardness shown in Fig. 10 are consistent with the analysis

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discussed the above.

(a) the nose of the projectile tested with a velocity of 853m/s

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with a velocity of 1401m/s

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(c) the shank of the projectile tested with a velocity of 853m/s

(b) the nose of the projectile tested

(d) the shank of the projectile tested

with a velocity of 1401m/s

(e) the tail of the projectile tested with a velocity of 853m/s

(f) the tail of the projectile tested with a

velocity of 1401m/s

Fig. 10. Microhardness results

4. Discussion Systematic investigations have been carried out to help understand the microstructural

evolution

processes

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the projectile penetration.

The

ACCEPTED MANUSCRIPT metallographic images of the specimens observed from optical microscope are applied to identify the metallographic structures and to propose the divisions of the affected area. In order to understand the microstructural evolution processes, the EDX and

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SEM methods are employed to identify the material compositions along the depth from the projectile surface, which are used to justify the zone divisions. In addition, the thickness of the MZ corresponding to the different regions of the projectile is

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obtained by the accurate measurements. Finally, the results of the microhardness tests

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provide approval of the microstructural classification and evolution processes proposed, as well as the thicknesses of the RZ determined by using the curve fitting method.

Therefore, the experimental results and the related analyses reveal that the

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mechanisms of the mass loss are the material softening due to shearing between the projectile and the target. The outer material bears a high temperature, resulting in the carbide precipitation, agglomeration or coalescence of cementite from the tempered

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reduction.

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sorbite, making the reduction in supersaturation, and finally leading a strength

4.1 Division of the affected surface Based on the microstructural features observed in various sections, it is revealed

that the failure of high-strength alloy steel 30CrMnSiNi2A during penetration is heavily dependent on the strain, temperature and strain rate induced by the interaction between projectile and target. According to the observation of the recovered projectiles, the affected surface layer can be subdivided into three sections along the

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ACCEPTED MANUSCRIPT depth from the exterior: the mixed zone (MZ), the refined zone (RZ) and the original zone (OZ). The existence of this division is ascribed to the temperature gradient and stress gradient along the depth generated by the friction and the interaction between

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projectile and concrete, as well as plastic deformations of projectile material itself. The OZ has the same microstructure as the projectile after heat treatment. The RZ is filled with ferrite covered with ε-carbide, which derives from the decomposition of

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tempered sorbite, resulting in grain refinement under a rapid cooling rate. It is

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confirmed that the MZ suffers tough and harsh conditions, filling with melted projectile material and concrete particles.

4.2 Evolution process during the penetration

As those physical quantities, such as stress, strain rate and temperature rise,

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decrease from the maximum at the top surface layer to approximately zero in the matrix, the structure undergoes different evolution processes at different depths from the top surface to the deep matrix. There is almost nothing changed on the material

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deep inside the projectile. However, the mechanical properties of microstructure on

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the edge of the OZ next to the RZ are enhanced by the low temperature tempering process, and tempered martensite with a high HRC obtained. In the RZ, the tempered sorbite OZ is gradually transformed to the RZ with refined grains by a certain temperature and pressure, resulting in the ferrite covered with ε-carbide around. With the increase of temperature and strain at a shallower depth, the MZ was formed in the surface due to combined effects of thermal softening, strain-hardening, abrasion action by concrete media. All these transformations are realized when

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ACCEPTED MANUSCRIPT temperature conditions are met. 4.3 Mechanism of the mass loss Material phenomena, such as softening, melting and phase transformation, happen

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to the surface of the projectile during penetration. According to analysis discussed above, it is confirmed that material failure during penetration undergoes these evolution processes. The key factor determining the failure is related to the projectile

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material performance at high temperature, high pressure and high strain rate. High

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strains with a high strain rate at high temperature that result in thermal softening, material flow, and eventually mass loss of a projectile are the mechanisms of the material failure during penetration. In other words, further increase on the striking velocity leads to the increase of the temperature, pressure and shear stress, eventually

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resulting in material flow, failure and the mass loss of projectile. In order to reduce the mass loss during penetration, several processing techniques could be adopted to upgrade

traditional

materials

used

for

projectiles,

e.g.

coating,

surface

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nanocrystallization, surface modification and high melting point material with

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wolfram. This will be valuable work for future research. 5. Conclusions

In this study, experimental work on a 60 mm diameter projectile penetrating a

concrete target at two velocities was conducted to investigate the mechanisms of the mass loss. Based on the analysis on microstructure images, the microstructure formation of the projectile surface layers after penetration is proposed. This covers the MZ, RZ and OZ as well as the related evolution of the transition zones. Experimental

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ACCEPTED MANUSCRIPT evidences and analysis of the transformation of each zone indicate that temperature, strain and strain rate play an important role in the mass loss. Thermal softening at a high temperature on the projectile material which is erased by the concrete aggregate

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is the main mechanism of mass loss during penetration. According to the analysis above, possible effective methods are proposed to reduce mass loss, which will be addressed in future work.

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6. Acknowledge

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This research work was sponsored by the National Natural Science Foundation of China (NSFC51278250), the Zijin Intelligent Program, Nanjing University of Science and Technology (2013_ZJ_0101) and Qing Lan Project of Jiangsu province. The authors would also like to thank the State Key Laboratory of Explosion Science and

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Technology (Beijing Institute of Technology) (KFJJ15-07M) for their great support on the research work presented in this paper. References

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[1] Tate A. A theory for the deceleration of long rods after impact[J]. Journal of the Mechanics and Physics of Solids, 1967, 15(6): 387-399. [2] Kennedy RP. A review of procedures for the analysis and design of concrete structures to resist missile impact effects[J]. Nuclear Engineering and Design, 1976, 37(2): 183-203. [3] Lundgren RG. High-velocity penetrators[C]. Presented at the American Institute of Aeronautics and Astronautics Missile Sciences Conference, Monterey, CA, 7-9 Nov. 1994. 1994, 1: 7-9. [4] Li QM, Chen XW. Dimensionless formulae for penetration depth of concrete target impacted by a non-deformable projectile[J]. International Journal of Impact Engineering, 2003, 28(1): 93-116. [5] Rosenberg Z, Dekel E. The penetration of rigid long rods–revisited[J]. International Journal of Impact Engineering, 2009, 36(4): 551-564. [6] Forrestal MJ, Frew DJ, Hanchak SJ, Brar NS. Penetration of grout and concrete targets with ogive-nose steel projectiles[J]. International Journal of Impact Engineering, 1996, 18(5): 465-476. [7] Jerome DM, Tynon RT, Wilson LL, Osborn JJ. Experimental observations of the stability and

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Striking

Initial

Final

Reduction

Initial mass

Residual

Mass loss

Penetration

number

velocity

length

length

percentage

of

mass of

percentage

depth (mm)

(m/s)

(mm）

(mm)

in length

projectile(kg)

projectile

(%)

(%)

（kg）

853

300

298.85

0.58

3.813

3.738

1.96

1670

2

1401

300

288.76

3.75

3.883

3.611

7.0

3200

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Table 2. Other chemical compositions in the steel investigated (wt%) Element

C

Si

Mn

Cr

Ni

Mo

S

P

Content

0.31

1.09

0.94

1.08

1.62

0.15

0.002

<0.005

V

Cu

Ti

0.07

0.089

0.0063

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Table 3. Thickness of the MZ covering on the residual projectile Striking

Thickness of MZ on

Thickness of MZ on

Thickness of MZ on

the

velocity(m/s)

the nose of the

the shank of the

the tail of the

projectile(µm)

projectile(µm)

projectile(µm)

projectile

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Number of

853

46.33

9.67

0.47

2

1401

10.98

8.89

62.74

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Mass loss of projectile significantly influences the penetration efficiency.
Plastic deformation and high temperature generate the microstructure evolution.
The evolution of the affected surface layer can be subdivided into three sections.

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Thermal softening, material flow and mass loss are the main failure mechanism.