Mass Loss and Penetration Depth of Hypervelocity Projectiles

Numerous investigations recorded a dramatic decrease in penetration depth at hypervelocity. The decrease is due to the severe mass loss of the projectile. The mechanics of mass loss are complicated, so the entirely theoretical model is still absent. In this paper, we derive a semitheoretical formula for mass loss by solving the equation of motion of the projectile during penetration. The result shows that the decrease in projectile mass at hypervelocity accords with the power law. Furthermore, we obtain a continuous formula for depth of penetration in the entire velocity domain. The theoretical results agree well with experimental data. The formula for the critical velocity of depth decreasing is also obtained as a by-product of the solving procedure.


Introduction
With the rapid development of hypervelocity air vehicles, the hitting speed of hypervelocity weapons has reached Mach 5-15 (Mach 1 � 340.3 m/s). ere will be intense interactions on the interface between the projectile and the target at such high velocities. Melting, blunting, and mass loss of the projectile will occur. ese phenomena weaken the projectile and diminish the terminal depth of penetration (DOP).
Numerous experiments have confirmed that projectile mass decreases dramatically under hypervelocity penetration. Forrestal and Piekutowski [1] used steel projectiles to penetrate aluminum targets and recorded the vanishing of the projectile at the impact velocity of 3075 m/s with the help of radiographs. Qian et al. [2] conducted penetration experiments with tungsten alloy projectiles and concrete targets.
e projectile was entirely eroded at the impact velocity of 3360 m/s. Li et al. [3] penetrated granite targets with alloy steel projectiles. e percentage of mass loss reached 88% when the impact velocity was 2378 m/s, and the velocity range of severe mass loss corresponded to that of DOP decreasing.
Projectile mass loss plays an essential role during the penetration and dominates the DOP in a specific range of velocity. Researchers have conducted numerous investigations on the mechanisms of mass loss. Silling and Forrestal [4] found a linear relationship between mass loss and the initial kinetic energy of projectiles. Klepaczko and Hughes [5] analyzed the wear at high sliding speeds and defined universal parameters to measure the wear intensity during penetration. Jones et al. [6] studied the work and heat during penetration and calculated the mass loss caused by surface melting. It is clear that many physical processes participate in the penetration process, leading to projectile mass loss. He et al. [7] combined several models and provided a formula with seven variables to estimate the mass loss. ese investigations were conducted on the condition that the impact velocity is below the transition velocity of semihydrodynamics, which is about 1000-1300 m/s. In this region, the DOP increases with impact velocity, though there is mass loss during penetration, and the projectiles maintain their primary forms after penetration. However, at hypervelocities, the dramatic decrease in DOP along with intense abrasion of projectiles will occur. It is evident that more complex physical processes occur during hypervelocity penetration, resulting in considerable difficulties in establishing accurate models to describe the process of mass loss. For engineering applications, a solution is to develop a semitheoretical model with unknown coefficients and to determine the coefficients by fitting experimental data.
In this paper, we analyze the penetration process of a cylindrical long rod projectile into a semi-infinite rock target and establish an equation of motion of the projectile. By solving the equation, we derive a formula for intense mass loss with one unknown coefficient. We further obtain the equations for DOP from low velocities to hypervelocities. Indepth analyses of the coefficients are carried out, and their physical meanings are clarified.

Dynamic Behaviors of Projectile and
Target during Penetration Figure 1 depicts the change of DOP with the impact velocity [3] (h is the DOP, L is the total length of the projectile, and υ 0 is the impact velocity). As shown in the figure, when the impact velocity is low, the DOP increases until the velocity reaches a critical value and then decreases dramatically with the velocity. After reaching the minimum, the DOP increases again with the velocity increasing. e change of the DOP is closely related to the states of projectile and target. erefore, it is necessary to understand the behaviors of projectile and target during penetration under different impact velocities.

Dynamic Behaviors of the Projectile.
With increasing the impact velocity, the interactions between projectile and target become more intense, and the behaviors of the projectile can be divided into four states as follows [3]: (a) Rigid State. When the impact velocity is relatively low, the deformation and mass loss of the projectile are negligible, and the projectile can be regarded as a rigid body. (b) Deformation State. When the pressure on the interface between projectile and target reaches the dynamic yield stress of the projectile, the projectile nose turns to the plastic state. Melting and mass loss also occur within a thin layer on the surface of the projectile nose. ese factors blunt the projectile nose and slightly weaken the penetration efficiency of the projectile. (c) Erosion State. With increasing the impact velocity, the interactions on the interface between projectile and target become more intense. e scope of yielding, melting, and mass loss of the projectile expands, leading to the failure of the structure of the projectile. In this state, the terminal DOP decreases with the impact velocity increasing. (d) Hydrodynamic State. When the impact velocity further increases, the projectile as a whole enters the plastic flow state and can be regarded as a steadystate fluid. e projectile mass is entirely lost at the end of the penetration.

Dynamic Behaviors of the Target.
With the increase in the impact velocity, the pressure on the target side of the interface also increases. e stress-strain state of the target in the near-penetration region can be seen as the one-dimensional strain state, and the induced stress waves are onedimensional plane waves [8]. Metal targets turn to plastic flow at the elastic limit. Concrete and rock targets, however, turn to a hardening state at the elastic limit, and the hardening is caused due to internal friction [9]. e target with internal friction can be modeled by a solid that consists of small spheres with cohesion and friction. e "spheres" here refer to the groups of crystals with strong mutual bonds in real rocks. Since the target is in the state of one-dimensional strain under strong impact, we can consider a model solid in a cylinder with rigid walls, where compression occurs only along the cylinder axis. With the increase in compression, the solid undergoes three states in sequence: (a) Elastic State. When the axial compression is small, friction is great, and cohesion is not broken. e deformation of the target obeys Hooke's law. (b) Internal Friction State. When the axial compression reaches the elastic limit, cohesion is broken, and spheres slide on each other. Friction in this state is not negligible, and, therefore, penetration resistance of the projectile contains the term of internal friction. (c) Hydrodynamic State. In this state, friction is negligible compared with great axial compression. e target can be considered as plastic fluid. e hydrodynamic term predominates in penetration resistance.
As we can see from the above analysis, both the projectile and the target undergo complex solid and fluid dynamic processes at hypervelocities. e terminal DOP is the result of the combination of various factors, which are not only involved in complex physical processes, but also affected by the randomness caused by the material inhomogeneity. It is difficult to establish an accurate model to predict the DOP for this condition, and it is unnecessary to do so for engineering applications. e appropriate approach is to analyze the factor that dominates the DOP and to establish a semitheoretical model.

Mechanisms of the Decrease in DOP
3.1. Influence of Projectile Yielding. Numerous experiments showed that when impact velocity reaches a critical value, the DOP does not increase with the velocity but decreases instead [1-3, 10, 11]. It is generally acknowledged that metal turns to plastic flow at the elastic limit. In this connection, the decrease in DOP may be due to the projectile yielding. If so, we can conveniently obtain the critical value of the impact velocity and the maximum DOP before decreasing.
We assume that the elastic limit of the projectile is larger than that of the target, and thus, the target turns to plastic before the projectile. When the projectile is still elastic, the pressure on the interface at the instant of impact is where ρ p and ρ t are, respectively, densities of the projectile and the target, D t is the shock wave velocity of the target, c p is the elastic longitudinal wave velocity of the projectile, and υ L and υ t are, respectively, particle velocities of the projectile and the target on the interface. e shock wave velocity D t is usually written as where a 1 and a 2 are coefficients related to materials and can be determined by fitting experimental data. Shemyakin [9] investigated the behavior of rock under dynamic loadings and concluded that the wave velocity in the vicinity of the loading source is approximately equal to elastic wave velocity, i.e. D t ≈ c t , where c t is the elastic longitudinal wave velocity of the target. Assuming that the initial particle velocity of the target equals the impact velocity, we can give the condition where the projectile yields, i.e., where υ y is the minimum impact velocity at which the projectile yields and σ yp is the dynamic compression strength of the projectile. Whiffin [12] conducted Taylor impact tests with mild steel projectiles of different sizes and confirmed that the dynamic compression strength of projectiles is constant (see Figure 2). Besides, Whiffin [12] obtained the relationships between the static and dynamic compression strength of steel and alloy, i.e., for steel and σ y σ s � 6.32 − 1.89lgσ s , for aluminum alloy, where σ s (MPa) and σ y (MPa) are static and dynamic compression strengths, respectively. We calculate υ y with different experimental data, and the results are listed in Table 1, where σ sp is the static compression strength of the projectile, and υ m is the critical velocity of decrease in DOP.
As shown in Table 1, velocities at which projectiles yield are much smaller than those at which DOPs decrease, indicating that projectiles have yielded before DOP decreasing. It is also worth noting that the critical velocities are nonlinearly related to the strength of the projectile and target. e analysis above confirms that the decrease in DOP is due to the combination of different factors.

Dominant Mechanism of the Decrease in DOP.
Existing investigations confirmed that blunting and mass loss of projectiles weaken the penetration efficiency of projectiles [4,7,[13][14][15][16][17], provided that impact velocities are less than the critical velocities. In this subsection, we analyze the influences of blunting and mass loss at hypervelocities, aiming to figure out the dominant mechanism of the decrease in DOP. Figure 3 depicts the variation of caliber-radius-head (CRH) and DOP with the impact velocity, where f c is the axial compression strength of targets. It is evident that the CRH value of the posttest projectiles decreases with the impact velocity and approaches a limit of 0.5, which represents a hemispherical projectile nose. Chen et al. [19] indicated that the limit of CRH defines a theoretical limit of projectile mass loss. As shown in Figure 3, DOP keeps increasing with the velocity after CRH reaches its limit. We can thus infer that the blunting of the projectile nose contributes little to the decrease in DOP. Figure 4 shows the variation of the ratio of posttest projectile mass with the impact velocity. As shown in the figure, with increasing the impact velocity, the residual projectile mass decreases slowly at first and then drops  rapidly at a critical value of the velocity. In Figure 4(a), the ratio of residual mass decreases from 90% to 20% when the impact velocity increases from 1600 m/s to 1800 m/s. In Figure 4(b), the ratio of residual mass decreases from 42% to nearly 0 when the impact velocity increases from 2860 m/s to 3080 m/s. It is worth noting that the velocity region in which DOP decreases is consistent with that in which the residual mass decreases dramatically (the region is stressed by a    hatched pattern in the figure). is consistency indicates that the decrease in DOP is due to the severe mass loss of the projectile. Figure 5 shows six posttest radiographs of targets and projectiles from Reference [1], and the corresponding DOPs are depicted in Figure 6 by points A-F, respectively. e radiographs with υ 0 � 781 m/s and υ 0 � 932 m/s show slight bending and blunting of the projectiles. Points A and B in Figure 6 indicate DOP increasing in this velocity region. When the velocity increases to 1037 m/s and 1193 m/s, large bending and shortening of the projectiles occur, leading to a dramatic decrease in DOP, as shown by points C and D in Figure 6. e surface of the projectile is extremely rough at υ 0 � 1193 m/s, which implies severe abrasion and mass loss of the projectile. At the velocities of 1802 m/s and 3075 m/s, abrasion and mass loss are more intense, the projectile bodies are broken, and the DOP increases again with the velocity increasing.
Recalling the dynamic behaviors of the projectile discussed in Subsection 2.1, we can now analyze the mechanics of DOP in different velocity ranges. In the rigid and deformation states, although yielding, blunting, and slight mass loss of the projectile have occurred, DOP still increases with the velocity. is is because the increase in the impact velocity compensates for the decrease in DOP caused by mass loss and blunting of the projectile. In this region, the projectile can be seen as a rigid body, since the DOP increases linearly with the velocity. With increasing the impact velocity, the scopes of abrasion, yielding, and melting expand. At a critical velocity, the projectile loses the capacity to maintain its primary form, and severe mass loss occurs. In this state, i.e., abrasion state, the increase in the impact velocity can no longer compensate for the decrease in DOP caused by mass loss; thus, DOP decreases dramatically with the velocity increasing. With a further increase in the velocity, the projectile turns to the hydrodynamic state and can be regarded as a steady-state flow. DOP increases again with the velocity increasing, because the residual mass is so small that the effect of mass loss is little in this region.

Theoretical Model of Mass Loss
To analyze the motion of projectiles in the erosion stage, we study the process of a projectile penetrating vertically into a semi-infinite target. As stated in Subsection 3.2, the shape of the projectile nose has changed to a hemisphere when severe mass loss happens. erefore, the original nose shape has little effect on the DOP in this stage. In this connection, we assume that a cylindrical projectile impacts the target orthogonally with the initial velocity υ 0 , taking the initial static target as the reference frame. During the penetration, the instant velocity of the projectile tail is υ p and the instant particle velocity of the target on the interface is υ t , as shown in Figure 7.
e particle velocity of the projectile nose relative to the projectile tail is On the interface of projectile and target, considering that shock wave velocities approximately equal to elastic wave velocities, we have [21] ρ p c p υ L � ρ t c t υ t .
Combining equations (6) and (7) yields e resistance force against penetration can be expressed as [20] where α s and β s are coefficients, and σ c is resistance force per unit area on the cross-section of the projectile. At medium and high velocities, we can prove that β s υ t ≫ α s ; thus, α s is negligible.
When the initial impact velocity, υ 0 , exceeds the critical velocity, υ m , the projectile enters the erosion state, where the equation of motion of the projectile is where A 0 is the cross-section area of the projectile, and l is the instant length of the entire residual projectile during the penetration. In fact, the particle velocity υ L equals the rate of projectile shortening, i.e., dl/dt � −υ L .Equation (10) can thus be written as Substituting equation (8) into equation (11) and integrating both sides yield where C is the undetermined coefficient, Taking into account the initial condition that υ p | l�L � υ 0 , we have It follows from equation (13) that the instant mass of the projectile, m, is provided that l/L � m/m 0 , where m 0 is the initial mass of the projectile. Equation (14) can be recast as where α � (ρ p /β s )(Z 2 /Z 1 )υ m . e projectile turns to the rigid state when υ p decreases to υ m , and thus, the residual mass of the projectile is It follows from equations (15) and (16) that α is the rate of mass loss when υ 0 > υ m . In the above solution procedure, α is determined by the penetration resistance and strengths of projectile and target. As we have known, however, there are at least six factors that have effects on mass loss. erefore, α can be regarded as an unknown coefficient that is determined by fitting experimental data.

DOP of Rigid Penetration.
When υ 0 ≤ υ m , the mass loss has little effect on DOP, and the projectile can be seen as a rigid body. For a rigid projectile, υ t � υ p , and thus, the equation of motion of the projectile can be expressed as where h ′ is the instant DOP during the penetration process. Substituting equation (9) into equation (17) yields the formula for DOP of rigid projectile: e coefficients α s and β s are determined as follows [20]: where n � 1.6-1.8, ε 0 is the limit of the shear strain of the target, τ s is the shear strength of the target, G is the shear modulus of the target, r 0 is the projectile radius, K c is the fracture toughness of the projectile, and ψ is the CRH value. For rock targets, β s υ 0 ≫ α s when υ 0 > 500 m/s, and thus, α s is negligible [3,20]. en, equation (18) can be simplified to

DOP of Erosion Penetration.
When the impact velocity, υ 0 , exceeds the critical velocity, υ m , the projectile turns to the erosion state, where projectile mass and DOP decrease dramatically. Because the instant velocity, υ p , keeps decreasing during the penetration process, the DOP consists of two parts, i.e., the erosion part when υ p > υ m and the rigid part when υ p ≤ υ m . For υ p > υ m , the equation of motion of the projectile can be expressed as Substituting equation (15) for the projectile mass in equation (21) yields e velocity decreases from υ 0 to υ m while DOP increases from 0 to h s1 ; thus, the DOP of the erosion stage can be obtained by integrating equation (21), i.e., When υ p decreases to υ m , the residual projectile keeps penetrating as a rigid body. is part is equivalent to the rigid penetration with the initial impact velocity υ m and the projectile mass m res . e DOP of this part can be obtained by substituting equation (16) for the projectile mass in equation (19), i.e., e terminal DOP is

Shock and Vibration
where λ c can be termed as "decrease coefficient." h s has the same form as equation (20). When υ 0 > υ m , λ c < 1, implying that DOP is less than that in the rigid state at the same impact velocity. In this connection, we can examine the validity of equation (16) by comparing theoretical and experimental results of DOP.

DOP of Hydrodynamic Penetration.
In the hydrodynamic state, the behaviors of projectile and target can be described as steady-state fluids, and the modified Bernoulli equation applies, i.e., [3] where H is the dynamic hardness of the target, and κ is the coefficient describing the state of the target. κ � 1 stands for the ideal liquid state of the target. e relationship between κ and the impact velocity is determined by Boltzmann's function [3]: , and ] is Poisson's ratio of the target. e DOP in the hydrodynamic state can be derived from equation (26), i.e., where λ � ���� � ρ p /ρ t and θ � ��������������� κ + (1 − κ/λ 2 )/M 2 * . We can see that θ ⟶ � κ √ as M * , i.e., υ 0 , increases. Besides, it follows from equation (27) that κ ⟶ 1 as υ 0 increases. Equation (28) can thus be simplified into Equation (29) is the DOP of ideal jet flow. e projectile length L is equivalent to the length of the jet flow.
In equation (28), the whole projectile turns to the hydrodynamic state. As shown in Figures 5(a) and 5(b), however, the projectile, though a large proportion of which is lost, still remains at the end of hypervelocity penetration. In this connection, there is no clear boundary between the erosion state and the hydrodynamic state. To obtain the continuous calculation formula for DOP, we assume that the erosion part of the projectile keeps penetrating in the form of fluid.
e equivalent jet length of the erosion part is Substituting equation (30) into equation (28) yields the DOP of the jet flow, i.e., Recalling equation (25), the total DOP when υ 0 > υ m is Equation (32) is valid for conditions, where impact velocities are greater than the critical velocity. e only unknown coefficient α can be determined by fitting experimental data. When the impact velocity is relatively low, h s dominates the terminal DOP; when the impact velocity is high, however, h f predominates.

Analysis of the Penetration Formulas.
If we rewrite the initial projectile mass as where ϑ is the coefficient related to the geometry of the projectile, then the formulas of h s1 , h s2 , and h f can be expressed in normalized forms that are independent of projectile size, i.e., It follows from equation (16) that e left-hand sides of equation (37) represent the proportions of residual mass and erosion mass, respectively. Equations (33)-(35) can be recast as where ϕ � 1/α. We can see that equations (38)  In general, one needs numerous experimental results to determine α and υ m by data fitting. However, we find that, in theory, one set of data is sufficient to determine all coefficients.
It follows from equation (37) that Substituting equation (41)  All parameters in equation (42) are either known or determined by one experiment at hypervelocity. In practice, one usually needs to conduct numerous experiments in a wide velocity range to determine υ m . With the help of equation (42), however, one can estimate υ m by several experiments at arbitrary hypervelocities.

Comparison with Experimental Results
Li et al. [3] conducted penetration experiments in a wide range of velocities. e experiments were divided into two stages. In the first stage, projectiles with a diameter of 10.8 mm were ejected towards targets with impact velocities between 1200 and 1810 m/s. In the second stage, projectiles with a diameter of 7.2 mm were ejected in the velocity range of 1830-4200 m/s. Projectiles of the two stages were ogivenose steel rods with a CRH value of 3.0 and a length-diameter ratio of 5. Targets were made of granite and were of the same size in two stages. e photos of posttest projectiles are shown in Figure 8, and the experimental and theoretical DOPs are shown in Figure 9(a) Comparing the photos of projectiles and the curves of DOPs, it is clear that DOPs decrease when dramatic mass loss of projectiles occurs.
Qian et al. [2] conducted penetration experiments with tungsten alloy projectiles and concrete targets. e impact velocities range from 1820 to 3660 m/s. e projectiles were flat-nose rods, whose diameter was 3.45 mm, and length was 10.5 mm. e experimental and theoretical DOPs are shown in Figure 9(b).
As for metal targets, 6061-T6511 aluminum targets were used by Piekutowski et al. [22]. VAR 4340, 3-CRH-nosed, steel rods were launched at velocities between 550 and 3000 m/s. e total length of projectiles was 71.1 mm, and the diameter was 7.11 mm. e results for experimental and theoretical DOPs are shown in Figure 9(c).
Forrestal and Piekutowski [1] conducted experiments with the same materials as in Piekutowski et al. [22]. However, the nose of projectiles was spherical rather than ogival. Figure 9(d) shows the experimental and theoretical DOPs of spherical-nose projectiles.
In general, we can see that theoretical results are in good agreement with experimental data. When υ 0 > υ m , the calculation formula describes well the transition of the projectile from the erosion state to the hydrodynamic state. Comparing Figure 9(c) with Figure 9(d), we can infer that the nose shape of projectiles has effects on the value of υ m . Blunter nose means larger resistance force acting on projectiles, which naturally results in more dramatic interactions and, consequently, smaller υ m for DOP decreasing. e calculated DOPs are kind of smaller than experimental DOPs in Figure 9(d), when the velocity is in the hydrodynamic region. is may be due to the decrease in density of the metal target under high temperature induced by severe interaction.
We can also validate equation (42) against experimental data in Figure 4(a). e initial projectile mass is 32.45 g, and the results are listed in Table 2. e mean value of calculated υ m is 1885.5 m/s, which is quite close to the experimental result, 1600 m/s.

Conclusions
A decrease in DOP as impact velocity increases has been observed in numerous penetration experiments. e decrease in DOP occurs at a critical velocity and, in general, is accompanied by a dramatic erosion of projectile mass, as well as the yielding and blunting of projectiles. To investigate the mechanism of DOP decreasing, we analyzed the relationship between the critical velocity and various factors.
We found that, before reaching the critical velocity, the yielding of projectiles has occurred, and the nose shape of projectiles has come to constant geometry, i.e., sphericity. However, the velocity of dramatic mass loss is nearly equal to the critical velocity of DOP decreasing. us, we can infer that the decrease in DOP is the result of the dramatic mass loss of projectiles. e projectile mass loss is the result of multiple factors, including yielding, melting, and wearing of the projectile. When the impact velocity is relatively low, the mass loss is too slight to reverse the DOP, and the DOP increases as the impact velocity increases. At a critical velocity, however, the severe interaction between the projectile and target violates the stability of the projectile body, leading to dramatic mass loss of the projectile and, consequently, the decrease in DOP with impact velocity increasing.
We established a semitheoretical model for mass loss of the projectile and obtained a power law of mass decreasing. en, a continuous formula for DOP at hypervelocity was derived considering the mass loss of the projectile. We compared the theoretical DOP with experimental results of steel projectiles penetrating different targets, including rock, concrete, and metal. e calculated DOP agrees with experimental results and describes well the transition of the projectile from the erosion state to the hydrodynamic state.
At last, the formula for the critical velocity υ m was obtained as a by-product of the solving procedure for DOP. One can predict the range of υ m with several experimental results at hypervelocities.