Impact perforation of aluminium alloy plates

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Abstract

The perforation of aluminium alloy plates subjected to impacts characterised as low-velocity (up to about 20 m/s) and moderate-velocity (20–300 m/s, approximately) are examined in this paper, wherein recent experimental data and some empirical equations have been compared. The threshold velocity for the normal perforation of metal plates focuses on cylindrical projectiles with various shaped impact faces, principally flat. A criterion is suggested in order to distinquish between low-velocity and moderate-velocity impacts.

Several new empirical equations have been developed in this paper. Recommendations are made for the use of particular empirical equations that are suitable for estimating the perforation energy of plates in the two impact velocity regimes which have been examined in this article. There is a paucity of experimental data which hinders the development of empirical equations, though surprisingly accurate predictions for perforation velocities are possible to achieve for some practical problems. Nevertheless, numerical studies are required to remove many of the restrictions on the validity of empirical equations, but these methods require a considerable amount of accurate experimental data on the dynamic material properties and failure criteria.

Introduction

Many articles have been written on the perforation of metal plating caused by the impact of missiles, or projectiles, (e.g., [1], [2], [3], etc.). A recent article [4] has discussed some of this work on the low-velocity (up to about 20 m/s) and moderate-velocity (20–300 m/s, approximately) perforation of steel plates struck by projectiles having cylindrical bodies and various nose shapes. This article discusses the data for aluminium alloy plating struck by rigid projectiles travelling within similar ranges of the initial impact velocities.

It was shown in Reference [4] that an estimate for the ratio between the time taken for a plastic disturbance to travel from the impact site to the plate boundary (th) and the time required for a projectile to perforate through the plate thickness (tp) isth/tp=(Φ/6)(R/H)2,whereΦ=ρVo2/σis Johnson’s damage number and where σ is a mean flow stress. Thus, if th << tp, then the entire plate would participate in the perforation process, i.e., the global deformations of a plate would be important throughout the response. On the other hand, if th >> tp, then the perforation process would be highly localised with insufficient time for global effects to develop. For example, th/tp = 0.016 if R/H = 10 for an aluminium alloy plate with ρ = 2720 kg/m3, σ = 272 MPa and Vo = 10 m/s, which gives Φ=103. This suggests that global effects (e.g., membrane forces) would be an important aspect of the response because they would have developed well before perforation had occurred. If Vo = 100 m/s for the same plate, then th/tp = 1.6, so that Eq. (1) suggests that both local and global effects would be important during the perforation process. At Vo = 1000 m/s, th/tp = 166, which implies that the perforation for this plate is highly localised because there is insufficient time for any global deformations to develop and for the disturbance to be transmitted away from the vicinity of the impact site through the action of the plastic hinge movement. This article is concerned primarily with the range of impact velocities which produce both global and local effects, but not with local effects alone which would introduce temperature effects and other phenomena that are not considered here.

The next two sections discuss the perforation of aluminium alloy plates struck by projectiles travelling at low-initial impact velocities (say up to about 20 m/s). Section 4 discusses the perforation due to moderate-impact velocities (20–300 m/s, approximately). A discussion and conclusions follow in Sections 5 Discussion, 6 Conclusions, respectively.

Section snippets

Low-velocity impact perforation of aluminium alloy plates

Langseth and Larsen [5] reported the results of impact tests on square plates made from two aluminium alloy materials (AA5083-H112 and AA6082-T6) when struck by missiles having blunt impact faces and travelling up to about 22 m/s. It is evident from Fig. 1 that the two sets of data do not collapse onto a single curve in the dimensionless Ωp–ψ space, despite having the same values of the test parameters ψ and S/d. It appears from Table 1 that the only major difference in these tests is the

Comments on the low-velocity impact perforation of aluminium alloy plates

The support conditions for the various plate specimens in the experimental studies reported in Table 1 vary between the different laboratories. Corran et al. [9] did explore the influence of this effect on the impact perforation of clamped circular plates which had a range of flexibilities at the supports related to the clamping pressure. They observed that the perforation velocities, Vo, for 2 mm thick mild steel plates increased about 13 per cent when a fully clamped support condition was

Perforation of aluminium alloy plates subjected to moderate-impact velocities

The experimental results of Crouch et al. [11] for aluminium alloy 2024-T351 circular plates, which are struck by missiles having G = 25g and with higher impact velocities up to 154 m/s, are also shown in Fig. 3. The values of Ωp are similar to the low-velocity data of Grytten et al. [10], but the associated value of S/d = 2.95 is significantly smaller and the impact velocity is much higher.

Iqbal and Gupta [14] have studied the perforation behaviour of aluminium alloy 1100-H12 circular plates

Discussion

Iqbal and Gupta [14] studied the perforation of aluminium alloy 1100-H12 plates struck by blunt, ogive and hemispherically nosed projectiles. The authors observed that hemispherical-nosed projectiles have the largest ballistic limit for the 0.5–3 mm thick monolithic plates, as shown in Fig. 8. The smallest perforation velocities were associated with the ogive-nosed and blunt-nosed missiles. There is a tendency for the ogive-nosed missiles to have smaller values of Ωp for the thinnest aluminium

Conclusions

Generally speaking, the experimental results reveal that flat-faced projectiles travelling at low- or moderate-impact velocities require the smallest impact energy and that hemispherically-tipped projectiles require the greatest energy to perforate aluminium alloy plating. This observation is reflected in the significant attention which has been given by the research community to developing empirical equations for the perforation of plating by flat-faced projectiles.

Eqs. (3), (8), (9) within

Acknowledgements

This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (Grant no.: K20902001780-10E0100-12510).

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