Rupture of thin ductile tubes by oblique impact of blunt missiles: experiments
Introduction
A measure of the ability of a thin metal tube to withstand missile impact is the ballistic limit (the minimum speed for perforation) or the speed for rupture (at which a fracture first passes through the entire thickness of tube wall). Previous experimental investigations of deformation and perforation of metal tubes have been restricted to impact by missiles (flat and domed noses) striking at a normal angle of obliquity 1, 2, 3, 4, 5. Those tests used mild steel tubes. The ratio between the missile diameter and the tube thickness, Dm/h was in the range 3–10. The ballistic limit for tubes that was established in those experiments was in the range from 20 m s-1 to over 300 m s-1. At the ballistic limit the tube ruptured; i.e. a crack extended through the wall.
While there have been experimental investigations of oblique impact on plates, no experimental investigation about oblique impact on tubes has been published. A guideline for the assessment of hazards to pipes from free-flying missiles was developed by the Impact Technology Department (ITD) at AEE Winfrith [6]. This guideline assumed that for oblique impact, the target thickness could be increased to the “shot-line” length to account for the increased path of the missile through the target; hence normal impact was thought to be a worst case scenario for safety calculations [6]. However, if a tube is thin (e.g. Dm/h>4) and rupture of the tube is taken as the critical condition for failure, this assumption has little experimental basis.
Oblique impact on thin aluminium and steel plates was investigated by Virostek et al. [7]. They observed that for a thin metal plate hit at an angle of incidence by a hemispherical-nosed missile, sliding of the missile nose along the target surface occurred prior to perforation. These experiments showed that for a hemispherical nose, the minimum perforation velocity for oblique impact was larger than that for normal impact.
The present investigation examines oblique impact and rupture of thin ductile tubes by non-deforming missiles. A series of experimental tests is reported in which a thin mild steel tube was struck at angles of obliquity up to 60° from normal by missiles with nose shapes described as flat, domed (the ratio of missile radius to nose radius is Rm/Rn=0.5) or hemispherical. The missiles were moderately large in comparison with the wall thickness of the tube and all the missiles had the same diameter and mass. The impact speed for rupture of the tubes was within the range from 100–320 m s-1. The local failure mechanism for rupture for each nose shape and various angles of obliquity are identified.
Section snippets
Apparatus
A compressed gas gun was used to propel the missiles. The single-stage high-pressure nitrogen gas gun was designed and built in the Department of Engineering, University of Cambridge. It is 3.5 m long, has a 28.7 mm internal diameter smooth bore barrel that is connected via a breech to a high pressure gas cylinder. Sabots made of Delron plastic carried the cylindrical missile down the barrel, thus enabling different missile sizes and shapes to be used. The sabot was stripped off the missile by a
Variation of speed for rupture with angle of obliquity
For tubes hit at an angle of obliquity by missiles with different nose shapes, the impact speed for rupture, Vr=1/2(Vh+Vl) is summarized in Table 2. The impact speed for rupture, Vr, is plotted against oblique angle of incidence in the axial plane in Fig. 2. Each datum point in Fig. 2 refers to a separate test.
The solid line in Fig. 2 represents the impact speed for rupture by a flat-nosed missile at angles of obliquity in the axial plane. It can be seen that for a flat-nosed missile, the speed
Factors affecting local deformation and rupture
It is noted that no deformation of the missiles was observed in any test so the missiles are considered to be rigid. When a tube is hit by a rigid missile, the segment of tube within the contact region must conform to the shape of the missile nose. Deformation in the contact patch may come in the form of surface plastic flow or shearing, bending and stretching of the tube wall or a combination of these effects. This combination depends on the missile nose shape as well as the angle of
Conclusions
- 1.
Rupture of a thin mild steel tube due to impact by a moderate size missile is the culmination of a process of plastic deformation. The initial rupture through the wall occurs either within or near the periphery of the region of local deformation; i.e. the crater. The failure type and the speed for rupture are strongly dependent on both missile nose shape and impact angle.
- 2.
For normal impact, the failure mode for rupture of a thin tube is dependent on the nose shape of the missile. For a
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2021, European Journal of Mechanics, A/SolidsCitation Excerpt :Shell formations used in military structures such as; shield, fuselage, boat hulls, tanks, containers, etc. are easily susceptible to ballistic loading and their impact behavior remains to be thoroughly understood. Research studies conducted on shell structures (Ma and Stronge, 1985; Palomby and Stronge, 1988; Zhang and Stronge, 1996, 1998; Colombo and Giglio, 2007; Manes et al., 2013; Gilioli et al., 2015; Fossati et al., 2020), explored the influence of material characteristics, target configurations and failure mechanisms involved in the perforation process during ballistic impact. Both experimental and numerical investigations have been carried out to ascertain the dependency of geometrical and material properties of projectiles and targets on the ballistic resistance of targets (Awerbuch and Bodner, 1977; Goldsmith and Finnegan, 1986; Gupta and Madhu, 1992, 1997; Madhu et al., 2003; Dey et al., 2007; Teng et al., 2007; Zhou and Stronge, 2008; Deng et al., 2012, 2013; Iqbal et al., 2010, 2015).
Effect of eccentricity and obliquity on the ballistic performance and energy dissipation of hemispherical shell subjected to ogive nosed
2021, Thin-Walled StructuresCitation Excerpt :Results revealed that obliquity has a significant effect on the impact speed of the rupture and it was decreased with an increase in the angle of incidence up to 50°, however with further increase in angle rupture speed increased. Zhang and Stronge [30] carried out a similar study for a thin mild steel tube and found that at 45° the speed of the rupture of the tube was minimum and it was 41% less than normal impact. When blunt projectile impacted on the thin tube at a large angle of obliquity 45° or 60° it was failed due to cutting and plugging depend on the nose shape and angle of obliquity.
Ballistic performance of monolithic and double layered thin-metallic hemispherical shells at normal and oblique impact
2021, Thin-Walled StructuresCitation Excerpt :It was found that at normal impact, the perforation process involved both shearing and global deformation in the tubes. Zhang and Stronge [4] described ballistic behavior of thin mild steel tubes subjected to oblique impacts with blunt, domed and hemispherical projectiles. The rupture velocity for the targets at higher impact angles was found to be decreased, when impacted with blunt projectiles, whereas in other cases the ballistic limit was found to be increased.
Energy dissipation in thin metallic shells under projectile impact
2016, European Journal of Mechanics, A/SolidsCitation Excerpt :The model was further employed (Zhang, 1998) for predicting minimum speed for rupture in 3.46 mm thick mild steel tubes against 41 g flat nosed projectile at varying angles of obliquity. It reproduced the same with an error of 13% for 0°, 26% for 60° and almost exactly for 45° obliquity in comparison to the actual results (Zhang and Stronge, 1998). It was observed that the minimum speed for rupture decreased with increasing angle of incidence until 45° and then it increased with further increase in the angle; in confirmation to experimental results.
An experimental-numerical investigation on aluminium tubes subjected to ballistic impact with soft core 7.62 ball projectiles
2013, Thin-Walled StructuresCitation Excerpt :Several of these works increase also the understanding of the behaviour of Al 6061 alloy, but they fail to take into account the use of a real bullet and they do not completely address the issue of the target shape under investigation, i.e. a tubular profile. Zhang and Stronge [14], and Zeinoddini et al. [15] addressed issues similar to the work presented in this article. In the first paper, analysing impacted blunt projectiles on mild steel tubes at various obliquity angles, their influence on the type of the rupture produced was studied.
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Present address: Taiyuan University of Technology, Taiyun, Shanxi 030024, People’s Republic of China