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Tribology of Rotating Band and Gun Barrel During Engraving Process Under Quasi-Static and Dynamic Loading

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Tribology of Rotating Band and Gun Barrel During Engraving Process Under Quasi-Static and Dynamic Loading Friction 2(4): 330–342 (2014) ISSN 2223-7690 DOI 10.1007/s40544-014-0061-3 CN 10-1237/TH RESEARCH ARTICLE Tribology of rotating band and gun barrel during engraving process under quasi-static and dynamic loading Bin WU*, Jing ZHENG, Qing-tao TIAN, Zhi-qiang ZOU, Xu-hua YU, Kai-shuan ZHANG Army Officer Academy PLA, No. 451 Huangshan Road, Hefei City, Anhui Province, 230031, China Received: 05 February 2014 / Revised: 05 June 2014 / Accepted: 23 July 2014 © The author(s) 2014. This article is published with open access at Springerlink.com Abstract: The engraving process of a projectile rotating band is one of the most basic research aspects in interior ballistics, which has not been thoroughly understood thus far. An understanding of this process is of great importance from the viewpoints of optimal design, manufacturing, use, and maintenance of gun and projectile. In this paper, the interaction of copper and nylon rotating bands with a CrNiMo gun barrel during engraving was studied under quasi-static and dynamic loading conditions. The quasi-static engraving tests were performed on a CSS-88500 electronic universal testing machine (EUTM) and a special gas-gun-based test rig was designed for dynamic impact engraving of the rotating bands. The mechanical behaviors of copper and nylon were investigated under strain rates of 10−3 s−1 and 2 × 103 s−1 using an MTS 810 and a split Hopkinson pressure bar (SHPB), respectively. Morphologies of the worn surfaces and cross-sectional microstructures were observed with scanning electron microscope (SEM) and optical microscope (OM). It was found that large deformation and severe friction occur during engraving. The surface layer is condensed and correlated with a hardness gradient along the depth from the top worn surface. The structure of the rotating band and gun bore, band material, and loading rate have great effects on band engraving. The flow stress-strain of the copper strongly depends on the applied strain rate. It is suggested that strain rate and temperature play significant roles in the deformation mechanism of rotating bands. Keywords: tribology; sliding friction; plastic deformation; rotating band; gun barrel; engraving; strain rate 1 Introduction cone to the muzzle and falls into two general categories: smoothbore and rifled. The former has a completely Ballistics is the study of the firing, flight, and effect of smooth and cylindrical interior surface, and the latter ammunition, which can be broadly divided into four contains lands and grooves machined axially with a distinct phases: interior, intermediate, exterior, and slight twist or helix, causing the projectile to rotate terminal. Interior ballistics deals with the motion of while in the bore. In this paper, rifled gun bore is a projectile that is still in the gun barrel. A gun barrel considered. is a long, slender tube that serves multiple functions. The primary function of a large caliber gun is to It often has three major internal regions: combustion accurately fire projectiles at high velocities towards a chamber, forcing cone, and bore (Fig. 1). The com- target. Above a certain size, semi-fixed rounds are bustion chamber is at the breech-end of the gun barrel impracticable because the entire assembly would be and it receives the complete round of ammunition. too heavy to be carried effectively. Therefore, separate The forcing cone is the tapered portion near the rear ammunition is loaded in this case, i.e., the projectile of the bore, connecting the combustion chamber and and the propelling charges are supplied and loaded the bore coaxially. The bore extends from the forcing separately. The projectile body has three main com- ponents that affect ballistics: bourrelet, rotating band, * Corresponding author: Bin WU. and obturating band (Fig. 2). The external diameter E-mail: [email protected] of the projectile is only slightly smaller than that of Friction 2(4): 330–342 (2014) 331 Fig. 1 Schematic diagram of rifled gun barrel. the projectile in the bore when high angles of elevation are applied. On certain projectiles, the obturating bands are made of plastic to provide forward obturation by preventing the escape of gas pressure from around the projectile. It is essential that the type of rotating band and its material should be selected properly based on the following considerations: (1) The engraving process should not degrade the ballistic properties after the rotating band has been engraved; (2) the band should be easy to affix and remain attached to the projectile body throughout the internal and external ballistic phases; (3) the band should not wear the bore exces- Fig. 2 A projectile with rotating band and obturating band. sively and compensate for normal wear in the bore. Typically, the rotating band is made of commercially the bore, which allows the projectile to slide axially pure copper, copper alloy (gilding metal, bronze, and within the bore. The bourrelet centers the forward brass), cupro-nickel alloy, etc. part of the projectile in the barrel and bears on the Upon preparation for firing a large caliber gun, the lands. The projectiles used in rifled guns of calibers projectile and propellant are loaded into the gun barrel greater than 20 mm have one or more external bands in two separate processes. The projectile is rammed towards the rear that are slightly larger than the into the gun barrel, and the rotating band contacts grooves but smaller than the combustion chamber. the lands and grooves in the forcing cone. When the The functions of a rotating band, also called driving gun is fired, the propellant is ignited by the primer, band, are as follows: (1) act as a rear bourrelet and and the burning propellant produces gaseous products help keep the projectile centered in the bore; (2) prevent of combustion that have very high pressure and the escape of combustion gases past the projectile; temperature. The propellant gases exert a force on (3) provide a repeatable “short-start force” that the base of the projectile, which begins its forward momentarily resists forward projectile motion while motion when the gases develop adequate pressure to the combustion process is just beginning; (4) engage overcome the “short-start force”. The rotating band rifling in the gun bore and transfer the rifling twist to and the obturating band (if present) will be engraved the projectile for creating projectile spin; and (5) retain to the shape of the rifling. 332 Friction 2(4): 330–342 (2014) Continued firing of a gun wears away materials selection process for slider materials such as red brass, from the forcing cone or bore. This happens either Al-bronze, and brass. Lisov investigated the wear because of the hot gases passing over the cone/bore mechanisms of 105- and 155-mm artillery projectiles’ at high velocities and/or by the projectile moving rotating bands theoretically and experimentally using down bore and interacting with the barrel walls. An variable parameters of internal ballistics [7]. Based on excessively worn bore cannot guarantee proper seating the measured strain of the barrel’s exterior surface, of the projectile. Thus, some of the expanding gases Andrews [8] found that high charges generally could escape from around the rotating band, thus degrade the rotating band by reducing the load the resulting in decreased muzzle velocity. The lands may rotating band applies to the 155 mm gun barrel during not engage the rotating band well enough to impart firing. The influence of rotating band construction on proper spin to the projectile. During loading and the gun tube loading was investigated experimentally initial stages of combustion, the rotating band is and numerically by Toivola et al. [9] and Keinänen et forcibly engraved into the rifling and exerts a radial al. [10]. They presented a practical analysis method force on the bore, which can contribute to wear as the for determining the tube inner wall pressure from projectile traverses the thermally damaged gun bore. outer wall strain measurements, and the generated The motion of a projectile in the gun barrel can be numerical results were in agreement with the measured divided into two successive stages. The first stage is firing results. Eleiche et al. [11] proposed the use of the engraving of a rotating band into the rifling in a glass-fiber reinforced polyamide as an alternative gun barrel, and the second stage is the projectile’s material for rotating bands and found that PA66 resin acceleration and movement along the gun bore until it with 2% GF reinforcement offers the ideal compromise; exits from the muzzle. The rotating band and the gun the rotating band withstands the associated plastic bore surface (lands and grooves) compose a friction deformation without failure at all sliding speeds. The pair, and the friction between them has different interaction between the projectile driving band and characteristics in each stage. At the beginning of the forcing cone of a weapon barrel in the course of the first stage, the projectile is still initially and is ramming was studied by Balla et al. [12]. It was found accelerated to a velocity of tens of meters per second that the ramming and extraction forces depend within a few microseconds. The projectile’s rotating remarkably on the degree of wear of the forcing cone band undergoes plastic deformation at high strain rates diameter. when it is engraved by the rifling. Thus, the engraving As mentioned above, although there are few process is essentially dynamic. The second stage published studies on the interaction between the features friction and wear under high sliding speeds, rotating band and the gun barrel, insight into the and surface melting is considered as the mechanism engraving process is in great demand in the field of of wear.
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