ASSESSMENT OF THE WEAR OF THE WORKING SURFACE OF THE REAKTIVE THIMBLE

ABSTRACT

The article considers the issue of assessing the condition of the inner surface of the reactive thimble during operation. The main factors affecting the working capacity of the inner surface of the reactive thimble during operation are analyzed. It was determined that the main reason for the loss of workability of the inner surface of the reactive thimble is the formation of various types of wear as a result of the temperature affecting the working surface. Analyzes were performed to assess the thermoerosive wear of the inner surface of the reactive thimble.

АННОТАЦИЯ

В статье рассмотрен вопрос оценки состояния внутренней поверхности реактивного барабана в процессе эксплуатации. Проанализированы основные факторы, влияющие на работоспособность внутренней поверхности реактивного наконечника в процессе эксплуатации. Определено, что основной причиной потери работоспособности внутренней поверхности реактивного наконечника является образование различных видов износа в результате температурного воздействия на рабочую поверхность. Были проведены анализы для оценки термоэрозионного износа внутренней поверхности реактивного наконечник.

Keywords: reactive thimble, wear of the thimble, surface layer, thermal condition.

Ключевые слова: реактивный наконечник, износ наконечник, поверхностный слой, тепловое состояние.

Introduction

In modern times, grenade launchers are accompanied by intense heating of the working surface of the reactive thimble during firing. If the temperature limit reached during firing is higher than the temperature resistance limit of the material, intensive wear of the working surface of the material will occur. This can lead to a violation of the direction of flight of the grenade during operation.

During the analysis of wear, the quality indicators of reactive thimble get different values. The working surface of the reactive thimble is mainly thermoplastic and thermoerosive wear.

The type of wear depends on a number of factors, of which the temperature of the reactive thimble surface before the next shot is crucial.

Thermoplastic wear is characterized by the existence of a critical firing mode, that is, such a combination of rate of fire and queue length (limit), in which the survivability of the barrel is exhausted almost instantly for a small number of shots (within one queue).

Since in critical mode the reactive thimble fails in one queue, which is unacceptable under operating conditions, therefore, the task of studying wear is reduced to determining the maximum length of the queue. This boundary should reflect that exceeding this mode will lead to the failure of the reactive thimble and the inability to conduct effective shooting [1].

However, it must be remembered that the maximum queue length should not be less than the required one, which is determined by the conditions of the effectiveness of the combat use of weapons.

Main part

For weapons with a tense firing mode, it is important to determine the condition under which intense thermoplastic wear of the reactive thimble bore will be practically absent. This firing mode is called normal. For a normal queue, it is also important to know the limit (the length of a continuous queue), exceeding which is impractical, since going beyond this limit leads to a sharp drop in the survivability of the reactive thimble. This limit lies 30... 45% below the limit limit and is determined by the number of shots fired from the weapon by the time the temperature in the surface layer of the cut, which is 20... 30% of the profile height, reaches such a value at which the maximum allowable stress of crushing the reactive thimble material is equal to the pressure of the belt. In this case, we consider a cross-section located at a distance of 2-3 calibers from the beginning of the initial part of the reactive thimble bore.

In case of thermoplastic wear, the survivability of the reactive thimble of  weapons is estimated by the ovality of the holes in the shields installed at a distance of 200 mm. It should not exceed 1.25 caliber [2].

Under normal firing conditions, the barrel will be considered worn out if the initial velocity of the projectile has decreased by 5-10%. The process of barrel wear in this case will be strongly associated with the "heat" of the barrel, which is called thermoerosive. It occurs in a certain sequence and is accompanied by the appearance of a grid of microcracks, which are then eroded into deep longitudinal grooves by the flow of hot gas and the leading belt of the projectile, followed by the staining of the rifling fields and chemical erosion of the metal of the barrel. This type of wear is called thermal erosion. Data analysis revealed three main mechanisms of erosion of gun barrels: mechanical, thermal and chemical. Mechanical erosion causes the destruction of the surface layer of the metal of the barrel under the impact of solid particles moving in the gas stream and the mechanical interaction of the leading elements of the projectile against the wall of the bore. It plays a major role in guns with low gas temperature and low ballistics. Thermal erosion is the wear of the bore surface under the influence of high temperatures. Chemical erosion is a change in the structure of the bore surface as a result of chemical interaction with the components of the gas stream, mainly CO, H₂O, N₂ and H₃, leading to the diffusion of carbon, nitrogen, hydrogen and other gases deep into the metal or the formation of low-temperature oxides. As a result of this interaction, the thermophysical and strength characteristics of the metal change, as well as the melting point of steel in the surface layer decreases.

In this case, thermal erosion wear can be represented as:

Where T₁- burning temperature of gunpowder; E-active energy of the process of wearing; A,K₁ və K₂-emprik əmsallar; T₀,T_m- initial and maximum temperature of the inner surface of the reactive thimble; λ,λ_x- thermal conductivity of metal layer and gunpowder gas; a- temperature conductivity of the metal layer; µ- gas viscosity; d-caliber; V- speed of grenade; ω-mass of gunpowder firing; W- diamteric wearing.

The semiempirical dependence of the general type of thermoerosive wear [3] relates the value of diametrical wear to the value of the critical temperature, the initial temperature state of the reactive thimble for the next shot, the thermal conductivity of the metal, the burning temperature of the powder, the caliber and muzzle velocity of the projectile grenade.

Formulas (1) are valid for any cross-section along the length of the bore in the temperature range Т_c < Т < Т_t. It is recommended to take the critical value of the Т_c temperature for gun steels above 850 K [3].

The rate of wear of the bore wall is more complex, especially in the range from the beginning of the caliber part to the location of the point of maximum wear, and depends on the opening time of the sections for heating by the gas flow during their passage by the bottom of the projectile and local values of instantaneous values of heat transfer coefficients and temperature factor.

A characteristic feature of the considered model of thermal erosion wear is the constancy of the temperature of the channel surface at which wear begins, as well as the presence of wear of the channel from the first shot.

Thus, analyzing the above, we can distinguish the following main types of reactive thimble wear of modern grenade weapons.

Picture 2. Model of distribution of temperature along the wall of reactive thimble

1. Thermal erosion wear. With sudden heating during the shot and subsequent rapid cooling, small cracks form on the surface of the channel, which flare up as a result of the subsequent thermal, chemical action of gases. Studies [3] show that the greatest thermal erosion wear is observed in this case in the breech at a section of 0.1 - 0.25 of the length of the rifled part. In this case, it is the thermal erosion wear of the initial section of the barrel that leads to the loss of the initial velocity of the projectile. For weapons using part of the gases discharged from the barrel for the operation of automation, survivability may be lost due to the wear of the exhaust vents, as this leads to a malfunction of the gun's automation or failure of the reactive thimble[1].

2. Thermoplastic wear. With intensive heating of the reactive thimble and intense firing mode, the barrel material at a depth of 1 – 1.5 thickness of the cut loses its strength properties. As a result of embedding or pressure of the copper lead belt on the combat face, the fields of the rifling are crumpled.

Thermoplastic wear is characterized by the failure of the projectile from the rifling and the violation of the correct stabilized flight of the projectile. With thermoplastic wear, the initial section of the reactive thimble, the area near the muzzle, wears out the fastest. At the beginning of the rifling, the projectile has the maximum acceleration associated with the maximum impact on the combat faces of the rifling, and the pressure and temperature at this moment reach a maximum.

3. Joint thermoplastic and thermal erosion wear. In automatic weapons, depending on the pace and length of the queue, there may be a zone between thermal erosion and thermoplastic wear, in which both wear is observed simultaneously. The transition criterion is the temperature of 800 K. Depending on the firing mode and ballistic characteristics, one or another type of barrel wear may prevail in this zone. In turn, as the rate of firing increases, intensive heating occurs, primarily in the working area of the rifling, due to the fact that the firing time is shortened, and the heat does not have time to spread to the entire thickness of the reactive thimble wall, concentrating in a thin inner layer. However, due to the increase in the temperature of the inner layer, the total amount of heat entering the wall decreases, and the wall temperature tends to the limit value.

Thus, based on the analysis of the factors affecting the wear of the reactive thimble, it was found that for a certain complex "gun-cartridge-projectile" with thermoplastic and erosive wear, the survivability of the reactive thimble is mainly determined by the level of heating during firing.

Reference:

1. S. Procházka and N. D. Ninh, Differences in Barrel Chamber and Muzzle Deformation during shot, Advances in Military Technology, Vol. 7, No. 2, December 2012.
2. Ercande_girmenci, M. HüsnüDirikolu (2012), A thermochemical approach for the determination of convective heat transfer coefficients in a gun barrel, Applied Thermal Engineering, 37, pg. 275-279.
3. Mehmet AKÇAY and M. Adil YÜKSELEN (2014), Unsteady thermal studies of gun barrels during the interior ballistic cycle with non-homogenous gun barrel material thermal characteristics, Journal of Thermal Science and Technology, 1300-3615.