Life cycle and performances progressing of artillery barrels

Abstract

The present researches of advancing the gun barrels and especially for artillery barrels, are dealing with materials other than steel or on combination between materials which are advanced and new innovative process for increasing of the performances and life cycle of all gun calibers. Considerable efforts were made for developing new technique, in addition to the investigation of new materials. In this research, 3-techniques are describes for increasing the mechanical properties of thick-walled cylinders which are subjected to high internal loads due to explosives. In autofrettage process, an elasticplastic state produces at the cylinder wall by loading of the barrel tube with a controlled inner pressure. In elastic process, the tube is unloading, this case does not recover the initial dimensions, therefore; the residual stress and strain stares can be determined. The practical data were obtained from a standard tension test specimen, collected from the D-30 gun barrel made out of an alloy steel. The experimental work was made in Artillery and Mortar Laboratory of Military Engineering College, Baghdad in 2003. A nonlinear constitutive equation was determined in order to establish the stress and strain states in the gun barrel.

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International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol: 11 No: 06 19
I J E N S
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Life Cycle and Performances Progressing Of Artillery Barrels
Dr. Eng. Mohammed J. Barzanjy
Mechanical Dept., College of Engineering, ERBIL, IRAQ
dr_m.barzanjy@yahoo.com
Abstract
The present researches of advancing the gun barrels and especially for artillery barrels, are dealing
with materials other than steel or on combination between materials which are advanced and new innovative
process for increasing of the performances and life cycle of all gun calibers. Considerable efforts were made
for developing new technique, in addition to the investigation of new materials.
In this research, 3–techniques are describes for increasing the mechanical properties of thick–walled
cylinders which are subjected to high internal loads due to explosives. In autofrettage process, an elastic-
plastic state produces at the cylinder wall by loading of the barrel tube with a controlled inner pressure. In
elastic process, the tube is unloading, this case does not recover the initial dimensions, therefore; the residual
stress and strain stares can be determined.
The practical data were obtained from a standard tension test specimen, collected from the D-30 gun
barrel made out of an alloy steel. The experimental work was made in Artillery and Mortar Laboratory of
Military Engineering College, Baghdad in 2003. A nonlinear constitutive equation was determined in order
to establish the stress and strain states in the gun barrel.
Keywords: Gun barrel, Autofrettage process, elastic-plasticity in thick wall cylinder.
Introduction
Autofrettage is an elastic-plastic technique used on thick walled metal tubes or pipes (cylinders) to
improve the durability of the part by creating a compressive residual stress at the bore. In this technique, the
cylinder is subjected to an internal pressure that is high enough to plastically deform the bore of the part but
not so high that it bursts the part is applied to the inside of the tube [1]. The result is that, after the pressure is
removed, the elastic recovery of the outer wall puts the inner wall into compression, providing a residual
compressive stress [2]. The applications are typically heavy walled tubes that experience cyclic loading and
are thus prone to fatigue. The main reason for using an autofrettage technique in cylinders is to increase its
fatigue life. The analysis of residual stresses and deformation in an autofrettaged thick-walled cylinder has
been given by Chen [3] and Franklin and Morrison [4].
Nowadays, in field of elastic-plastic thick-walled cylinders (tubes) deformation achievements', more
technological methods are used. They are being: the hydrostatic method, the mechanical method the ballistic
method [5]. Applications where these methods are used include diesel engine components such as fuel rails
and fuel lines, hydraulic cylinders, oil field components and components used in the production of urea. The
major application is in military components such as gun barrels.
Present paper present some theoretical guidelines of above specific methods applied to artillery
barrels during manufacturing field, as well as experimental data are obtained after using an autofrettage
technique.
The Hydrostatic Method
This method is firstly used for the artillery barrels autofrettage. It consists in loading a bore of gun
barrel with a controlled internal pressure which amount is greater than that one corresponding to an elastic
limit state, therefore; generates in the barrel wall an elastic-plastic state ( the inner layers of the barrel are
plasticized more strong). After the load is removed from the barrel (perfect elastic state), this does no longer
the tube come back to an initial dimensions. In fact, determines the well appearance of the deformation state
and residual stresses which have a good stability with time. When a barrel is subjected several times with
constant autofrettage pressure to the loading-unloading cycle, better evaluation of residual state is
recommendable [5].

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The stresses that are generated by the loading during shooting inside the artillery barrel overlap with
the permanent stresses, thus, one obtains lower total stresses than in the case when these residual stresses are
not present. Theoretically, the working pressure is increased up to the value of the autofrettage pressure
without the appearance of other residual deformations in the material. Practically, the working pressure is
established under the autofrettage pressure for safety reasons. Therefore, the working pressure in an
autofrettaged barrel can be increased by (50-150) % according to the geometry and to the degree of elastic-
plastic deformation if it comprised with the normal one. There can appear axial stresses in the hydrostatic
method of elastic-plastic deformation according to the way of ensuring the sealing. These axial stresses have
maximum values in the case when the sealing is made with plugs installed at the end of the barrel. This
procedure involves residual deformations, an addition is necessary for the final processing and its values
depending on the desired degree of elastic-plastic deformation. Usually, the degree of deformation is
(1-2) %. Under these conditions, the processing addition for the internal diameter (d) is about (0.019-0.02d).
Some advantages of this method are: (1) according to the thickness of the barrel wall, the ability of
differentiate autofrettage process on different sections; (2) - the possibility of autofrettaging the gun barrels
with a variable internal bore profile. While the main disadvantage is related to the high-pressure installation
and to the sealing's accomplishment.
The Mechanical Method
In this method, an elastic-plastic deformation of the tube (barrel) is produced as a consequence of the
passing of a calibrated mandrel (pushing or pulling) through its channel with a diameter bigger than the
diameter of the barrel. In the hydrostatic method, the autofrettage pressure was applied as a control
procedure, while here; autofrettage is conditioned by the tightening between the barrel and the mandrel. In
order to achieve the movement of the mandrel, a force that will be able to counteract the friction forces
between the mandrel and the barrel is needed. The formula which helps us determine the advancement of the
mandrel is given by the relation [6]: F=πdlμp …….. (1)
Where μ- Is the sliding friction coefficient.
p - Is the contact pressure equivalent with the pressure corresponding to the hydrostatic method.
Special materials and lubricants are used for reducing the resistant force by decreasing the sliding
friction coefficient. The main advantages of the mechanical autofrettage method are related to the simplicity
of the installation and to the improvement of the quality of the internal superficial layer as a consequence of
the tamping of the material. The following disadvantages can be enumerated:
(1) High dimensional precision before applying the procedure.
(2) The impossibility of autofrettaging the barrels in the areas where their channel is profiled or deformed.
The Ballistic Method
The ballistic method consists in a special shooting with the barrel that is to be autofrettaged. A
pressure develops during the shooting that is greater than the service pressure with the desired value and
distribution. In this method, it necessary that the barrel bore diameter is smaller than its nominal diameter
when the autofrettage shooting is applied. From the internal ballistic calculations, the quantity of powder,
the (propelled, projectile, sealing plugs) masses are determined so as to obtain the desired variation law for
the autofrettage pressure [7].
After the autofrettage shooting, the efficiency of method is controlled by measuring the residual
deformations. The autofrettage shooting can be repeated, with other parameters in case the desired
parameters are not obtained. It concludes that this autofrettage method is easier to achieve and it can be
taken into consideration as an alternative to the procedures described above.
Analyzing the three procedures of autofrettaging the artillery barrels, taking into consideration the
advantages and disadvantages of each of them and considering the possibilities of making the installations,
without resorting to imports, all the options are oriented to the hydrostatic procedure, the most known,
which was described before.

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Results and Discussion
In elastic-plastic technique, the information about material properties is necessary. Therefore it may
be understand and determine stress-strain curves which obtained by experiments. The mechanical properties
of the material are evidenced on the characteristic diagram obtained at the axial tension test. The mechanical
characteristics which necessary to the calculations are the following:
σ
e
- The limit of elasticity.
E - Modulus of elasticity.
The breaking elongation.
Calculations were determined on specimens of standard tension test sampled from D-30 gun barrel
made out of an alloy steel. During the calculations of stress-strain analysis, the modulus of elasticity and the
limit of elasticity are appearing directly. But the breaking elongation gives qualitative information in the
possibilities of applying the procedure. In artillery barrels manufacturing, 10% is a value of the breaking
elongation. When applying the autofrettage procedure, the deformation does not exceed 2%, the conclusion
is that the safety reserve is considerable. Up to the elasticity limit, the material is behavior as linear elastic
according to Hooke’s Law ( σ = Eε ). The stress curve branch of the characteristic diagram, which is higher
than the elasticity limit, is polynomial approximated. The average values of the mechanical characteristics
obtained from 10 specimens are as follows:
1- E = 2.08 E5 MPa
2- σ
e
= 843 MPa
3- The elasticity limit deformation: ε
e
= 4.13 E-3.
To make the uniaxial stress-strain curve more compatible with the applied method it is necessary to idealize
the stress-strain curve [6]. In the present study, it has proposed a particular model by approximating the
nonlinear portion of stress-strain curve with the following polynomial function:
72
110926.0110243.011 












−+













−+−=−
eeeo
σ
σ
σ
σ
σ
σ
ε
ε
The constitutive equations of the material are not linear due to the difficulties as determining the
states of stress and deformation. Therefore; there was an importance to determine the characteristic diagram
for the elastic-plastic stress-strains. The theory of plastic flow is preferred in order to analyze the states of
stress and deformations when elastic-plastic stress-strain appears in long enough thick-walled cylinders. In
above theory, the constitutive equations are incrementally formulated, as a differential connection between
deformations and stresses. The coefficients that interfere with this connection are dependent on the state of
stress that is reached. Because the constitutive equations are complex, the solution is done numerically.
In order for the effect from the ends not to be felt in the median section of the models, their height
was taken of 130mm. The dimensioning was made according to the available force F
max.
= 0.2 MN. In order
to achieve a maximum pressure of approximately 0.8 MPa, the diameter of the plunjer resulted. The body of
the pump was dimensioned from the tensile strength conditions and according to these ones the interior
diameter of the trial models resulted too. A mixture of glycerin with 10% water was used as a working fluid.
This combination ensures a compressibility coefficient of approximately 0.214 E-9 Pa
-1
. There have been
tried two models, one with the external diameter of 74 mm and another with the external diameter of 85 mm
made of alloyed steel OHN03-MFA. We also followed the behavior of the models under loading, strain
gauges were applied on the lateral surface and the internal and external diameters were accurately measured,
in the median plane of the model. The experimentation of the models was done by means of a device
assembled on the trial machine ZD-20, which develops a compression force of 0.2 MN. During the testing
the internal pressure applied to the model was followed, and it was considered proportional to the applied

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loading and the circumferential and axial deformations registered at the strain gauges and measured by
means of the strain tension bridge. After reaching the maximum pressure corresponding to each model the
unloading was done up to the atmospheric pressure.
Conclusions
From the optimization numerical and experimental results, the following conclusions can be derived:
(1) The optimum radius of elastic-plastic junction in autofrettage tube does not differ significantly for
elastic-perfectly plastic material compared with those obtained using an elastic-plastic with strain
hardening material's model.
(2) The maximum deviations from the average values is not higher than ( 2% ).
(3) In hardness test, there have been obtained values between 104 HRB and 109 HRB.
(4) The values in point (3) above were proved useful as comparing the experimental data with the numerical
results.
(5) The theory of plasticity is not fully exploited because it takes a considerable effort for understanding and
implementing the techniques for an elastic-plastic state.
References
[1] T.Z.Blazinski, "Applied elasto-plasticity of solids", Hong-Kong: Macmillan, 1983.
[2] GH Majzoobi, GH Farrahi, AH Mahmoudi, "A finite element simulation and an experimental study of
autofrettage for strain hardened thick-walled cylinders", J. Mater. Sci. Eng. A., vol.359, 2003.
[3] PCT. Chen, "Stress and deformation analysis of autofrettaged high pressure vessels", ASME special
publication, vol. 110, PVP. New York, 1986.
[4] G.J. Franklin, JLM. Morrison, "Autofrettage of cylinders", vol. 174, pp. 947-974, 1960.
[5] P. Lixandru, Gh. Barsan, "A mathematical model of the elasto-plastic state in the thick-walled tubes",
The annual symposium of the institute of solids mechanics, Romanian academy, Bucharest, Romania,
1994.
[6] L.M. Kachanov, "Fundamentals of the theory of plasticity", Mir publishing house, Moscow, 1974.
[7] GH. Barsan, P. Bechet, M. Barbu, "A basic theoretical model foe elastic-plastic stress analysis of the
thick-walled tubes subjected to an internal pressure", ICNT '07, Brno, Czech Rep., 2007.