This paper discusses results from a series of trials carried out to determine the effect of the projectile driving band on the stress applied to a 155mm gun barrel during firing. The interference between the driving band and gun barrel can apply significant loads to the barrel and, in extreme cases, lead to premature cracking and failure of the barrel. Strain gage data from firing trials has been used to characterize the external strain from firing different projectiles and charges to identify potential problems and provide information for fatigue analysis. Very high band strains were routinely measured under “oiled bore” conditions, i.e., after barrel cleaning and also during the first one to three rounds of a serial following a long pause in firing, such as at the start of a day’s firing. In general, the strain associated with the driving band was seen to decrease with increased charge zone, barrel wear, and, at higher charge zones, distance along the barrel. In the majority of tests fired at maximum charge, there was no strain peak associated with the driving band in the forward part of the barrel. In conjunction with these experimental observations, a laboratory study has been carried out on the effect of a narrow pressure band on the deformation of a thick-walled tube. An apparatus was constructed in order to pressurize a known length of a smooth-bore cylinder. Sealing width at the edges of the band was minimized to reduce edge effects, and an oversize pressurized “cap” was used to ensure that the bandwidth remained constant during the experiments. Spacer disks were used to vary the bandwidth and also to adjust the cylinder position relative to the band. Measured external strains on the tubes were compared to calculations based on analytical solutions for step pressure changes and are shown to be in good agreement.
Analytical solutions for shrink-fit compound gun tubes were used to study the effects of the liner/sleeve interface diameter, the radial interference, and the effect of machining tolerances on tube strength. Finite element analyses were then done of a midwall cooled compound gun tube where a loose-fitting inner tube (liner) is permanently deformed by means of hydraulic pressure to lock it to the outer tube (sleeve). The interaction between the liner and sleeve was modeled with contact elements. The effect of machining subsequent to the hydraulic autofrettage was taken into account. Simulations were first done for smooth tubes with initial clearance of varying magnitude and, second, for the case where the liner has axial semi-circular cooling channels machined on its outer surface. Manufacturing tolerances were found to be much less critical for the hydraulic autofrettage than with the shrink-fit option. The interface diameter seems to be a relatively insensitive parameter. Relatively large initial clearance between inner and outer tubes can be tolerated. The hydraulic autofrettage option therefore seems better than the shrink-fit only option for compound gun tubes. It was demonstrated that the effect of the cooling channels on the stresses in the tube is significant and substantially weakens the inner tube. However, it is still possible to produce a workable design.
This paper describes a range of hydraulic fatigue and pressure tests carried out on gun barrels and ordnance components in support of weapons research and development programs. Cyclic testing of representative sections of large caliber guns has been routinely carried out to determine safe fatigue life for operational use. Ultrasonic techniques have been developed for mapping multiple cracks within the gun barrels by which periodic examinations of barrels during testing have been used to build up histories of crack initiation and growth. In relatively unworn barrels multiple cracks, initiated along each rifling groove, are shown to form in an extremely stable array that can grow to several millimeters depth, in agreement with calculated stress intensity factors, before one crack accelerates to dominate final failure. The development of localized erosion from extensive firing is shown to significantly affect the subsequent hydraulic fatigue cycling by generation of only one or two cracks as well as the prior removal of heavily prestressed bore material. Tests on fume extractor sections of barrels show that cracking initiates in the jet holes and only grows to the bore at late stages. Introduction of different levels of autofrettage acts to bias the radial location of the cracks. An extensive experimental program of work has been carried out in support of design studies on mid-wall cooled gun barrels exploring a number of different configurations for construction of compound tubes and the results of numerical simulations of the assembly process have been compared with strain gauge measurements during the experimental procedure on scale tubes. Subsequent hydraulic cycling was used to determine the fatigue implications of the processing route.
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