A comparative study to map the residual strain/stress states through the walls of autofrettaged thick-walled steel cylinders has been conducted with neutron diffraction, Sachs boring and the compliance methods. Test samples with different wall thickness ratios were prepared to have significant amounts of reverse yielding due to the Bauschinger effect. All three methods indicate that the autofrettage action primarily influenced the hoop stresses, which varied rapidly close to the bores from compressive to tensile within the first half of the wall thickness. Quantitatively, results from the neutron diffraction and Sachs boring techniques compare favourably across large regions of the cylinder walls, while the compliance results showed different features. The existence of reverse yielding close to the cylinder bores has been sensed to different magnitudes and distances from the bores. Plastically yielded material regions identified from the neutron diffraction results correlated well with theoretical modelling.
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.
Realistic material models have been developed on the basis of the experimental investigation of reverse loading with actual Bauschinger effect and implemented into a two-dimensional finite element computer program. The developed program is capable of treating the elastoplastic deformation behavior of thick-walled cylinders during both loading and unloading phases. Strain hardening may occur during loading, and reverse yielding may occur during unloading at a yield strength significantly reduced due to the Bauschinger effect. Three different models for the reverse hardening are presented. Strain hardening during reverse yielding may have a different slope than for forward loading, and it may also be nonlinear. The intended application is for autofrettage analysis of thick-walled cylinders. Being a numerical solution, it will also be very useful for finite element analysis of residual stress experimental procedures and also in the determination of more accurate stress intensity factors for autofrettaged cylinders that had undergone reverse yielding due to the Bauschinger effect.
A comparative study to map the residual strain/stress states through the walls of autofrettaged thick-walled high-strength steel cylinders has been conducted with neutron diffraction, Sachs boring, and the compliance methods. Test samples with different wall thickness ratios were prepared to have significant amounts of reverse yielding due to the Bauschinger effect. In an effort to explain observed differences in the hoop stress results, the crack compliance experiment was simulated with finite elements. Several residual stress fields were introduced in the finite element models. A theoretical finite element (FE) model, which is capable of accurately modeling the highly nonlinear reverse yielding of the material, was able to accurately predict the crack compliance strain measurements.
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