| Summary: | High pressure vessels such as gun barrels are autofrettaged in order to increase their operating
pressure and fatigue life. Autofrettage causes plastic expansion of the inner section of the
cylinder – setting up residual compressive stresses at the bore after relaxation. Subsequent
application of pressure has to overcome these compressive stresses before tensile stresses can
be developed, thereby increasing its fatigue lifetime and safe working pressure.
A series of Finite Element (FE) models of hydraulic autofrettage were created, to establish
the correct boundary conditions required and means of developing accurate but
computationally efficient models. Close agreement was observed between the solutions
obtained from the developed models and those from existing analytical and numerical
models. These initial models used a simplistic bi- linear stress-strain material representation;
this deficiency was then addressed through the development of two means of creating radial
position dependent non-linear material behaviour within FE, crucial for accurate prediction of
residual stresses.
The first utilised a method of altering the elastic properties of the material to achieve nonlinear
stress-strain response. This provided accurate results that compared well with existing
methods, but was unable to be used in simulation of swage autofrettage due to its elastic
nature. The second method achieved non- linear behaviour through direct manipulation of the
stress and plastic strain states of the FE model at a fundamental level. This was hence
suitable for arbitrary loading procedures, including swage autofrettage.
A swage-like model that applied deformation via a band of pressure was developed, to
investigate the influence of localised loading and shear stresses that result on the residual
stress field.
A full model of swage autofrettage was then developed, which was optimised on the basis of
accuracy and solution effort. It was then used to investigate the effects of various mandrel
and contact parameters on the creation of residual stresses. The model is suitable for use in
future optimisation studies of the swage autofrettage procedure.
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