Thermal Relaxation in Autofrettaged Cylinders
An experimental study of the loss of bore expansion and change of residual stresses in autofrettaged cylinders resulting from internal heating combined with external cooling provides information useful in the design of pressure vessels operating at high temperature, Two-foot long cylinders were heated internally to bore temperatures up to 950°F and simultaneously cooled externally to produce a temperature difference of as much as 725°F from bore to outside surface. Reduction of the autofrettage bore expansion and reduction of residual stresses resulted, because the thermal stresses added to the residual stresses and exceeded the lowered yield strength at elevated temperature, permitting relaxation to occur.
The data reveals that under certain temperature conditions a considerable portion of the autofrettage induced bore expansion and the associated residual stresses can be lost in a few minutes when external cooling occurs. The experimental results indicate that partial overstrain in autofrettage may be preferable to full overstrain in order to minimize the loss in residual stress.
KeywordsFatigue Furnace Convection Resid Photography
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