Experimental verification of the autofrettage process for thick-walled tubes

The success of the autofrettage process applied to thick-walled tubes is de termined in industry via a control chart representing the measured permanen t diametral expansions versus the yield stress of the material. The widely used acceptance limits are due to Hill's analysis of the problem. An experi mental investigation showed that the above limits might lead to refusal of a substantial number of autofrettaged tubes. By considering the separate ef fects of hardening, the Bauschinger factor, reverse yielding, anisotropy an d geometrical eccentricity, the need for a more realistic material model is demonstrated. A model incorporating the first three factors, found in the literature, is used to suggest new acceptance limits. Autofrettage experime nts on a relatively large number of tubes indicated that the new control li mits might render refusals acceptable.