IN SEARCH OF A NEW GOVERNING FAILURE CRITERION FOR TORISPHERICAL HEADS

摘要

The failure criterion that governs the present design rules of formed heads in Section VIII/Div. 2 of the ASME B&PV Code limits crown displacement upon a single application of pressure, and it does so by means of a calculated collapse pressure. One problem with the rules is that they require greater head thicknesses than those of Section VIII/Div. 1. Another problem is that recent test data taken for two formed heads have shown no signs of head displacements or deformations that could have rendered the heads unserviceable at pressures substantially higher than the calculated collapse pressures. Any link between the calculated collapse pressures and the failures of the test heads could not be established. The main thesis proposed in the paper is that such a link does not exist for torispherical heads in general. The reason for this is investigated theoretically and attributed to the geometric strengthening of heads, as they deform. It is recommended that the reasons for limiting displacements or deformations for a single application of pressure be reviewed and, if needed, a new failure criterion defined, or other failure modes, such as tensile plastic instability (burst), be considered. The latter mode is described and options for its evaluation are presented in this paper. The protection of torispherical heads against the failure modes of low-cycle fatigue and incremental plastic growth upon cyclic loading, which are controlled by shakedown, is not covered in this paper but is left to future investigations.