Comparison of Methods for Calculating Stress Intensity Factors for the Thread of a Pressure Vessel Closure and of a Gun Breech Ring

Non-mandatory Appendix D of Section VIII, Division 3 of the ASME Boiler and Pressure Vessel Code provides a method for calculating the stress intensity factors for the region of a thread root of a threaded closure. This method involves calculation of the distribution of stress acting on a plane normal to the axis of the thread. This distribution is fitted with several different cubic equations for different regions and the coefficients of these cubic equations are entered into an equation to calculate the distribution of stress intensity factor for each region. The values of stress intensity factor for each region after the first one are shifted to obtain a continuous distribution. In a paper to be presented at the August 2002 ASME Pressure Vessel and Piping Conference (Kendall 2002) the author compared the stress intensity factors calculated by the above Code method with those determined by Neubrand and Burns, 1999, using a weight function method. In Kendall, 2002, the stress intensity factors for this same closure design were calculated using the Code method and also calculated using a proposed modification of this method. The results showed slightly better agreement for the proposed modification of the Code method. This paper will report the details of these calculation methods and the results from Kendall, 2002. It will also give a comparison of the stress intensity factor results of these methods for a thread of a typical gun breech ring, and a comparison of the calculated fatigue crack growth lives.