Optimizing thread-root contours using photoelastic methods

The research work described in this paper was part of a program which was conducted to determine stress distributions in a complex multicomponent pressure vessel and in several redesigned versions of the same vessel. Three-dimensional photoelasticity methods were selected for the analyses when it was determined that a number of highly stressed internal areas were inaccessible for evaluation of stresses by electrical-resistance strain gages and uncertainties regarding load transfer between mating components made an analytical determination impractical. The results of the stress determinations in the original vessel indicated that the highest stresses were located in the small fillets at the roots of the sectored threads which were used to permit quick and convenient assembly of the parts. Results of a prototype fatigue study verified the importance of these stresses when it was determined that fracture initiated in this high-stress area. Because of design specifications and the nature of conditions under which the vessel operates, stress reductions had to be accomplished using thread form or rootcontour changes. Since three-dimensional studies are time consuming and expensive, two-dimensional methods were selected to evaluate a number of changes before final evaluation was made in a three-dimensional model. Comparisons between results from the two- and three-dimensional studies indicated that the percent improvements indicated in the two-dimensional studies were roughly obtained in the three-dimensional vessels.