Photoelastic study of interrupted pressure vessel frames

Abstract

The objective of this thesis is to obtain an understanding of the variation in stress on the cardinal axes of a circular penetration with elliptical reinforcements in a simulated stiffened cylindrical pressure vessel. The size and location of the circular penetration is such that the continuity of one of the ring frames has been destroyed. The biaxial compressive loading of a pressure vessel has been obtained by superimposing the stress distribution obtained on the free boundary with a loading parallel to the ring frames representing hoop stress and the stress distribution obtained with a loading at right angle to the ring frames representing longitudinal stress. By a systematic variation of the reinforcement geometry and the weight of reinforcement materials, the relationship between the stress concentration factors, geometry and weight of material has been investigated. The photoelastic method of experimental stress analysis was chosen as the means of carrying out the investigation. The standard crossed circular polariscope was used in studying the isochromatic fringe patterns. The two-dimensional models were constructed with PS-5 material from Photoelastic, Incorporated. All models were built upon a basic foundation of 4" x 4" x 1/4" material with a central 1-1.4" diameter circular hole. The effects of the various geometries and weights of reinforcement were investigated by using three basic families of geometrical shapes in which a sequence of five steps in the reinforcement thicknesses were made for each family. For the particular series of geometries investigated, the best geometry for the reinforcement is the circular family which consisted of models 3,6,9,12, and 15. This series shows the lowest stress concentration factors for a given thickness or weight of reinforcement material. A comparison was made between the stress obtained by a strain gage and the stress obtained by photoelastic analysis. the result showed close agreement between the two techniques of experimental stress analysis. The complicated interrelationships if all the variables that ultimately specify the geometries associated with a penetration in a stiffened cylindrical pressure vessel indicate that today each specific configuration must be analyzed on an individual basis. To attempt to interrelate such parameters as pressure vessel thickness, frame size, frame shape, frame spacing and penetration size in order to obtain an all inclusive design procedure for optimum reinforcement would appear to require an extensive model testing program. These results might then serve as a basis from which to derive design procedures.