טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentGelfeld Irene
SubjectInvestigation of the Autofrettage Process in
Thick-Walled Bores
DepartmentDepartment of Mechanical Engineering
Supervisor Professor Emeritus Jehuda Tirosh
Full Thesis text - in Hebrew Full thesis text - Hebrew Version


Abstract

Autofrettage is a metal fabrication technique which goal is to increase durability of the final product. The tube is subjected to internal pressure of sufficient magnitude to enlarge the bore and in the process the inner layers of the metal are stretched beyond their elastic limit. This means that the inner layers have been stretched to a point where the steel is no longer able to return to its original shape once the internal pressure in the bore has been removed. Although the outer layers of the tube are also stretched the degree of internal pressure applied during the process is such that they are not stretched beyond their elastic limit. The reason why this is possible is that the stress distribution through the walls of the tube is non-uniform. Its maximum value occurs in the metal adjacent to the source of pressure, decreasing markedly towards the outer layers of the tube. Because the outer layers remain elastic they attempt to return to their original shape; however, they are prevented from doing so completely by the now permanently stretched inner layers.  It is important to set a high work pressure for autofrettaged cannons in order to develop modern ammunition with high kinetic capabilities. This study will present an analytical solution for the stress field which develops in an autofrettaged cannons before and during the shooting. The analytical solution is obtained by using non linear Von-Mises yield criteria in plain stress and plain strain condition. This solution will be compared with a standard Tressca solution which is extensively used. Nowadays it is possible to create a numerical simulation which can provide a reliable solution for the residual stress field in an autofrettaged cannons. However this requires expensive FE software and an appropriate hardware to support it. In this study we have developed a simple code which provides an almost immediate solution to the residual stress field in autofrettaged cannon. In addition this study will present recommendations for an efficient design in the aspect of determining the optimal level of autofrettage for varied bore geometries, calculating the decline in defense of autofrettage due to machining, and the improvements that may be obtained by cooling the External volume of the bore during the shooting process. The results will be compared to an FE solution which validates the reliability of our solution.