Theoretical And Experimental Investigations For The First-Order And Second-Order Mechanical Autofrettage Technique Of Thick-Walled Cylinders

The mechanical autofrettage of thick—walled cylinders is a sort of practical and valuable technique used to enhance its elastic strength of thick—walled tube and increase its fatigue life. In this thesis, the second—order mechanical autofrettage technique is put forward and studied for the frist time. It will expand the use of mechanical autofrettage technique on different—bore tubes, cut down the push force and further tap the potentialities of current equipments.The theoretical analysis methods on the first—order and second—order mechanical autofrettage technique of thick—walled cylinders, with reflecting the material characteristics, are studied systematically in this thesis. By use of the elastoplastic principal stress method and the method of weighted residuals, the elastoplastic stress, strain, displacement, permanent bore enlargement, excess, degrees of autofrettage and push force etc. key problems in the first-order and second—order autofrettage are calculated, a lot of important conclusions are given. From the results, it can be seen that the influence of the material strain hardening and Bauschinger effect should be considered while thearetical calculating. Based on the theoretical analysis, theimitating experiments of the first—order and second—order autofrettage process proved the validity of the theory, the feasibility of the second—order autofrettage technique itself. This kind of technique can meet the autofrettage requirements of tubes with different bore and material.Based on the energy principal method, an analytical method is provided for studing calculation of the push force and for shape design of the mandrel. This research method reveals the change law of the displacement modes influenced by the change of cone. It may be used for reference for further studing autofrettage technique.From the view of microscosmic point, on account of mechanical property experimental results of macroscopic materials and observation of microcosmic structure, we discussed the plastic deformable mechanism of gun autofrettage tube, the reasons for Bauschinger effect and macroscopic mechanical property of materials, a complex autofrettage technique assumption of eliminating Bauschinger effect is presented. It may be used for reference in developing new kind of steel and enhancing the elastic strength of autofrettage tubes.Finally, after analysis of measuring the residual stress in autofrettage thick—walled tubes, the corresponding formulae and the formulae of measuring residual stress using Sachs method and X-ray method have been presented.The results obtained in present thesis can provide valuable references in theory and experiment for development and practice of the first—order and second-order autofrettage techniques.