A study of the autofrettage process



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Over the past few decades, modern industrial processes have required higher and higher operating fluid pressures, and use of these high pressures have necessitated development of the auto-frettage procedure. This procedure consists of subjecting a thick-walled tube or vessel to such high internal pressure that plastic yielding in circumferential tension occurs on the inside for a significant portion of the wall thickness. When the auto-frettaging pressure is released, residual compressive circumferential stresses exist inside the wall (and for a significant portion of the wall thickness). These stresses are of opposite sign to those caused by subjection of the tube or vessel to subsequent working pressures. The inside circumferential stresses are then much lower than they would be had the tube or vessel never been autofrettaged. Use of the autofrettage procedure thus permits the use of higher pressures and insures a longer fatigue life than may be obtained by any other method. This thesis presents a thorough study of the theory and practice of the autofrettaging procedure, and also presents a comparison of the residual strains obtained with electric resistance strain gages during typical routine autofrettaging of high pressure tubes with the residual strains predicted by use of formulas derived for autofrettaging procedures.