Design Method for Autofrettaged Thick-Walled Cylinders with Outside Diameter Discontinuities

摘要

A design method has been developed to determine the fatigue life of thickwalled autofrettaged cylinders containing outside diameter (OD) notches which act as failure initiation sites. The method uses local strain analysis for fatigue crack initiation and fracture mechanics analysis for crack propagation to failure. The method is relatively easy to program and thus can be used to optimize cylinder designs with respect to OD initiated failures. The computer program developed requires only minimal input to estimate life: cylinder dimensions, notch depth and root radius, internal pressure, and material yield strength. Other material properties (low cycle fatigue data, fracture toughness, and crack growth law) and permanently stored. The program calculates the elastic stress concentration factor using a Neuber diagram as a default. If the elastic kt is known from other sources, this feature of the program can be overridden. The stress concentration factor is used to calculate notch root stresses from which local strains are estimated. Once the local strains are known, crack initiation life is estimated using the stored low cycle fatigue data. A crack is then assumed to exist and a power law is integrated to determine crack propagation life to failure. The total life is the sum of the initiation and propagation lives. The applicability of the design method is demonstrated by using it to predict the total fatigue lives of existing cylinder designs with measured fatigue lives. The method's predictive capability is very good to conservative. In no case was life substantially overestimated. In addition, a design example is presented.