| A mathematical model which predicts transient cyclic stress-strain behavior of low alloy, high strength steel has been developed. The model has been successfully applied to both uniaxial and proportional multiaxial (thick-walled cylinder) cyclic loadings.;Fatigue life prediction methods for thick-walled cylinders usually involve full scale testing or reduced scale triaxial fatigue testing of the same metal to develop a cyclic pressure vs. cycles to failure relationship. Here, a mathematical model that can make accurate fatigue life predictions for thick-walled cylinders has been developed and its accuracy is verified by comparison with full scale triaxial fatigue test data. The model defines the threshold crack size, the nonarbitrary initiation crack size, and the critical crack size, and computes the fatigue initiation life, the fatigue propagation life and the total fatigue life.;It has been commonly accepted that once a thick-walled cylinder has been autofrettaged, no further plastic deformation will occur when the same pressure is applied again. This is found not to be true. Instead, the plastic-elastic boundary may propagate during each pressure cycle and lead to complete plastic deformation to the outer diameter after a certain number of cycles. This phenomenon has been observed experimentally by many researchers in the past when tangential strains at the outer diameter were found to increase during repeated autofrettage but no explanation has been given for this "strain walking." The present research suggests a new autofrettaging process that results in much higher residual compressive stresses but with less than one tenth of the diametral dilation at the bore than obtained with traditional autofrettaging methods. |
