An Improved Method for Recovering Residual Stress From Strain Measurements in Cylinders and Rings

Abstract

The traditional method for determining the residual stress from experimental strain readings in axisymmetric configurations can produce large discrepancies in stress predictions, particularly radial stress. By numerical calculation of autofrettage residual stress in a long thick cylinder and subsequent numerical modeling of release of axial stress during the cutting of a short ring sample, a potential preexisting radial and hoop residual stress field is calculated. These stress values are converted to radial and hoop strains at a number of discrete radial locations. Numerical strain values are then randomized to enforce a standard deviation some 25% higher than that for a typical experimental procedure. Pairs of randomized, discrete strain values are used to predict associated residual stresses using the traditional method. This produces wide scattering of predicted stress values, particularly radial stress, and showed high sensitivity to assumed Poisson’s ratio. A simple, alternative strain-stress analysis procedure (ASSAP) is proposed. ASSAP enforces equilibrium requirements and essential stress free boundary conditions at the bore. ASSAP is shown to improve prediction of radial residual stress by around an order of magnitude in the near-bore region, and to effectively eliminate the sensitivity to Poisson’s ratio. The predicted radial stress profile is of sufficient quality to define the associated hoop stress profile. The predicted radial and hoop stress profiles are in close agreement with the original numerical solutions for the ring. Numerical and ‘recovered’ bore hoop stresses are within 1.4%. This work also demonstrates a significant limitation of methods that involve the cutting of axially short ring samples from a long, autofrettaged cylinder. Release of axial stress during cutting creates a reduction in compressive bore hoop stress. Such a discrepancy would be very significant if ring, rather than long-cylinder, values were used in fatigue lifetime calculations.