Standard Test Method for Estimating the Approximate Residual Circumferential Stress in Straight Thin-walled Tubing

Abstract

4.1 Residual stresses in tubing can be detrimental to its future mechanical performance. Such stresses can, for example, make the tubing more susceptible to stress corrosion cracking when exposed to certain environments. 4.2 High residual stress gradients are common at the surface of metal tubing, typically caused by cold drawing, peening, grinding, etc. However, details of these gradients are not resolved by this test method. 4.3 Residual stresses in new thin-walled tubing are very sensitive to the parameters of the fabrication process, and small variations in these parameters can produce significant changes in the residual stresses. This test method provides a convenient means for estimating and comparing the residual stresses in specimens from each heat or heat lot produced. 4.4 This test method approximates the stress distribution through the wall thickness as being linear. It uses the Hatfield and Thirkell formula3, as later modified by Sachs and Espey4, to provide a simple method for calculating the approximate residual circumferential stress at the tubing surface. This approximation is usually reasonable for thin-walled tubing, in which the wall thickness does not exceed one tenth of the outside diameter. Even in cases where the approximation does not reveal local details, experience has shown that the approximate stresses estimated by this test method frequently serve as useful indicators of the susceptibility to stress corrosion cracking of the tubing of certain metal alloys when exposed to specific environments.5 4.5 Because of the linear approximation of the residual stress distribution in the tubing, the results of this test method should not be used for design, manufacturing control, localized surface residual stress evaluation, or other purposes without supplementary information that supports the application. 4.6 This test method is primarily used to estimate residual fabrication stresses in new thin-walled tubing between 19 mm (0.75 in.) and 25 mm (1 in.) outside diameter and 1.3 mm (0.05 in.) or less wall thickness. While measurement difficulties can be encountered with smaller or larger tubing, in principle, there does not appear to be any theoretical size limitation on the applicability of this test method.