4.1 Residual stresses in tubing may be detrimental to the future performance of the tubing. Such stresses may, for example, influence the susceptibility of a tube to stress corrosion cracking when the tube is exposed to certain environments.

4.2 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. See, for example, Table 1, which shows the residual stresses measured by this practice in samples from successive heats of a ferritic Cr-Mo-Ni stainless steel tube and a titanium condenser tube. This practice provides a means for estimating the residual stresses in samples from each and every heat.

4.2.1 This practice may also be used to estimate the residual stresses that remain in tubes after removal from service in different environments and operating conditions.

4.3 This practice assumes a linear stress distribution through the wall thickness. This assumption is usually reasonable for thin-walled tubes, that is, for tubes in which the wall thickness does not exceed one tenth of the outside diameter. Even in cases where the assumption is not strictly justified, experience has shown that the approximate stresses estimated by this practice frequently serve as useful indicators of the susceptibility to stress corrosion cracking of the tubing of certain metal alloys when exposed to specific environments.

4.3.1 Because of this questionable assumption regarding the stress distribution in the tubing, the user is cautioned against using the results of this practice for design, manufacturing control, localized surface residual stress evaluation, or other purposes without supplementary information that supports the application.

4.4 This practice has primarily been 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 may be encountered with smaller or larger tubes, there does not appear to be any theoretical size limitation on the applicability of this practice.

1.1 A qualitative estimate of the residual circumferential stress in thin-walled tubing may be calculated from the change in outside diameter that occurs upon splitting a length of thin-walled tubing. This practice assumes a linear stress distribution through the tube wall thickness and will not provide an estimate of local stress distributions such as surface stresses. (Very high local residual stress gradients are common at the surface of metal tubing due to cold drawing, peening, grinding, etc.) The Hatfield and Thirkell formula, as later modified by Sachs and Espey,² provides a simple method for calculating the approximate circumferential stress from the change in diameter of straight, thin-walled, metal tubing.

1.2 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.