An investigation of residual stresses in tubing

Simpson, George Bleet (1976) An investigation of residual stresses in tubing. PhD thesis, University of Glasgow.

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Residual stresses fall naturally into two categories; macrostresses and microstresses. An indication of the origins of both, types is given. The presence of residual macrostresses in a body can be recognized by the occurrence of distortions on making cuts in the body. In bodies of regular geometry, the distortions which occur on gradual removal of uniform layers from one surface can often be used to assess the magnitude of the residual stresses in the removed material. Over the years, many techniques based on this principle have been developed. The Sachs method (ref.5) for the determination of triaxial axisymmetric residual stresses in solid bars, hollow cylinders and tubes, is one such technique, A brief account of the method and the assumptions on which it is based is presented, A number of important refinements are described. The residual stresses in thin-walled cylinders and tubing have been the subject of many experimental investigations, Although some studies of quenching and machining stresses have been carried out, the residual stresses produced during tube-drawing have attracted the most attention. Recent work on this topic is briefly reviewed, and the origins of residual stress in drawn tubing are discussed. Although it has long been realised that tubing drawn under commercial conditions almost always exhibits some degree of eccentricity, only Knights (82) has suggested that this should be taken into account when determining residual stresses in such tubing. The same author has also suggested that significant circumferential variations of the residual stresses may occur in drawn tubing, possibly associated with plastic bending at entry or exit from the drawing die. The present work was carried out to determine whether circumferential variations of residual stress existed in drawn tubing, to establish the effect of eccentricity on the determination of residual stress by the Sachs method, and to evaluate the bending deflection method (which is the usual alternative to the Sachs technique) in the presence of eccentricity and possibly suggest some improvements. The results obtained from an experimental investigation of the longitudinal and circumferential variation of the deformations produced on cutting drawn tubing to length suggest the presence of additional residual stresses not previously reported. The terms 'type A' and 'type B' residual stresses are introduced to distinguish between normal residual stresses and the additional residual stresses. 'Type B' longitudinal residual stresses exist as alternate regions of tension and compression round the circumference of drawn tubing and 'type B' circumferential residual stresses exist as opposing bending stresses in adjacent lengths. Both appear to be completely relieved in what are normally regarded as 'long' specimens (i,e, for the tubing used, in specimens with length/diameter ratios less than 5). 'Type B' longitudinal residual stresses may possibly be associated with imperfect lubrication during drawing. The origin of 'type B' circumferential residual stresses is not clear. The determination of circumferential and radial 'type A' residual stresses in a uniform cylinder by the Sachs technique is based on the assumption that the relief of these stresses in part of the cylinder (by layer removal) is equivalent to the application of a uniform pressure to the remainder. Since the application of a uniform pressure to an eccentric cylinder produces circumferential variations of stress on both boundaries, the Sachs technique cannot readily be applied where eccentricity is appreciable. Experimental results suggest that the Sachs technique can be applied in practice to determine the circumferential and radial 'type A' residual stresses over about 80% of the wall thickness of drawn tubing with I.D./O.D. =0.941 and an initial wall thickness variation less than +/-6% provided that these residual stresses do not vary circumferentially, Where the circumferential stresses applied by layer removal are compressive, the procedure may be complicated by the occurrence of elastic pre-buckling deformations of a circumferential lobar form, and a stage may even be reached at which plastic buckling takes place, The determination of longitudinal 'type A' residual stresses in the presence of eccentricity is complicated by the development of longitudinal shear stresses as the variation of wall thickness is increased by layer removal. The possibility of applying a simple modification of the Sachs analysis which takes account of wall thickness variation, but not shear stresses, is examined. The limitations of the Sachs method in the presence of eccentricity are discussed. Although the bending deflection method has certain advantages over the Sachs technique for the determination of 'type A' longitudinal residual stresses in the presence of eccentricity, its usefulness is restricted since no satisfactory method of determining the coexistent circumferential residual stresses is available, A method which incorporates a recently developed technique for the determination of local residual stress (91,100), is proposed and is shown to have considerable advantages.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Additional Information: Adviser: R Fletcher
Keywords: Mechanical engineering
Date of Award: 1976
Depositing User: Enlighten Team
Unique ID: glathesis:1976-73853
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 14 Jun 2019 08:56
Last Modified: 14 Jun 2019 08:56

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