Measurement of the Residual Stress Tensor in a Compact Tension Weld Specimen

Abstract

Neutron diffraction measurements have been performed to determine the full residual stress tensor along the expected crack path in an austenitic stainless steel (Esshete 1250) compact tension weld specimen. A destructive slitting method was then implemented on the same specimen to measure the stress intensity factor profile associated with the residual stress field as a function of crack length. Finally deformations of the cut surfaces were measured to determine a contour map of the residual stresses in the specimen prior to the cut. The distributions of transverse residual stress measured by the three techniques are in close agreement. A peak tensile stress in excess of 600 MPa was found to be associated with an electron beam weld used to attach an extension piece to the test sample, which had been extracted from a pipe manual metal arc butt weld. The neutron diffraction measurements show that exceptionally high residual stress triaxiality is present at crack depths likely to be used for creep crack growth testing and where a peak stress intensity factor of 35 MPa√m was measured (crack depth of 21 mm). The neutron diffraction measurements identified maximum values of shear stress in the order of 50 MPa and showed that the principal stress directions were aligned to within~20°of the specimen orthogonal axes. Furthermore it was confirmed that measurement of strains by neutron diffraction in just the three specimen orthogonal directions would have been sufficient to provide a reasonably accurate characterisation of the stress state in welded CT specimens.

Quantification of residual stresses in electron beam welded fracture mechanics specimens

“…It was assumed that these orientations were aligned to the directions of the principal strains, which allowed the stresses to be determined using Equation (2) (Hutchings et al, 2005). Residual strain measurements on a similar specimen in eight directions were performed by Traore et al (2013) to show that the principal stress components were closely aligned to the specimen's axes.…”

Determination and mitigation of the uncertainty of neutron diffraction measurements of residual strain in large-grained polycrystalline material

“…This approach reduced the residual stress acting along the axis of the reference cylinder to an insignificant level. The magnitude of residual stresses remaining in the 6 mm reference cylinders was assessed to be low on the basis of numerical studies of Repper et al (2012) and application of the closed form analysis described by Traoré et al (2013), assuming an initial residual stress field with a wavelength of the order of the pipe thickness. The reference cylinders were mounted vertically in a three-jaw chuck on the sample table, allowing measurements in the pipe axialhoop plane at six positions midway between the cuts.…”

Plasticity in the Contour Method of Residual Stress Measurement

“…In the former case the plate was rigidly fixed at both extreme (lengthwise) ends of the plate, whereas for the latter case just one end was fixed. Asymmetric clamping is of particular interest because this is a requirement for slitting measurements which can be combined with a contour stress analysis [8]. The contour cut was simulated by incrementally removing mesh elements equal to the width of the cut (assumed to be 0.5 mm) in an elastic-perfectly-plastic analysis under plane stress conditions.…”

“…5) [8]. The two destructive techniques were applied in tandem and only one side of the sample was maintained during the cutting to fulfil the slitting restraining condition.…”

“…The level of rigidity imposed by "restraining" displacements at a line or along a plane in an FE analysis is rarely reproducible in practice. 'Finger' clamp tools are sometimes used to locate the test component during the real cutting process [8], but the function of these is to stop global movement of the test component on the machine bed rather than restricting cut face opening/closure (which can involve large forces). Restraint of cut face opening/closure can be approached perhaps by use of 'fitted bolts', but the degree of restraint is uncertain unless quantified by complex FE analysis simulating the interaction of the bolts with the test component.…”