Foroogh Hosseinzadeh

Slitting and Contour Method Residual Stress Measurements in an Edge Welded Beam

Welding is known to introduce complex three-dimensional residual stresses of substantial magnitude into pressure vessels and pipe-work. For safety-critical components, where welded joints are not stress-relieved, it can be of vital importance to quantify the residual stress field with high certainty in order to perform a reliable structural integrity assessment. Finite element modeling approaches are being increasingly employed by engineers to predict welding residual stresses. However, such predictions are challenging owing to the innate complexity of the welding process (Hurrell , Development of Weld Modelling Guidelines in the UK, Proceedings of the ASME Pressure Vessels and Piping Conference, Prague, Czech Republic, July 26–30, 2009, pp. 481–489). The idea of creating weld residual stress benchmarks against which the performance of weld modeling procedures and practitioners can be evaluated is gaining increasing acceptance. A stainless steel beam 50 mm deep by 10 mm wide, autogenously welded along the 10 mm edge, is a candidate residual stress simulation benchmark specimen that has been studied analytically and for which neutron and synchrotron diffraction residual stress measurements are available. The current research was initiated to provide additional experimental residual stress data for the edge-welded beam by applying, in tandem, the slitting and contour residual stress measurement methods. The contour and slitting results were found to be in excellent agreement with each other and correlated closely with published neutron and synchrotron residual stress measurements when differences in gauge volume and shape were accounted for.

A Novel Cutting Strategy for Reducing Plasticity Induced Errors in Residual Stress Measurements Made With the Contour Method

The contour method (CM) has emerged as a valuable technique for the measurement of residual stresses (RS). The method involves cutting the sample in which residual stresses are to be measured, using wire electric discharge machining (EDM), and measuring the deformation that occurs on the newly created surface, which can be related to the residual stresses that existed beforehand. The contour method provides a full 2-D map of the stresses acting in a direction normal to the plane of the cut. It is ideally suited to measurements in power plant welds since, unlike diffraction-based techniques, it is not affected by microstructure gradients, and it is well suited to thick section components. However, as with other mechanical strain relief techniques, it is prone to errors arising from plasticity when residual stresses close to the yield strength of material are encountered. This paper describes contour method measurements in an AISI Type 316L austenitic steel three-pass. A novel cutting and restraint strategy is applied in an attempt to reduce plasticity errors following optimisation studies simulating the cutting process using the finite element (FE) method.

Measurements of Residual Stress in a Welded Compact Tension Specimen Using the Neutron Diffraction and Slitting Techniques

The residual stress field in a compact tension specimen blank extracted from a non-stress-relieved thick section butt weld has been measured using neutron diffraction and the slitting method. Significant triaxial residual stresses were found in the specimen that is normally assumed to be stress free. Moreover the level of stress was sufficient to make a significant contribution to the crack driving force in creep crack growth tests. The benefits of using more than one measurement technique in such investigations are demonstrated.

Through thickness residual stresses in large rolls and sleeves for metal working industry

Abstract There is little experimental knowledge about the initial state of through thickness residual stresses in rolls and sleeves for the steel rolling industry. This is surprising bearing in mind the impact that residual stress has on the performance of the roll and sleeve materials in the highly aggressive loading environments of the metal working industry. Previous work has been confined to measurement of very near surface residual stresses and numerical predictions of residual stress distributions. In the present paper through thickness residual stress measurements were carried out using a deep hole drilling technique on a series of rolls and sleeves representative of those used in the rolling industry. Different features of the manufacturing processes used in their production are shown to influence the magnitude and distribution of the residual stresses. It is also shown that the measurements can be used, together with a finite element analysis, to determine the volumetric distribution of the residual stresses.

Evaluation of Errors Associated with Cutting-Induced Plasticity in Residual Stress Measurements Using the Contour Method

Cutting-induced plasticity can lead to elevated uncertainties in residual stress measurements made by the contour method. In this study plasticity-induced stress errors are numerically evaluated for a benchmark edge-welded beam to understand the underlying mechanism. Welding and cutting are sequentially simulated by finite element models which have been validated by previous experimental results. It is found that a cutting direction normal to the symmetry plane of the residual stress distribution can lead to a substantially asymmetrical back-calculated stress distribution, owing to cutting-induced plasticity. In general, the stresses at sample edges are most susceptible to error, particularly when the sample is restrained during cutting. Inadequate clamping (far from the plane of cut) can lead to highly concentrated plastic deformation in local regions, and consequently the back-calculated stresses have exceptionally high values and gradients at these locations. Furthermore, the overall stress distribution is skewed towards the end-of-cut side. Adequate clamping (close to the plane of cut) minimises errors in back-calculated stress which becomes insensitive to the cutting direction. For minimal constraint (i.e. solely preventing rigid body motion), the plastic deformation is relatively smoothly distributed, and an optimal cutting direction (i.e. cutting from the base material towards the weld region in a direction that falls within the residual stress symmetry plane) is identified by evaluating the magnitude of stress errors. These findings suggest that cutting process information is important for the evaluation of potential plasticity-induced errors in contour method results, and that the cutting direction and clamping strategy can be optimised with an understanding of their effects on plasticity and hence the back-calculated stresses.

Residual Stress Measurement of a Ferritic Bead on Plate Benchmark Test Specimen Using the Contour Method

This paper presents new measured residual stress data for a ferritic weld bead on plate benchmark specimen. The contour residual stress measurement method was applied because it could provide an informative map on the plane of interest capable of quantifying local variations in residual stress associated with phase transformations. The quality of the contour cut was optimised by carrying out initial cutting trials to tune the electro-discharge machining cutting parameters. The risk of plasticity during the cutting procedure was minimised by using an “embedded crack” contour cutting configuration, clamping with fitted bolts and a large diameter cutting wire in order to reduce the crack opening and stress concentration at the cut tip. The results of the contour method measurement are in reasonable overall agreement with published stresses measured by neutron diffraction. However, certain features in the results suggest that significant plasticity did occur during the contour cut despite the specific measures taken to minimise this effect.

Residual stresses in a machined and shrink fitted assemblies

A shrink-fit is a semi-permanent assembly system, widely used in industry, which is designed to resist the relative movement or transmit torque between two components. This done by creating high radial pressures at the interface of the constituent parts. Examples include fitting of gears to shafts to transmit torque and shrink fitted hard wearing sleeves to shafts for steel rolling mills. This paper presents measured and predicted residual stresses from a variety of shrink fitted assemblies, including aluminium alloys, stainless steel and cast irons. In a number of cases the sleeves shrink fitted to the shaft were subjected to prior machining or prior quenching to introduce initial residual stresses in the sleeve. Residual stresses were measured using a combination of deep-hole drilling (DHD) and incremental centre hole drilling (ICHD) techniques. The ICHD method provided near surface residual stress measurements to depths up to about 1mm. The DHD method measured through thickness residual stresses along radial lines through the outer sleeve and into the shaft up to depths of 400mm. Results are shown for a variety of assemblies and reveal residual stress that are significantly different from both simple analysis and finite element studies. For example, prior machining introduced near surface compressive residual stresses, that is then expected to be counteracted by the near surface tensile shrink fit residual stresses. However, measurements reveal that this is not the case.

Measurement of Residual Stresses in Large Engineering Components

Residual stresses play an important role in the strength and life of the components in service. Furthermore, full knowledge of the residual stress field through the whole component is crucial to understanding how residual stresses impact on failure [1]. In the present investigation through thickness residual stress measurements are carried out using the deep hole drilling technique in large components used in the steel rolling industry.

Should Residual Stresses Be Taken Into Account in Structural Integrity Assessment of Offshore Monopiles

A key challenge in the Offshore Wind industry is assuring the life-cycle structural integrity of wind turbine foundation monopiles. This is due to harsh environmental aspects as well as the loading regime (i.e. constant exposure to wave and wind forces introducing both fatigue and corrosion damage). Welding is a widely used joining technique for the manufacturing of offshore monopile structures. However, this is an aggressive process that introduces high levels of residual stress, which in turn may lead to reduced fatigue life, corrosion cracking resistance and accelerated degradation mechanisms. This study presents evidence that a measurement-informed strategy could be used towards developing a more reliable structural integrity assessment procedure for offshore monopile structures by taking into account the effect of residual stresses. A welded mock-up, 90 mm thick, 2600 mm wide and 800 mm long plate, was fabricated using a typical double-V welding procedure following current industrial practice. The contour method of residual stress measurement was employed to map residual stresses in the welded mock-up as well as in the CT specimens extracted from the weld region of the plate for future fatigue tests. Residual stress measurement results show that the mock-up plate contained tensile residual stresses above yield in the core of the weld, while the extracted CT specimens had lower though still significant residual stress levels. These results indicate that if the initial residual stresses are not carefully considered during fatigue or corrosion cracking tests, the results from the CT specimens alone will likely result in misleading structural life estimations.

Residual stress measurement in a stainless steel clad ferritic plate using the contour method

In pressure vessels stainless steel weld-overlay cladding is a widely used technique to provide a protective barrier between the corrosive environment and the ferritic low alloy base metal. While the cladding layers enhance corrosion resistance, the induced residual stresses due to the deposition of weld layers are of major concern. It is of paramount importance to understand how residual stresses interact with service loading when the vessel is pressurized. Therefore, knowledge of the initial residual stresses due to cladding is an essential input for structural integrity assessment of pressure vessels. In the present paper the Contour Method was conducted to measure residual stresses in an austenitic steel cladded plate that was fabricated from a ferritic steel base plate with three layers of austenitic stainless steel weld metal cladding deposited on the top surface. The Contour Method was chosen for various reasons. First, it provides a full 2D variation of residual stresses over the plane of interest. Second, it is not limited by the thickness of components or microstructural variations and finally it should potentially capture the variation of residual stresses in each individual weld beads and due to the possible phase transformation in the ferritic base material. The map of longitudinal residual stresses was measured by sectioning the test component along a transverse plane at mid-length. The measured residual stresses were in good agreement with published results in the open literature.