Treatment Of Residual Stresses Research Articles

Mean stress correction in fatigue design under consideration of welding residual stress

The fatigue strength of welded steels is affected by the applied load mean stress and the residual stress in the vicinity of the weld. The mean stress correction in fatigue design concepts used for welded structures commonly distinguishes between three subjective generalized residual stress conditions, “low, medium, and high” tensile residual stress. This qualitative treatment of residual stress leads to imprecise evaluation of residual stress effects, in particular when compressive residual stress is present or high-strength steels are applied. The objectives of the underlying study are to emphasize the interaction of load mean stress with residual stress and to provide an approach for the combined treatment of those stress components in the nominal stress concept. The principles of mean stress and residual stress effects on fatigue are presented and discussed. Furthermore, the role of residual stress relaxation is emphasized and cyclically stabilized local residual stress is combined with mean stress to effective mean stress. The fatigue design concept of local endurance limits and effective mean stress is introduced for the quantitative evaluation of residual and mean stress effects. Finally, the effective mean stress approach is applied to longitudinal stiffeners made from different steel grades containing various residual stress conditions. It is shown how design S-N curves can be adjusted based on quantitative effective mean stress. Finally, an improved bonus factor concept based on effective mean stress is presented, which allows a mean stress correction under consideration of the residual stress condition.

Finite Element Analysis of Residual Stress in Welded Joints of T91 Steel

The pressure equipment is the key equipment in the petrochemical industry, natural gas chemical industry and coal chemical industry. Without safe and reliable pressure equipment, advanced process industry cannot be realized. It is important of the quality assurance of pressure vessel in order to prevent the failure of pressure vessel, especially some rupture accidents. One of the key reasons for quality assurance of pressure vessel is the treatment of residual stress in welded joint. In this paper, the working stress and welding residual stress of the thick plate of pressure equipment material and the welding joint of pressure equipment have been measured and analyzed successively by means of actual stress measurement and finite element simulation analysis. The influencing factors of welding stress are introduced, and the control of residual stress is analyzed and some suggestions are put forward in this paper.

Welding residual stresses in a strip of a pipe

Residual stress was measured through the thickness of a strip sample of a girth welded pipe. The motivation behind this measurement was to investigate whether the segment test, commonly used in validation of Engineering Critical Assessment results, is representative of full girth welds in terms of the welding residual stress. A major rationale was to try to understand how much residual stress will be relieved due to the removal of the strip sample from the pipe. The reported results in this paper are based on aJoint Industry Project (JIP) led by DNV GL on the treatment of residual stress on pipes undergoing high plastic deformation. The paper presents measured residual stresses data in the hoop and axial direction of the pipe. In the current study, the measured residual stresses in the segment sample are found to be similar to those at similar full girth welds but showed more compressive stresses in the weld cap, more tensile stresses in the weld root, and a similar profile through the thickness. This re-distribution of residual stress can be attributed to the release of the “far field” bending stresses when the pipe was sectioned into the strip in both the hoop and axial directions.

Redistribution of welding residual stress following high plastic deformation in seamless pipes

This paper summarises the findings of a Joint Industry Project (JIP) led by DNV GL on the treatment of residual stress on pipes undergoing high plastic deformation. The paper presents measured residual stresses and some of the full-scale test results supported by Finite Element Analysis (FEA) for the seamless pipes before and after high plastic deformation on selected girth welds. Several FEA models of the reeling simulation have been developed including mapping an initial axial residual stress (transverse to the weld) profile onto a seamless girth-welded pipe. The initial welding residual stress distribution used for mapping was based on a number of through thickness measurement along the circumference of the girth welds. The predicted residual stresses after reeling simulation was subsequently compared with experimental measurements. It is evident that FEA models can predict welding residual stress re-distribution during the reeling process, but some differences with measurements are present through the thickness.

Evaluation of the Influence of Residual Stresses on Ductile Fracture

In this work, the significance of residual stresses on ductile fracture is investigated by a set of experiments that are analyzed by finite element simulations. The treatment of residual stresses as expressed in fracture assessment procedures such as R6 is believed to be very conservative for ductile materials, when fracture occurs at high primary loads. Earlier numerical studies have reinforced this belief. This is supported in the current study. Tests on notched 3PB specimens with and without residual stresses were conducted on two ferritic steels. The residual stresses were introduced by applying a compressive preload on notched specimens. The tests were designed to achieve crack initiation at load levels around the plastic limit load. The crack growth in the tests was measured by a compliance method and by color marking of the crack surface. The crack tip driving force J was evaluated numerically for specimens with and without residual stresses. The experimental results show that the residual stresses clearly contribute to J at low primary loads. However, this contribution diminishes as the primary loads increase. The experimental results were also compared with results evaluated using the R6 procedure. These comparisons revealed overly high conservatism in R6 for cases with residual stresses compared to the ones for cases without residual stresses where less conservatism was evident.

Developments in the Treatment of Residual Stresses in Welded Components

A long-term UK research programme on environmentally assisted cracking (EAC), residual stresses [1, 2] and fracture mechanics [3, 4] was launched in 2004. It involves Rolls-Royce plc and Serco Technical Services, supported by UK industry and academia. The residual stress programme is aimed at progressing the understanding of residual stresses and on the basis of this understanding manage how residual stresses affect the structural integrity of plant components. Improved guidance being developed for the treatment of residual stresses in fracture assessments includes the use of stress intensity factor solutions for displacement controlled loading as opposed to the more commonly used load controlled solutions. Potential reductions in crack driving force are also being investigated in relation to (i) utilizing a residual stress field that has “shaken-down” due to operational loads, (ii) introducing a crack progressively as opposed to instantaneously, and (iii) allowing for the fact that a crack may have been initiated during the life of a component as opposed to being present from the start-of-life. This paper describes some of these latest developments in relation to residual stress effects

Fatigue crack growth performance of peened friction stir welded 2195 aluminum alloy joints at elevated and cryogenic temperatures

The effects of various surface treatments on fatigue crack growth and residual stress distributions in friction stir welded 2195 aluminum alloy joints were investigated. The objective was to understand the degree to which residual stress treatments can reduce fatigue crack growth rates, and enhance fatigue life of friction stir welded components. Specimens were fabricated from 12.5 mm thick 2195-T8 aluminum plate, with a central friction stir weld along their length. Residual stresses were measured for three specimen conditions: as-welded (AW), welded then shot peened (SP), and welded then laser peened (LP). Crack growth rate tests were performed in middle-cracked tension specimens under constant amplitude load for each of the three conditions (AW, SP, LP) and at three temperatures (room, elevated, and cryogenic). At room and elevated temperature, crack growth rates were similar in the AW and SP conditions and were significantly lower for the LP condition. At cryogenic temperature, it was difficult to discern a trend between residual stress treatment and crack growth rate data. Laser peening over the friction stir welded material resulted in the fatigue crack growth rates being comparable to those for base material.

Some challenges in the treatment of residual stress in flaw tolerance calculations—A TAGSI view

The UK Technical Advisory Group on the Structural Integrity of Nuclear Plant (TAGSI) has recently reviewed the methodologies for combining residual stresses with primary stresses. The review was focused on those situations where secondary stresses are large compared to the primary stresses and where, in the case of ductile materials such as stainless steel, procedures predict low margins of safety, contrary to perception and practical experience. The paper reports the main points arising from the review. Proposals are made for new terminology to describe residual stress data and profiles, namely, judgemental means and upper bounds, and it is recommended that, in future, residual stress data are presented in both these forms. Factors influencing conservatism in R6 analyses are discussed and include the methods for combining primary and secondary stresses and the way the defect is modelled. The choice of fracture toughness parameter is also important. The review showed that no codes other than R6 have improved advice on the treatment of secondary stresses. Potential alternative approaches to reduce conservatism are those based on (i) strain-based methods and (ii) uncracked body analyses to explore stress redistribution through shakedown.

Treatment of residual stress in failure assessment procedure

The effect of residual stress on component failure has been investigated using the distributions from current failure assessment procedures, and a residual stress profile simple to apply with less conservatism has been proposed for the weld geometries of T-plate and tubular T-joint. The stress intensity factors (SIFs) in the two weld geometries under various types of loads have been calculated using the Green’s function method. The Green’s functions were determined not only for the T-plate but also for the tubular T-joint with the built-in ends. The use of a linear (bending) stress profile, derived from an analysis of measured residual stress distributions in T-plate and tubular T-joints, has been examined. The profile was validated with experimentally measured residual stress distributions in two materials, a high strength and medium strength ferritic steel and two geometries, a T-plate joint and a tubular T-joint for crack lengths up to half the plate or pipe thickness. Whereas the recommended residual stress distributions are geometry and material specific, it is shown that a simplified linear bending profile provides a possible guideline, applicable to a range of materials and geometries, where detailed information on weld procedures or residual stress profiles are unavailable.

비드의 용용상태를 고려한 가스메탈 아크용접의 3차원 열탄소성 변형 해석

Welding is essential in ship production since welding is very popular method for joining two or more metals. However, welding causes residual stress and distortion and these give a bad influence to the structure strength and assembly of ship blocks. Therefore, prediction and treatment of residual stress and distortion is a key to accuracy control in shipyard. In this paper, a computational procedure, based on thermal-elastic-plastic 3-dimensional FEA, has been suggested to simulate butt and fillet welding process. In the simulation process, temperature distribution at each time step is obtained by heat transfer analysis and then thermal deformation analysis is done with obtained temperature distributions to find the residual stress and distortion. In heat transfer analysis, enthalpy method is used to realize phase change at melting temperature. Also element birth and death method is used to simulate adding of weld metal in both heat transfer analysis and thermal elastic plastic analysis. The proposed procedure is verified by related researches and the results show good agreement with those of related researches.

The Treatment of Residual Stress in Fracture Assessment of Pressure Vessels

The treatment of weld residual stress in the fracture assessment of cylindrical pressure vessels is considered through partitioning the stress into membrane, bending, and self-balancing through-wall components. The influence of each on fracture behavior is discussed. Stress intensity factor solutions appropriate to each type of stress are presented. Short-range, medium-range, and long-range stress categories are identified according to simple rules relating the effect of increasing crack length to stress intensity factor and ligament net stress. Proposals are made on how the stress intensity factor from these stress types may be incorporated into a Kr, Lr-based fracture assessment.

A Study of Mechanical Stress Relief (MSR) Treatment of Residual Stresses for One-Pass Submerged Arc Welding of V-Grooved Mild Steel Plate

The heat-transfer and thermal stress distributions in one-pass submerged arc welding (SAW) were numerically determined using the finite element method (FEM) for a V-grooved rectangular steel plate in which the weld preparation was filled during welding. A two-dimensional non-linear heat-transfer analysis was performed for a transverse section of the plate. This was followed by a thermo-elasto-plastic transient thermal stress analysis, assuming plane strain to be constant for the same model section. The same stress model was used to simulate a mechanical stress relief (MSR) treatment of the plate. This has frequently been used in the fabrication of large pressure vessels instead of post-weld heat treatment (PWHT). In this way its effect in reducing the residual stress in the welded plate was investigated. MSR was simulated by enforcing a constant displacement loading in the welding direction. The solution of the thermal stress analysis showed that it was possible for the residual stress around the weld centre to be accurately estimated by accurate modelling of the dilution of the filler metal in the fused zone of the base metal. The conclusion derived from the MSR simulation was that it could quantitatively predict the effect of reducing the residual stress in the welded plate. Only limited experimental data were available. The mechanism of stress reduction was plastic straining in regions of high residual stress. The amount of stress reduction at the weld centre had a linear relationship to the magnitude of the external stress relieving load. The numerical results for the MSR simulation agreed fairly well with experimental ones obtained from the MSR test performed on a welded plate.

Recommendations on the treatment of residual stresses in PD6493 for the assessment of the significance of weld defects

The British Standards Institution document PD6493 (1980) “Guidance on some methods for the derivation of acceptance levels for defects in fusion welded joints” is currently under revision. The fracture section of the new document uses two-parameter failure assessment diagrams and permits assessments of the significance of weld defects at three levels of complexity. This paper presents new procedures for the treatment of secondary stresses for level 2 and level 3 assessments in PD6493. Comparisons are also made between the proposed procedures and the CEGB R6 procedures which are based on similar principles to the revised PD6493 document. Some validation studies are also reported in this paper.

Propagation of fatigue cracks from notches

AbstractThe traditional design approaches to fatigue at notches, based on stress level–endurance relationships, are briefly reviewed. It is shown, by considering crack propagation from notches and invoking a change in control mode from notch plasticity to crack-tip plasticity, that a critical stress condition can be obtained which must be exceeded if the crack is to propagate to failure. The traditional techniques are then reinterpreted and explained by this propagation method. An example is given of crack growth from a sharp defect at a weld toe. It is shown that the integration of an elastic fracture mechanics growth law can reproduce stress range–cycles to failure data for this situation. There are, however, complexities of stress analysis and crack shape. A simple treatment of residual stresses affecting the threshold and slow–growth regimes, shows some promise as a technique for accounting for residual stresses.MST/70