Residual Stress Effects on Cleavage Fracture

2007 ◽  
Author(s):  
A. H. Sherry ◽  
K. S. Lee ◽  
M. R. Goldthorpe ◽  
D. W. Beardsmore
Keyword(s):  
Fracture Toughness ◽  
Residual Stress ◽  
Residual Stresses ◽  
Cleavage Fracture ◽  
Driving Force ◽  
Compact Tension ◽  
Pressure Vessels ◽  
Crack Driving Force ◽  
Stress Effects ◽  
Definition Of

It is recognised that the driving force for the initiation and propagation of defects in materials may, under some circumstances, include contributions from both externally applied loads such as internal pressure in pressure vessels and piping and secondary stresses such as weld residual stresses. For non stress-relieved welds, residual stresses can provide a significant proportion of the crack driving force. This paper describes the results obtained from an experimental programme aimed at extending the understanding of residual stress effects on cleavage fracture. The paper describes the preparation and testing of standard and preloaded compact-tension specimens of an A533B pressure vessel steel at its Master Curve reference temperature. Standard tests on compact-tension specimens provide fracture toughness data which are broadly consistent with the conventional three-parameter Weibull model, with Kmin = 20 MPa√m and an exponent of about 4. The preloaded compact-tension specimens included a high level of tensile residual stress at the crack location. Fracture toughness data obtained using the test standards from these specimens fall significantly below the standard specimen data, since the contribution from residual stresses is ignored. However, when due account is taken of the residual stress on the crack driving force using a correct definition of the J-integral, the distributions of fracture toughness data from both specimen types are found to overlay each other. The definition of J used in this paper allows residual stress effects on fracture to be accounted for in a single fracture parameter.

Measurement and Modelling of Residual Stresses in Fracture Toughness Specimens Extracted From Large Components

2008 ◽  
Author(s):  
S. J. Lewis ◽  
S. Hossain ◽  
C. E. Truman ◽  
D. J. Smith ◽  
M. Hofmann
Keyword(s):  
Fracture Toughness ◽  
Residual Stress ◽  
Residual Stresses ◽  
Driving Force ◽  
Large Scale ◽  
Structural Integrity ◽  
Mechanical Load ◽  
Crack Driving Force ◽  
Neutron Diffraction Technique ◽  
Fracture Specimens

A number of previously published works have shown that the presence of residual stresses can significantly affect measurements of fracture toughness, unless they are properly accounted for when calculating parameters such as the crack driving force. This in turn requires accurate, quantitative residual stress data for the fracture specimens prior to loading to failure. It is known that material mechanical properties may change while components are in service, for example due to thermo-mechanical load cycles or neutron embrittlement. Fracture specimens are often extracted from large scale components in order to more accurately determine the current fracture resistance of components. In testing these fracture specimens it is generally assumed that any residual stresses present are reduced to a negligible level by the creation of free surfaces during extraction. If this is not the case, the value of toughness obtained from testing the extracted specimen is likely to be affected by the residual stress present and will not represent the true material property. In terms of structural integrity assessments, this can lead to ‘double accounting’ — including the residual stresses in both the material toughness and the crack driving force, which in turn can lead to unnecessary conservatism. This work describes the numerical modelling and measurement of stresses in fracture specimens extracted from two different welded parent components: one component considerably larger than the extracted specimens, where considerable relaxation would be expected as well as a smaller component where appreciable stresses were expected to remain. The results of finite element modelling, along with residual stress measurements obtained using the neutron diffraction technique, are presented and the likely implications of the results in terms of measured fracture toughness are examined.

Developments in the Treatment of Residual Stresses in Welded Components

2011 ◽  
Vol 681 ◽  
pp. 73-78
Author(s):  
Steve K. Bate ◽  
Ian Symington ◽  
John Sharples ◽  
Richard Charles ◽  
Adam Toft ◽  
...  
Keyword(s):  
Residual Stress ◽  
Residual Stresses ◽  
Driving Force ◽  
Structural Integrity ◽  
Research Programme ◽  
Residual Stress Field ◽  
Crack Driving Force ◽  
Stress Effects ◽  
Environmentally Assisted Cracking

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

Effects of Weld Geometry on Residual Stress and Crack Driving Force for Centerhole Control Rod Drive Mechanism Nozzles: Part I — Weld Residual Stress

2005 ◽  
Author(s):  
Wentao Cheng ◽  
David L. Rudland ◽  
Gery Wilkowski ◽  
Wallace Norris
Keyword(s):  
Residual Stress ◽  
Residual Stresses ◽  
Driving Force ◽  
Nuclear Regulatory Commission ◽  
Crack Driving Force ◽  
Control Rod ◽  
Drive Mechanism ◽  
Weld Geometry ◽  
Weld Residual Stress ◽  
Regulatory Commission

The U.S. Nuclear Regulatory Commission (NRC) has undertaken a program to assess the integrity of control rod drive mechanism (CRDM) nozzles in existing plants that are not immediately replacing their RPV heads. This two-part paper summarizes some of the efforts undertaken on the behalf of the U.S.NRC for the development of detailed residual stress and circumferential crack-driving force solutions to be used in probabilistic determinations of the time from detectable leakage to failure. In this first paper, the finite element (FE) simulations were conducted to investigate the effects of weld geometry on the residual stresses in the J-weld for a centerhole CRDM nozzle. The variables of weld geometry included three weld heights (weld sizes) and three groove angles for each weld height while keeping the same weld size. The analysis results indicate that the overall weld residual stress decreases as the groove angle increases and higher residual stress magnitude is associated with certain weld height. The results also reveal that the axial residual stresses in the Alloy 600 tube are very sensitive to the weld height, and that the tube hoop stresses above the J-weld root increase with the increasing weld height.

Displacement Controlled Stress Intensity Factor Solutions for Structural Integrity Assessments of Welding Residual Stress Distributions

2008 ◽  
Author(s):  
Adam Toft ◽  
David Beardsmore ◽  
Colin Madew ◽  
Huego Teng ◽  
Mark Jackson
Keyword(s):  
Residual Stress ◽  
Stress Intensity Factor ◽  
Stress Intensity ◽  
Intensity Factor ◽  
Residual Stresses ◽  
Weight Function ◽  
Driving Force ◽  
Welding Residual Stress ◽  
Crack Plane ◽  
Crack Driving Force

Within the UK nuclear industry the assessment of fracture in pressurised components is often carried out using procedures to calculate the margin of safety between a lower-bound fracture toughness and the crack driving force. Determination of the crack driving force usually requires the calculation of elastic stress intensity factor solutions for primary loads and secondary loads arising from weld residual stresses and/or thermal stresses. Within established UK assessment procedures weight function solutions are available which allow the stress intensity factors to be calculated from the through-wall opening-mode stress distribution in an uncracked component. These weight-function solutions are generally based on models where either no boundary condition is applied, or where one is applied at a distance either side of the crack plane that is very long compared with the crack size and wall thickness. Such solutions do not take into account any reduction in the stress field that might occur as the distance from the crack faces increases. Weld residual stress fields may often be expected to reduce in this manner. A separate, earlier study has shown that the stress intensity factor for a cracked plate loaded in displacement control decreases substantially as the loading plane is moved closer to the crack plane. It would therefore be expected that a similar reduction in stress intensity factor would be obtained for a residual stress analysis when displacement boundary conditions are imposed at a distance relatively close to the crack plane. This paper describes an investigation of the differences, particularly in terms of a reduction in calculated stress intensity factor, which may arise from application of displacement controlled stress intensity factor solutions, as compared with load controlled solutions, when considering weld residual stresses. Consideration is also given as to how new displacement controlled stress intensity factor solutions could be developed by modification of existing load controlled solutions.

The Effects of Constraint and Compressive or Tensile Residual Stresses on Brittle Fracture

2009 ◽  
Author(s):  
Simon Kamel ◽  
Robert C. Wimpory ◽  
Michael Hofmann
Keyword(s):  
Fracture Toughness ◽  
Residual Stress ◽  
Brittle Fracture ◽  
Driving Force ◽  
High Strength ◽  
Element Analysis ◽  
Crack Driving Force ◽  
Temperature Fracture ◽  
Crack Tip Constraint ◽  
Two Parameter

Residual stress is a key feature in components containing defects which can affect the crack driving force and alter the crack tip constraint to give a modified fracture toughness. In this paper experimental and numerical investigations are performed on ‘C’ shape fracture mechanics specimens, extracted from a high strength low alloy tubing steel, to examine the effects of constraint and tensile or compressive residual stress on brittle fracture. The residual stress is introduced into the specimens by a tensile or compressive mechanical pre-load to produce, respectively, a compressive or tensile residual stress in the region where the crack is introduced. Neutron diffraction measurements are performed on the pre-loaded specimens prior to introduction of a crack, and compared with predictions of the residual stress from finite-element analysis, using tensile properties derived at room temperature. Fracture toughness tests are carried out on the as-received (non-preloaded) and pre-loaded specimens and the effect of residual stress on crack driving force and constraint is evaluated using the two-parameter J-Q approach.

Provisional Results for an Experimental Investigation Into the Effect of Combined Primary and Secondary Stresses When Considering the Approaches of R6 and the Recently Developed g() Method

2011 ◽  
Author(s):  
Peter James ◽  
Paul Hutchinson ◽  
Colin Madew
Keyword(s):  
Fracture Toughness ◽  
Residual Stress ◽  
Experimental Investigation ◽  
Stress Field ◽  
Driving Force ◽  
Residual Stress Field ◽  
Crack Driving Force ◽  
Material Characterisation ◽  
Point Bend ◽  
Initial Results

Engineering components, particularly those containing weldments, may contain small crack-like defects that experience combinations of primary and secondary stresses during service. A new function, g(), has been introduced previously to quantify the influence of plasticity interaction under combined primary and secondary loading on a components crack driving force. This paper compares g() with experiments performed to consider g() over a range of plasticity values. This experimental programme was performed on scalloped notch three point bend specimens that had experienced a pre-compression to induce a residual stress field before being tested to failure over a range of temperatures (−150, −90 and −50 °C). Samples which did not undergo a pre-compression were also tested to provide an estimate of the materials fracture toughness at the temperature in question. Through analysing the experimental results it is clear that further material characterisation is required. This paper, therefore, only presents the initial results at this stage. However, as a pessimistic interpretation of the results has been made, and since both the existing R6 and the g() plasticity interaction parameters are acceptable, the experiments provide useful validation to both methods.

The Influence of Residual Stresses on Small Through-Clad Cracks in Pressure Vessels

1984 ◽  
Vol 106 (4) ◽  
pp. 383-390 ◽  
Author(s):  
H. G. deLorenzi ◽  
B. I. Schumacher
Keyword(s):  
Residual Stresses ◽  
Pressure Vessel ◽  
Driving Force ◽  
Base Material ◽  
Pressure Vessels ◽  
Plastic Behavior ◽  
Material Interface ◽  
Crack Driving Force ◽  
Elastic Plastic ◽  
Stress Relieving

The influence of cladding residual stresses on the crack driving force for shallow cracks in the wall of a nuclear pressure vessel is investigated. Thermo-elastic-plastic analyses were carried out on long axial through-clad and sub-clad flaws on the inside of the vessel. The depth of the flaws were one and three times the cladding thickness, respectively. An analysis of a semielliptical axial through-clad flaw was also performed. It was assumed that the residual stresses arise due to the difference in the thermal expansion between the cladding and the base material during the cool down from stress relieving temperature to room temperature and due to the subsequent proof test before the vessel is put into service. The variation of the crack tip opening displacement during these loadings and during a subsequent thermal shock on the inside wall is described. The analyses for the long axial flaws suggest that the crack driving force is smaller for this type of flaw if the residual stresses in the cladding are taken into account than if one assumes that the cladding has no residual stresses. However, the analysis of the semielliptical flaw shows significantly different results. Here the crack driving force is higher than when the residual stresses are not taken into account and is maximum in the cladding at or near the clad/base material interface. This suggests that the crack would propagate along the clad/base material interface before it would penetrate deeper into the wall. The elastic-plastic behavior found in the analyses show that the cladding and the residual stresses in the cladding should be taken into acocunt when evaluating the severity of shallow surface cracks on the inside of a nuclear pressure vessel.

Development of an Alternative Approach to the Acquisition of Fracture Toughness in Laboratory Specimens Containing Residual Stress

2010 ◽  
Author(s):  
R. G. Hurlston ◽  
J. Sharples ◽  
A. H. Sherry
Keyword(s):  
Fracture Toughness ◽  
Residual Stress ◽  
Residual Stresses ◽  
Pressure Vessels ◽  
Relaxation Methods ◽  
Edge Notch ◽  
Material Fracture ◽  
Crack Tip Constraint ◽  
Conservative Values ◽  
Welding Processes

Welding is an essential process in many industries and is used for both the production and repair of nuclear plant, notably pressure vessels and piping. However, traditional welding processes can cause large amounts of residual stress to be generated within the structure. Current methodology for evaluating fracture toughness from specimens containing residual stresses, e.g. BS7448, relies heavily on engineering judgement. This can result in inaccurate, albeit generally conservative, values of fracture toughness being used in defect assessments. The aim of the work presented in this paper is to investigate the use of constraint based fracture mechanics to quantify ‘unique material fracture toughness’ from laboratory specimens containing residual stresses using the ‘apparent fracture toughness’ values derived from standard fracture toughness testing. This is achieved using an analytical knowledge of the effect of residual stress on crack-tip constraint and, if incorporated into fracture toughness methodology, remove the need for unreliable residual stress relaxation methods when using weld coupons for fracture toughness assessments. A novel mechanical method for generating residual stresses in single edge notch bend specimens has been assessed analytically. In this paper, computational analysis of low and high constraint bend specimens, each with and without residual stress, is used to demonstrate the principle and validity of the proposed method.

Effects of Weld Geometry on Residual Stress and Crack Driving Force for Centerhole Control Rod Drive Mechanism Nozzles: Part II — Circumferential Cracked K-Solutions

2005 ◽  
Author(s):  
David L. Rudland ◽  
Wentao Cheng ◽  
Gery Wilkowski ◽  
Wallace Norris
Keyword(s):  
Residual Stress ◽  
Finite Element ◽  
Residual Stresses ◽  
Driving Force ◽  
Finite Element Models ◽  
Operating Pressure ◽  
Crack Driving Force ◽  
Control Rod ◽  
Weld Geometry ◽  
Circumferential Crack

The U.S.NRC has undertaken a program to assess the integrity of CRDM nozzles in existing plants that are not immediately replacing their RPV heads. This two-part paper summarizes some of the efforts undertaken on the behalf of the U.S.NRC for the development of detailed residual stress and circumferential crack-driving force solutions to be used in probabilistic determinations of the time from detectable leakage to failure. In this second paper, the weld residual stresses from the first paper were mapped onto detailed fracture mechanics finite element models that contained different length circumferential cracks in the CRDM centerhole nozzle. In each case, the cracks were unpinned after application of the operating pressure and temperatures and the K-solutions extracted. The results from these analyses suggest that the crack-driving force for a circumferential crack at the root of the J-weld slightly increases with increasing weld height but appears to be insensitive to bevel angle.

Residual Stress Effects on Ductile Tearing

2006 ◽  
Author(s):  
A. H. Sherry ◽  
M. R. Goldthorpe ◽  
J. Fonseca ◽  
K. Taylor
Keyword(s):  
Residual Stress ◽  
Residual Stresses ◽  
Driving Force ◽  
Structural Integrity ◽  
Driving Forces ◽  
Internal Stresses ◽  
Engineering Structures ◽  
Numerical Work ◽  
Crack Driving Force ◽  
Integrity Assessment

Residual stresses are internal stresses generated during the fabrication and/or operation of engineering structures. Such stresses can provide the major element of the driving force for crack initiation and growth. Structural integrity assessment procedures, provide guidance for the assessment of defects located within regions of high residual stress. However, such guidance may be conservative where the defect develops progressively during service. This paper describes recent experimental and numerical work aimed at quantifying such conservatisms and providing improved guidance for undertaking more realistic analyses. The results demonstrate that pre-loaded compact-tension specimens provide a useful means for studying the behaviour of cracks within residual stress fields. The magnitude of calculated crack driving forces due to residual stresses is influenced by the approach used to introduce cracks into the stress field, with progressive cracks providing lower levels of crack driving force than instantaneously introduced cracks. The J R-curve associated with cracks under primary or combined primary + secondary loading can apparently be rationalized when the total crack driving force is calculated using methods that take proper account of the influence of prior plasticity on the J-integral. However, it is noted that due to differences in the form of the crack-tip stress and strain fields for static and growing cracks, such values of J may be path dependent and influenced by the magnitude of the growth increment.

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