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.

Load History Effects on Crack Driving Force for Cracks in Residual Stress Fields

2008 ◽  
Author(s):  
Richard Charles ◽  
David W. Beardsmore ◽  
Huaguo Teng ◽  
Chris T. Watson
Keyword(s):  
Fracture Toughness ◽  
Residual Stress ◽  
Driving Force ◽  
Thermal Loading ◽  
Stress Fields ◽  
Residual Stress Field ◽  
Element Analysis ◽  
Crack Driving Force ◽  
History Effects ◽  
Material Fracture

Fracture mechanics assessments of engineering components and structures containing defects are made by comparing an estimate of the crack driving force KJ with an effective fracture toughness KJc. The assessments must account for the combined effect of primary loads, such as internal pressure in pressurised components, and secondary stresses arising from welding and/or thermal loading. Elastic-plastic finite element analysis, or simplified methods set out in standard assessment procedures, can be used to estimate the crack driving force KJ as a function of the applied primary load on the component. The effective fracture toughness KJc should take account of the material fracture toughness and the crack tip constraint. For the assessment of defects in weld residual stress fields, it is usually assumed that the defect is inserted into the as-welded stress distribution in such a way that traction free crack surfaces are created simultaneously at all positions on the crack faces. However, it may be beneficial to take account of any relaxation in the residual stress field that might arise during proof-testing or in-service cyclic loading, and to consider a more gradual, progressive introduction of the defects. These benefits could, in principle, result in a reduction in the crack driving force. This paper describes work that has been undertaken to provide estimates of the crack driving force KJ for a fully-circumferential defect in a circumferential repair weld in a cylindrical pipe. Calculations have been carried out to establish KJ for a number of cases where different pressure overloads are applied to the uncracked pipe and different methods of crack insertion are applied. Estimates of the margin of safety on fracture toughness and pressure loading were calculated. At the outset, it was assumed that the fracture toughness of relevance for the defects is the material fracture toughness KJc* derived from strain free, high constraint fracture toughness specimens. No allowance was made for constraint effects associated with the finite geometry or initial strains in the pipe. The values of KJ were derived from values of J calculated using the JEDI post-processing code; this allows for initial inelastic strains present in the model prior to the start of the crack insertion process.

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.

Stored Energy of Plastic and Elastic Origin and Recrystallization

2008 ◽  
Vol 571-572 ◽  
pp. 143-148
Author(s):  
Krzysztof Wierzbanowski ◽  
Andrzej Baczmanski ◽  
Jacek Tarasiuk ◽  
Paul Lipiński ◽  
Alain Lodini
Keyword(s):  
Residual Stress ◽  
Plastic Deformation ◽  
Dislocation Density ◽  
Stress Field ◽  
Driving Force ◽  
Recrystallization Process ◽  
Stored Energy ◽  
Residual Stress Field ◽  
Orientation Distributions ◽  
Deformation Models

Stored energy is generally considered as a main driving force of recrystallization process. After plastic deformation a high dislocation density and residual stress field remain in a material. Both quantities are at the origin of the stored energy and we call them as the “plastic” and “elastic” parts of this energy. Their orientation distributions can be determined using diffraction and deformation models. Both components of the stored energy are studied in the present work. Their distributions and characteristics are studied for f.c.c. and b.c.c. materials.

Modelling Fatigue Crack Growth in a Residual Stress Field

2006 ◽  
Author(s):  
A. J. Price ◽  
P. Tsakiropoulos ◽  
M. R. Wenman ◽  
P. R. Chard-Tuckey
Keyword(s):  
Fracture Toughness ◽  
Residual Stress ◽  
Fatigue Crack ◽  
Stress Distribution ◽  
Stress Field ◽  
Crack Growth ◽  
Nuclear Reactor ◽  
Element Model ◽  
Residual Stress Distribution ◽  
Residual Stress Field

Tensile residual stresses can have a detrimental affect on the safe operating limits of components. In most cases, these residual stress fields can be relieved through various treatments but in many cases it is not realistic to expect the complete elimination of these stresses. When considering the Reactor Pressure Vessel (RPV) located within a Nuclear Reactor Plant (NRP), knowledge of fatigue and fracture within a residual stress field is essential in support of safety cases. This research has investigated the behaviour of flaws that lie within a residual stress field with emphasis on fracture toughness through a series of fracture toughness tests. Alongside this experimental series, a finite element model has been created to predict the stress distributions prior to fracture. To enable an accurate simulation of the residual stress field distribution before loading to fracture it is important that the introduction of a fatigue crack is accurately modelled. This paper details several methods of introducing a fatigue crack into a simulation. During this research it has been shown that the introduction of a crack in progressive stages will lead to a better representation of the residual stress distribution prior to fracture. It has been shown that it is essential to use experimentally determined crack front shapes for the final stage of crack growth as this shape can significantly alter the residual stress distribution.

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.

Fracture Margins for Growing Cracks in Weld Repairs

2005 ◽  
Author(s):  
Michael C. Smith ◽  
Peter J. Bouchard ◽  
Martin R. Goldthorpe ◽  
Didier Lawrjaniec
Keyword(s):  
Residual Stress ◽  
Fracture Mechanics ◽  
Driving Force ◽  
Driving Forces ◽  
Linear Elastic Fracture Mechanics ◽  
J Integral ◽  
Residual Stress Field ◽  
Crack Driving Force ◽  
Linear Elastic ◽  
Elastic Fracture

The residual stress field around a single-pass weld filling a slit in a thin rectangular plate has been simulated using both 2D ABAQUS and 3D SYSWELD finite element models, with good agreement between the two codes. Through-wall cracks of varying lengths have been inserted into the plate along the weld centre-line, and the non-linear crack driving force due to residual stress evaluated using three formulations of the J-integral: the standard ABAQUS J, the G-theta approach coded into SYSWELD, and a modified J-integral, Jmod, that retains its path independence under non-proportional loading. Cracks were introduced into the FE meshes either simultaneously (all crack flank nodes released in the same step) or progressively (crack opened in small increments from mid-length to tip). The results were compared with crack driving force estimates made using linear elastic fracture mechanics (LEFM) and the R6 procedure. The crack driving forces predicted by all three J–formulations agree well for simultaneous opening, showing that the crack driving force rises to a peak for a crack length equal to the weld length, and falls for longer cracks. Linear elastic fracture mechanics gives a good estimate of the crack driving force for very short defects (confirming the absence of elastic follow up), but is conservative for longer defects, overestimating the peak driving force by 20%. The R6 estimates, which incorporate plasticity corrections, are more conservative than LEFM, overestimating the peak crack driving force by up to 60%. The crack driving force for a progressively opened crack is much lower than for simultaneous opening, indicating that there may be considerable excess pessimism in conventional assessments of defects of this type.

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

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.

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