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.

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.

Thermal Simulation of an Arbitrary Residual Stress Field in a Fully or Partially Autofrettaged Thick-Walled Spherical Pressure Vessel

2008 ◽  
Author(s):  
M. Perl
Keyword(s):  
Residual Stress ◽  
Finite Element ◽  
Stress Field ◽  
Pressure Vessel ◽  
Stress Fields ◽  
Equivalent Temperature ◽  
Residual Stress Field ◽  
Element Analysis ◽  
Spherical Pressure ◽  
The Ideal

The equivalent thermal load was previously shown to be the only feasible method by which the residual stresses due to autofrettage and its redistribution, as a result of cracking, can be implemented in a finite element analysis, of a fully or partially autofrettaged thick-walled cylindrical pressure vessel. The present analysis involves developing a similar methodology for treating an autofrettaged thick-walled spherical pressure vessel. A general procedure for evaluating the equivalent temperature loading for simulating an arbitrary, analytical or numerical, spherosymmetric autofrettage residual stress field in a spherical pressure vessel is developed. Once presented, the algorithm is applied to two distinct cases. In the first case, an analytical expression for the equivalent thermal loading is obtained for the ideal autofrettage stress field in a spherical shell. In the second case, the algorithm is applied to the discrete numerical values of a realistic autofrettage residual stress field incorporating the Bauschinger effect. As a result, a discrete equivalent temperature field is obtained. Furthermore, a finite element analysis is performed for each of the above cases, applying the respective temperature field to the spherical vessel. The induced stress fields are evaluated for each case and then compared to the original stress. The finite element results prove that the proposed procedure yields equivalent temperature fields that in turn simulate very accurately the residual stress fields for both the ideal and the realistic autofrettage cases.

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.

Load and Crack History Effects on Fracture

2007 ◽  
Author(s):  
J. K. Sharples ◽  
P. M. James ◽  
L. A. Higham ◽  
P. M. Wood ◽  
H. Teng ◽  
...  
Keyword(s):  
Residual Stress ◽  
Driving Force ◽  
Driving Forces ◽  
Normal Operation ◽  
Proportional Loading ◽  
Crack Driving Force ◽  
History Effects ◽  
Weld Residual Stress ◽  
Assessment Procedures ◽  
Critical Process

Assessments of the integrity of structures containing defects or cracks require estimates to be made of the elastic-plastic crack driving force (CDF) parameter J. This is the characterising parameter that controls the intensity of the fields of stress and strain close to the tip of a crack. Such estimates of J are inherently made in assessment procedures such as R6, Revision 4 [1]. Engineering components are typically subjected to load cycles, often with significant variations in magnitude. Normal operation cycles or overload (by a proof pressure test for example) may cause a re-distribution of weld residual stresses. A defect can be present at fabrication or develop during operation due to a sub-critical process such as fatigue or stress corrosion cracking. In these two cases, it is reasonable to suppose that the actual crack driving forces are different; since the development of a defect in a region of weld residual stress, in conjunction with additional primary loading, can cause significant non-proportional loading of the crack tip. The objective of the work described in this paper is to provide more accurate estimates of the crack driving force parameter for defects subjected to combined primary and secondary stresses, taking into account the effects of loading hisotory. The eventual aim is to reduce uncertainty in assessments of plant integrity, and to clarify advantage that can be taken from a reduction in crack driving forces due to weld residual stress resulting from overload, operational cycles and the progressive introduction of sub-critical defects. Finite element analyses and R6 calculations are undertaken and compared to examine the effects of inserting a crack at different times during the life of an engineering structure.

scholarly journals Numerical Reconstruction of Residual Stress Fields from Limited Measurements

2014 ◽  
Vol 996 ◽  
pp. 243-248
Author(s):  
Harry E. Coules ◽  
David J. Smith ◽  
Karim H.A. Serasli
Keyword(s):  
Residual Stress ◽  
Stress Field ◽  
Measurement Data ◽  
Stress Fields ◽  
Residual Stress Field ◽  
Element Analysis ◽  
Numerical Reconstruction ◽  
Incomplete Measurement ◽  
Stress States ◽  
Measurement Strategies

By finding stress states which are consistent both with any existing experimental measurements and with elasticity theory, residual stress fields can often be reconstructed from incomplete measurement data. We discuss such methods of residual stress reconstruction, their implementation using finite element analysis, and the measurement strategies which enable them. In general, reconstruction of residual stress fields must be formulated as an inverse problem, which can usually be solved using stress basis functions. However, prior knowledge of the form of the residual stress field and/or underlying eigenstrain distribution often allows the problem to be reduced such that inverse methods become unnecessary, greatly simplifying the analysis. Two examples of when residual stress field reconstruction can be simplified in this way are given.

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

A Novel Fracture Mechanics Specimen to Investigate the Effect of Residual Stress and Crack-Tip Constraint on Fracture

2011 ◽  
Author(s):  
Simon Kamel ◽  
Tiyaporn Vanagosoom ◽  
Prakash Shanthenu ◽  
Catrin M. Davies ◽  
Kamran M. Nikbin
Keyword(s):  
Fracture Toughness ◽  
Residual Stress ◽  
Finite Element Analysis ◽  
Crack Tip ◽  
Residual Stress Field ◽  
Element Analysis ◽  
Load Line ◽  
Low Constraint ◽  
Crack Tip Constraint ◽  
Fracture Toughness Specimen

A novel fracture toughness specimen design is presented for investigating the effects of residual stress and crack tip constraint on fracture. The specimen design, denoted here as an SC(T) specimen, is a hybrid of a blunt-notched C(T) specimen and an SEN(T) specimen. The SC(T) specimen is mechanically pre-compressed on the C(T) load line to introduce a residual stress in the ligament ahead of a blunt notch. Following pre-compression, a crack is introduced into the tensile residual stress field. The SC(T) specimen can then be loaded either on the C(T) or SEN(T) load line to examine the effects of residual stress under high constraint or low constraint conditions respectively. Finite element analysis is performed to examine the sensitivity of the specimen dimensions on crack tip constraint and to demonstrate the introduction of residual stress into the specimen.

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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