Engineering Doctorate – Nuclear Materials Development of Advanced Defect Assessment Methods Involving Weld Residual Stresses If using an image in the background fill the area as shown, leaving a cream strip at the bottom for the Serco logo. Headings to be set all in lower case as is the new company style, using a large font to follow the same style used in the new company brochure. Presented by Robert Hurlston

Content Introduction Problem Definition Basis for Project Project Plan Evaluating Fracture Toughness in Weld Specimens Shortfalls in Methodology Basis for Project Two Parameter Fracture Mechanics Effect of Residual Stress on Constraint Evaluation of Unique Material Toughness Work to Date Project Plan Summary

Introduction It is essential that structural integrity of reactor pressure vessels in pressurised water reactors can be ensured Fracture toughness of materials within the structure are commonly used in failure assessments This can be difficult to evaluate where weld residual stresses are present The aim of this project is to: assess the applicability of constraint based fracture mechanics to quantify 'unique material fracture toughness' in laboratory specimens containing residual stresses using the 'apparent fracture toughness' values derived from standard fracture toughness testing

Problem Definition

Evaluating Fracture Toughness BS7448 is the British Standard containing methodology for experimental evaluation of critical fracture toughness in metallic materials Pre-cracked bend or compact tension specimens are tested in displacement controlled monotonic loading at a constant rate of increase in stress intensity factor Data obtained is used to determine plane strain fracture toughness (K, CTOD, J)

Residual Stress Modification Part II of BS7448 is designated to describing methods for defining critical fracture toughness in areas of welding residual stress Addresses two issues: To define suitability of weld notch placement To define protocol for modification of residual stress This is generally done in order to reduce residual stress to a ‘negligible’ level via local compression of material at the crack tip

Local Compression Residual stress shall be considered acceptably low provided that: The fatigue crack can be grown to an acceptable length The fatigue crack front is acceptably straight

However, it has become apparent, through research, that these methods can often have the opposite effect Modifying driving force and crack-tip constraint Furthermore, triaxiality introduced via local compression can affect constraint, which can significantly influence measured fracture toughness It is assumed that the compression reduces all residual stresses to low and uniform levels such that any remaining residual stress has no effect on fracture

Basis For Project

Constraint Based Approach to Fracture Mechanics Elastic-plastic crack-tip fields can be characterised via a two parameter approach J describes the crack tip driving force and T or Q (used in this project) describes crack tip constraint This forms the basis of two parameter fracture mechanics, where toughness is expressed as a function of constraint in the form of a J-Q locus The approach allows enhanced ‘apparent’ fracture toughness associated with shallow cracks to be used via constraint matching Allows the high levels of conservatism associated with use of deeply cracked fracture toughness specimens to be relaxed

Constraint Work into the effects of constraint has mostly focussed upon understanding and predicting the role of specimen/defect geometry When the plastic zone at the crack tip is infinitesimally small compared to all other characteristic lengths and is embedded in an elastic field small scale yielding conditions exist Q is essentially 0 Loss of constraint occurs where the plastic zone at the crack tip is in contact with or near a traction free surface or plastic strain caused via gross deformation

Crack Tip Stress Fields Constraint is calculated by comparing the crack tip stress distributions generated under small-scale yielding conditions and in real geometries O’Dowd and Shih provide an approximate expression, where Q is the correction factor characterising this difference:

J annulus Qσ0 Small-scale yielding Finite geometry

J-Q Locus J The Q stresses calculated can now be used to construct a load line in J-Q space Q

RKR Model When making fracture assessments, it is usually assumed that crack tip conditions in a standard fracture toughness specimen approximate high constraint This is considered to be conservative as crack tip constraint is likely to be lower in the structure being assessed Where fracture depends on the crack tip stress, effective (constraint corrected) fracture toughness, Jc, can be calculated by solving equations of the form: Ritchie, Knott and Rice provide a simple framework for its implementation

J annulus margin B C Qσ0 Small-scale yielding A Finite geometry

Constraint corrected J (Jc) RKR Model The RKR model can be used to calculate Jc at all points along the J-Q loading line to produce a Jc-Q locus The point at which the loading line intersects this locus is the corrected failure point for the specimen or component with given geometry J*c is the materials fracture toughness J Constraint corrected J (Jc) J*c Q

Effect of Residual Stress and Biaxial Loading on Constraint It has been shown in a number of studies that crack tip constraint is strongly influenced by both residual stress and biaxial loading Xu, Burdekin and Lee (figure) report similar findings

Correcting Weld Fracture Toughness The main objective for this project is to demonstrate the applicability of a unique material (Jc-Q) fracture toughness curve where weld residual stresses are present within the material Given knowledge of the effect of residual stresses present on constraint (from FE) it will be possible to correct measured weld fracture toughness data to find the unique (SSY) material toughness value This: Removes the necessity of relaxing residual stresses in laboratory specimens Ensures that residual stress is only accounted for once in any subsequent failure assessment

Work to Date

Finite Element Modelling Side edge notched bend specimens modelled with cracks of a/W = 0.2 and a/W = 0.4 (where W = 50mm) Residual stresses generated using a novel adaptation of out-of-plane compression

Using constraint based fracture mechanics (described previously): Loading lines can be plotted for both geometries, with and without residual stress Their associated fracture toughness curves can be plotted using RKR Fracture toughness curves collapse onto one another

Validation Experimental work is planned to validate these results Fracture toughness values to be obtained for each of the modelled cases Agreement between simulation and experiment would allow a model to be developed for implementation of this methodology for use in acquisition of weld fracture toughness

Summary Current BS7448 methodology for acquisition of fracture toughness in welds relies too heavily upon engineering judgement Use of constraint based fracture mechanics model is proposed to correct for weld residual stresses using (FE) knowledge of their effect on constraint when evaluating fracture toughness It is anticipated that preventing the need for stress relaxation before testing will provide significant benefits when evaluating weld fracture toughness

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