welding defect assessment residual stress

1. Presented by
Robert Hurlston
Engineering Doctorate – Nuclear Materials
Development of Advanced Defect Assessment
Methods Involving Weld Residual Stresses

2. Content
 Introduction
 Problem Definition
– 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

3. 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

4. Problem Definition

5. 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)

6. 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

7. 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

8.  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

9. Basis For Project

10. Constraint Based Approach to Fracture Mechanic
 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

11. 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

12. 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:
    ij
ij
ij Q
J
r
J
r 






 0
0
*
0 ,
/
,
/ 


13. 

J
r /
0

J annulus
Small-scale yielding
Qσ0
Finite geometry

14. J-Q Locus
 The Q stresses calculated can
now be used to construct a
load line in J-Q space
Q 0
J

15. 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
     
*
0
*
0
0
*
0 /
/
/ c
c
c J
r
Q
J
r
J
r 





 

 



16. 

f



c
c
J
r 0

J
rc 0
 J
r /
0

*
0
c
c
J
r 
B
C
A
margin
J annulus
Qσ0
Small-scale yielding
Finite geometry

17. 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
0
J
Q
J*c

18. 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
0
20
40
60
80
100
120
140
160
180
200
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5
Qc (Case 3)
P-SENT
SENT
CT
P-CT
Fracture
toughness,
K
JC
(MPa√m)

19. 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

20. Work to Date

21. 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
-400
-200
0
200
400
600
800
0 5 10 15 20 25 30 35 40
x ahead of notch (mm)
Opening
mode
stress
(MPa)
-400
-200
0
200
400
600
800
0 5 10 15 20 25 30 35 40
x ahead of notch (mm)
Opening
mode
stress
(MPa)

22.  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
0
50
100
150
200
250
300
350
-0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2
Q
J
(Nmm
-1
)
0.2 no RS
0.4 no RS
0.2 RS
0.4 RS
Jc (SSY)
Jc (0.2)
Jc (0.4)
Jc (0.2 RS)
Jc (0.4 RS)
Jc Closed Form

23. 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

24. 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

25. Questions???