the philosopht of constraint correction
TRANSCRIPT
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THE PHILOSOPHY OF CONSTRAINT CORRECTION
by
Christian Thaulowa, Erling stbyb, Brd Nyhusb, Vigdis Oldenb and Zhiliang Zhangb
Abstract
The Failure Assessment Diagrams (FAD) used in BS 7910:1999 (Guide on methods
for assessing the acceptability of flaws in fusion welded structures) represent high
structural constraint applications. The standard gives literature references forconstraint correction methods, based on T and Q, but none of these are included in
the standard. There is evidently a need to present a framework for a practicalapplication of constraint corrections. The paper presents constraint correction
parameters and demonstrates the JQM Approach with reference to a 690-steel. With
the increased use of FE calculations in the industry, a method for direct calculations,with high accuracy and low costs, is presented.
presented at
2nd International Symposium on High
Strength Steel
23-24 April 2002, Stiklestad, Norway
a The Norwegian University of Science and Technologyb SINTEF Materials technology
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Cracks
All materials will contain cracks or defects. The question is: When will cracks be of
practical interest? Under which conditions will cracks influence upon the behaviour of
structures and components? When can we ignore the existence of cracks?
Structural engineers normally judges the capacity or ultimate strength of a structure
on the basis of a load-deflection diagram, where the maximum load or plastic collapse
load is considered as the limit. The next step then is to impose a partial safety factor
on this limit load combined with minimum tensile eleongation requirements.
If we now introduce cracks in the structure, this can influence the load bearing
capacity, Figure 1, either by brittle fracture, ductile tearing, plastic collapse or
combinations of these failure modes.
Traditional structural design compares the design stress with the flow properties of thematerial, which is normally taken to some fraction of the yield stress. A material is
assumed to be adequate if its strength is greater than the expected applied stress. In
fracture mechanics there are two structural variables, design stress and flaw size, and
fracture toughness replaces strength as the material resistance property. Fracture
mechanics quantifies the critical combination of these three variables.
In fabrication with steel and aluminium, the welded joints represents the most critical
region. This is where cracks normally appears and regions of the weld metal or the
heat affected zone can have low toughness. The weld metal, heat affected zone and
the base material will have different material properties, and this mismatch in strength
will influence the failure conditions. The effect of material mismatch on fracture
depends upon the crack size, the location of the crack, the strength mismatch and the
fracture toughness.
For cracks located at the fusion line in steel weldments, weld metal overmatch is
recommended if ductile behaviour can be guaranteed. If, however, brittle fracture can
occur, evenmatch seems most favourable in order to avoid brittle fracture initiation
from the heat affected zone Thaulow et al (7, 8).
In the welding of high strength steels, the probability of even- or undermatch
increases, Figure 2, and it will be of importance to quantify the effects of mismatch.
In this paper we first shortly introduce the principle of constraint and transferability.
We the presents the JQM Approach and show how the approach can be applied for
the 690-steel investigated in the PRESS project. At the end we introduce effective
ways of applying constraint corrections, and the new company LINKftr.
Constraint
The starting point in fracture mechanics analysis is to consider a crack of a certain
size located in a component or specimen. An external load is applied and thecomponent is loaded until it fails. During loading a plastic zone develops from the
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crack tip, and at a certain load net section yielding occurs as the plastic zone reaches
the through thickness surface.
As long as the plastic zone at the crack tip is limited compared with the geometry of
the component or specimen, socalled small scale yielding, a single parameter fracture
mechanics approach can be applied. K, J or CTOD characterizes the crack tipconditions and can be used as geometry independent fracture criterion.
The geometry dependence under linear elastic conditions for five standard fracture
mechanics geometries are plotted in Figure 3. The pure tensile specimens, DENT and
CCT, have the lowest constraint , while specimens dominated by bending have the
highest constraint. Standard fracture mechanics testing procedures are based on the
specimens with high constraintin order to reproduce the worst case conditions.
However, the single parameter fracture mechanics breaks down in the presence of
excessive plasticity, and fracture toughness will now depend on the size, geometry
and mode of loading.
McClintock (1) was one of the first to examine the near crack tip stress field under
fully plastic conditions for various specimen geometries and non-hardening materials,
Figure 4. For small scale yielding, the maximum stress is approximately three times
the yield stress, while a centre cracked panel under tension is incapable of maintaining
significant triaxiality. These effects are, however, less severe when strain hardening is
taken into account. We notice that the DENT specimen, with low constraint under
linear elastic conditions, Figure 3, now reach high stresses because of the interference
between the two fields of deformation.
The history of constraint is how to deal with crack tip stresses under fully plastic
conditons. The aim is to find a parameter that characterize the stress-strain fields, so
that results from one test geometry can be transferred to another geometry.
One approach has been to restrict the application of fracture mechanics to high
constraint since single-parameter fracture mechanics may be approximately valid in
the presence of significantly plasticity, provided the specimens maintains a relatively
high level of triaxiality. Most laboratory fracture mechanics specimens, as three-point
bending and compact tension, represent this high triaxiality conditions.
A more basic approach has been to define the crack tip triaxialtity as the ratio betweenthe hydrostatic stress, or first invariant of the stress tensor, which does not cause any
plastic deformation, and the Mises effective stress, which is the square root of the
second invariant of the deviatoric stress being responsible for plastic flow. This
parameter has been extensively applied to describe ductile crack initiation and growth.
There are a number of mathematical models for void growth and coalescence, where
the two most widely referenced models were published by Rice and Tracy (2) and
Gurson (3). They found an exponential dependence of the void growth rate on the
stress triaxiality, 0/ssh . Here hs is the hydrostatic stress, and 0s is the yield stress.
The yield stress has later, Needleman and Tvergaard (4), been substituted with es ,
the Mises effective stress.
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Brocks and Schmitt (5) has intoduced the parameter h for the ratio, and proposed this
ehh ss /=
as the second parameter needed, in addition to J, to quantify the geometry dependence
of ductile crack growth. They also argue that the obvious disadvantage, that it is afield quantity and requires 3D elastic-plastic FE-solutions, can be overcome by
extrapolation schemes and inexpencive computer power.
Another constraint parameter is the T-stress, Larsson and Carlsson (9), Du and
Hancock (10). This is a non-singular linear elastic stress component parallell to the
crack. The T-stress charcterizes the local crack tip stress field for linear elastic
material, and the global in-plane constraint of a specimen with respect to
predominantly local small scale yielding conditions. It has however been argued that
the T-stress also can be applied under plastic conditions, Betegon and hancock (11).
T increases or lower the hydrostatic stress by
( ) Tfr
KIh
3
1
2+= f
ps
The idea of adding a second term has been taken over in elastic-plastic fracture
mechanics by defining the so-called Q parameter, O`Dowd and Shih (12, 13)
0
0
s
ss=
-
=TyyyyQ
The solution for yys is obtained by FE calculations.
The Q parameter, like the T stress, is supposed to characterize the geometry
dependent constraint. Both quantities affect the hydrostatic stress in the same way, i.e.
negative values lower, positive values raise the hydrostatic stress.
The Failure Assessment Diagrams (FAD) used in BS 7910:1999 (Guide on methods
for assessing the acceptability of flaws in fusion welded structures) represent high
structural constraint applications. The standard gives litterature references for
constraint correction methods, based on T and Q, but none of these are included in
the standard: "The FADs represent high structural constraint applications. When
toughness is measured using standard procedures, it is possible to modify the FAD to
account for lower constraint. Alternatively, it is possible to maintain the use of a high
constraint FAD and account for lower structural constraints using appropriate test
geometries." There is evidently a need to present a framework for a practical
application of constraint corrections.
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JQM-Approach
The JQM Approach quantifies the crack tip stress fields in dependence of geometry
(size, crack depth, global geometry and mode of loading), the Q parameter, and
material (yield strength and hardening exponent), called M, Figure 5, Zhang et al(14), Thaulow et al (15). The Approach is based on the exsisting J-Q theory and the
RKR brittle failure criterion , but is further developed to take material mismatch into
account.
The stress field is expressed with three terms
)12()( 1_01_00,0 bqsqsss +++ == Mij
Q
ij
TM
ijij fMfQ
where b=0 for mismatch ratio m=s s0 2 0 1_ _ 1 (weld metal overmatch), and b=1
form
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The evolution of constraint for the three test specimens as function of applied J, is
presented in Figure 10. When we add the mismatch effect, for cracks located at the
fusion line, the constraint increases with weld metal overmatch, Figure 11. At a
certain load ductile crack growth can be experienced. The effect of limited ductile
crack growth has been examined, stby et al (18), and an increase in constraint is
observed, Figure 12. The constraint effect on ductile crack growth, J-R curves, hasbeen further evaluated, Zhang et al (19) and Nyhus et al (20).
In order to establish material resistance curves, a large testing program has been
performed, Nyhus et al (21). The lower bound toughness results from the heat
affected zone shows that the toughness increases significantly as the constraint is
reduced, Figure 13. The M parameter is not included in this calculation because it is
close to evenmatch conditions.
By comparing the applied- and resistance curves, Figure 14, we can now determine
the critical conditions for brittle fracture.
We can now select a structural component of interest, introduce a crack, and calculate
the constraint and check if brittle fracture will take place.
Discussion
At present stage FE calculations are needed in order to calculate the constraint. But
two approaches have been suggested to make the calculations simpler, more effective
and less time consuming.
The first is an engineering or simplified approach where the need for calculations is
reduced to a minimum. Polynoms for a range of typical stress-strain curves are
calculated in beforehand and presented for practical use.
The other approach is the so-called direct calculations. The 3D crack geometry is
represented by a so-called linespring FE element. This element is introduced in a shell
FE analysis of a structure at critical locations.
A new company, named LINKftr as, has been established with the aim to develop
software for direct calculations. The LINKftr concept is to link detailed crack tip
calculations with the structural response, with the linespring as the transfer-element,Figure 15. The introduction of linespring elements will not influence on the
calculation capacity for the shell element geometry; hence, high accuracy can be
obtained at low costs, Figure 16. And not to forget: the routines will be easy to use.
Acknowledgement
This work is a part of the research project PRESS (Prediction of Structural Behavior
on the Basis of Small Scale Testing), with financial support from the Norwegian
Research Council and EU. The authors wish to thank colleagues from industry and
research institutes for the close cooperation.
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Conclusions
Single-parameter fracture mechanics breaks down in the presence of excessive
plasticity, and a second term has to be added to the standard K, J or CTOD inorder to quantify the constraint.
The J-Q-M Approach quantifies the crack tip stress fields in dependence ofgeometry (size, crack depth, global geometry and mode of loading), the Q
parameter, and material (yield strength and hardening exponent), called M.
Three simple specimen geometries have been selected to cover a wide range ofconstraint: SENB (a/W=0.5), SENB (a/W=0.2) and SENT (a/W=0.2). Both
calculations and test results reveal that the specimens are good candidates for
future standardization with respect to constraint corrections.
The constraint correction procedures must be easy to perform, have high accuracyand low costs. A new company, LINKftr, has been established with the aim to
develop software for direct calculations.
References
(1) McClintock, F.A. "Plasticity Aspects of Fracture." Fracture: An Advanced
Treatise, Vol. 3, Academic Press, New York, 1971, pp.47-225
(2) Rice, J.R. and Tracy, D.M. "On the ductile enlargement of voids in triaxial stress
fields." Journal of the Mechanics and Physics of Solids, Vol. 17, 1969, pp.201-217
(3) Gurson, A.L. "Continuum Theory of Ductile Rupture by Void Nucleation and
Growth: Part 1-Yield Criteria and Flow Rules for Porous Ductile Media." Journal of
Engineering Materials and Technology, Vol. 99, 1977, pp.2-15
(4) Needleman, A. and Tvergaard, V. "An Analysis of Ductile Rupture in NotchedBars." Journal of Mechanics and Physics of Solids, Vol. 32, 1984, pp.461-490.
(5) Brocks, W. and Schmitt, W. "The Second Parameter in J-R Curves: Constraint or
Triaxiality?" Second Symposium on Constraint Effects, ASTM STP 1244, 1994
(6) Anderson, t.L. "Fracture mechanics. Fundamentals and applications" 2nd edition,
1995, CRC Press, Florida, USA.
(7) Thaulow,C. Effect of weld metal over- and undermatch on fracture resistance of
pipeline girth welds. Open seminar on Deep Water Pipelines and Flowlines, , 21October 1999, Trondheim, Norway
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(8) Thaulow,C., Hauge, M, Zhang,Z.L.,Ranestad,. and Fattorini,F.: On the
interrelationship between fracture toughness and material mismatch for cracks located
at the fusion line of weldments. Engineering Fracture Mechanics, 64 (1999) pp.367-
382.
(9) S.G. Larsson, A.J. Carlsson, Influence of non-singular stress terms and specimen
geometry on small-scale yielding,Journal of the Mechanics and Physics of Solids 21
(1973) 263-277
(10) Z.Z. Du, J. W. Hancock, The effect of non-singular stresses on crack-tip
constraint,Journal of the Mechanics and Physics of Solids 39 (1991) 555-567
(11) C. Betegn, J.W. Hancock, Two-parameter charaterisation of elastic-plastic
crack-tip fields, Journal of Applied Mechanics 58 (1991) 23-43
(12) ODowd N. P. and Shih C. F., Family of crack-tip fields characterised by atriaxility parameter: Part I - Structure of fields.J. Mech. Phys. Solids, 39, 989-
1015, (1991).
(13) ODowd N. P. & Shih C. F., Family of crack tip fields characterised by a
triaxility parameter-Part II. Fracture applications,J. Mech. Phys. Solids, 40, 939-963
(1992).
(14) Zhang, Z.L., Hauge, M. and Thaulow, C.: "Two Parameter Characterisation of
the Near Tip Stress Fields for a Bi-Material Elastic-Plastic Interface Crack", Int.
Journal of Fracture, 79:65-83, 1996.
(15) Thaulow, C., Zhang, Z.L., Ranestad, . and Hauge,M., J-Q-M approach for
failure assessment of fusion line cracks: two material and three material models.
ASTM STP 1360, Fatigue and Fracture Mechanics: 30th Volume. St.Louis. June 1998.
(16) Olden, V. "Notch tensile testing og high strength steel weldments." 2nd
International Symposium on High Strength Steel, 23-24 April, 2002, Verdal, Norway
(17) Nyhus, B., Polanco, M., Knagenhjelm, H.O. and hauge, M. "A more efficient
engineering critical assessment for pipes based on testing of single edge notch tension
specimens." 6th International Pipeline Conference&Exhibition, Merida, Mexico,November, 2001
(18) stby, E , Nyhus, B, Thaulow, C, Olden, V. and Zhang,Z.L. "The effect of
geometry and ductile crack growth on the near-tip constraint level."
2nd International Symposium on High Strength Steel, 23-24 April, 2002, Verdal,
Norway
(19) Zhang Z.L.,Thaulow,C. and degrd J.A Complete Gurson Model Approach
for Ductile Fracture. Engineering Fracture Mechanics. 67, 155-168, 2000
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(20) Nyhus,B., Zhang, Z.L. and Thaulow, C. "Normalisation of Material Crack
Resistance Curves by the T Stress." 2nd International Symposium on High Strength
Steel, 23-24 April, 2002, Verdal, Norway
(21) Nyhus, B and stby, E. "SENT Testing of High Strength Steel." 2nd
International Symposium on High Strength Steel, 23-24 April, 2002, Verdal, Norway
(22) Chiesa, M., Nyhus, B., Skallerud, B. and Thaulow, C "Efficient Fracture
Assessment of Pipelines. A Constraint Corrected SENT Specimen Approach",
Engineering Fracture Mechanics, 68, 527-547, 2001
(23) Chiesa, M., Skallerud, B. and Thaulow, C. "Line spring elements in a yield
strength mismatch situation with application to welded wide plates", Engineering
Fracture Mechanics, 68, No 8, 2001
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Figure 1 Load vs displacement of a structural component. The normal structural
design (no-crack assumption) is schematically compared when cracks are included
(brittle fracture, ductile tearing, plastic collapse).
Figure 2 Distribution of yield strength for base material (BM) and weld metal (WM)
for two classes of steel.
Brittlefracture ?
No-crack assumption
Global displacement
Globalloa
d
With possible cracks
450 690MP
BM BM WMWM
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Figure 3 Linear elastic solutions for standard fracture mechanics test specimens. The
figure is based upon Anderson (6)
Low constraint
High constraint
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Figure 4 Plastic deformation pattern in small scale yielding (a) and slip line patterns
under fully plastic conditions in three fracture mechanics test geometries. The
estimated local stresses are based on the slip line analyses of McClintock (1), and
apply only to non-hardening materials. From Anderson (6).
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n Geometry specimen crack
loading
n Mismatch strength
hardening
general
J-Q theory J-M
J-Q-M theory
WM
BM or HAZ
Figure 5 The JQM theory
Figure 6 The JQM theory. The actual geometry and mismatch is always compared
with a reference solution, representing small scale yielding and homogeneous
material.
Same failure
condition
Jref, Q=0, M=0Japplied, Q, M
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Figure 7 Fracture toughness as a function of the constraint
FractureToughness
Constraint Q+M
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Figure 8 Chemical composition and mechanical data for the 690-steel investigated in
the PRESS project.
Figure 9 The three standard test specimen geometries examined in the PRESS
project
FRACTURE
TOUGHNESS
[J,K,CTOD]
GEOMETRY / CONSTRAINT [Q]
SENB (a/W = 0.5)
SENT clamped
(a/W=0.2)
SENB (a/W = 0.2)
690 MPa steel
C Si Mn P S Al N Cu Mo Ni Cr V Nb Ti B
.16(.17)
.430(.36)
1.20(1.35)
.013(.015)
.001(.003)
.039(-)
.008(.008)
.035(.037)
.31(.43)
.34(1.00)
.425(1.00)
.043(-)
.026(-)
.003(-)
.0002(-)
1.0 mm
FL
Notched tensile testing Stress-strain curves
Chemical composition
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Figure 10 Evolution of constraint in the three test specimens
Figure 11 The effect of mismatch on the constraint. m=1.3 represents 30% weld
metal overmatch when HAZ is considered as the critical material (or reference
material, Figure 6).
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Figure 12 Effect of ductile crack growth on the crack tip stress and the constraint.
Figure 13 Lower bound fracture toughness for the 690-steel with the crack located at
the fusion line.
Ductile crackgrowth initiation
Increase in local crack tipconstraint due to ductilecrack growth
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Figure 14 Comparison between the crack driving force and the fracture toughness
test results for the three specimen test geometries
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Figure 15 The LINKftr concept. Detailed crack tip calculations are linked to the
structural response with linespring as the transfer-element.
Figure 16 Application of direct fracture calculations based on linespring elements.
Procedures are developed by LINKftr.
3D FE
calculations
Shell elements FE
calculations
with line spring
Analytical
equations
(CrackWise)
Accuracy
Costs
Costs
Accuracy
LINKftr
THE LINK BETWEEN LOCAL FAILURE AND STRUCTURAL RESPONSETHE LINK BETWEEN LOCAL FAILURE AND STRUCTURAL RESPONSE
LINKfailure LINKtransfer LINKrespons
=
B
B
A
A
B
B
A
A
Q
Q
Q
Q
q
q
q
q
DDDD
DDDD
DDDD
DDDD
2
1
2
1
2
1
2
1
44434241
34333231
24232221
14131211
)()(
h
aD
ep
ijLine Spring Tangent stiffnessmatrix
BAiq , Generalized displacements atnodes A and B
iQGeneralized Force at nodes A
and B, in tension and in
bending ( N,M),