Application of Fracture Mechanics to Characterized cracking and fracture of weldments

Submitted by

Amit Jain

2009 MED 3494

Under the guidance of

Prof. R.K.Pandey

Applied Mechanics Department

Indian Institute of Technology, Delhi-110016

November 2010

Contents

1. Introduction 3

2. Fracture Testing Of Weldments 6

2.1 Specimen Design and Fabrication 6

2.2 Notch Location and Orientation 7

2.3 Fatigue Pre-cracking 10

2.4 Post-Test Analysis 10

3. Case study 12

4. References 14

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1. Introduction

The advent of high toughness structural materials necessitated a corresponding research in the

area of welding-- the process which is extensively used in fabrication. The specific properties

that characterize a good weld are the homogeneity of the weldment and matching strength of

the weld joint to that of the base metal. While these properties are not difficult to achieve in

the weld metal zone, in the zone lying between the weld zone and the base metal, termed as

the heat affected zone (HAZ), control of property is often difficult to achieve. The successive

heating and cooling of the HAZ during multi pass welding could result in a microstructure

which is prone to brittle fracture (Lancaster lq80). In fact, a number of catastrophic failures,

resulting in huge losses in terms of personnel and financial have been reported to have

originated from the HAZ of weld joints.

In order to evaluate the health of a weld joint, a number of test procedures have been evolved.

Depending on the convenience and mutual agreement between the manufacturer and the

consumer, conventional tests, like the 'w' bend test for assessing the ductility, and controlled

thermal severity test (CTS) for assessing tendency towards cold cracking are generally

carried out. These tests, however, serve to qualitatively assess the weld joint as a whole. For a

more objective and scientific assessment of the weld joint, the composite comprising three

zones that constitute the weld joint, viz., the base metal, the HAZ and the weld metal, have to

be individually assessed with regard to their tendency to cracking. It is now widely

recognized that any weld joint will invariably contain defects. Defects could arise due to

improper welding e.g., voids, slag entrapments, or under-cutting. An incipiently embrittled

HAZ microstructure may also be considered as a defect. During service, all the defects in a

weld composite are candidate sites for cracks to initiate. Once initiated, the cracks grow

through a component, following a path of least resistance, leading to failure. A recent tool in

analyzing and characterizing the behavior of cracks is fracture mechanics. The theoretical

and functional basis of fracture mechanics has been sufficiently well developed in order for it

to be applied to the prediction and prevention of service failures in engineering components.

Despite the precautions taken during weld fabrication, mechanical imperfections such as

geometric misalignment or defects still prevail in weldments. These defects in the form of

notches or sharp cracks can lead to premature or unexpected failure of weld joints. Their

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critical size and shape, however, depends on the combination of loading, material

microstructure and joint configuration. The problem has attracted the attention of internal

organizations such as the International Institute of Welding (IIW) to institute Commission X

with the assignment of studying "Significance of Defects with Regard to Brittle Fracture".

The engineering approach to solving the weld defect problem is not to eliminate all flaws,

since this approach would be prohibitively uneconomical if not technically unattainable.

Instead, the effort has been to utilize analytical methods to quantitatively assess the allowable

flaw dimensions that a structure could sustain without endangering its integrity and

performance. Considerable progress has been made in predicting the effects of flaw size on

brittle fracture, ductile fracture and fatigue failure of structural components that are

homogeneous. Weldments do not fall into this category because the material properties in the

WM (weldment), HAZ (heat affected joint) and BM (base metal) are different and the

dimensions of these zones can differ by orders of magnitude. This presents additional

difficulties to both the stress and failure analysis.

A view taken in fracture mechanics is to analyze the effects of flaws in terms of the welded

structure for the intended service conditions. The objective is to understand how flaws affect

the susceptibility of the weld joint to fracture under conditions that involve highly nonlinear

and irreversible local deformation. The linear elastic fracture mechanics (LEFM) theory

relying on the concept of energy release rate or critical stress intensity factor K lc and crack

opening displacement COD are not applicable since they have no meaning in the presence of

gross yielding. Even more questionable is the empirical approach of measuring the so called

material toughness 1 of weldments by machining a sharp notch in or near a welded joint and

load it to failure. The COD bend test or the notched tension test are often used for this

purpose. The test data will invariably contain much scatter as the results will be sensitive to

load type and location of the flaw. Attempts have also been made to apply the path

independent 2 J-integral for characterizing weldments. They are subject to the same

criticisms because the inherent behavior of welded joints is not path independent but rather

depend significantly on the load path.

In view of the above comments, each weld joint must be regarded as a structural component

designed to carry its specific load. The way with which the microstructures of WM, HAZ and

BM affect load transmission is reflected through the macroscopic material parameters. The

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assessment of weld joint behavior depends on the advancement of an approach that could

translate the uniaxial data of homogeneous specimens to a nonhomogeneous and multiaxial

state of stress or energy. The rate of change of area under the true stress and true strain curve

is regarded to be essential because it represents the energy dissipated in a unit volume of solid

to damage material. This quantity is known as the strain energy density function dW/dV and

will be used as a criterion for analyzing the failure of weldments with initial defects.

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2. Fracture Testing Of Weldments

2.1 Specimen Design and Fabrication:

The underlying philosophy of the Welding Institute guidelines on specimen design and

fabrication is that the specimen thickness should be as close to the section thickness as

possible. Thicker specimens tend to produce more crack tip constraint, and hence lower

toughness (See Chapters 2 and 3 of [1]). Achieving nearly full thickness weldment

specimens often requires sacrifices in other areas. For example if a specimen is to be

extracted from a curved section such as a pipe, one can either produce a sub size rectangular

specimen which meets the tolerances of the existing ASTM standards, or a full thickness

specimen that is curved. The Welding Institute recommends the latter.

If curvature or distortion of a weldment is excessive, the specimen can be straightened by

bending on either side of the notch to produce a "gull wing" configuration, which is

illustrated in Fig. 2.1. The bending must be performed so that the three loading points (in an

SENB (single edge notched bend) specimen) are aligned.

Figure 2.1(a): The gull-wing configuration for weldment specimens with excessive curvature

Figure 1.1(b): Single edge notched bend (SENB) specimen.

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Fabrication of either a compact or SENB weldment specimen is possible, but the SENB

specimen is preferable in nearly every case. The Welding Institute recommendations cover

both the rectangular and square section SENB specimens. The appropriate choice of

specimen type depends on the orientation of the notch.

2.2 Notch Location and Orientation:

Weldments have a highly heterogeneous microstructure. Fracture toughness can vary

considerably over relatively short distances. Thus it is important to take great care in locating

the fatigue crack in the correct region. If the fracture toughness test is designed to simulate an

actual structural flaw, then the fatigue crack must sample the same microstructure as the flaw.

For a weld procedure qualification or a general assessment of a weldments’ fracture

toughness, location of the crack in the most brittle region may be desirable, but it is difficult

to know in advance which region of the weld has the lowest toughness. In typical C-Mn

structural steels, low toughness is usually associated with the coarse grained HAZ and the

inter-critically reheated HAZ. A micro-hardness survey can help identify low toughness

regions because high hardness is often coincident with brittle behaviour. The safest approach

is to perform fracture toughness tests on a variety of regions in a weldment.

Once the microstructure of interest is identified, a notch orientation must be selected. The two

most common alternatives are a through-thickness notch and a surface notch, which are

illustrated in Fig. 2.2. Since full thickness specimens are desired, the surface notched

specimen should be square section (BXB), while the through thickness notch will usually be

in a rectangular (B X 2B) specimen.

For weld metal testing, the through-thickness orientation is usually preferable because a

variety of regions in the weld are sampled. However, there may be cases where the surface

notched specimen is the most suitable for testing the weld metal. For example, a surface

notch can sample a particular region of the weld metal, such as the root or cap, or the notch

can be located in a particular microstructure, such as unrefined weld metal.

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Figure 2.2 Notch orientation in weldment specimens

Notch location in the HAZ often depends on the type of weldment. If welds are produced

solely for mechanical testing, for example as part of a weld procedure qualification or a

research program, the welded joint can be designed to facilitate HAZ testing. Figure

2.3 illustrates the K and half-K preparations, which simulate double- V and single- V welds,

respectively.

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Figure 2.3: Special weld joint designs for fracture toughness testing of the heat affected zone (HAZ).

The plates should be tilted when these weldments are made, to have the same angle of attack

for the electrode as in an actual single- or double-V joint. For fracture toughness testing, a

through-thickness notch is placed in the straight side of the K or half-K HAZ.

In many instances, fracture toughness testing must be performed on an actual production

weldment, where the joint geometry is governed by the structural design. In such cases, a

surface notch is often necessary for the crack to sample sufficient HAZ material. The

measured toughness is sensitive to the volume of HAZ material sampled by the crack tip

because of the weakest link nature of cleavage fracture.

Another application of the surface notched orientation is the simulation of structural flaws.

Figure 2.4 illustrates HAZ flaws in a structural weld and a surface notched fracture toughness

specimen that models one of the flaws. Figure 2.4 demonstrates the advantages of allowing a

range of a/W ratios in surface notched specimens. A shallow notch is often required to locate

a crack in the desired region, but existing ASTM standards do not allow a/W ratios less than

0.45. Shallow notched fracture toughness specimens tend to have lower constraint than

deeply cracked specimens. Thus there is a conflict between the need to simulate a structural

condition and the traditional fracture mechanics approach, where a toughness value is

supposed to be a size independent material property. One way to resolve this conflict is

through constraint corrections.

Figure 2.4 Test specimens with notch orientation and depth that matches a flaw in a

structure.

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2.3 Fatigue Pre-cracking:

Weldments that have not been stress relieved typically contain complex residual stress

distributions that interfere with fatigue pre-cracking of fracture toughness specimens. Tensile

residual stresses accelerate fatigue crack initiation and growth, but compressive stresses

retard fatigue. Since residual stresses vary through the cross section, fatigue crack fronts in

as-welded samples are typically very non-uniform.

The first method bends the specimen in the opposite direction to the normal loading

configuration to produce residual tensile stresses along the crack front that counterbalance the

compressive stresses. Although this technique gives some improvement, it does not produce

acceptable fatigue crack fronts.

The R ratio in fatigue cracking is the ratio of the minimum stress to the maximum. A high R

ratio minimizes the effect of residual stresses on fatigue, but also tends to increase the

apparent toughness of the specimen. In addition, fatigue pre-cracking at a high R ratio takes

much longer than pre-cracking at R = 0.1, the recommended R ratio of the various ASTM

fracture testing standards.

The only method evaluated that produced consistently straight fatigue cracks was local

compression, where the ligament is compressed to produce nominally 1% plastic strain

through the thickness, mechanically relieving the residual stresses. However, local

compression can reduce the toughness slightly. Towers and Dawes concluded that the

benefits of local compression outweigh the disadvantages, particularly in the absence of a

viable alternative.

2.4 Post-Test Analysis:

Correct placement of a fatigue crack in weld metal is usually not difficult because this region

is relatively homogeneous. The microstructure in the HAZ, however, can change dramatically

over very small distances. Correct placement of a fatigue crack in the HAZ is often

accomplished by trial and error. Because fatigue cracks are usually slightly bowed, the

precise location of the crack tip in the centre of a specimen cannot be inferred from

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observations on the surface of the specimen. Thus HAZ fracture toughness specimens must

be examined metallographically after the test to determine the microstructure that initiated

fracture. In certain cases, post-test examination may be required in weld metal specimens.

Figure 2.5 illustrates a procedure for sectioning surface notched and through-thickness

notched specimens. First, the origin of the fracture must be located by the chevron markings

on the fracture surface. After marking the origin with a small spot of paint the specimen is

sectioned perpendicular to the fracture surface and examined metallographically. The

specimen should be sectioned slightly to one side of the origin and polished down to the

initiation site. The spot of paint appears on the polished specimen when the origin is reached.

Figure 2.5: Post-test sectioning of a weldment fracture toughness specimen to identify

the micro-structural that caused fracture.

In addition to sectioning the specimen, the amount of coarse-grained material at the crack tip

must be quantified. For the test to be valid, at least 15% of the crack front must be in the

coarse-grained HAZ. The purpose of this procedure is to pre qualify steels with respect to

HAZ toughness, identifying those that produce low HAZ toughness so that they can be

rejected before fabrication.

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5. Case study

For centre-cracked strip the fracture mechanics parameters such as the J-integral, crack opening displacement and load-point displacement can be written as the following equations.

Where a is the crack length and W the specimen width, h1, h2, and h3 the function of parameters a/W and strain hardening exponent n. P is the total load per unit thickness and PO the reference load per unit thickness.

Results:

Table 1 and Fig. 3.2 show the values of h1, h2 and h3 computed for a centre-cracked specimen with a/W = 0.4 by fully plastic finite element program.

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Figure 3.1: centre cracked welded strip

(1)

(2)

(3)

(1)-(3)

Figure 3.2: Comparison of values of h1,h2, and h3for centre-cracked strip in plane

stress problem.

The values of h1, h2 and h3 from [5] are compared with those from EPRI report. In Fig. 3.2, the dashed lines express the results interpolated from an EPRI table and the solid lines are plots of the numerical results from Table 1. It is shown from Fig. 3.2 that the present solutions are smaller than those reported by the EPRI approach. The differences between the two series of lines are within six per cent. It is proved that the fully plastic FEM program used here is available.

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References

[1] T.L. Anderson, “Fracture Mechanics: Fundamentals and Applications”, 2nd edn, 1991.

[2] D. Broek, “Elementary engineering fracture mechanics”, 2nd edn, 1990.

[3] J F Lancaster, “Metallurgy of welding” 3rd edn (London: George Allen and Unwin)

chap. 4, 1980

[4] F Watkinson, P H Bodger, J D Harrison, “The fatigue strength of welded joints in high strength steels and methods for improvement”. Proc. Conf. Fatigue of welded structures (Brighton: Welding Inst.), 1970

[5] J. X. ZHANG, Y. W. SHI and M. J. TU, “Studies on the fracture mechanics parameters of weldment with mechanical heterogeneity”, Department of Mechanical Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi Province, China, 2003

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