[48]Efects of Notch Position of the Charpy Impact Specimen on the Failure Behavior in Heat Affected...

8/11/2019 [48]Efects of Notch Position of the Charpy Impact Specimen on the Failure Behavior in Heat Affected Zone(2008)

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j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / j m a t p r o t e c

Effects of notch position of the Charpy impact specimen

on the failure behavior in heat affected zone

Y.C. Jang a, J.K. Hong b, J.H. Park b, D.W. Kim a, Y. Lee a,

a Department of Mechanical Engineering, Chung-Ang University, Seoul 156-756, Republic of Koreab Korea Institute of Machinery and Materials, Changwon, Kyungnam 641-101, Republic of Korea

a r t i c l e i n f o

Keywords:

Notch position

Charpy impact test

Energy absorption

Failure simulation

a b s t r a c t

Experimental and numerical studies were performed to examine the effects of notch posi-

tion on the failure behavior and energy absorption when the Charpy V-notch impact test is

made at 1 C. Carbon steel plate (SA-516 Gr. 70) with thickness of 25 mm usually used for

pressure vessel was welded by Shielded Metal-Arc Welding method and specimens were

fabricated from the welded plate. The Charpy impact tests were then performed with speci-

mens having different notch positionsvarying withinHAZ. A series of 3-D FE analysis which

simulates theCharpy test arecarried outas well. TheFE analysis takes into account the het-

erogeneous mechanical properties in HAZ. Results reveal that the absorbed energies during

impact test depend significantly on the notch position. Experimentally measured energy is

in agreement with computed one when the notch is positioned by 1.5 mm from the fusion

line.

2007 Elsevier B.V. All rights reserved.

1. Introduction

The Charpy V-notch test is a standardized high strain-rate

test which can measure the amount of energy absorption of

material. This test was first proposed more than a century

ago (Russel, 1898; Charpy, 1901). This absorbed energy is a

measure of a given materials toughness and acts as a tool to

study brittle-ductile transition, depending upon the test tem-

perature. With this test, one can evaluate reliability of weld

joint component and/or structure based on measured energy

absorption of material (specimen) and understanding defor-mation and failure process during test.

The specimen is composed of three parts, weld, heat

affected zone (HAZ) and base material. These have different

mechanical properties. Consequently it causes stress mis-

match between them. Hence, stress field at the ahead of

notch is significantly dependent upon the position of V-

notch along weld, HAZ and base material and subsequently

Corresponding author.E-mail address:[email protected](Y. Lee).

the energy that the specimen absorb during impact test is

different.

In this light, Hong et al. (2007) performed the Charpy V-

notchtest with notchpositionvaried withinHAZ andreported

the absorbed energy is influenced by notch position relative

to various microstructures and is reduced as notch position

closes to base material. Moltubakk et al. (1999) studied the

influence of notch positioning on the fracture behavior exper-

imentally with 3-point bending test and calculated Weibull

stress distribution in HAZ with 2-D FE analysis.

Tvergaard and Needleman (1988) performed FE analysisto investigate brittle-to-ductile failure transition for differ-

ent weld joint under condition of plane strain deformation.

They studied failure behavior of weld, HAZ and base material

(HY100) while moving the notch position in the weld. They

reported energy for crack propagation is very sensitive to the

relative location of notch in the weld and brittle failure might

occur as notchcloses to HAZ. Tvergaard and Needleman (1986)

0924-0136/$ see front matter 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.jmatprotec.2007.11.272
mailto:[email protected]://localhost/var/www/apps/conversion/tmp/scratch_2/dx.doi.org/10.1016/j.jmatprotec.2007.11.272http://localhost/var/www/apps/conversion/tmp/scratch_2/dx.doi.org/10.1016/j.jmatprotec.2007.11.272mailto:[email protected]


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also examined theeffect of rate sensitivityon thefailure mode

with2-D FE analysis. Eberleet al.(2000) carriedout 2-DFE anal-

ysis of the Charpy V-notch specimen to calculate JR-curve for

German standard steel StE 460. The computed JR-curve was

then compared with experimentally measured one.

For homogeneous material, full three-dimensional FE anal-

ysis of the Charpy V-notch test was presented first time by

Mathur and Needleman (1994). Tvergaard and Needleman(2004)presented full three-dimensional FE analysis to inves-

tigate the sensitivity to where the notch across the HAZ layer

and comparison made with test specimen where the notch is

cut on the face parallel to the surface of the test piece.

Many studies presented so far have performed FE analy-

sis with assuming that HAZ is of homogeneous mechanical

properties. However, HAZ has various microstructures and

consequently different mechanical properties. In addition,

reproducibility of the Charpy V-notch test for the study of

notch position has been always problematic because test

results usually exhibits a large scatter. Note it is almostimpos-

sible to control alignment of the notch position in micrometer

scale during test. Hence, a full 3-D FE analysis which can sim-ulate quantitatively the effect of non-uniform distribution of

mechanical properties in HAZ on energy absorption is highly

desirable.

In this study, we divided HAZ into three regions based on

experimentally measured Vickers hardness and distributed

the mechanical properties in HAZ and the notch is positioned

in the three regions accordingly. A series of three-dimensional

FE analysis is then carried out to capture the effect of not

position. Computed absorbed energy is compared with exper-

imentally measured one and issue on the location of notch in

the Charpy V-notch test specimen is discussed. The material

used in this study is SA-516 Gr.70.

2. The Charpy test, welding condition andspecimen

2.1. The Charpy test

The Charpy test was performed according to standard test

methods for notch bar impact testing of metallic materials

(ASTM E 23-02). The total length of the specimen is 55mm and

the rectangular cross-section area is 10 mm 10 mm. Speci-

men has a V-shaped notch with a flank angle of 45 and depth

of2 mm. The tip radiusof notch is0.25mm. The radiusof strik-

ingedge is 8 mm.Before test, specimenis positioned upon twoanvils with a span of 40 mm.

2.2. Welding condition

The SA-516, carbon steel plate (C: 0.18%, Si: 0.3%, Mn: 1.15%,

P: 0.014%, S: 0.03%, Cu: 0.17%, Ni: 0.31%, Cr: 0.02%, Mo: 0.098%,

V: 0.026%, Nb: 0.016%) generally used for pressure vessel, was

chosen for this study. The chemical compositions of SA-516

Gr. 70 steel is given inTable 1.The material has yield strength

of 360 MPa, ultimate strength of 540 MPa and an elongation of

34%.

The welded joint with thickness of 25mm plate is

fabricated with Shielded Metal-Arc Welding (SMAW)

Table 1 Yield stress and ultimate tensile strengthassigned to finite elements analysis

Weld W-HAZ C-HAZ B-HAZ Base material

y(MPa) 480 630 500 400 360

u(MPa) 720 940 740 620 535

Fig. 1 Schematic of cross-section of welded plate.

method. The welding condition of the specimen is

as follows: current = 110170 A, voltage = 3035 V, travel

speed= 1215cm/min and inter-pass temperature = 54 C.

Maximum heat input was 1617 kJ/cm and a half-K weldgroove instead of V groove was used to distribute material

properties in HAZ to thickness direction of welded plate.

2.3. Specimen

Once welding and subsequent cooling is finished, specimens

(marked in dashed line) are then sampled as shown in Fig. 1.

The specimens marked in dashed line as rectangular shape is

paralleling to the surface of welded plate and is taken at the

location of 1/4t(tis the thickness of welded plate).

The specimens are prepared such that the notch is located

at different positions. To study the effect of notch position in

HAZon theabsorbed energy, wehavethree types of specimens

having different notch positionsin HAZ (Fig. 2). In other words,

the location of notch varies within HAZ.

3. Numerical simulation

3.1. Finite element analysis

Three-dimensional finite element analysis was conducted

using ABAQUS, a commercial FEA code (Version 6.6-1). An half

of thespecimen was analyzed due to geometrical symmetry of

the specimen and loading condition. The three-dimensional

mesh of specimen is shown in Fig. 3.Since we focus on thevariation of absorbed energy of specimen, significant mesh

refinement is given around the area that considerable plastic

flow occurs. Element type for the specimens is C3D8R, but ele-

ment type for transition mesh region is C3D4. The number of

elements used is 39,000. The edge length of smallest element

at the notch root is 250 m.

For arbitrary crack growth simulation, Needleman (1987)

adopted a cohesive force model in which the fracture charac-

teristics of the material are embedded in a cohesive surface

traction-displacement separation relation for all elements in

the material. But obtainingthe material parameters character-

izingthe cohesivesurface separation law is quite complicated.

Crack growth simulation was alsoperformed using an element


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Fig. 2 Specimens with different notch positions in HAZ. NT-1, NT-2 and NT-3 indicate that notch is located 0.5, 1.0 and

1.5 mm from fusion line.

Fig. 3 Representative finite element mesh of the Charpy

test specimen.

removing method (Tvergaard, 1982)which requires informa-

tion on a prescribed stress or strain around the crack tip toevaluate whether or not the crack will grow further. When

the crack tip passes a material point occupied by an element

ahead of the initial crack tip, the element is assumed to van-

ish, which is realized by reducing the magnitude of the nodal

forces to be proportional to the fraction of the length that the

crack tip has traversed in the failed element. In this study,

we use the element removing method for simulating crack

propagation. During the removing step, no additional load

increments are applied, and no other elements vanish. This

guarantees that the reduction of internal force to zero has no

effect on thecomputed crack path. Thestrikingedge has initial

velocity of 6 m/s.

3.2. Hardness variation in the HAZ

Generally, the grains on the heat affected zone adjacent to

the fusion line are more coarsening compared with those of

other areas, and therefore they have lower toughness values

(Taillard et al., 1995; Devillers et al., 1993).However, accord-

ingto several surveillance test reports (Hong, 1997), toughness

of the HAZ was reported to be higher than that of base

material. This implies the mechanical properties might be

dependent of welding condition and chemical components of

material. Therefore, we must measure the mechanical prop-

erties directly through performing a series of tensile test (or

compressive test if necessary), but we cannot perform the

test for HAZ since we are in trouble in making specimen

suitable for the test. Note that width of HAZ is so narrow

that one cannot make specimen for tensile test. For the rea-

son, we rely on an alternative which yields the mechanical

property.

The alternative is that we first measure the hardness of

base material, HAZ andweld andthen calculateyield strength

of themfrom the measured hardness. The hardnessmeasured

across base material, HAZ and weld is shown inFig. 4.This

hardness configuration points out the mechanical properties

in HAZ is quite heterogeneous. A correlation between Vickers

hardness,Hvand yield strength,yused in this study is in the

followings.

Hv =Cy (1)

Fig. 4 Measured Vickers hardness at weld, HAZ and base material.


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Fig. 5 HAZ is divided into three regions for finite element analysis. Two cases are considered. W-HAZ is a region adjacent

to the weld fusion line, C-HAZ is a center region of HAZ and B-HAZ is a region adjacent to the base material.

Cis proportionality constant. Rationale for this relation is as

follows. It has been found that the measured hardness of

base material is equal to the measured yield strength of base

material multiplied by a constant 3. Ultimate tensile stress, unecessary for failure simulation, is also assigned accordingly

with referring toKim and Yoon (1998).Mechanical propertiesemployed in the study are summarized inTable 1.

As strain rates of material increase, its yield strength

increases as well. For many common metals this effect starts

to play a role in the deformation behavior and crack propaga-

tion. This study applies yield stress ratio, /0. is flow stress

and0 is static yield stress. In mild steel, yield stress ratio is

in the range 2.02.8 (Folch and Burdekin, 1999).In this study,

yield stress was assumed 2.0.

3.3. Notch position

To investigate the effect of inhomogeneous distribution of

mechanical properties in HAZ on the crack propagation behav-ior and absorbed energy of specimen during test, HAZ with

width of 2 mm is divided into three regions which have dif-

ferent mechanical properties as shown inFig. 5.The width of

each region (W-HAZ, C-HAZ and B-HAZ) is different.

Two cases are considered according to different width of

each region. In Case I, the width of W-HAZ is 1.0mm and

C-HAZ and that of B-HAZ is 0.5 mm, respectively. In Case II,

the width of W-HAZ and C-HAZ is 0.5 mm, respectively and

that of B-HAZ is 1.0 mm. Notch position for each case is des-

ignated as NT-1, NT-2 and NT-3, corresponding to the notch

positions inFig. 5. Yield stresses calculated by using Eq. (1)

and ultimate tensile stresses are assigned to the elements

belonging to the regions (W-HAZ, C-HAZ and B-HAZ) desig-nated above. If equivalent plastic strain at an element reaches

Fig. 6 Energy absorption experimentally measured is

compared with computed one in terms of notch positions

(NT-1, NT-2 and NT-3) at HAZ (seeFig. 5).

a prescribed failure strain, ductile damage initiation criterion

(DUCTCRT) is triggeredat an element andthen the magnitude

of the elemental nodal forces is reduced to be proportional

to the fraction of the length that the notch tip has traversed

in the failed element. The failure strain of the each mate-

rial are assumed as follows; base material= 0.3, W-HAZ = 0.26,C-HAZ = 0.27, B-HAZ = 0.29 and weld= 0.28.

4. Results and discussion

4.1. Energy absorbed during impact

Fig. 6 compares experimentally measured absorbed energy

with computed one in terms of the notch position (NT-1, NT-2

andNT-3)at HAZ. When thenotch is adjacent to thefusionline

(i.e., NT-1), the energy absorption measured is larger than cal-

culated one. This indicates the ultimate tensile strength and

yield stress of specimen adjacent to the fusion line is higherthan those setto finite element model. Especially, much larger

difference is observed in Case I, in comparison with Case II.

The difference of the absorbed energy between the NT-1 of

Case I and NT-1 of Case II is approximately 60 J.

But the difference reduces when notch is located at NT-2.

In the NT-3 of Cases I and II, the energy absorption measured

is similar to calculated one. In NT-1 of Case I, crack passes

through W-HAZ (which has higheruandy) but, in Case II, it

does not pass through W-HAZ. The crack of the NT-3 of Cases

I and II, passes through base material as well. This result indi-

cates the difference of the absorbed energy depends on crack

growth path as well. A detail explanation for this is given in

the following section. For Cases I and II, a good agreement isnoted when notch is located at NT-3. These results imply that

the notch might be located near the base material when we

makes specimen for the Charpy impact test.

4.2. Crack propagation

Fig. 7 illustrates the contour of ductile damage initiationof ele-

ments being deformed and three stages for crack propagation

in deformed state. If DUCTCRT is 1.0, the element is about

to be failed and then crack starts propagating. If DUCTCRT

is zero, the elements do not reach the prescribed failure

strain as yet. It illustrates 2-D configuration of crack propa-

gation for Case I. If whole HAZ is assigned as homogeneous


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Fig. 7 Contour of ductile damage initiation criterion and equivalent plastic strain. NT-1, NT-2 and NT-3 specify that CL(center line) of notch is located at 0.5, 1.0 and 1.5 mm from fusion line (seeFig. 5).

mechanical properties, crack propagation directionis straight.

However, when inhomogeneous mechanical properties are

assigned, the direction of crack propagation is notstraight any

more.

Fig. 7(a) shows when notch is located between W-

HAZ and C-HAZ (i.e., NT-1 in Case I), the crack passes

through C-HAZ region and propagates toward the B-HAZ

region and base material. Fig. 7(b) (i.e., NT-2 in Case I)

andFig. 7(c) (i.e., NT-3 in Case I) also show that the crack

progresses toward the B-HAZ and base material. However,

the amount of crack growth direction turned toward the

B-HAZ region and base material is different, as can beshown.

Fig. 7(d) illustrates the direction of maximum stress tri-

axiality (negative pressure stress/von Mises stress) and the

equivalent plastic strain distribution at the notch. Whenstress

triaxiality is large, failure starts easily. At notch tip, the stress

triaxialilty of element to impact direction is the maximum.

Regardless of homogeneous material and inhomogeneous

material, the maximum stress triaxiality is toward impact

direction. However, the crack growth direction is strongly

dependent of the distribution of equivalent plastic strain. This

is because B-HAZ and C-HAZ region has loweru(UTS) andy(yield stress) than W-HAZ. Initially the crack does not go along

the impact direction but its propagation direction becomes

coincident with the impact direction after the crack tip meets

the base material.

5. Concluding remarks

Experimental and numerical studies have been made in

the present work to examine the effect of inhomogeneous

mechanical properties in HAZ on energy absorption. Notch

position in HAZ was dividedinto three sub-regions which have

different mechanical properties. Vickers hardness test on the

specimen alsoshows the mechanical properties varyto a greatextend. The results of 3-D FE analysis showed the energy that

specimen absorbed during impact test is strongly dependent

of the relative notch position in HAZ. The energy absorption

experimentally measured is in agreement with computed one

when the notch of the specimen is displaced by 1.5 mm from

the fusion line.

Acknowledgement

Y. Lee wishes to acknowledge the financial support from the

Korean Science and Engineering Foundation (R01-2006-000-

10358-0 (2006)).


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[48]Efects of Notch Position of the Charpy Impact Specimen on the Failure Behavior in Heat Affected...

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