Evaluation of fatigue damage

Evaluation of fatigue damage at welded tube joint under

cyclic pressure using surface hardness measurement

Xu Chen *,Shuang-Mei Zhao

Department of Process Equipment and Machinery,School of Chemical Engineering,Tianjin University,Tianjin 300072,PR China

Received 9August 2004;accepted 27August 2004

Available online 11November 2004

Abstract

In this paper,fatigue tests were conducted on 1Cr18Ni9Ti stainless steel and its welded joints were subjected to cyclic inner pressure.During the fatigue tests,the HL hardness of base metal (BM),weld metal (WM)and heat a?ected zone (HAZ)were measured.The results showed that 1Cr18Ni9Ti was a cyclic hardening material and the WM was a cyclic softening material.Moreover,the fatigue damage evolution of BM was a power relationship and the fatigue damage evolution of WM and HAZ were a linear relationship.Applying Chaboche ?s nonlinear continuum fatigue damage model and studying on evolution of HL hardness,a damage parameter based on HL hardness and a fatigue damage accumulation model was proposed.

Keywords:Welded joints;Fatigue life;Hardness;Fatigue damage

1.Introduction

Welding has been widely employed in industry as one of the most used methods for connecting compo-nents.But due to the heterogeneity induced from welding,base metal (BM),weld metal (WM)and heat af-fected zone (HAZ)have di?erent mechanical behaviors,which makes welded joints complicated under local stress–strain conditions [1,2].Welded joints are subjected to various forms of cyclic loading in practical applications and fatigue failure is common.For pressure vessels and piping,etc,the strength of the welded joints determines the strength of the whole structure.Thus,welding is a major factor in the fatigue lifetime reduction of components [3].

*Corresponding author.Tel.:+862227408399;fax:+862287893037.

E-mail address:xchen@https://www.360docs.net/doc/d46237410.html, (X.

Chen).

Engineering Failure Analysis 12(2005)

616–622

https://www.360docs.net/doc/d46237410.html,/locate/engfailanal

There is an abundance of studies on the fatigue behavior of welded joints.Some recent work is included in[4–8].A comparative evaluation of the low-cycle fatigue behavior of type316LN BM,316WM and 316LN/316weld joints was carried out at a high temperature by Valsan et al.[6].Zhao et al.[7]studied the interaction and evolution of short fatigue cracks in the weld joint of1Cr18Ni9Ti stainless steel via rep-lica observations.Cheng et al.[8]studied the low-cycle fatigue crack initiation behavior of the WM,HAZ and BM in the weld joint of16MnR pressure vessel steel.In order to detect the fatigue damage of the com-ponents,many nondestructive evaluation techniques have been adopted in the?eld.The fatigue damage accumulation process around a notch was studied using a noncontact digital image measurement system [9].Relation between magnetic properties and fatigue substructure in fatigued SM490YA steel was inves-tigated[10].An IR(IR)thermography technique,as a nondestructive evaluation technique,was applied to investigate the fatigue damage of reactor pressure vessel(RPV)steels during20and1000Hz fatigue test [11].Because surface plastic deformation usually determined fatigue damage and its evolution,Okazaki et al.studied the Vickers hardness of welded joints under low-cycle fatigue.

Damage accumulation theory is an important one for fatigue analysis.Damage refers to the reduction of the e?ective working section of specimen or structure under cyclic loading because of the increasing prop-agation of micro-crack.The damage variables describing the damage state of the materials should not only be able to re?ect damage mechanism,but also be sensitive to damage process for easy measurement and has mathematical characteristic.

Based on continuum damage mechanics,the nonlinear continuum fatigue damage model proposed by Chaboche and Lesne[12]has achieved great success in describing uniaxial fatigue.The fatigue damage was considered as the function of maximum stress and mean stress de?ned as follows:

d D?fer M; r;DTd N;e1T

N f

1=1àa

;e2T

N f?

1àa

r Mà r

Me rT

àb

;e3T

a?1àa

r M

r b

kàb

;e4T

Me rT?M0e1àb rT;e5Twhere M0,b,b,a are material constants,k is the slope of the fatigue stress versus life curve,r b is the ulti-mate strength,r M is the maximum stress and r is mean stress.

In this paper,the hardness evolution of1Cr18Ni9Ti stainless steel welded joints during cyclic internal pres-sure was measured and a damage parameter was de?ned in terms of the HL hardness of the surface.A fatigue damage accumulation model was proposed based on Chaboche?s nonlinear continuum fatigue damage model.

2.Specimen and experiments

The base material used in this investigation was a solution-treated1Cr18Ni9Ti stainless steel,which was acquired as a round bar with a diameter of40mm.The chemical composition of the material was(wt%): 0.065C,1.34Mn,0.95Si,0.03S,0.03P,8.74Ni,17.54Cr,and0.41Ti.The room temperature mechanical properties of BM,as appeared on the mill sheet,are given in Table1.

The welded joint was made by use of a shielded metal-arc welding process with an argon atmosphere environment.A special?ux cored electrode of Austenite102(A102)with a diameter of3.2mm was used.

X.Chen,S.-M.Zhao/Engineering Failure Analysis12(2005)616–622617

The chemical composition of A102was (wt%):0.08C,I8.1Cr,9.0Ni,0.50Mo,l.5Mn,0.9Si,0.035P,0.03S,and 0.5Cu.The electrode had ultimate tensile strength of 550MPa,elongation of 20%,and yield strength of 270MPa.Notice that the BM carries higher strength (Table 1)than the electrode material.

A 10mm wide,9mm deep circumferential notch was ?rst machined at the center of a 1Cr18Ni9Ti stain-less steel bar cut to the specimen length,and the notch was ?lled with WM.The specimen was then ma-chined to the shape shown in Fig.1,with an outer diameter of 32mm and inner diameter of 30mm.The specimen had a 10mm long WM section at the center of the gage length.The inner and outer surfaces of the specimen were honed.This design was able to avoid bending deformation because of residual stresses produced in welding [13].

In the experiments,hydraulic pressure was used which provided a pressure as high as 50MPa.The high-pressure oil from the oil pump discharged automatically through the relief valve.The thin-walled tubular specimen was subjected to cyclic alternating internal pressure.The loading was stress-controlled with a fre-quency of 1Hz.Strain and pressure were measured with the strain gauge and pressure sensor,respectively.The data are collected on line through the A/D data collector.

Hardness test had been seen as an important approach for studying material mechanical behavior.In this paper,the HL hardness during the cyclic loading was measured and a damage parameter was de?ned in terms of HL hardness.In order to obtain continuous experiments,a portable hardness tester,HLN-11A by the Beijing Time New Technology corp.was preferred.

The hardness of BM,WM and HAZ were measured during the fatigue test.Moreover,the BM,WM and HAZ of welded specimen were measured axially point-by-point to study the hardness distribution in axial direction and evolution trend with the loading cycles.

3.Experiment results and analysis

Table 2shows the fatigue life of BM specimen (B for short)and welded specimen (W for short)under various stress amplitudes,from which it can be seen that fatigue life decreases as stress amplitude

increases.

Fig.1.Geometry of specimen:1–WM section;2–BM section;3–sealing ring;4–?ange.

Table 1

The mechanical properties of 1Crl8Ni9Ti stainless steel

b (MPa)

s (MPa)w (%)d (%)E (GPa)G (GPa)m HB 590295695620378.20.299160618X.Chen,S.-M.Zhao /Engineering Failure Analysis 12(2005)616–622

The curves of hardness of BM,WM and HAZ in welded component to cycle number is shown in Fig.2and the ?tting curve was obtained by the least squares method.It was shown that the hardness of BM in-creased at the initial cyclic stage and then decreased,but that of WM and HAZ seemed to drop continu-ously during the whole cyclic loading.According to the results of Okazaki et al.[14],cyclic hardening material showed an initial hardness increase while cyclic softening material presented a hardness decrease.The conclusion came that BM was a cyclic hardening material while WM and HAZ were cyclic softening materials.The conclusion was same as the observation of the stress amplitude under strain controlled low-cycle fatigue test for same BM and WM [13].The axial distribution for hardness of specimen W5is given in Fig.3.It was shown that HAZ had the smallest hardness,and that WM had highest hardness.

Suppose that the hardness change was related to the fatigue damage.Thus,based on the hardness the damage was de?ned as:

D ?1àHL HL max ;e6T

where HL was HL hardness at a certain cycle.HL max was the maximum HL hardness during fatigue test.The damage evolutions of BM,WM and HAZ,respectively,simulated using the least squares method were correlated in Fig.4.It was shown that the fatigue damage evolutions of WM and HAZ were a power rela-tionship with cycle number and the fatigue damage evolution of BM was a linear relationship with cycle number.When fatigue failure occurred,namely N =N f ,the damage values for the three sections came equal.Thus,the fatigue damage of welded joints can be characterized by the fatigue damage evolution of BM.

Table 2

Fatigue life test results

Specimen no.

Stress range D r (MPa)Fatigue life N f Bl

43036670B2

360126600W4

335121388W5

38548060W8

39533600W945324

040

Fig.2.The ?tting curve for hardness of BM,WM and HAZ to cycle number.

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619

Based on Chaboche?s model,a nonlinear continuum fatigue damage model was proposed as

d D?D a

r M

a r s

d N;e7T

a?1à1àh

r M

r b c

;e8T??symbolizes:

h x i?

x when x>0;

0when x>04:

TFig.4.The damage evolution?tted curve for BM,WM and

HAZ.

Fig.3.The axial distribution of the hardness.

620X.Chen,S.-M.Zhao/Engineering Failure Analysis12(2005)616–622

When D =D c ,N =N f ,The integral of Eq.(7)for D from D =0to D =D c gave:

N f ?11àa r M a r s

àb D 1àa c ;e10TD ?D c N N f 1=1àa :e11T

a ,h ,

b ,

c ,a material constants,D c is critical damage value,r s is yiel

d strength,r M is th

e maximum stress,r b is ultimate strength.

Based on the test results fatigue failure was de?ned as when the hardness value decreased 10%of the maximum hardness.According to the above fatigue failure de?nition D c =0.1.The material constants can be calculated from the damage evolution curves of BM specimen and welded specimen as a =4.314,b =8.621,c =8.126and h =1.061.

The predicted fatigue life by Eq.(10)and the experimental one are presented in Fig.5and show a good correlation between prediction and experiment within a scatter of a factor of two.Thus,the damage accu-mulation model proposed in this paper can predicted the fatigue life of 1Cr18Ni9Ti stainless steel tube and its welded joints under inner pressure well.

4.Conclusions

In this paper,fatigue experiments and hardness tests were conducted on 1Cr18Ni9Ti stainless steel tube and its welded joints under cyclic internal pressure.The results obtained from this study were listed as follows:

1.The hardness tests showed that 1Cr148Ni9Ti stainless steel was a cyclic hardening material and the welded metal and HAZ were cyclic softening materials.The hardness of HAZ was the smallest and that of WM was greater than the hardness of BM.The fatigue damage evolution of BM was a power rela-tionship and that of WM and HAZ were a linear

relationships.

https://www.360docs.net/doc/d46237410.html,parison of predicted life and experimental life.

X.Chen,S.-M.Zhao /Engineering Failure Analysis 12(2005)616–622621

622X.Chen,S.-M.Zhao/Engineering Failure Analysis12(2005)616–622

2.Applying Chaboche?s nonlinear continuum fatigue damage model and studying evolution of HL hard-

ness,a damage parameter based on HL hardness and a fatigue damage accumulation model was pro-posed to predict the low-cycle fatigue life and the damage evolution of1Cr18Ni9Ti stainless steel and its welded joints.

3.The method presented for measuring material fatigue damage with a portable HL hardness tester can be

extended to predict the fatigue life of other structures and components.Hardness test appears to provide

a useful method of investigating the stress–strain behavior during fatigue.

Acknowledgements

The authors gratefully acknowledge?nancial support for this work from National Natural Science Foundation of China and the Teaching and Research Award Program for Outstanding Young Teachers in Higher Education Institutions of MOE,P.R.C.

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