residual stress effect on fatigue strength of non-load-carrying cruciform welded joints of sm570q...

20 RESIDUAL STRESS EFFECT ON FATIGUE STRENGTH OF NON-LOAD-CARRYING CRUCIFORM WELDED JOINTS OF SM570Q STEEL FOR WELDED STRUCTURES

1 INTRODUCTION

The fatigue strength of welded joints is usually evaluatedby testing under an applied stress ratio of R = 0.However, high tensile residual stresses generally existin real welded structures. Consequently, as is wellknown, the fatigue strength of large-scale welded mem-bers does not vary with the stress ratio [1, 2]. This insen-sitivity to stress ratio arises because of the presence ofhigh tensile residual stress. A tensile residual stress hasthe effect of increasing the effective mean stress. Shake-down occurs easily in welded members, because thesum of the tensile residual stress and the applied stressexceeds the yield strength of material. As a result, thereal maximum stress in a large-scale welded memberbecomes equal to the yield strength regardless of stressratio. This condition corresponds to a high positive stressratio and therefore represents more severe loading thanR = 0. This is important in tests on small-scale speci-mens that do not contain high tensile residual stresssince the resulting fatigue life may be over-estimated.

In this study, the simulation of high tensile residual stressby cycling down from the fixed maximum stress equalto yield strength (σmax = σy testing [3-6]) was applied tonon-load-carrying cruciform welded joints [7]. The aim

was to investigate the possibility that, with this tech-nique, the fatigue performance of large-scale structurescould be reproduced using small-scale specimens. Tothis end, comparative tests were carried out on small-scale specimens under σmax = σy loading and wider spec-imens designed and fabricated to ensure that they didcontain high tensile residual stress under R = 0.

2 EXPERIMENTAL PROCEDURES

The tests were performed on cruciform joint specimensconsisting of plates with fillet welded transverse non-load-carrying attachments, as shown in Fig. 1 and 2.They were fabricated from 20 mm thick JIS SM570Qrolled steel plate. The chemical composition and

RESIDUAL STRESS EFFECT ON FATIGUESTRENGTH OF NON-LOAD-CARRYING

CRUCIFORM WELDED JOINTS OF SM570Q STEELFOR WELDED STRUCTURES

A. Ohta, Y. Maeda, N. SuzukiNational Institute for Materials Science (Japan)

ABSTRACT

It is common to evaluate the fatigue strength of welded joints by testing simple specimens incorporating the welddetail of interest, usually at R = 0. However, such specimens do not usually contain significant residual stress,whereas yield-magnitude tensile residual stresses generally exist in real welded structures. As a result, the effectivemaximum stress in the fatigue loading is equal to yield, giving more severe loading than R = 0. Thus, unsafe esti-mates of fatigue lives may be obtained from small-scale specimens at R = 0. In this study, the technique of apply-ing fatigue loading that cycles down from yield to simulate the effect of high tensile residual stress (σmax = σy test-ing) was applied to non-load-carrying cruciform welded joints in 20 mm thick SM570Q steel. Comparative tests wereperformed on narrow specimens under σmax = σy loading and wider specimens, fabricated to ensure that they did con-tain high tensile residual stress, under R = 0. The results were in good agreement, confirming that the fatigue per-formance of large-scale welded structures can be reproduced using the σmax = σy test technique.

IIW-Thesaurus keywords: Fatigue strength; Residual stresses, Welded joints; Cruciform joints; Fillet welds; MMAwelding; Nonload carrying; Fatigue tests; Lifetime; Computation; Weld toes; Carbon manganese steels; Influencingfactors; Practical investigations.

Welding in the World, Vol. 46, n° 11/12, 2002

IIW-1581-02 (ex-doc. XIII-1921-02) recommended forpublication by Commission XIII “Fatigue of welded com-ponents and structures”

Fig. 1. Welding sequence.(a) two passes welding, (b) three passes welding.

RESIDUAL STRESS EFFECT ON FATIGUE STRENGTH OF NON-LOAD-CARRYING CRUCIFORM WELDED JOINTS OF SM570Q STEEL FOR WELDED STRUCTURES 21

mechanical properties are given in Tables 1 and 2,respectively. The 0.2% proof strength reported in themill sheet was 606 MPa.

The non-load-carrying cruciform welded joints weremade by manual arc welding with covered electrodes inthe flat position. The welding sequence is shown inFig. 1. The weld line was perpendicular to the rollingdirection of the material. Details of test specimens areshown in Fig. 2.

The joints containing high tensile residual stress (Fig. 2a)were made by the following procedure. The attachmentswere first welded in position with a gap approximately70 mm long in the middle left unwelded. The weldingsequence was as shown in Fig. 1a. After cooling down

to room temperature, the gap was gouged and thenwelded (referred to as the “slit part” in Fig. 2a) using thesequence in Fig. 1b. Finally, both edges were machinedto make the specimen width 250 mm with the slit weldlocated at the middle width.

The 40 and 10 mm wide specimens (Fig. 2b and c) wereextracted from 1,500 mm wide panels welded with con-tinuous fillet welds, without gaps, using the sequenceshown in Fig. 1b. These were then machined into spec-imens with the required widths.

The residual stress distributions along the width of thespecimens and towards the weld toe were measuredby mechanically cutting around strain gauge rosettesfixed to the specimen. The residual stress normal to the

Platethickness

Element (wt%)

(mm) C Si Mn P S Ni Cr Mo V Cu Al Ceq1 PCM2

20 0.12 0.25 1.45 0.014 0.004 0.01 0.029 0.055 0.042 0.01 0.023 0.39 0.21

Table 1. Chemical composition of materials.

(a) Joint containing residual stress (b) Joint of 40 mm width (c) Joint of 10 mm widthFig. 2. Fatigue specimens.

Tensile properties1

Plate thicknessYield strength Tensile strength Elongation

Charpy absorbed Vickers hardness(mm)

at 0.2% offset (MPa) (MPa) (%)energy2 (J) (HV 98N)

20 606 669 35 302 212

Table 2. Mechanical properties of materials.


weld, that is parallel to the loading direction to be usedin the fatigue tests, was calculated by Eq. (1).

σr = –E (εy + νεx ) (1)1 – ν2

The residual stress distribution through the thickness [8]was measured with strain gauges by gradual removal ofsurface layers, as shown in Fig. 3. Strain gauges werebonded at the location of the weld toe after removal ofthe weld (Fig. 3a), and the surface on the opposite sidewas gradually machined. From the relationship betweenthe depth of cutting and the measured strain, the resid-ual stresses were calculated according to Eq. (2).

When the thickness was reduced by half, additionalgauges were bonded to the machined surface (Fig. 3b),and Eq. (3) was used to determine the residual stresses.

σ1 = E {2ε – t – z dε – 3(t – z) ∫z

0

ε dz} (2)2 dz (t – z)2

σ2 = E {2ε’ – t – 2z’ dε – 3 (t – z’) ∫z’

0

ε’ dz’ }4 dz’ 2 {(t /2) – z’ }2

+ 3E (t – z’){ ∫t/2

0

ε dz – 2ε’t/2} – 2 ∫t/2

0σ1 dz (3)

1 – ν2 4 (t – z)2 t t

In these equations E is Young’s modulus, v is Poisson’sratio, ε is the strain measured when the surface layerwas cut to depth z, t is the original plate thickness, ε‘ isthe strain measured when the opposite side surfacelayer of the remaining half-plate thickness is cut to depthz’ and ε’t/2 is the strain measured when the plate is cutto depth t/2.

Axial load fatigue tests were performed in ambient airusing various electro-hydraulic machines with capaci-ties of 1.5MN, 1MN, 500kN, 400kN and 300kN. Theloading patterns used were as illustrated in Fig. 4. Inthe usual test with R = 0, the minimum stress, σmin, isfixed at 0 while the maximum stress, σmax, depends onthe stress range Δσ, such that σmax = Δσ. On the otherhand, in the test simulating the presence of high tensileresidual stress [3-6], σmax is kept constant at σy whileσmin depends on Δσ, such that σmin = σy - Δσ. Both formsof loading were used in the tests on the narrower spec-imens (Fig. 2 b and c), while the wide specimens (Fig.2a) were only tested with R = 0. Tests were also per-formed at R = 0 on base metal specimens, for compar-

ison. The waveform was sinusoidal and the test fre-quency was 10-50Hz in all tests.

3 RESULTS AND DISCUSSIONS

As expected, the specimens failed by fatigue crackingfrom a weld toe. Fig. 5 shows the fracture surface ofone of the joints containing high tensile residual stress.Beach marks show that the fatigue crack initiated in thecentral part of the weld, where the slit weld was located.This part of the weld was made using the 3-pass pro-cedure, as also used for the narrower specimens.

Figures 6 to 9 show the residual stress distributions.Fig. 6 shows the distribution along the weld line, 5 mmaway from weld toe; Fig. 7, the distribution towards weldtoe; Fig. 8, the distribution through plate thickness onthe weld toe section. Fig. 9 shows the estimated resid-ual stress distribution through plate thickness at the weldtoe in the middle of specimen width.

These results confirmed that the joint fabricated toensure the presence of residual stress did indeed con-tain tensile residual stresses of yield strength magni-tude at the weld toe. Thus, this specimen contained thelevel of residual stress that exists in real large-scale

Fig. 3. Procedure of residual stress measurement across the plate thickness.

Fig. 4. Comparison of loading patternsfor R = 0 and σmax = σy tests.


Fig. 5. Fracture surface of one of the jointscontaining high tensile residual stress.

(a) Joint containing residual stress (b) Joint of 40 mm widthFig. 6. Residual stress distribution along weld line 5 mm apart from weld toe.

(a) Joint containing residual stress (b) Joint of 40 mm widthFig. 7. Extrapolation of residual stress at weld toe after picking up 10 mm with bar

from the middle width of specimen.

(a) Joint containing residual stress (b) Joint of 40 mm widthFig. 9. Estimated residual stress in weld toe section of middle width of specimen.

Fig. 8. Residual stress distribution through platethickness in weld toe section.


welded structures. It was also found that the 40 mmwide specimens contained fairly high tensile residualstress, up to around one third yield, at the weld toe.

Fig. 10 shows the fatigue test results obtained from thewide welded specimens containing residual stress andthe base metal, both tested at R = 0. The bands in thisfigure represent the 90% confidence interval. The fatiguelimit obtained by the small-sample staircase method withsix specimens was 52 MPa, corresponding to anendurance of around 2 × 107 cycles.

Fig. 11 shows the fatigue test results obtained from thenarrower specimens. The 90% confidence intervalsenclosing the results from the wide specimens contain-ing high tensile residual stress, from Fig. 10, are alsoshown. It can be seen that the open data plots (resultsfor σmax = σy tests) for the small specimens are in goodagreement with those obtained from the wider speci-mens. The fatigue limits for the small specimens fromthe σmax = σy tests were also the same as that for thewide specimen (52 MPa). These results show that theσmax = σy testing of small specimens is an effectivemethod for obtaining S-N data that could serve as abasis for the design of large welded structures contain-ing high tensile residual stresses.

The solid data plots (results obtained at R = 0) in Fig. 11are also in agreement with those from the wider speci-

mens, except near the fatigue limit. This means that thesum of the applied stress and the tensile residual stressexceeded the yield strength in the higher stress rangeregion.

The fatigue limits for the 40 mm and 10 mm wide spec-imens at R = 0 were 55 MPa and 65 MPa, respectively.Thus, a higher fatigue limit was obtained from the nar-rower specimens, which contained the smaller tensileresidual stresses, as shown in Figs. 8 and 9. This meansthat the maximum stress near the fatigue limit wasslightly below the yield strength in the tests on the nar-rower specimens at R = 0. Consequently, similar esti-mates of the fatigue limit may be obtained from both thenarrow specimens and the wider ones containing hightensile residual stress tested at a lower mean stress inthe present welded joint design. However, this will notalways be the case and σmax = σy testing is consideredto be a more reliable method for establishing the fatiguelimit for a real structure. It may be noted from the pre-sent results that the technique does not produce over-conservative values.

Other support for the σmax = σy test for determining thefatigue limit comes from previous studies [3-6, 9]. In par-ticular, it was found that the reduction in the magnitudeof the fatigue limit with increase in applied stress ratioextended to higher values than R = 0.5. Consequently,lower values were obtained from σmax = σy tests if thecorresponding stress ratio exceeded 0.5. The same wasfound in tests on low strength steels if the maximumapplied stress under R = 0.5 exceeded yield. In otherwords, even tests at the relatively high stress ratio of R= 0.5 can give non-conservative estimates of the fatiguelimit in the presence of very high tensile residual stress.

In another case [4], it was shown that the fatigue limitobtained from welded specimens not containing hightensile residual stress tested at a small mean stress washigh compared with that for a real welded structure (115compared to 70 MPa). Thus, again, the σmax = σy testprovides safer and more realistic results.

Referring back to the present results, it will be notedthat the number of cycles corresponding to the fatiguelimit was around 2 × 107 cycles. This results in a lowerfatigue limit than one coincident with the more usual 5-10 × 106 cycles. The same was found previously fortransverse butt welds tested with σmax = σy [6].Furthermore, σmax = σy tests under random loading gavelives similar to those predicted by Miner’s rule whenused in conjunction with the S-N curve for σmax = σy con-stant amplitude test results [6]. This was true even forloading spectra in which most of the stress ranges werelower than the constant amplitude fatigue limit obtainedwith σmax = σy. This suggests that the constant amplitudefatigue limit obtained with σmax = σy is also applicablefor random loading. This contrasts with the usual designapproach of extrapolating the S-N curve beyond thefatigue limit at a shallower slope [10] to give a lowereffective value. Thus, S-N curves produced either fromspecimens containing high tensile residual stress orusing the σmax = σy test technique may be suitable fordirect application with Miner’s rule for cumulative dam-age calculations.

Fig. 10. Fatigue test results for joint containingresidual stress.

Fig. 11. Fatigue test results for small specimens.


4 CONCLUSION

Comparative fatigue tests were performed on transversenon-load carrying cruciform joints in three widths of20 mm thick steel plate. The widest (250 mm) was spe-cially designed to embody high tensile residual stressand they were tested with R = 0. The two narrower spec-imens (10 and 40 mm) with lower residual stresses inthem were tested with σmax = σy. The following conclu-sions were drawn:

a) The test results were in good agreement, confirmingthat the σmax = σy test technique simulates the effect ofhigh tensile residual stresses in small-scale weldedspecimens.

b) The same relatively low fatigue limit, correspondingto N = 2 × 107 cycles, was also obtained from all threetypes of specimen.

c) However, additional tests on small specimens atR = 0 over-estimated the fatigue limit relevant to weldedjoints containing high tensile residual stress.

d) The results complemented previous ones from otherweld details obtained under both constant and variableamplitude loading in demonstrating the value of the σmax

= σy test as an effective method for producing safe S-Ndata from small specimens that are applicable in thedesign of large-scale welded structures.

ACKNOWLEDGEMENTS

The authors wish to thank the members of theCommittee for Fatigue of Welded Joints in NIMS for thesupport on this data sheet project, and Dr. S.J. Maddox,TWI, Cambridge, UK for his assistance with the editingof this paper.

REFERENCES

1. Fisher J.W., Fatigue Strength of Welded A514 SteelBeams, Fatigue of Welded Structures 1, The WeldingInstitute (1971), 135-148.

2. Gurney T.R. and Maddox S.J., A Re-Analysis of FatigueData for Welded Joint in Steel, Welding ResearchInternational 3 (1973), 1-54.

3. Ohta A., Maeda Y., Mawari T., Nishijima S. andNakamura H., Fatigue Strength Evaluation of Welded jointsContaining High Tensile Residual Stresses, Int.J. Fatigue8 (1986), 147-150.

4. Nakamura H., Nishijima S., Ohta A., Maeda Y., UchinoK., Kohno T., Toyomasu K. and Suya I., A Method forObtaining Conservative S-N data for Welded Structures, J.Testing & Evaluation 16 (1988), 280-285.

5. Ohta A., Mawari T. and Suzuki N., Evaluation of Effectof Plate Thickness on Fatigue Strength of butt Welded jointsby a Test Maintaining Maximum Stress at yield Strength,Engng. Fracture Mech. 37 (1990), 987-993.

6. Ohta A., Maeda Y. and Suzuki N., Fatigue Strength ofButt-Welded Joints under Constant Maximum Stress andRandom Minimum Stress Conditions, Fatigue FractureEngineering Materials Structures 19 (1996), 265-275.

7. Data Sheet on Fatigue Properties of Non-Load-CarryingCruciform Welded Joints of SM570Q Rolled Steel for WeldedStructure – Effect of Residual Stress – 90 (2002), 1-9.

8. Kawada Y., Taira S. and Tada Y., Manual for StressMeasurements, Ohm Press, Tokyo (1972), 363-366 (inJapanese).

9. Ohta A., Suzuki N. and Maeda Y., Shift of S-N Curveswith Stress Ratio, Welding in the World, in press.

10. Haibach E., The Allowable Stresses under VariableAmplitude Loading of Welded Joints, Proc. Conf. FatigueWelded Structures 2, The Welding Institute, Cambridge(1971), 328-339.

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