RESEARCH ON THE INFLUENCE OF SAGGING AND CONTINUOUS UNDERCUT ON THE CAPACITY OF BUTT-WELDED JOINT

International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),

ISSN 0976 – 6359(Online), Volume 5, Issue 8, August (2014), pp. 07-19 © IAEME

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RESEARCH ON THE INFLUENCE OF SAGGING AND CONTINUOUS

UNDERCUT ON THE CAPACITY OF BUTT-WELDED JOINT

Vladimir Stojmanovski1, Zoran Bogatinoski

2, Viktor Stojmanovski

3

1(Centre for Research, Development and Continuous Education – CIRKO, Inspection Body for

Pressure Vessels, Metal Structures and Cableways, Skopje, Macedonia) 2(Professor, Ss. Cyril and Methodius University in Skopje, Faculty of Mechanical Engineering,

Skopje, Macedonia) 3(Associate Professor, Ss. Cyril and Methodius University in Skopje, Faculty of Mechanical

Engineering, Skopje, Macedonia)

ABSTRACT

The behavior of butt-welded joint with imperfection of the outer contour due to sagging and continuous undercut has been analyzed in this paper. The analysis was done by testing and numerical investigation using Finite Element Analysis.

For the testing, the standard probes have been made from material S235JR that is mostly used for the production of welded structures. Sagging and continuous undercut on both sides of the testing plates have been simulated in the welded joint in order to evaluate the imperfection.

Research presented in this paper is directed in gaining acknowledgement and experience for analysis of the welded structures and their usage in design, construction, production and testing. In that manner the real picture of stress distribution is going to be acquired and this will contribute in the design of structures with decreased factor of safety leading to less expensive and yet safe structures which is the common interest of the companies that construct, produce and assemble welded structures.

The purpose of this paper is to endorse the influence of the sagging and continuous undercut on the capacity of the welded joint in order to make appropriate judgment for the safety. Keywords: Butt-Weld, Continuous Undercut, FEA, Imperfection, Material Testing, Sagging.

I. INTRODUCTION

Due to discontinuities from various imperfections found on the outer contour of the welded joint (such as sagging and continuous undercut), there is irregular stress distribution at the joint with

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elevated stress peaks. The influence of these peaks cannot be precisely estimated during the calculation of the joint. In practice, the solution of the problem, in order to prevent the existence of such imperfections, lays in establishment of rigorous criteria prescribed by the regulation. Sometimes there is a question whether these rigorous criteria are reasonable due to the fact that they directly influent the costs of the welded structure. On the other side, in some separate cases as far as there is discontinuity, it is very likely that during the reparation the situation might worsen, particularly if there is a location where the reparation is hard to be made. Considering these facts, in some cases, it is necessary to make judgment whether there is need to make reparation on the discontinuities found on the outer contour during the examination.

In this paper has been analyzed the behavior of butt-welded joint with imperfection of the outer contour due to sagging and continuous undercut.

The probes used for the tensile, bending and toughness tests are standard and they are produced from the plates [2] made from material S235JR that is mostly used for the production of welded structures. Chemical composition and mechanical properties have been obtained by analyzing the material. Appropriate welding technology for the probes has been adopted according to EN499 and E7018 according to AWSA 5.2. the technology has been verified and appropriate WPQR certificate has been issued.

Sagging and continuous undercut on both sides of the testing plates (probes) were simulated at the welded joint. Static examination of the basic material S235JR and the welded joint are made. The joints are analyzed by FEA in their real dimensions of the model and the imperfections with the ALGOR software [4]. Such analysis has shown the stress distribution of the joint.

Research presented in this paper is directed in gaining acknowledgements and experience for analysis of the welded structures and their usage in design, construction, production and testing. In that manner is going to be acquired the real picture of stress distribution that will contribute in the design of structures with decreased factor of safety leading to less expensive and yet safe structures that are the common interest of the companies which project, produce and assemble welded structures. II. BASIC MATERIAL

Models (probes) analyzed in this paper are produced from material S235JR. The material has been tested in the laboratory and properties of material gained from the test are presented in Table 1 and Table 2.

Table 1: Chemical Composition of the material

Chemical element (%)

C Si Mn P S Cr Ni Al Cu Nb Ti Mo V B Cekv

0,11 0,08 0,58 0,013 0,012 0,03 0,02 0,043 0,03 0,02 0,01 0,01 0,01 0,0 0,213

Table 2: Mechanical Properties of the material

Dimensions Fm (N)

Tension Bending Toughness

Reh (Mpa)

Rm (MPa)

A5 (%)

Reh/Rm α

(mm) α (0)

Ρ (J)

9,5x24,6 L=118 L0=90 120980 366 517 31,5 0,71 Ø40 180 112

t= + 200C

International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976

ISSN 0976 – 6359(Online), Volume 5, Issue 8, August (2014), pp. 0

Tension test graph is presented on Figure 1.

Fig. 1: Tension test graph for material S235JR

From the performed tests, it can be concluded that chemical composit

properties, the elongation, bending and toughness meet the requirements of the standard EN 10025 for the quality of the material S235JR. III. WELDING TECHNOLOGY

Welding of the plates was performed with TIG welding procedure (141) for the rootARC welding procedure for filling and finish. ARC welding was performed with basic electrode type E424 32 X5 according to EN499 and E7018 according to AWSA 5.2 [3].

Fig. 2: Welding order: 1. Root

Professionally qualified welder who possesses valid certificates performed the welding. The

prescribed welding technology was verified and the WPQR certificate has been issued IV. THE PROBES FOR THE TESTING

From the Basic material using the presc

created for the purpose of the test. The characteristic imperfections (sagging and continuous


6359(Online), Volume 5, Issue 8, August (2014), pp. 07-19 © IAEME

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Tension test graph is presented on Figure 1.

Tension test graph for material S235JR

From the performed tests, it can be concluded that chemical compositproperties, the elongation, bending and toughness meet the requirements of the standard EN 10025 for the quality of the material S235JR.

Welding of the plates was performed with TIG welding procedure (141) for the rootARC welding procedure for filling and finish. ARC welding was performed with basic electrode type E424 32 X5 according to EN499 and E7018 according to AWSA 5.2 [3].

Welding order: 1. Root -TIG (141), 2. Filling -ARC (111), 3. Finish -

Professionally qualified welder who possesses valid certificates performed the welding. The prescribed welding technology was verified and the WPQR certificate has been issued

THE PROBES FOR THE TESTING

From the Basic material using the prescribed welding technology appropriate plates are created for the purpose of the test. The characteristic imperfections (sagging and continuous


From the performed tests, it can be concluded that chemical composition, mechanical properties, the elongation, bending and toughness meet the requirements of the standard EN 10025

Welding of the plates was performed with TIG welding procedure (141) for the root weld and ARC welding procedure for filling and finish. ARC welding was performed with basic electrode type

-ARC (111)

Professionally qualified welder who possesses valid certificates performed the welding. The prescribed welding technology was verified and the WPQR certificate has been issued

ribed welding technology appropriate plates are created for the purpose of the test. The characteristic imperfections (sagging and continuous



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undercut) are simulated at the joint. The imperfection models used in the further tests are presented in Table 3

Table 3: The Models (probes) used for the tests

Probe mark

1. Welded joint with grinded face and root

2.1

2. Weld with significant sagging on one side

2.6.

3. Weld with continuous undercut on both sides

2.7.

IV.1. PERMITTED DEVIATION OF THE IMPERFECTIONS

The characteristic imperfections according to ISO 6520, depending on the level of quality of the weld are presented in Table 4.

Table 4: Permitted deviation of the imperfections according to ISO 6520

Appearance of the

imperfection t (mm)

Boundary values of the imperfection for the level of quality

D C B

2.6.

> 3

Small sizes h≤0,25 t

no max 2 mm for probe 2.6 h≤2,375 mm


no max 1 mm for probe 2.6 h≤0,95 mm


no max 0,5 mm for probe 2.6 h≤0,475 mm

2.7.

> 3

h≤0,2 t no max 1 mm for probe 2.7

h≤1,9 mm

h≤0,1 t no max 0,5 mm

for probe 2.7 h≤0,95 mm

h≤0,05 t no max 0,5 mm

for probe 2.7 h≤0,475 mm

IV.2. VISUAL EXAMINATION OF THE WELDED JOINTS

Visual examination and dimensional control of the welded joints are performed in order to

evaluate the imperfections. The results from the dimensional control of the imperfections are presented in Table 5.

Table 5: Results from the dimensional control Joint with significant sagging

2.6.

a1

(mm) b1

(mm) a

(mm) b

(mm) c

(mm)

7,6 2 14 1 1

Joint with continuous undercut

2.7.

a1

(mm) b1

(mm) a

(mm) b

(mm) c

(mm) c1

(mm)

6,4 2 16,4 2 2 2

t h

t h



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IV.3. RADIOGRAPHIC CONTROL OF THE JOINTS

Welded probes – plates are radiography tested. The radiograms are presented in Table 6

Table 6: Radiograms

Welded joint with grinded surfaces

Mark Model Radiogram

2.1.

Grinded face and

grinded root

2.6.

Significant sagging

2.7.

Continuous undercut

The defects are clearly recognizable on the radiogram. V. TESTING OF THE WELDED JOINTS

Tensile, bending and toughness tests are performed in order to examine the effect of the

imperfection V.1 TENSILE TEST

The tensile test was performed on both the basic material and the samples without (2.1) and with imperfections (2.6 and 2.7). The tensile test graph for the basic material is presented on figure 2. The results from the tensile tests for the models 2.1, 2.6 and 2.7 and for the basic material marked with 2. are presented in Table 7.

Table 7: The results from the tensile test

Test probe

Probe dimension

A0 (mm2)

Rp0,2 (N/mm2)

Breaking force

Fm(N)

Rupture stress

Rm (N/mm2) Location of rupture

2. 9,5 x 24,6 233,70 366 120980 517 ∆5=31,5%

2.1 9,2 x 24,2 220,80 386 123290 558 Basic material

2.6 8,8 x 24,2 212,96 393 126580 594 Basic material

2.7 8,8 x 24,5 215,60 355 119520 554 Zone of the

temperature influence



12

All of the welded plates have equal nominal thickness. Therefore, at the location of the joint, the widths of the probes are approximately equal. The cross sectional area at the location of the rupture changes due to the different size (depth of the imperfection). At the welded structures where the joints are butt-welded the area of the cross-section has to be constant. For the calculation of the butt-welded joint according to the existing standards, the thickness of the weld is permanently considered as equal to the thickness of the basic material. Then, for different nominal cross section, for defining of the capacity, for comparison most relevant element is the breaking force Fm (N) acquired from the test. The intensity of the breaking force is presented on Figure 3.

Fig. 3: The breaking force Fm(N)

V.2 BENDING TEST

The results from the bending test of the basic material (signed with 2.), the welded joint with no imperfection (signed with 2.1) and welded joints with appropriate imperfections (signed with 2.6 and 2.7) are presented in Table 8

Table 8: Results from the bending test

Test probe

Type of test

Dimension (mm)

Diameter of bend former

(mm)

Shoulders distance (mm)

Bending angle (0)

2. RBB/FBB 10x30 Ø40 70 180

2.1 RBB/FBB 10x30 Ø40 70 180 / 180

2.6 RBB/FBB 10x30 Ø40 70 180 / 180

2.7 RBB/FBB 10x30 Ø40 70 180 / 180

V.3. TOUGHNESS TEST

The results from the toughness test of the basic material (signed with 2.), the welded joint with no imperfection (signed with 2.1) and welded joints with appropriate imperfections (signed with 2.6 and 2.7) are presented in Table 9. The test was performed according to EN875



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Table 9: Results from the toughness test

Probe mark Dimension of the probe a x10 x 55

Temperature (0C)

A0 (cm2)

Deformation energy (J)

Calculated toughness (J/cm2)

Location of the groove

Type of

groove 1 2 3 Sr. 1 2 3 Sr.

2. V 9,2x10x55 +20 0,74 78 138 120 112 105 187 163 151 Middle of the weld

V

2.1 V 9,2x10x55 +20 0,736 144 104 90 112 195 141 122 152

Middle of the weld

V

2.6 V 11,0x10x55 +20 0,88 191 294 207 230 217 334 235 262 Middle of the weld

V

2.7 V 12,5x10x55 +20 1,0 262 294 295 283 262 294 295 283 Middle of the weld

V

According to the analysis of the results of the toughness test it can be concluded:

- Obtained values of the toughness material meet the requirements for the toughness of the material S235JR at the temperature of +200C. Minimum required is 27 J

- Since the groove of the probe is located in the middle of the joint, discontinuities 2.1, 2.6 and 2.7 do not influence the toughness.

VI. FINITE ELEMENT ANALYSIS OF THE IMPERFECTIONS

Plane strain models for the analysis of the considered cases were used. The analysis was

performed with ALGOR software. The models are loaded with Fsr=65KN, force that delivers stress condition close to the yielding.

The Yield stress measured from the tensile test is used as load criteria in the Finite Element Model. For the material S235JR the load is Rp=366 N/mm. According to EN 10025, the material has minimum yield stress Reh=235 N/mm2. In such case the proper value is the one measured from the tensile test Rp(0,2)=366 N/mm2. The Young modulus for all the analyzed cases is 2,1x105 N/mm2. In the Finite Element Model, at the locations of the imperfections the finer mesh has been used. [4].

VI.1. STRESS DISTRIBUTION DUE TO THE IMPERFECTIONS

VI.1.1. WELDED PLATE WITHOUT IMPERFECTIONS (GRINDED FACE AND ROOT – MODEL 2.1) The real model and the Finite Element Model are presented on Figure 4.

Fig. 4: The real model and the Finite Element Model for the case 2.1


ISSN 0976 – 6359(Online), Volume 5, Issue 8, August (2014), pp. 0

The stress distribution is presented on Figure 5

Fig. 5: The stress distribution for the case 2.1 (no imperfection)

VI.1.2. WELDED PLATE WITH SAGGING (MODEL 2.6) The real model and the Finite Element Model are presented on Fi

Fig. 6: The real model and the Finite Element Model for the case 2.6


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The stress distribution for the case 2.1 (no imperfection)

WELDED PLATE WITH SAGGING (MODEL 2.6)

The real model and the Finite Element Model are presented on Figure 6.

The real model and the Finite Element Model for the case 2.6


The stress distribution for the case 2.1 (no imperfection)

The real model and the Finite Element Model for the case 2.6



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Fig. 7: The stress distribution for the case 2.6 (sagging)

VI.1.3. WELDED PLATE WITH CONTINOUS UNDERCUT (MODEL 2.7) The real model and the Finite Element Model are presented on Figure 8.

Fig. 8. The real model and the Finite Element Model for the case 2.7



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Fig. 9: The stress distribution for the case 2.7 (continuous undercut)

VI.2. ANALYSIS OF THE RESULTS FROM THE FEA

Stress distribution at the welded joint for different cases (models) is presented on Figures 5, 7 and 9. At sufficient distance from the welded joint there is steady stress condition. If the stress distribution is analyzed, it may be concluded that at the locations where the continuity is interrupted (imperfection) there is existence of the stress peak. These peaks are close to the values of the yield stress of the basic material. Analyzing the results of the FEA, the following may be summarized:

- The Imperfections (defects) have significant influence on the stress distribution, - When the model is loaded with the force Fsr, near the defects (discontinuities), the stress

achieves the values close or equal to the yield stress of the material, - The verification of the proper modeling is the fact that far enough from the weld there is

steady stress that in fact is the stress delivered when the force Fsr is divided by the cross-sectional area of the plate (the probe).

VII. COMPARATIVE ANALYSIS AND THE REVIEW OF THE RESULTS

The results from the tensile test of the basic material S235JR and the welded plates with

various imperfections are presented in table 7. For defining of the capacity of the joint the most proper element for comparison is the value of the breaking force Fm. The value of the breaking force graphically is presented in Figure 30. The Material S235JR is characterized by good strength properties and good weldability. The results from the tensile test are:

- Yielding stress Rp0,2=366 N/mm2, - Rupture stress Rm=517 N/mm2, - Breaking elongation ∆5=31,5%.

According to the provided strength and deformation properties and the chemical composition

it can be concluded that the material S235JR for the butt-welded profiles (2.1, 2.6 and 2.7), completely fulfills the requirements of the standard EN 10025.



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The breaking force from the tensile test of the joint with grinded face and root (2.1) is Fm

(2.1)=123290 N. This force is approximately equal with the breaking force of the basic material Fmom.=120890 N. The variation is within the tolerant limits. The rupture occurred in the basic material.

From the bending test of the probe 2.1 has been obtained bending angle of 1800. It means that in both cases the criteria of the used bending test standard has been fulfilled. The results from the toughness test of the probe 2.1 are presented in the table 14. The measured values for the toughness fulfill the requirements for the material S235JR at the temperature of +200C. Minimum required value is 27J

Permitted depth of the sagging depending on the level of quality is presented in Table 10 and the measured dimensions of the weld with sagging are presented in Table 11.

Table 10: Permitted values of the sagging Level of quality

D C B

Depth of the sagging h (mm) 2,375 0,95 0,475

Depth of the sagging h (mm) - maximum allowed 2,00 1,00 0,50

Table 11: Measured dimensions of the sagging

Probe Depth of the sagging h (mm)

2.6 1,00

From the visual examination and dimension control can be concluded that welded probe 2.6 does not fulfill the criteria for the level of quality C and B.

From the tensile test of the welded probe 2.6, the measured breaking force is Fm(2.6)=126580

N. By comparing the results from the basic material (probe 2.) and the weld without imperfections (grinded face and root - probe 2.1) can be summarized:

- The breaking force Fm

(2.6) is superior than breaking forces of the basic material and the probe 2.1

- The rupture occurred at the basic material - The stress concentration due to imperfection of the outer contour (Figure 7) does not

influence the capacity of the joint in the static loading condition. This is due to superior ductility of the material S235JR.

From the bending test of the probe 2.6 (Table 8) the bending angle of 1800 has been

measured. It proves that in both cases the required criteria from the bending test standard has been fulfilled.

From the toughness test of the probe 2.6, (Table 9) have been measured toughness values that fulfill the requirements of the toughness of the material S235JR on +200C (probes 2.6)

Permitted depth of the continuous undercut depending on the level of quality is presented in Table 12 and the measured dimensions of the excess metal are presented in Table 13.

Table 12: Permitted values of the continuous undercut

Level of quality D C B

Depth of the root h (mm) 1,90 0,95 0,475

Depth of the root h (mm) - maximum allowed 1,00 0,50 0,50



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Table 13: Measured dimensions

Probe Depth of the root h (mm)

2.7 2,00

From the visual examination and dimension control can be concluded that welded probe 2.7

does not fulfill the criteria for the level of quality D, C and B. From the tensile test of the welded probe 2.7, the measured breaking force is Fm

(2.7)=119540 N. By comparing the results from the basic material (probe 2.) and the weld without imperfections (grinded face and root - probe 2.1) can be summarized:

- The cross sectional area of the probe 2. is A0

(2)=233,70mm2, and of the probe 2.7 is A0

(2.7)=215,60mm2. It gives A0(2.7)=0,92A0

(2). After performing the reduction Fm(2.7)=130000

N. Reduced capacity of the probe 2.7, Fm(2.7) is superior than capacity of the probe 2.1. This

proved that the decreasing of the carrying capacity of the model 2.7 Fm(2.7)=119520 N is result

of decreased cross-sectional area due to the continuous undercut. - The rupture occurred in the zone of the thermal influence. This is result of the location of the

zone of thermal influence where by undercut the cross-section is decreased. - The stress concentration due to discontinuity of the outer contour (Figure 9) does not

influence the capacity of the welded joint in the condition of static loading.

From the bending test of the probe 2.7 and both the root in compressed zone and the root in tension zone the requirements for the bending tests are fulfilled. In both cases the bending angle of 1800 has been achieved.

From the toughness test of the probes 2.7 (table 12) are obtained values for toughness that meet the requirements for the toughness of the material S235JR at +200C. (Probe 2.7) VIII. CONCLUSIONS

From the FEA, the experiments and the analysis of the results from the research it may be concluded:

• The material S235JR of the probes with butt-welds (2.1, 2.6 and 2.7) completely fulfills the requirements of the standard EN10025. The material has good weldability and due to increased ductility is less sensitive to the stress concentration.

• Superior sagging (probe 2.6) depending on the depth can influence the capacity of the joint in static (and presumably in dynamic conditions). The depth of the sagging fulfills the permitted value for class of quality D (h=1 mm < 2mm), but does not fulfill the permitted limits for the class B (h=1mm > 0,5 mm) and h=1 mm = 1mm (class C).

• The high continuous undercut (probe 2.7) depending on the depth has influence of the weld capacity. The depth of the weld with continuous undercut is higher than the permitted value for D, C and B level of quality.

• In the static loading conditions, during the quality assessment of the welded joints in the aspect of imperfections that cause discontinuity of the outer contour can be allowed certain violation of the imperfection dimensions compared to the permitted values of the standard ISO 6520.

• During the quality assessment of the welded joints it should be considered the level of stress at the weld, the kind of the stress and the kind of the loading of the structure.

• During the quality assessment of the welded joints, particularly the dynamically loaded joints, the material of the welded structure must be considered. That is due to the fact that different materials have different sensibility of the stress concentration.



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• Having in mind the knowledge gained from this paper, in certain cases the certain welds may be judged positive even when there are present some imperfections, particularly imperfections in the outer contour. For delivering such judgment, the person must have good understanding of materials, welding, construction, design, calculation etc.

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