Scholarly article on topic 'Elastic-plastic behaviour of welded joints during loading and unloading of pressure vessels'

Elastic-plastic behaviour of welded joints during loading and unloading of pressure vessels Academic research paper on "Materials engineering"

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Procedia Structural Integrity
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{"elastic-plastic behaviour" / "welded joints" / "pressure vessel" / "static loading-unloading" / "structural integrity"}

Abstract of research paper on Materials engineering, author of scientific article — Simon Sedmak, Mahdi Algool, Aleksandar Sedmak, Uros Tatic, Emina Dzindo

Abstract In this paper elastic-plastic behaviour of welded joints during loading and unloading of pressure vessel has been analysed. Two stage pressuring process has been applied in previous experimental investigation and simulated using the finite element method. The effect of residual stress and strain has been analysed.

Academic research paper on topic "Elastic-plastic behaviour of welded joints during loading and unloading of pressure vessels"

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Structural Integrity

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Procedia Structural Integrity 2 (2016) 3546-3553

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21st European Conference on Fracture, ECF21, 20-24 June 2016, Catania, Italy

Elastic-plastic behaviour of welded joints during loading and unloading of pressure vessels

Simon Sedmaka, Mahdi Algoolb, Aleksandar Sedmakc, Uros Tatica, Emina Dzindoa

a Innovation Centre of Faculty of Mechanical Engineering, 11000 Belgrade, Serbia

b University ofSirte, Sirte, Libya c Faculty ofMechanicalEngineering, Universityof Belgrade, 11000 Belgrade, Serbia

Abstract

snthis paper elastic-plastic; bbhaviour of welded joints dpring loading and unloading of pressure vessel hits been analysed. Two stage pressuring process lias ngev applied in previous experimental investigation and simulated using the finite element method. The effect of residual stress and strain has been analysed.

© 2016,PRC)STR (Procedia Structural Integrity) Hosting by Elsevier Ltd. All rights reserved. Peer-reeiew under respensibiHty of the ScientificCommittee of ECF21.

Kjgwordg.'elastic-plastic behaviour; welded joins; pressure vene^ static loading-unloading; !tructurag integrity

1. Introduction

The penstock built during the late seventies in the scope of reversible hydro power plant "Bajina Basta" (HPP BB) requined innovative design and extensive experimental reseaech to verifg its structural integrity, [Sedmak et al.

(20 ir)q

To most important aspect os design was the decision to produce one penstock instead of two, as would be re-quised if a mmiM ^tmmictuer^l sleel had been used. For orLly one penstock the application of structural steel of yield strength levml 700 MPa was inevitable. Thiu requirement was satisfied by HT80, weldabte, quenched and tempered, tow alloy high strength (HSLA) steel, with ultim^e tensile strength above 800 MPa. Anyhow, selection of this, IKLA oteel opened a new problem. Namely, the plate thickness in the penstock most stressed part was calculoted to be slightly above 47 mm, which was the upper limnit in plate fabricarion, [Sedmak and ^dmak (1995), Sedmak et al. (22011)] Therefore, two full scale prototypes of this penstock; were mSde in orded to gmther the data about its mtegrity, one tested in the static loading-unloading sequence, and the othea one impact loading (explosion). The

2452-3216 © 2016, PROSTR (Procedia Structural Integrity) Hosting by Elsevier Ltd. All rights reserved.

Peer-review under responsibility of the Scientific Committee of ECF21.

10.1016/j.prostr.2016.06.442

overall behavior of a welded penstock under load was analyzed based on this approach, allowing an evaluation of crack significance and "fitness-for-purpose" assessment.

In this paper elastic-plastic behaviour of welded joints during loading and unloading of pressure vessel has been analysed by using the Finite Element Method (FEM) to simulate experimental results, briefly presented as well, whereas more details are given in [Tatic et al.].

2. Pressure vessel full-scale model

The most important data for the full-scale model of the penstock are given. The welded joints, longitudinal (L) and circular (C), as shown in Fig. 1, were produced by shielded metal arc welding (SMAW) and submerged arc welding (SAW) processes. Typical chemical composition of SM 80P steel plates and its weld metals is given in Table 1, and mechanical properties in Table 2.

Fig. 1. Instrumentation and specimens sampling in the penstock model static pressure test Table 1. Chemical composition of SM 80P steel and of MAW and SAW weld metals

Element

SM 80P 0.10 0.30 0.90 0.01 0.008 0.24 0.48 1.01 0.47 0.03 0.0016 0.5

Weld MAW 0.06 0.53 1.48 0.011 0.005 - 0.24 1.80 0.43 - - -

metal SAW 0.07 0.37 1.87 0.01 0.011 - 0.44 0.13 0.73 - - -

Table 2. Mechanical properties of SM 80P steel and of MAW and SAW weld metals

Material Direction Tensile Charpy impact test

Y.S., MPa U.T.S., MPa Elongation vE-40, J vTrs, °C

SM 80P rolling 794 - 755 804 - 834 24 - 29 156 - 224 -92

cross rolling 794 - 755 795 - 834 22 - 23 60 - 147 -58

Weld metal MAW 722 810 22 99 -5

SAW 687 804 23 78 -18

Figure 2 shows the instrumentation on the developed model mantle, with the scheme of specimens cutting [Tatic et al.].

Pressurizing of the model had been performed in two stages. In the first loading (FL) stage the pressure reached 90.2 bar (at = 399 MPa), corresponding to working pressure, then model was held under pressure of 73.5 bar for two hours. After unloading (UL), model was tested by the pressure of 120.6 bar (oi=533 MPa) in the second loading (SL) stage, close to the total working and water hammer load.

Fig. 2. Typical relationships between pressure and strain

3. Results and discussion

Finite element analysis (ABAQUS) of full-scale model of penstock has been performed and presented in the following form: Von Misses stresses distribution, (FL-UL, SL-UL), Von Misses stress-strain curves (FL-UL, SL-UL), Von misses stress-pressure curves (FL-UL, SL-UL), Pressure-von Misses strain curves (FL-UL, SL-UL) and Hoop stresses-strain curves (FL-UNL, SL-UL).

3.1. Pressure vessel without RS

Figure 3 shows, the von Misses distribution of finite element model for first load as calculated in ABAQUS software, the highest stresses was in weld joint (LSI SAW), and the base metal at that same side. This concentration of stresses is due to the geometry of model.

Fig. 3. von Misses stresses distribution of FE model of first load, (P =14.5MPa)

As indicated in figure 4, the plastic strain initiate just in the weld joint (LS1 SAW), this behavior is due to the lower yield strength of joint and its location in the stress concentration region.

Fig. 4. plastic deformation of FE model (FL-UNL, P=14.5MPa).

As the internal pressure increased in the second load of FE model the level of von Misses stress will increased, and the distribution of stress has been not changed compared to the first load except the behavior of weld joint (LS1 SAW), which is has stress lower than base metal at that side of stress concentration region due to the effect of initiation of plasticity as indicated in figure 5.

Fig. 5. von Misses stresses distribution of FE model of second load, (P=18.5MPa)

As illustrated in figure 6, the level of von Misses stresses have been exceeded yielding of base metal and weld joints at that side of stress concentration region and the plasticity initiated and spreads in base metal and weld joints in this area.

Fig. 6. plastic deformation of FE model (SL-UNL, P=18.5MPa).

3.2. Results without RS (FL-UNL, SL-UNL).

Figure 7 illustrates the behavior of von Misses stress-strain curve of weld joint (LSI) for FL-UN and second load-unload, this behavior showed the linearity of stress-strain curve of loading and unloading behavior for first and second load.

Figure 8 shows the behavior of von Misses stresses with loading and unloading, as the pressure increasing the von Misses stresses increasing until the yield point of the weld joint, then the changing of Von Misses stresses will be lower, for unloading the behavior will be linear, until the effect of residual stresses then will be non-linear.

Fig. 7. Von Misses stress-strain behaviour LSI SAW without RS as calculated in ABAQUS.

Fig. 8, Von Misses-Inner Pressure behaviour of WM LSI SAW as calculated in ABAQUS.

The behaviour of von Misses strain with inner pressure as calculated in ABAQUS is illustrated in figure 9, this behaviour showed linearity during loading and unloading with a little bit changing during plasticity.

The behaviour of hoop stress-strain curve as indicated in figure (5-8), the yielding for first load starts at 13.34 MPa of inner pressure (531.5 MPa of hoop stresses), while for second load the plastic deformation initiated at 14.8 MPa of inner pressure (586.1 MPa of hoop stresses).

Fig. 9. Pressure-Von Misses strain of LSI

Fig. 8, Von Misses-Inner Pressure behaviour of WM LSI SAW as calculated in ABAQUS.

3.3. Results for FL (with RS).

Figure 11 shows von Misses stresses distribution, the highest stresses has been in weld joints at the stress concentration side, due to the effect of initial residual stresses (40% of yield strength) stresses and the shape of geometry of model. Figure 12 shows the initiation of plasticity after FL, in LSI, due to the lower yield point and its location. Figure 13 shows von Misses distribution in SL, with the maximum in weld joints at the concentration stresses side with a considerable increasing of von Misses stresses in base metal at that side.As the inner pressure increased for second load the plastic strain initiated in the other weld joints at the shorter side CMAW, LS3 SEW

Fig. 11. Von Misses stresses distribution of FE model for first load with RS (P=11.2MPa)

Fig. 12. plastic strain of WM LS1 SAW after FL (P=11.2MPa).

m Max. Principal

(Avg: 75%)

— +l,611e-03

+l,477e-03

+l,342e-03

— +l,20Se-03

— +l,074e-03

_ +9,396e-04

+8,054e-04

— +6,712e-04

— +5,369e-04

_ +4,027e-04

+2,685e-04

+l,342e-04

_ +0,000e + 00

—————

Fig 13. Von Misses distribution of FE model for SL with RS (P=14.4MPa).

Fig. 14. Plastic strain of WM LSI SAW after SL (P=14.4 MPa)

3.4. Results for weld metal LS1 SEW, (FL-UNL, SL-UNL, with RS)

The behavior of von Misses stress-strain curve of weld joint LSI SAW with residual stresses is similar to the behaviour without residual stresses, but it yields at lower level of inner pressure due to the effect of residual stresses as illustrated in figure 15.

The behavior of hoop-stress-strain curve of weld joint LSI SAW with residual stresses showed that, the plastic strain for first load was in direction of axial stresses not in circumferential direction (there is no plasticity for first load in hoop stress-strain curve) as indicated in figure 16, this behaviour is due to the shape of the geometry model (angle of 5°), which is exerted more compression in axial direction.

total von Misses strain%

total hoop strain, %

Fig. 15. von Misses stresses-strain curve of WM LSI SAW with RS as Fig. 16. Hoop stresses-strain of WM LSI SAW with RS as

calculated in ABAQUS. calculated in ABAQUS

4. Conclusions

Based on the results presented here and in more details in [1], one can conclude the following:

• For the higher heat input ainicis always higher, while amis higher in WM, but smaller in HAZ, so the higher heat input is somewhat better.

• The HAZ of microalloyed steel has greater resistance against cracks than the WM, being quite different comparing e.g. to the behaviour of microalloyed steels welded joints.

• High stress levels for initiation of stable crack growth suggest the possibility that the welded structure can operate safely even in the presence of relatively large surface cracks.

• The integrity of heterogeneous welded joints is not affected by the presence of surface cracks because overmatching plays a protecting role, which consists in a small plastic deformation of weld metal even at high loads causing fracture of parent metal. The latest conclusion holds at low temperatures, as well.

Acknowledgements

We acknowledge the support for this investigation by Ministry for Education, Science and Technological Development, Republic of Serbia, project TR 174004.

References

Algool, M., 2015. Initial Plastic Deformations and Residual Stresses Influencing the Welded Joint Behaviour in the Presence of Cracks, Doctoral

Thesis, University of Belgrade, Faculty of Mechanical Engineering. Sedmak, A., Sedmak, S., Milovic, Lj., 2011. Pressure Equipment Integrity Assessment by Elastic-Plastic Fracture Mechanics Methods, published by DIVK.

Sedmak, S., Sedmak, A.,1995. Experimental investigation into the operational safety of a welded penstock by a fracture mechanics approach,

Fatigue and Fracture of Engineering Materials and Structures 18(5), 527-538. Tatic, U., Sedmak, S., Durdevic, A., Sedmak, A., Bakic, R., Numerical Modelling of Full'Scale Penstock Model Testing, 3rd International Conference High Strength Steels for Hydropower Plants.