Numerical simulation of fatigue crack growth in friction stir welded T joint made of Al 2024 T351 alloy Academic research paper on "Materials engineering"

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Procedia Structural Integrity 2 (2016) 3065-3072

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

Numerical simulation of fatigue crack growth in friction stir welded

T joint made of Al 2024 T351 alloy

Abubakr Kredegha, Aleksandar Sedmaka,Aleksandar Grbovica, Nenad Milosevica, Darko

Danicicb

"Faculty ofMechanicalEngineering, University of Belgrade, Swrbia, bRB Kolubarci, EUS, Serbia

Abstract

The Extended Finite Element Method (xFEM) has been applied to simulate fatigue crack growth in an AA2024-T351 Twelden joint, 5 mm thick, made by friction stir welding. The; ABAQUS and Morfeo software has been used. Tensile fatigue loading (mem dress 10 MPa, stress ratio R=0) is applied to Tjoints with a configuration suitable for reinforced panels wCck both skin and the web (reinforcement or stiffened th made of a high strength AA2024-T351. Crack is introduced in one edge of the; skin base materiaL The properties of materials in the areas of roikts ond geometry measeyes of Tjotnt are adopted from available experiments. Followino numerical results are obtained: creak fuont coordinates (x, y, s) and stress intensity eactors (Ki, Kii, Km and Kef) distribution along the crack tip, ar weil as ahe Satigue life estima(ion eor every crack propagation step. Tde main objective odthis research is to better understand fabigue behaviour of friction stir welded T joint of AA2024-T351.

© 2016, PROSTR (Procedia Structural Integrity) Hosting by Elsevier Ltd. All rights reservee. Peer-reekw under respensibiOty of Shs S^entSc Committee mf ECF21.

Keywords/friction stir welding; aluminum alloys; fatigue crack growth; extended finite element method; fatigue life

1. Intro duction

The large scale use of welding for joining of aerospace structures has long being inhibited by the difficulty of productkan of Al alloys welds with high fatigue strength, especially in the case of 2XXX and 7XXX series. These types of aluminum, alloys are usually perceived as non-weldable due to limited porosity and microstructure during solidificatiun mthe fusion zone. There is eko a substantia los s in the mechanical properties as related to the base material. The Welding Institute (TWI) came up with Friction Stir Welding (FSW) in 1991 as a process for joining Al alloys in the solid state, providing good mechanical properties and avo(ding aforementioned problems, Thomas (19955). Thh concept behind FSW can bo perceived as very simple, but stiïl a bit complex whom applied to produce T

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.2016.06.383

joints. Different alternatives to produce T joints using FSW are shown in Figure 1, and more details are presented in Djurdjevic (2015), Zivkovic (2015) and Zivkovic (2015).

-------- 1

ííj:

(el Ifh

Figure 1. Different alternatives to produce T joints using FSW (a) T- joint in two parts, (b) T- joint in three parts without penetration, (c) T- joint in three parts with complete penetration, (d) T- joint in two parts with partial penetration, (e) T- joint in three parts with partial penetration, (f) T- joint in three parts with partial penetration.

The T joint specimens were produced from two flat plates, 5 mm thick. Numerical simulation of the tensile test of T joints using the non-linear finite element code ABAQUS were performed, in order to improve the understanding of the behavior of this type of joint. ABAQUS software and Morfeo are used to display the results of the growth of cracks in FSW 2024-T351 welded joints in all regions. Tensile fatigue load stress is applies, with a ratio of the stress intensity R = 0 with maximum stress 10MPa. The properties of materials in the areas of joints and geometry measures of FSW joint are adopted from available experiments

2. Material properties in Friction Stir Welding (FSW)

This paper presents the analysis of the crack propagation in friction stir welded T joint of two plates (5 mm thick), made of aluminum alloy 2024-T351. Four different zone of welded joint are shown in figure 2.The mechanical properties of the materials are defined for each of these zones with values and are shown in Tables 1 and 2.The constant Paris law (C and m) are taken from Ali's experiments same value for all zones, C=2.02345*10-10 cycles -1, m=2.94, (Golestaneh, A. F, Materials and Design 2009, Golestaneh, A. F, Suranaree Journal of Science and Technology 2009, Zivojinovic, D., 2011).

Fig. 2. The transverse cross section in as-welded FSW 2024-T351 Al Alloy T joint and the mapping of boundaries between macrostructural

zones.

Table 1. Material properties in Friction Stir Welding of Al Alloy 2024-T351

Fsw regimes PZ HAZ TMAZ NZ

Young's modulus 68000 68000 68000 68000

Poisson's ratio 0.33 0.33 0.33 0.33

Yield stress (MPa) 370.00 484.00 272.00 350.00

Hardening constant 770.00 719.00 800.00 -

Hardening exponent 0.086 0.05546 0.1266 -

Hardness (Hv1) 132.00 167.00 118.00 142.00

Table 2. Stress-strain data of FSW zones

PZ HAZ TMAZ NZ

Stress (MPa) Strain Stress (MPa) Strain Stress (MPa) Strain Stress (MPa) Strain

20 0.0003 25 0.0004 50.34 0.00070 30.43 0.00044

40 0.0006 35 0.0006 75.86 0.00123 51.30 0.00080

45 0.0009 58 0.00100 106.90 0.00160 69.56 0.00120

90 0.0014 83 0.00126 131.03 0.00200 91.30 0.00150

125 0.0021 95 0.00150 186.21 0.00310 130.43 0.00210

220 0.0034 130 0.00200 268.96 0.00450 186.95 0.00320

300 0.0050 175 0.00280 331.03 0.00570 286.96 0.00430

320 0.0058 280 0.00438 331.31 0.00550

440 0.0084 330 0.00558

487 0.0120 480 0.00898

540 0.01166

3. Numerical model of Aluminum 2024-T351 T-joint

The structure of two friction stir welded joints consists of different stages for numerical simulation of crack growth within the structure:

1. Create 3D model (shape and dimensions), Fig. 3.

2. Defining the materials, mechanical properties for all different zones.

3. Introducing the initial crack within the structure, including its shape and location.

4. Introduce the loading including its intensity, type and location within the structure.

5. Defining the boundary conditions.

6. Generating the final mesh, the mesh must be refined around the initial crack and in the regions were the crack expected to grow.

7. Analyzing the results obtained. All analyzed results will be introduced in the following tables and graphs. 4. Results and discussion

All simulation analyzes are performed using ABAQUS/Morfeo software. The calculations obtained including stress intensity factors and crack growth data given as a function of load cycles N and crack length are shown in Table 3, as well as in Figures 4 and 5, respectively

The stiffeners (stringers) indicate redistribute load, and increasing of the structural life of the material welded structure, at the same time stress intensity factors decrease when the crack reaches the stringer compared to unreinforced welded structure. Faster crack growth occurs after load cycles number of cca 70000, as shown in the change of the curve slope in Figure 5. During the propagation of the crack through the structure, change of its direction can be clearly seen after the crack propagation reaches the stringer, it grows vertically within the stringer and horizontally within the base material as it is shown in Figure 4. This is related to shear stresses within the structure leads to two additional fracture modes introduced by their stress intensity factors (Kn,Km) Stress intensity factors distribution with crack propagation steps for all modes (Mode I , Mode II , Mode III) can be seen in Table 3. The structure will maintain its integrity since the stress intensity factors is still smaller than the critical stress intensity factors (fracture toughness).

Table 3 Numerical data: stress intensity factors changes with crack growth.

STEP No. x coord. y coord. z coord. Keff KI KII Kiii

STEP 1 32.5 0.416625 84.9999 58.5599 58.5439 1.12209 0.061025

STEP 2 31.501 0.000823 84.9616 70.8492 70.7704 -1.59187 0.017724

STEP 3 30.5011 0.00094 84.9682 82.4329 82.367 0.51373 0.045845

STEP 4 29.5018 0.001465 84.9622 93.7761 93.4894 -0.08391 0.054293

STEP 5 28.5027 0.002259 84.9581 104.499 104.176 0.01841 0.013896

STEP 6 27.5034 0.002846 84.9536 114.94 114.576 0.013571 -0.00807

STEP 7 26.504 0.003378 84.9488 125.261 124.858 0.012528 -0.02873

STEP 8 25.5047 0.003911 84.9439 135.579 135.145 0.008668 -0.03672

STEP 9 24.5052 0.004395 84.9388 146.051 145.593 0.023812 -0.02466

STEP 10 23.5053 0.004428 84.9334 156.409 155.932 0.211431 -0.051

STEP 11 22.5053 0.004479 84.9254 166.581 166.074 0.584981 -0.06266

STEP 12 21.5058 0.004918 84.9104 177.697 177.13 0.681217 -0.01119

STEP 13 20.5089 0.007565 84.8879 189.138 188.523 -0.32102 0.199115

STEP 14 19.5117 0.009874 84.869 202.204 201.587 -0.45661 0.272152

STEP 15 18.5174 0.01481 84.8549 215.592 214.953 -0.18062 0.23008

STEP 16 17.5262 0.022327 84.8428 229.389 228.687 0.013278 0.11272

STEP 17 16.5378 0.032502 84.8312 243.842 243.035 0.048874 0.081264

STEP 18 15.5407 0.035124 84.8194 259.715 258.648 0.184532 0.197215

STEP 19 14.5409 0.035463 84.8073 273.194 271.991 0.657063 0.430872

STEP 20 13.5436 0.037953 84.792 286.13 284.001 1.06159 -2.24119

STEP 21 12.5554 0.04787 84.7723 298.415 295.753 1.66492 -2.17528

STEP 22 11.5614 0.053052 84.7429 310.527 307.224 1.20834 -3.53541

STEP 23 10.5696 0.059875 84.7068 322.066 317.388 1.28617 -5.0629

STEP 24 9.58024 0.069132 84.6635 335.489 329.433 3.80428 -4.43084

STEP 25 8.58906 0.076462 84.5971 351.496 342.527 3.45848 -6.49716

STEP 26 7.60567 0.089946 84.5158 366.723 351.891 3.23582 -9.0617

STEP 27 6.65912 0.008802 84.3886 386.01 361.632 3.12656 10.7498

STEP 28 5.70687 0.390046 84.4829 386.997 347.851 5.99755 -26.5449

STEP 29 4.78826 0.090744 84.2006 384.366 335.805 7.14517 -11.2182

STEP 30 3.86851 0.139307 84.1365 382.047 322.055 9.37619 -11.5448

STEP 31 3.00176 0.000684 83.8674 370.6 298.578 5.2282 5.96545

STEP 32 2.3017 0.004785 83.7541 375.052 301.754 -5.14371 25.9782

STEP 33 1.69232 0.000193 83.6815 368.653 279.84 -19.1624 1.91199

STEP 34 1.20936 -0.0386 83.6885 394.601 331.668 14.5665 25.2557

STEP 35 0.378063 0.05357 83.5783 408.819 347.192 -6.88665 34.5386

STEP 36 -0.77646 -0.41444 83.7738 421.028 377.083 -12.9916 66.6564

STEP 37 -1.53781 0.292509 83.7039 435.628 414.529 -2.07667 34.8246

STEP 38 -2.29268 0.948357 83.6994 437.403 417.769 11.725 7.50726

STEP 39 -2.02817 3.12092 84.0581 461.41 447.037 34.4692 33.2347

STEP 40 -4.90219 -0.9542 83.9546 547.863 494.851 -2.266 68.6755

STEP 41 -5.62922 -1.665 83.1775 613.818 529.835 -67.8504 66.0472

STEP 42 -6.67147 -1.29927 83.7366 670.702 630.018 56.8647 12.69

STEP 43 -7.50136 -2.49964 83.908 756.572 716.132 -23.9424 45.9295

STEP 44 -8.46558 -2.42606 83.9594 764.929 685.254 -169.564 -51.5869

STEP 45 -9.30015 -2.43906 84.4381 772.018 693.016 54.6571 -112.84

STEP 46 -10.1741 -2.43339 84.5713 905.292 811.831 19.779 -149.08

STEP 47 -10.9894 -2.48126 84.7469 991.938 857.934 -35.2377 -171.168

Fig. 4. Stress intensity factors vs x-y coordinates of crack.

50 45 40 35 30 25 20 15 10 5 0

•- —•— -—« --• __•— NUMBER OF CYCLES

10000 20000 30000 40000 50000 60000 70000 80000 90000

Fig. 5. Number of cycles versus crack propagation steps.

The following illustration is given the distribution of Von Mises stresses in the structure in several steps including step (0) as shown in Figure 6a-f.

Fig. 6 (a-f). Von Mises stresses for reinforced plate with FSW T-joint. a. Step 0; b. Step 12; c. Step 22; d. Step 33; e. Step 44; f. Step 46.

5. Conclusions

The main results and conclusions of the work presented in this paper are as follows:

• Numerical simulation can be used to determine the right time to withdraw a cracked component from operation, before unstable crack propagation occurs.

• The crack propagates in different directions (base material and stringer) because of shearing stresses in the structure and redistribution of stress intensity factors.

References

Thomas, W.M., Nicholas, E.D., Needham, J.C., Murch, M.G., Templesmith, P., Dawes, C.J., 1995. Friction Stir Butt Welding, Int.Patent App. PCT/GB92/02203 and GB Patent App. 9125978.8,Dec. 1991. U.S. Patent No. 5,460,317.

Djurdjevic, A., 2015. Friction Stir Welding of Aluminum alloy T joints, (in Serbian) D.Sc. thesis, University of Belgrade, Faculty of Mechanical Engineering.

Zivkovic, A., Djurdjevic, A., Sedmak, A., Tadic, S, Jovanovic, I., Djurdjevic, Dj., Zammit, K., 2015. Friction stir welding of aluminium alloys - t joints, Structural Integirty and Life 15(3), 181-186

Zivkovic, A.,Djurdjevic, A., Sedmak, A., Dascau, H., Radisavljevic, I., Djurdjevic, Dj., 2015. Friction Stir Welding of T-joints, Proc. 3rd IIW South-East European Welding Congress, Romania.

Golestaneh, A. F., Ali, A., Zadeh, M., 2009. Modelling the fatigue crack growth in friction stir welded joint of 2024-T351 Al alloy. Materials and Design 30, 2928-2937.

Golestaneh, A. F., Ali, A., Voon , W. S., Faizal, M., & Mohammadi, M. Z., 2009. Simulation of fatigue crack growth in friction stir welded joints in 2024-T351 Al alloy. Suranaree Journal of Science and Technology 16, 35-46.

Zivojinovic, D., Djurdjevic , A., Grbovic, A., Sedmak , A., Rakin, M., 2014. Numerical modelling of crack propagation in friction stir. Procedia Materials Science 3, 1330-1335.