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Fatigue & Fracture of Engineering Materials & Structures doi: 10.1111/j.1460-2695.2008.01234.x Fatigue behaviour of friction stir welded AA2024-T3 alloy: longitudinal and transverse crack growth M. T. MILAN, W. W. BOSE FILHO, C. O. F. T. RUCKERT and J. R. TARPANI∗ Department of Materials, Ae...

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Fatigue & Fracture of Engineering Materials & Structures doi: 10.1111/j.1460-2695.2008.01234.x

Fatigue behaviour of friction stir welded AA2024-T3 alloy: longitudinal and transverse crack growth M. T. MILAN, W. W. BOSE FILHO, C. O. F. T. RUCKERT and J. R. TARPANI∗ Department of Materials, Aeronautics and Automotive Engineering, Engineering School of S˜ao Carlos, University of S˜ao Paulo, Av. Trabalhador S˜ao-Carlense, 400, Centro, CEP. 13.566-590, S˜ao Carlos-SP, Brazil Received in final form 20 April 2007

A B S T R A C T The fatigue crack growth properties of friction stir welded joints of 2024-T3 aluminium

alloy have been studied under constant load amplitude (increasing-K), with special emphasis on the residual stress (inverse weight function) effects on longitudinal and transverse crack growth rate predictions (Glinka’s method). In general, welded joints were more resistant to longitudinally growing fatigue cracks than the parent material at threshold K values, when beneficial thermal residual stresses decelerated crack growth rate, while the opposite behaviour was observed next to K C instability, basically due to monotonic fracture modes intercepting fatigue crack growth in weld microstructures. As a result, fatigue crack growth rate (FCGR) predictions were conservative at lower propagation rates and non-conservative for faster cracks. Regarding transverse cracks, intense compressive residual stresses rendered welded plates more fatigue resistant than neat parent plate. However, once the crack tip entered the more brittle weld region substantial acceleration of FCGR occurred due to operative monotonic tensile modes of fracture, leading to non-conservative crack growth rate predictions next to K C instability. At threshold K values non-conservative predictions values resulted from residual stress relaxation. Improvements on predicted FCGR values were strongly dependent on how the progressive plastic relaxation of the residual stress field was considered. Keywords aluminium alloy; crack growth rate prediction; fatigue; friction stir welding; residual stress. NOMENCLATURES

∗

a = crack length AA2024-T3 = high-strength aluminium alloy grade d = slot aperture da/dN = crack growth rate E = plane-stress Young’s modulus E′ = plane-strain Young’s modulus EL = elongation at fracture FCGR = fatigue crack growth rate(s) FSW = friction stir welding h(x,a) = weight function HAZ = heat-affected zone K C = critical stress intensity factor K MAX = maximum applied stress intensity in fatigue K Ir = residual stress intensity factor in mode I of crack opening

Correspondence: J. R. Tarpani. E-mail: [email protected]

526

 c 2008 The Authors. Journal Compilation  c 2008 Blackwell Publishing Ltd. Fatigue Fract Engng Mater Struct . 31, 526–538

FATIGUE BEHAVIOUR OF FRICTION STIR WELDED AA2024-T3 ALLOY

527

K rx , K ry = residual stress intensity factor distribution on x and y directions L = length of generic test piece L 0 = original gage length M = location of strain-gage R = stress ratio R′ = effective stress ratio RA = reduction in area at fracture S = engineering, nominal, remote or gross stress S, L(x), T(y) = three main orthogonal metallographic axes or directions TP = test piece TMAZ = thermo-mechanically affected zone UTS = ultimate tensile strength W = width of generic test piece WEDM = wire electro-discharge machine YS = yield strength Z(a) = influence function K (th) = range of stress intensity factor in fatigue (threshold value) ε = strain ε M = strain measured at M position ν = Poisson’s coefficient σ rx , σ ry = residual stress distribution on x and y directions INTRODUCTION

In recent years, friction stir welding (FSW), a solid-state joining technique, has been considered a potential technique to replace conventional riveting operations and fusion welding methods (e.g. laser and electron beam) in aircraft manufacture. However, the weld still results in a continuous medium for crack propagation and hence, knowledge of the fatigue and fracture properties of such classes of materials is vital if a damage-tolerant design is adopted. Advantages of FSW process include: design simplification (easy periodical inspection, less macro stressconcentrators), low distortion, poreless welding process (less micro-stress concentrators), static strength as high as 80–100% of parent material, improved fatigue performance and better load distribution. FSW disadvantages comprise: lack of extensive data on mechanical properties, continuous crack propagation medium, mismatching between plastic properties of weld and parent metals and residual stress effects. Residual stresses are invariably present in welded structures after fabrication. They are likely to affect mechanical and corrosion properties of the materials and therefore influence the in-service performance of structural components. The effects of residual stresses on fatigue crack propagation have been reported by several authors such as Itoh et al.,1 Bussu and Irving2 and Milan and Bowen.3,4 Based on Parker’s superposition principle,5 they concluded that tensile residual stresses increase the crack growth rate due to increasing effective stress ratio (R′ ).

On the other hand, compressive residual stresses reduce the fatigue crack growth rate (FCGR) by decreasing the effective stress ratio. Additionally, residual stresses were found to affect initiation fracture toughness values (K C ) of aluminium alloys.6,7 Transverse cracks present an even more complex situation, inasmuch as the defect propagates from the parent material towards the weld region. The crack tip intersects different microstructures and distinct intrinsic fatigue cracking behaviours can be expected. This paper presents data obtained from studying the fatigue crack resistance of FSW joints of aeronautical grade AA2024-T3 high-strength alloy containing either longitudinal or transverse cracks. The effective stress ratio method (Glinka’s R′ )8 was employed to predict FCGR in the welded alloy taking into account the residual stress intensity factor calculated by the slitting or cut compliance method9 and parent material fatigue properties. Predicted crack growth rates are then compared to experimental values. The authors expect to contribute to the still limited body of knowledge regarding FSW materials by providing useful fatigue crack growth data for both safe-life and damage-tolerant designs. INCREMENTAL SLITTING AND WEIGHT FUNCTION METHODS

The cut compliance or incremental slitting method is a helpful technique to determine both the near surface and through thickness residual stress profiles. It is based on

 c 2008 The Authors. Journal Compilation  c 2008 Blackwell Publishing Ltd. Fatigue Fract Engng Mater Struct . 31, 526–538

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M . T . M I L A N et al.

the fact that when a cut, simulating a crack, is incrementally introduced into a part, the residual stresses are relieved, causing the part to deform. Such deformation can be sensed by strain gauges attached at specific positions of the part (Fig. 1) and the residual stress intensity factor profile can be derived.9,10 Assuming a sufficiently narrow slot (d ≪ a), linear elastic fracture mechanics (LEFM) can be employed to establish a relationship (Eq. 1) between the measured strains, ε, and the corresponding residual stress intensity factor in opening mode, K Ir :9 K Ir (a) =

E ′ d εM , Z(a) d a

(1)

where ε M is the measured strain at the back face position M during the cutting procedure, a is the slot length, E′ , the generalized form of the Youngs modulus (E′ = E for plane-stress, and E′ = E/1 − ν 2 for plane-strain conditions), and Z(a) the ‘influence function’ that depends on the test-piece geometry, cut plane location and strain measurement position. Here Z(a) is considered as being independent of the residual stress profile. For a rectangular plate, where L > 2W , and taking strain measurements at the back face (position M), Z(a) is given as follows:11 - for a/W < 0.2 (shallow crack):

Z(a) =

−2,532 (W−a)1.5



1 − 25 ·

  5.926 · 0.2 −



a W

a W

2

2 − 0.2 ·  − 0.288 · 0.2 −

a W



+1



stress distribution in the flawless test specimen, σ r :12 K Ir (a) =

a

h(x, a) · σr (x) · d x

(4)

0

Insofar as the weight function h(x,a) is universal for a given crack geometry, one can determine the inverse weight function, so that σ r can be determined from K Ir (i.e. inverse path). Therefore, by employing the slitting technique and the incremental stress method (inverse weight function) it is possible to obtain the original residual stress profile in the uncracked body, as well as the residual stress profile redistribution due to crack growth. MATERIAL AND TEST SPECIMENS

AA2024-T3 3.2 mm-thick plates were friction stir welded along the rolling direction (Fig. 2). Welding parameters are proprietary to the Brazilian aircraft manufacturer, Embraer S/A. Basic tensile properties of the parent alloy are provided in Table 1 for both longitudinal and transverse directions. Tri-dimensional views of, respectively, chemically etched and unetched microstructures of the material tested are shown in Fig. 3. Full-plate-thickness test pieces with in-plane dimensions of 60 × 120 mm2 were wire electro-discharge machined according Figs. 4 and 5 for, respectively, longitudinal and transverse crack growth specimens. No post-weld heat treatments were applied to the FSW plates, so that all experiments were carried out after all natural aging had ceased.

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