Fatigue Analysis of Wing-Fuselage Lug section of a Transport Aircraft

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Fatigue Analysis of Wing-Fuselage Lug section of a Transport Fatigue Analysis of Wing-Fuselage Lug section of a Transport Aircraft XV Portuguese Conference on Fracture, PCF 2016, 10-12 February 2016, Paço de Arcos, Portugal Aircraft 1 1 2* K.Shridhar , B.S.Suresh M.Mohan Kumar.turbine Thermo-mechanical modeling of a1,, high pressure blade of an 1 K.Shridhar , B.S.Suresh M.Mohan Kumar.2* Department of Mechanical Engineering, B M S College of Engineering, Bangalore- 500 019. airplane gas turbine engine Fatigue & Structural Integrity Structural Technologies Division 500 019. Department of Mechanical Engineering, BGroup, M S College of Engineering, Bangalore1 1

2

CSIR-National Laboratories, Bangalore 560 017. Fatigue & StructuralAerospace Integrity Group, Structural Technologies Division a CSIR-National Aerospace Laboratories,b Bangalore 560 017.c * Corresponding author: Email-Id: [email protected], Phone: +91-(0)80- 2508 6325 a * Corresponding author: Email-Id: Phone:de +91-(0)802508 6325 Pais, 1, 1049-001 Lisboa, Department of Mechanical Engineering, Instituto [email protected], Técnico, Universidade Lisboa, Av. Rovisco Portugal b IDMEC, Department of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, Portugal Abstract c CeFEMA, Department of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, Abstract Portugal 2

P. Brandão , V. Infante , A.M. Deus *

Lug joints are special type of pin joints, which are widely used for joining various parts of an aircraft, especially for joining Lug are special type service, of pin joints, widely used for to joining various parts an aircraft, for joining wingsjoints to fuselage. During the lugwhich type are joints are subjected fatigue loading andofcomplete loadespecially transfer takes place wings tothe fuselage. During service, the lug type joints are subjected tostress fatigue loading andand complete load transfer takeslead place through pin. At the pin and lug interface, the combination of high concentration fretting could potentially to Abstract through the pin. and At the pincrack and lug interface, the combination of highBecause stress concentration andthe fretting could potentially leadare to crack initiation then propagation under cyclic loading. of this reason wing-fuselage lug joints crack initiation and fracture then crack propagation under cyclic loading. Because reason the wing-fuselage lug joints are considered as most critical components incomponents the aircraft structure. To of appraise the safety level of lugs under working During their operation, modern aircraft engine are subjected to this increasingly demanding operating conditions, considered as fracture critical components inSuch the structure. To appraise the safety levelmodel oftypes lugs working conditions, crack growth and residual life data areaircraft required.In the present work,toaundergo computational for estimating the especiallyfatigue themost high pressure turbine (HPT) blades. conditions cause these parts different ofunder time-dependent conditions, fatigue crack growth and residual life datathe arefinite required.In present a computational forbe estimating the residual fatigue lifeof ofwhich attachment lugs been proposed. Theelement pin isthe assumed towork, havewas larger stiffness than the lug. The is degradation, one is creep. A has model using method (FEM) developed, in model order to able to pin predict residual fatigue lugs has beendata proposed. pin isfor assumed to have astress larger stiffnessbythan the lug. Theloaded pin is assumed to be fitlife in of theattachment lughole zero clearance and no The frictional restraint. Initially concentration in the the creep behaviour of HPT with blades. Flight records (FDR) a specific aircraft, provided aeffects commercial aviation assumed be fit in the lughole zero and clearance and nodata frictional restraint. Initially stress inthethethroughloaded lug were to determined byto applying analytical and numerical methods. Stress intensity factor forconcentration the loaded lug with company, were used obtainwith thermal mechanical for three different flight cycles. In pin order toeffects create 3D model lug were determined by analysis, applying and numerical methods.Both Stress factor for the methods pin lug been with throughthe-thickness emanating from theaanalytical lug hole has scrap been determined. analytical and numerical have used were for needed for the FEM HPT blade was scanned, and itsintensity chemical composition andloaded material properties obtained.the The dataintensity that from wasfactor. gathered was fed into FEM model and different simulations weremethods run, firsthave with a simplified the-thickness emanating the lug hole has beenthe determined. Both andlugnumerical been used for3D obtaining stress Further, fatigue crack growth life foranalytical the cracked subjected to constant amplitude cyclic rectangular block in order toFurther, bettercrack establish the model, and then theofreal 3D obtained from blade scrap. The obtaining theestimated stress shape, intensity factor. fatigue crack growth life forwith the cracked lug mesh subjected to constant amplitude cyclic loading was using the Walker’s growth model. Also the effect different radius ratios (lugthe geometry) on the overallof expected terms of displacement wasmodel. observed, particular the trailing edgeused. of the(lug blade. Therefore loading was estimated using the Walker’s crack growth Alsointhe effectsoftwares ofat different ratios geometry) onsuch the a number cycles tobehaviour failure is in studied. For the FE analysis MSC Nastran/Patran haveradius been model of cancycles be useful in theisgoal of predicting turbine blade life,Nastran/Patran given a set of FDR data. have been used. number to failure studied. For the FE analysis MSC softwares © 2018 The Authors.Published by Elsevier B.V. © 2019 The Authors. Published by Elsevier B.V. B.V. © 2016 The Authors. Published Elsevier B.V. © 2018 The Authors.Published byby Elsevier This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility ofthetheCC Scientific Committee of PCF 2016. This is an and openpeer-review access article under BY-NC-ND licenseunder (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection under responsibility of Peer-review responsibility of 2018 the SICE 2018 organizers. Selection and peer-review under responsibility of Peer-review under responsibility of the SICE organizers. Selection and peer-review under responsibility of Peer-review under responsibility of the SICE 2018 organizers. Keywords: High Pressure Turbine Blade;life,MSC Creep; Finite Element Method; 3D Model; Simulation. Keywords:Lug joint,Fatigue crack growth Patran/Nastran Keywords:Lug joint,Fatigue crack growth life,MSC Patran/Nastran

2452-3216© 2018 The Authors. Published by Elsevier B.V.

This is an open article Published under thebyCC BY-NC-ND 2452-3216© 2018access The Authors. Elsevier B.V. license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection under responsibility of Peer-review responsibility of the SICE 2018 organizers. This is an and openpeer-review access article under the CC BY-NC-ND licenseunder (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection and peer-review under * Corresponding author. Tel.: +351responsibility 218419991. of Peer-review under responsibility of the SICE 2018 organizers. E-mail address: [email protected]

2452-3216 © 2016 The Authors. Published by Elsevier B.V.

Peer-review under responsibility of the Scientific Committee of PCF 2016.

2452-3216  2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection and peer-review under responsibility of Peer-review under responsibility of the SICE 2018 organizers. 10.1016/j.prostr.2019.05.046

K. Shridhar et al. / Procedia Structural Integrity 14 (2019) 375–383 K. Shridhar et al./ Structural Integrity Procedia 00 (2018) 000–000

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1. Introduction In aerospace systems the lugs represent an essential type of joint since they connect wings to fuselage, engines to engine pylons, flaps, ailerons and spoilers to wings. During service, the lug type joints are subjected to cyclic loading and all loads are transferred through the pin. At the interface between pin and lug, the combination of high stress concentration and fretting could potentially lead to crack initiation and then crack growth under cyclic loading. In this study, we mainly concentrated on wing-fuselage lug joints for the reason that wing-fuselage lug attachments are considered as most critical fracture components in the aircraft structure. In the pin loaded lug joint, the combination of high stress concentration could potentially lead to appearance of the crack initiation and then crack growth under cyclic loading. To appraise the safety level of lugs under working conditions, fatigue crack growth and residual life data are required. J.C. Ekvall, [1] developed the relationship between the elastic stress concentration factor Kt and lug geometry based on the results of the finite element analysis. Schijve and Hoeymakers, [2] studied lugs with through-thethickness and corner crack. The authors developed an empirical relationship based on experimental data for crack growth and stress intensity factor. Also a number of methods have been developed to relate fatigue crack growth to the maximum stress intensity factor and stress intensity factor range. Elber, [3] recognized that the stress ratio has impact on the crack extension, and proposed the relationship for crack growth rate based on the effective stress intensity factor. Walker, [4] suggested the two-parameter driving force model for the crack growth investigation. modified the Paris crack growth law by introducing the maximum stress intensity factor and the stress ratio instead of the stress intensity factor range. R. J. Grant and J. Smart, [5] described a numerical study of the stress distribution in pin-loaded lugs. The effects of the ratio of lug width (w) to hole diameter (d), the ratio of the distance of the hole centre from the free end of the lug(H) to hole diameter and showed how does the type of fits between lug hole and pin affects the magnitude and position of the maximum stress around the hole in the lug. Slobodanka Boljanovic, [6] proposed engineering procedures for estimating the strength of aircraft lugs subjected to cyclic tensile loading. The residual strength of through the- thickness damaged lugs is modeled by introducing the Walker’s model..Mirko Maksimovic, [8], established the stress intensity factor relations for lugs with through the thickness crack and for lugs with semi elliptical surface crack using finite element results. M. Naderi, [9] utilized Extended Finite Element Method XFEM, to model fatigue crack growth of attachment lugs. Crack growth and fatigue life of single throughthickness and single quarter elliptical corner cracks in attachment lug were estimated and then compared with the available experimental data. Also author reported that the Walker’s crack growth model yields conservative fatigue life for all lug cases studied in the investigation. Nomenclature α β σnom σbr σ σmax L,w, H, t Ri Ro P a ai af da dv γ,C,m da/dN

Loading angle in degree Lug taper angle in degree Nominal stress MPa Bearing stress MPa Stress MPa Maximum stress MPa Length, width, height and thickness of the lug respectively, mm . Inner and outer radius of the lug mm Applied load,N Crack length, mm Initial crack length in mm Final crack length in mm Edge length of elemental at crack tip, mm Crack opening displacement, mm Material constants Crack growth rate, mm/cycle



E G K KIC ∆K ∆Kth R kt Ysum N F

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Young’s modulus GPa Strain energy release rate, N/mm Stress intensity factor, MPa√m Fracture toughness, MPa√m Stress intensity factor range, MPa√m Threshold Stress intensity factor, MPa√m Stress ratio Stress concentration factor Geometrical correction factor Number of cycles Force at the crack tip, Grid point force N

2. Problem Description Lug joints are special type of pin joints, which are widely used for joining various parts of an aircraft, especially for joining wings to fuselage. During service, the lug type joints are subjected to fatigue loading and complete load transfer takes place through the pin. At the pin and lug interface, the combination of high stress concentration and fretting could potentially lead to crack initiation and then crack propagation under cyclic loading. Because of this reason the wing-fuselage lug joints are considered as most fracture critical components in the aircraft structure. To appraise the safety level of lugs under working conditions, fatigue crack growth and residual life data are required. The work investigates the effect of different radius ratios (lug geometry) and loading angle on the stress distribution of pin loaded lug joint. Further, fatigue crack growth life of the lug with through-the-thickness crack emanating from the lug hole subjected to constant amplitude cyclic load is estimated. Table 1.Material properties [10][11] Aluminum 2024-T351 Young’s modulus

73.1MPa

Poisson’s ratio

0.33

Yield strength

324MPa

Ultimate strength

469MPa

Ultimate bearing strength

814MPa

Fracture toughness KIC

46MP√m Steel

Young’s modulus

210MPa

Poisson’s ratio

0.3

Fig1. Specification of lug with through the thickness crack

In the present work, a lug of width 100mm, length 150mm and thickness 15mm made of Aluminum 2024- T351 alloy material is considered because of its good fatigue strength. The outer radius is fixed to 50mm and the inner bore radius is varied to get different radius ratios i.e. 1.5,2 and 3. The lug is loaded through the pin with an axial tensile load of 50KN (along the length of the lug) and load is transferred from the pin to the lug using gap elements. For lug aluminum alloy is considered and pin is made of steel material. Mechanical properties and material constants are as listed in table 1.

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3. Stress analysis of aircraft lug A geometrical non-linear static stress analysis has been carried out for the lugs with different radius ratios to identify location, magnitude of maximum stresses and to investigate the effect of radius ratio on stress concentration factor. The primary intension of stress analysis is to identify maximum tensile stress location because it is the possible location of crack initiation. 3.1. Effect of radius ratio on stress concentration. The pin loaded lug is analyzed for 1.5, 2 and 3 radius ratios by varying inner radius of the lug to check the effect on stress concentration.  A non-linear static stress analysis will be carried out to find maximum stress (σmax) and to identify the maximum stress location.  The SCF will be found using the relation: 𝜎𝜎��� �1� 𝑘𝑘� � 𝜎𝜎�� Where,

𝜎𝜎�� �

𝑃𝑃 2𝑅𝑅� � �

 The Kt found using FE analysis will be verified by analytical relation [1] : �

�������� � ���� 𝛽𝛽 𝑅𝑅� � � � 1� 𝑘𝑘� � �2��5 � 135 𝑅𝑅�

�2� 

�3�

4. Stress intensity factor solutions of cracked pin loaded lug. In general, geometry of notched structural components and loading is too complex for the stress intensity factor (SIF) to be solved analytically. The SIF calculation is further complicated because it is a function of the position along the crack front, crack size and shape, type of loading and geometry of the structure. In this work analytical results and FEM were used to perform linear fracture mechanics analysis of the pin-lug assembly. Based on the stress intensity factors, fatigue crack growth life predictions have been investigated. The lug dimensions are defined in Fig 2.

Fig 2.General parameters of lug with through the thickness crack



K. Shridhar et al. / Procedia Structural Integrity 14 (2019) 375–383 Author name / Structural Integrity Procedia 00 (2018) 000–000

a)

379 5

From FE analysis SIF for different crack lengths are calculated by considering MVCCI method.

b) SIF for the same crack lengths are calculated using analytical relations [8]: 𝐾𝐾 � 𝑌𝑌��� . 𝜎𝜎𝜎 √𝜋𝜋𝜋𝜋 𝑌𝑌��� � ���

��

Where, �.�2 � �� � � � � � �

��

� �

��

���

� �

���

𝑤𝑤 � 2𝑅𝑅� 2

2𝑅𝑅� 2𝑅𝑅� � � ��.22 � ��.�� � � � �.�� � � 𝑤𝑤 𝑤𝑤 ��

���

�

�

𝑈𝑈 � ���� � �

�

� ����

𝑈𝑈 � �.�2 � �.�2 �

2𝑅𝑅� � 2𝑅𝑅� � � �.2� � � 𝐻𝐻 𝐻𝐻 �

� � �.�2� � � �.���������

c) Stress intensity factors found using FE analysis are verified by analytical values. d) Crack length v/s Stress intensity factor graph is plotted.

���

���

��� ����

����

5. Fatigue life estimation using Walker’s crack growth model. There are many equations that can be used to determine the number of cycles to failure. Paris Equation is the most widely used. The only problem is that it is mainly applicable for very thin components or shells. Hence, the Walker Equation is used, as it gives accurate results for slightly thicker components that cannot be analysed by the Paris Equation. a) Fatigue crack growth life is estimated using Walker’s equation, 𝑑𝑑𝑑𝑑 � 𝐶𝐶��� � 𝑅𝑅�� 𝐾𝐾��� �� 𝑑𝑑𝑑𝑑

[C =1.16Χ10-9 m = 3.477 � � �.�2�]

��2�

Fatigue life of lug can be evaluated by integrating the crack growth rate relationship

𝑑𝑑 � �

��

��

𝑑𝑑𝑑𝑑 𝐶𝐶��� � 𝑅𝑅�� 𝐾𝐾��� ��

b) Crack length v/s Number of cycles is plotted.

����

K. Shridhar et al. / Procedia Structural Integrity 14 (2019) 375–383 K. Shridhar et al./ Structural Integrity Procedia 00 (2018) 000–000

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6. Results and Discussions A geometrical non-linear static stress analysis has been carried out to estimate the stress concentration factor for different radius ratios. An attempt is made to investigate the effect of loading angle on the stress concentration factor. Further, fatigue crack growth life of the lug with through the thickness crack emanating from the lug hole has been evaluated and stress intensity factor for varied crack lengths is evaluated using analytical relations and FE analysis. MVCCI method has been used in the determination of stress intensity factor through FE analysis. 6.1. Determination of stress concentration factor. The stress concentration factors for radius ratios 1.5,2 and 3 have been found by both FEA and analytical approach. Results are listed in table 2. Table 2. Analytical and FEA stress concentration factor results

Sl.no

Ro/Ri

1 2 3

1.5 2 3

Ro in mm 50 50 50

Ri in mm 33.34 25 16.67

Max stress Applied FEA (MPa) stress (MPa) Kt 209 50 4.2 181 66.66 2.72 171 100 1.71

Analytical Kt 4.4 2.75 1.72

Analytical and FEA results are in good agreement with each other. From the above data it was observed that as radius ratio increases, stress concentration factor reduces. A maximum stress concentration factor 4.2 was observed for radius ratio 1.5 and minimum stress concentration factor 1.71 was observed for radius ratio 3.Stress concentration factor v/s radius ratio plot as shown in fig 3(a). From the fig 3(b) it is observed that maximum stress concentration occurs at a location approximately perpendicular to the loading direction.

Kt

a

5,0 4,5 4,0 3,5 3,0 2,5 2,0 1,5 1,0 0,5

b Analytical FEA

1

1,5

2

2,5

3

3,5

Ro/Ri Fig 3.(a) Ktv/s Radius ratio; (b) Stress distribution plot for radius ratio 3 loaded in axial direction.

6.3. Estimation of stress intensity factor. Radius ratio 1.5 fig 4(a): For the 2mm crack, SIF observed was 13MP√m.For 4mm crack it shows significant rise by 30%.Further, for each 2mm crack increments average 15.5% rise was observed. The SIF for 12mm crack was 30.1MP√m. Radius ratio 2 fig 4(b):For the 2mm crack, SIF observed was 10.9MP√m.For 4mm crack it shows significant rise by 28%.Further, for each 2mm crack increments average 9.1% rise was observed. The SIF for 20mm crack was 27.9MP√m. Radius ratio 3 fig 4(c):For the 2mm crack, SIF observed was 9.1MP√m.For 4mm crack it shows significant rise by 18.6%.Further, for each 2mm crack increments average 3.3% rise was observed. The SIF for 20mm crack was 14MP√m.

7 381

Author name / Structural Integrity Procedia 00 (2018) 000–000 K. Shridhar et al. / Procedia Structural Integrity 14 (2019) 375–383



From the above analysis it is observed that SIF values are more critical for lesser radius ratios and shows significant change for smaller changes in crack length. a

b

35

20 15 10

FEM

5 0

Analytical 0

2

4

6

8

10

12

20 15 10

FEM Analytical

5 14

0

Crack length in mm

c

30 25

25 SIF inMPaѵm

SIF in MP√m

30

b

16

0

5

10 15 Crack length in mm

20

d

14 SIF in MPaѵm

12 10 8 6

FEM

4

Analytical

2 0

0

2

4

6

8

10

12

14

16

18

20

22

Crack length in mm Fig 4, (a) SIF v/s Crack length for radius ratio 1.5 ;(b) SIF v/s Crack length for radius ratio 2; (c) SIF v/s Crack length for radius ratio 3.; (d) Von Mises stress plot of radius ratio 1.5 for 10 mm crack length.

Fig 4(d) shows Von Mises stress distribution plot of lug(radius ratio 1.5 )with 10mm crack length. It shows how critical is the stress distribution around the crack tip. In this case a maximum stress 682MPa was observed at the crack tip. 6.4. Fatigue life estimation of damaged lug The Fatigue crack growth life of the pin loaded lug joint with through the thickness crack emanating from the lug hole subjected to constant amplitude cyclic loading (R=0;Pmax=50KN) has been estimated by employing the Walker’s crack growth model. Radius ratio 1.5: From the fatigue crack growth curve it was observed that crack growth is stable up to 8 mm crack length and an average crack growth rate of 3Χ10-5 mm/cycle was observed. There after crack growth rate i.e. slope of the curve goes on increasing rapidly for the next incremental crack lengths. From 8-16 mm average crack growth rate observed was 1Χ10-4 mm/cycle. Radius ratio 2: It was observed that crack growth is stable up to 8 mm crack length and an average crack growth rate of 1Χ10-5 mm/cycle was observed. There after crack growth rate i.e. slope of the curve goes on increasing

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rapidly for the next incremental crack lengths. From 8-24 mm average crack growth rate observed was 5Χ10-5 mm/cycle. Radius ratio 3: From the data it was observed that crack growth rate is slow and gradually increasing when compared other two cases. Between 2 to 12 mm crack growth rate observed was 6Χ10-6 mm/cycle. Further, from 12 to 32 mm crack growth rate was 1Χ10-5 mm/cycle. From the fig 5 it is clear that for the given loading lug with radius ratio 3 has higher fatigue life and lug with radius ratio 1.5 has lesser fatigue life. From this it can be concluded that fatigue crack growth rate is higher in lugs with lesser radius ratios.

Fig 5. Crack length v/s Number of cycles

7. Conclusions In the present work, stress analysis has been carried out by MSC Patran/Nastran to analyze the stress distribution for a pin loaded lug and also identify the maximum tensile stress location which may be the prone region of developing fatigue cracks. The effect of lug geometry i.e. radius ratios and the direction of loading on the stress distribution were also studied. The SIF for progressive crack lengths has been determined for the different radius ratios of the lug geometry through analytical and numerical approaches. Further, fatigue crack growth life of the pin loaded lug joint with the crack emanating from the lug hole subjected to constant amplitude cyclic loading has been estimated by employing the Walker’s crack growth model. The following conclusions can be drawn from the present work:    

As radius ratio increases, stress concentration factor reduces. A maximum SCF of4.2 was observed for radius ratio 1.5 and minimum SCF of 1.71 was observed for radius ratio 3. Stress concentration is maximum at the location perpendicular to the loading direction and it shows exponential decay as we move along the net section towards the lug edge. SIF values are more critical for lesser radius ratios and shows significant change for smaller changes in crack length. The number of cycle to failure is as follows:  For the radius ratio =1.5, the maximum number of cycles to failure is seen to be 2, 94,910 cycles.  For the radius ratio= 2, the number of cycles to failure is 7, 76,180 cycles.



K. Shridhar et al. / Procedia Structural Integrity 14 (2019) 375–383 Author name / Structural Integrity Procedia 00 (2018) 000–000



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 For the radius ratio=3, the number of cycles to failure is found to be 36, 34,386cycles. For the given loading, lug with radius ratio 3 has higher fatigue life than lug with radius ratio 1.5. From this it can be concluded that fatigue crack growth rate is higher in lugs with lesser radius ratios.

Acknowledgements The authors thank Director, NAL, and the Head, STTD, NAL, Bangalore for their support and encouragement during this work. The Principal and Head, Department of Mechanical Engineering, B.M.S College of Engineering, is thanked for their support during the course of this work. References J.C. Ekvall., 1986.Static strength analysis of pin-loaded lugs. Journal of Aircraft 23,pp.438-443. Schijve.j.,Hoeymakers., 1979.Fatigue crack growth in lugs.Fatigue of Engineering Materials and Structures1,pp.185-201. Elber.W.,1971.The significance of fatigue crack closure, In: Damage tolerance in aircraft structure. American Society for Testing and Materials, pp.230-242. Walker.E.K.,1970.The effect of stress ratio during crack propagation and fatigue for 2024-T3 and 7076-T6 aluminum, In: Effect of environment and complex load history on fatigue life.American Society for Testing and Materials,pp.1-14. R.J. Grant., J. Smart.and P. Stanley,.1994.A parametric study of the elastic stress distribution in pin-loaded lugs.,Journal of Strain Analysis 29,pp.299–307. Boljanovic Slobodanka., Maksimovic Stevan., 2014.Fatigue crack growth modeling of attachment lugs.International Journal of Fatigue Boljanovic. S., Maksimovic. S., Carpinteri.A.,2013. Fatigue life evaluation of damaged aircraft lugs.Scientific Technical Review63, pp.3-9. Maksimovic M., Nikolic-Stanojevic V., Maksimovic K., Stupar.S.J., 2012.Damage tolerance analysis of structural components under general load spectrum.Technical Gazette 19, pp. 931-938, Naderi M., Iyyer N.,2015.Fatigue life prediction of cracked attachment lugs using XFEM. International Journal of Fatigue77, p.186-193. J. Gilbert Kaufman.,2001.Fracture Resistance of Aluminum Alloys. United States of America: ASM International,pp.98-103. Michael Bauccio.,1993.ASM Metals Reference Book.Third edition, ASM International,Materials Park.