PSI - Issue 14

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

377

3

E G K

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

K IC ∆K

∆K th

R k t N F

Stress ratio

Stress concentration factor Geometrical correction factor

Y sum

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 K IC

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.