PSI - Issue 14

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

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K. Shridhar et al./ Structural Integrity Procedia 00 (2018) 000–000

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 K t and lug geometry based on the results of the finite element analysis. Schijve and Hoeymakers, [2] studied lugs with through-the thickness 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 through thickness 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

Loading angle in degree Lug taper angle in degree Nominal stress MPa Bearing stress MPa

β

σ nom

σ br

σ

Stress MPa

σ max L,w, H, t

Maximum stress MPa

Length, width, height and thickness of the lug respectively, mm .

R i R o Inner and outer radius of the lug mm P Applied load,N a Crack length, mm a i Initial crack length in mm af Final crack length in mm da γ,C,m Material constants da/dN Crack growth rate, mm/cycle dv

Edge length of elemental at crack tip, mm Crack opening displacement, mm