PSI - Issue 5

Ahmed Bahloul et al. / Procedia Structural Integrity 5 (2017) 430–437 Ahmed Bahloul et al. / Structural Integrity Procedia 00 (2017) 000 – 000

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could evaluate FCG life of mechanical structures with an acceptable confidence level, still remains among a challenging point in several industrial sectors. Lug type joint is considered as the most critical engineering component in the aerospace industry. It is used generally to assemble components with other mechanical structures such as wings to fuselage and spoilers to wings. Since lug type joints are a very sensitive component in the aeronautical industry, several researches [Kim et al.(2003), Mikheevskiy et al. (2012), Baljanovic and Maksimovic (2014), Naderi and Iyyer (2015) and Bahloul et al. (2017) ] have dealt with the problem of fatigue crack growth for these structures. The present paper aims at developing an engineering approach for FCG life prediction of 7075-T6 Aluminum alloy cracked lug component taking into account the effect of residual stress distribution around the crack tip and material dispersions. The XFEM was implemented for FCG modeling. The RC-SIF parameter is proposed to consider the effect of residual stress distribution near the crack tip and the MCS is used to determine the iso-pobabilistic a-N curves corresponding to 5%, 50% and 95% of reliability. A comparison between the suggested approach and available experimental data is performed. 2.1. Numerical procedure for FCG life prediction In fatigue fracture analysis, traditional empirical models examine the FCG rate within the framework of linear elastic fracture mechanics (LEFM). However, a crack tip plastic zone can be almost developed when a growing crack occurs in ductile materials. This plastic zone’s size varies depending on various parameters such as specimen thickness, crack length, applied load, yielding stress …etc. It is generally admitted that the local strains/stresses, located in this zone, control the FCG mechanism. It was showed [Noroozi et al.(2007)] that the relationship between the stress intensity factor and the stresses/strains field near the crack tip is often affected by residual stresses generated by reversed plastic deformations. Since stress intensity factor is defined as a driving force parameter for predicting crack growth path, FCG rate and fatigue life, it is necessary to quantify the residual stress impact in terms of SIF. Using the weight function method, the residual stresses can be converted to residual stress intensity factor K res as follows: = ∫ ( , ) = =0 (1) Where ( , ) are the residual stress in the vicinity of crack tip and the weight function expression [21], respectively. ( , ) = √2 ( 2 − ) ⌊1 + 1 (1 − ) 1⁄2 + 2 (1 − ) 1 + 3 (1 − ) 3⁄2 ⌋ (2) The coefficient 1 , 2 3 are dependent on the cracked component geometry. It was assumed that for a positive stress ratio, only the maximum stress intensity factor is affected by the crack tip residual stress distribution, without significant changes in the minimum stress intensity factor [Noroozi et al.(2007)]. Therefore, the residual-corrected stress intensity factor (RC-SIF) can be written as: ∆ = ∆ + (3) Hence, the number of load cycles for each step of crack propagation can be determined as follows: ∆ = ∆ ∆ (4) 2. Computational Engineering Approach