Issue 46

M. El Habiri et alii, Frattura ed Integrità Strutturale, 46 (2018) 34-44; DOI: 10.3221/IGF-ESIS.46.04 35 Fatigue phenomenon undergoes several stages and from an engineering point of view it is convenient to divide the fatigue life of a structure into three stages [10]: fatigue crack initiation, stable of crack growth and unstable crack growth. Fatigue crack initiation presents the main stage in fatigue life which has been extensively studied [11, 12]. Different approaches will be used for estimation of fatigue crack initiation life has been applied by many researchers [13, 14], which is based essentially on stress concentration factor and local stress-strain concepts. Others researchers have used the equivalent strain-energy density method to predict fatigue crack initiation [12, 15]. These works assumed that crack growth part of fatigue life is small comparatively to the initiation fatigue life. In initiation phase, fatigue life is linked strongly to metallurgical, geometrical, loading parameters and stress state without external loading (presence of residual stress). The main loading parameter under constant applied load is stress ratio (mean stress effect) investigated by several researchers, principally in stable crack propagations on some materials [16, 17]. Effect of stress ratio on fatigue crack initiation from hole without residual stress was investigated by Fujczak [18]. It is noticed that an increase in the R-ratio decreases the number of cycles to initiate a fatigue crack. In study conducted by Ranganathan et al [19], crack initiation phase has been considered in the estimation of total fatigue life when short crack growth approach has been applied. The results on fatigue crack initiation of 2024 T351 Al-alloy shows an increasing in initiation life with increasing in stress ratio and maximum remote stress in absence of residuals stresses. Fatigue life depends essentially on zone of compressive residuals stresses around the hole. These stresses decrease the effect of external applied stress field and tend to delay crack growth rate and increase the fatigue life. To quantify cold expansion effects on fatigue crack initiation and propagation, Amrouche et al. [20] have conducted experimental and numerical investigation on 6005 Al-alloy plate. It was found that the compressive residual stresses induced by the cold expansion process are the important parameter for the improvement of fatigue initiation life. Also, it was shown that the fatigue initiation life increases with increasing in the degree of cold expansion and depend on compressive residuals stress zones [20, 21]. In study conducted by Gopalakrishna et al. [22], two cold expansion techniques were applied to generate compressive residuals stress around hole, as split-sleeve with taper pin and split-sleeve with ball. It was concluded that split-sleeve with tapered pin technique gives higher compressive residuals stress comparatively to split-sleeve with ball technique. The improvement in fatigue life was also found to be maximal at 5% degree of expansion, at which the fatigue life is 5.3 times higher than the non-expanded holes. The same tendency was confirmed by others researchers [22-25]. In experimental fatigue investigation of Chandawanich and Sharpe [23] of cold worked hole in 7075 T6 Al-alloy plate, they consider initiation stage corresponded to crack approximately 0.1 mm long. The fatigue initiation lives are too large comparatively to the non-cold-worked hole for all level of applied external load. Additionally to the changes of compressive residuals stress along the plate thickness from entrance face to exit face in cold expansion process, distribution of compressive residuals stress and fatigue lifetime is affected also by variation of plate thickness [26]. In combined numerical/experimental study, Chakherlou and Vogwell [27] have investigated the effect of residual stress induced by cold expansion process of hole on fatigue life failure of 7075 T6 Al-alloy plate. A 3D finite element model was used to simulate cold expansion. In FE modelling, 8-node linear isoparametric cubic elements were used for the pin and the plate. The fatigue results show the improvement in fatigue life in cold expanded hole of specimens comparatively to the drilled specimens. The improvement in fatigue life is almost 10 times. Liu et al [28] have used cold expansion with a split sleeve to produce residuals stress fields. They have performed numerical simulations using 3D finite element modelling. The friction effect was included in this investigation by using the elastic coulomb friction with a friction coefficient  =0.1 also primary included by Chakherlou and Vogwell [27]. Also, Liu et al. [28] have studied the effect of cold expansion on prediction of fatigue life using respectively SWT model [29] and WB model [30]. It was concluded that WB model give a better result than the SWT model. Kurhade et al. [31] conducted 3D finite element analysis for plane stress, where the effect of strain hardening was investigated on the distribution of residuals stress induced by new method called “CsSmPCx process: sleeve with split mandrel and pilot cold expansion” on 7050-T7451 aluminium alloy. From this process the residual stresses along the plate front and back edges decreases with some rise in the tensile stresses away from the hole edge. As for the increase in fatigue amelioration of hole with cold expansion process, Liu et al. [32] reported that, the fatigue life was improved by a factor from 1.5 to 5.0 witch depend on the expansion level. Also, in work of Semari et al. [6], the effects of residuals stresses induced by ball in cold expansion process on fatigue life of 6082 T6 are given by experimentally and numerically investigation (3D finite element analysis). Improvement in fatigue life by expansion was shown under increasing of degree of expansion. The aiming of this paper is to evaluate the residual stress field induced by cold expansion process around hole in the first hand. On the other hand, we investigate the effects of compressive residuals stresses around the hole on improving of fatigue crack initiation life using local strain approach in 2024 T351 of Al-alloy plate. Also, the effect of stress ratio is highlighted .