The effect of electroless Ni–P coatings on the fatigue life of Al 7075-T6 fastener holes with symmetrical slits M.A. Rahmat a , R.H. Oskouei b , R.N. Ibrahim a,⇑ , R.K. Singh Raman a,c a Department of Mechanical & Aerospace Engineering, Monash University, Clayton, VIC 3800, Australia b School of Computer Science, Engineering & Mathematics, Flinders University, Bedford Park, SA 5042, Australia c Department of Chemical Engineering, Monash University, Clayton, VIC 3800, Australia article info Article history: Received 25 October 2012 Received in revised form 1 February 2013 Accepted 4 February 2013 Available online 20 February 2013 Keywords: Electroless nickel plating Fastener holes Fatigue abstract In this paper, the fatigue behaviour of Al 7075-T6 fastener holes with symmetrical through slits was stud- ied. The holes were coated with electroless nickel (EN) plating with a high phosphorous content of 10– 13 wt% and a thickness of 40 lm. Uncoated open-hole, EN coated open-hole, uncoated bolted hole and EN coated bolted hole specimens were fatigue tested. Bolted samples were clamped with a high tightening torque of 7 Nm. The established S–N curves showed 282–1348% improvements in the fatigue life due to the combined effect of EN coating and bolt clamping, depending on the level of maximum alternating stress. Excellent adhesion was observed between the coating and the aluminium substrate along the crack path. Tensile tests results showed a considerable reduction of 54% in the ductility of the coated material while both the yield and ultimate strengths were found to slightly increase by approximately 6% in comparison with the uncoated aluminium alloy. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction High strength aluminium alloys, mainly from the 7xxx series such as Al 7075-T6, are widely used in aircraft structures because of their high specific strength [1,2]. A majority of aircraft structural elements are designed to have fastener holes for mechanical joints. These joints are usually critical areas which are susceptible to fati- gue failure in operational conditions. When contemplating fatigue failures, most of the crack initiations are found to occur at the sur- face of the material usually due to localised strains from inten- tional or unintentional stress concentrated regions such as designed notches and corrosion pits [1]. Therefore, effective ap- proaches to prevent or delay crack initiations at the notch surfaces are of considerable importance. Manipulating the resultant inter- nal stress of the material to be in a compressive state would assist in resisting remote tensile loads, therefore increasing the critical stress threshold needed to initiate a crack [3,4]. Previous research explored cold-expansion as a method of increasing fatigue life [5–7]. The studies mainly concluded that cold-expanded holes exhibit longer fatigue lives compared to spec- imens with plain holes. This is attributed to the compressive resid- ual stresses that cold-expansion introduces to the deformed and strain hardened region. However, it is noted that the tangential residual stresses have an uneven distribution around the hole, with the most compressive occurring at the hole edge near the mid- plane and the least compressive at the entrance face. This causes the cold expanded holes to initiate fatigue cracks in the entrance face vicinity. Literature reports a positive relationship; where an increase in bolt tightening torque gives rise to the normal force exerted to the joint; resulting in more compressive stresses around the hole. Fatigue crack initiations are thus delayed, therefore increasing resistance to fatigue failure [8–12]. Numerical simulations per- formed by Chakherlou et al. [8] revealed that the fatigue life of alu- minium bolted plates was improved due to the presence of internal compressive stresses around the plate hole attributed from the clamping force. However, this was not the case for fatigue tests conducted with countersunk fasteners as reported by Liu et al. [9] since a reduction in fatigue life was found as the clamping force of the countersunk fasteners was increased. In addition, another numerical and experimental comparison study [11] considered the friction coefficient in the simulation of steel bolted plates and the authors found a slight increase in the fatigue life as the coefficient of friction between the washer and surface of the plate increased. Abazadeh et al. [13] investigated the effect of bolt clamping force on the fracture strength and stress intensity factor of an alu- minium holed plate with edge cracks. It was concluded that the symmetrical edge cracks, which are in the region with the highest stress intensity factor, decreases their stress intensity as the bolt clamping force is increased. These minute cracks or notches are perhaps seen as the most dangerous for aircraft structural designs. 0142-1123/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijfatigue.2013.02.007 ⇑ Corresponding author. Tel.: +61 3 99051982. E-mail address: raafat.ibrahim@monash.edu (R.N. Ibrahim). International Journal of Fatigue 52 (2013) 30–38 Contents lists available at SciVerse ScienceDirect International Journal of Fatigue journal homepage: www.elsevier.com/locate/ijfatigue