PSI - Issue 14

L. Chikmath et al. / Procedia Structural Integrity 14 (2019) 922–929 Chikmath/ Structural Integrity Procedia 00 (2018) 000–000

923

2

Outer radius of the lug Load ratio Regions of separation

Mean stress Yield stress Ultimate stress Shear stress

Thickness of the lug

1. Introduction Many high technology structures such as those in aerospace are of large scale. They are made in modules to ease the manufacturing process and to facilitate easy assembly and dis-assembly. Consequently joints become inevitable in connecting these structural components[Moisseieff et al., 1944].Distinctly lug joints with fasteners play the requisite role in connecting major parts of the components by acting as primary load carrying members [Hsu, 1981]. Their geometrical configurations are susceptible to stress concentrations with usage and failures of such lug joints may lead to catastrophe. Hence their structural integrity is of prime concern from design point of view [Kathiresan et al., 1986]. Particularly there is need for betterment of fatigue life of these lug joints with fasteners. One of the conventional, popular and economical technique for improving the fatigue life is cold working of holes [Champoux, 1986]. Objective in cold working of holes is to cause compressive residual hoop (tangential) stresses around the circumference of the hole which reduce the effect of tensile hoop stresses on the critical location during remote loading [Forgues et al., 1993]. This cold working of fastener holes by mechanical means was found to be cost effective and increase the fatigue life by a factor of two to seven [De Matos et al., 2005]. In mechanical means of cold working, a large size mandrill is driven into the hole and withdrawn leaving plastically stretched material around the hole. This has been experimentally shown to improve fatigue life of the plain hole [Ball et al., 1998]. Boeing aircraft industry developed a split sleeve cold expansion process during late 1960. This retarded the fatigue crack initiation and crack growth at critical holes. The method of inducing this compressive residual stress by radial expansion (cold working) of fastener holes is still being used in most of the aircraft industry [FTI, 2002]. This is due to the benefit of fatigue life of a structural component which would lead to optimum operational and maintenance cost. Later advanced method of cold expansion was developed by Chakherlou [Chakherlou et al., 2003] to create uniform compressive residual stress around the circumference of the hole. This was achieved by a tapered pin with a mating tapered split sleeve. Earlier contributions carried out mainly focused on modelling of residual stresses with help of finite element methods using two dimensional or three dimensional procedures[Forgues et al., 1993, Poussard et al., 1995 and Kang et al ., 2002]. Further hole profile and variation in strains on both lateral surfaces were studied experimentally. Budiansky [Budiansky, 1959] developed analytical models to evaluate the cold-worked hole considering non-linear response at unloading for finite dimensions. This theory was later modified to get solutions on fatigue analysis of these holes [Guo, 1993]. Further Leon [Leon, 1998] studied the phenomenon of cold working of holes and stated that the main interest allied with this cold working is advancement of the fatigue life that in turn reduces the unscheduled maintenance, increased duration between inspection intervals which ultimately cut the maintenance cost with increasing the aircraft readiness. Over the years it was technologically shown that interference fit bolts where the diameter of the pin is larger than that of the hole, leads to improvement in fatigue life. This was intuitively known since push and clearance fits transfer the bolt load over part of the pin-lug interface only. Whereas for interference fits, full contact is maintained over considerable extent of maximum load level in the fatigue cycle. However, the only minor drawback in interference fit is the large initial stress at zero load level which leads to large mean stress in the fatigue cycle [Venkataraman, 1966]. There were several attempts to develop techniques of decreasing the mean stress levels. Cold working of the holes in which a large size mandrel is inserted into the hole which causes the stress levels to go beyond yield and later withdrawing the mandrel leading to plastically stretched material around the hole boundary is one of the methods for this purpose [Dupret et al., 1996]. In this paper, the effect of cold working process on lug joint has been studied by