PSI - Issue 14

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

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computational methods up to the level of finding out the fatigue life of cold worked holes along with interference fit and their beneficial effects are demonstrated. Based on the approach of cold working of holes from literature, this paper models and analyses this process in the critical lug joint configuration with fasteners and later subjects it to remote static and fatigue loading along with interference pins. Two dimensional numerical pre- and post- processing is carried out using finite element software with four node quadrilateral elements in MSC PATRAN [MSC PATRAN, 2006]. Non-linear analysis is carried out to account for plastic deformation of the material in MSC NASTRAN [MSC NASTRAN, 2005]. The lug joint configuration analysed is shown in Fig.1. Cold working is done by introducing a mandrel whose diameter (λ c ) is higher than that of the hole and withdrawing it later. Later interference fit pin is inserted with various misfit parameter (λ). The separation of pin from the hole of the lug is handled effectively through inverse technique which is described in later stages of the paper. From the stress analysis, the stress distribution around the pin-hole interface is studied and critical locations are identified. The impact of various parameters is integrated on which fatigue crack initiation analysis is carried out. This is done using Basquin's equation with Morrow's mean stress effect and Miner's rule [Ince et al., 2011 and Miner, 1945]. Also the remaining life of these cold worked holes in lug joint is brought out in this paper that would help in applying damage tolerance design concepts to extend the design lives for their aging fleets.

2. Methodology 2.1 FE modelling

Certain geometric dimensions in the lug joint configuration shown in Fig.1 are frozen from the literature [Chikmath et al., 2017]. The Ro/Ri is fixed at 2.5 and length and width of the lug joint to 200mm and 100mm respectively. The lug material is Aluminium alloy T6-6061 and pin is assumed to be rigid. This nearly corresponds to a steel pin in an aluminium lug where in the ratio of modulus of pin (Ep/E) to lug is equal to three [Moisseieff et al., 1944]. The pin-hole interface is assumed smooth with zero friction ( → 0) . Since the model is symmetric about x-axis, symmetric model is modelled using four node quadrilateral elements as shown in Fig.2. The number of nodes and elements are also finalised from the same literature mentioned above.

Fig 1 : Typical lug joint Fig 2 : Symmetric FE model of lug joint A large mandrel (λ c ) is inserted in the hole of the lug. The lug material is chosen to be elasto-plastic (bi-linear) [Hsu et al., 1975]. Then the interference fit with misfit parameter (λ) is inserted in the hole. This phenomenon is pictorially shown in Fig.3. In interference fit, initially there is full contact between the pin and the lug. With the application of pin load, the compression increases at point B with decrease at point A (Fig.4). At higher load levels, pin separates at point A from the pin-hole interface. This separation region escalates symmetrically around the pin. This constitutes to a moving boundary value problem with changing separation/contact regions. This was earlier handled by iterative methods [Harris et al., 1970].This iterative techniques consumed enormous computational time and were found to be non-economical. Later in the geometric design where the progression of separation/contact regions are known a priori, it was shown in the literature [Rao, 1978 and Mangalgiri et al., 1984] that the inverse formulation is beneficial over iterative approach. This paper adopts the inverse approach in handling the issues in the present problem.