| dc.description.abstract | Mechanical joints are most commonly used to connect various components of large?scale structures. They have to perform the function of proper load transfer between components, and their structural integrity is of crucial importance. These joints, causing discontinuities in the structure, are locations of stress concentration and potential sites for initiation of failure. It is extremely valuable to develop methods and procedures to numerically model and analyse these joints under all possible geometric, material, and loading parameters and to correlate numerical analysis with experimental data.
To evaluate joint performance under operational conditions, it is necessary to analyse these joints under both static and cyclic loading. Interference?fit pins are often used to enhance the fatigue life of these joints. This results in partial contact or separation at the pin–plate interface, which changes continuously with load level and therefore requires contact stress analysis. Additionally, operational load levels may result in local material yielding, introducing further non?linearity. Efficiently combining these two sources of non?linearity and developing analysis procedures applicable to lug joints form the subject matter of the present study.
This thesis deals with non?linear contact stress finite?element analysis of lug joints under cyclic loading and correlates certain numerical parameters with experimental data. A pin?in?plate configuration is analysed to simulate load transfer through lug joints. The lug and the pin are assumed to be under plane?stress or plane?strain conditions, and the pin?plate interface is assumed to be smooth. The pin can be in an interference, push, or clearance fit, where the pin diameter is greater than, equal to, or less than the hole diameter, respectively. However, the majority of the numerical analysis is carried out on interference?fit lug joints.
For any of the above fits, the pin–plate configuration enters a state of partial contact or separation beyond a certain load level, leading to a non?linear contact stress problem even under linear elastic material assumptions. Moreover, yielding due to interference or applied loading introduces material non?linearity. To address both sources of non?linearity simultaneously, a software package has been developed in this thesis based on an incremental–iterative approach that accounts for changes in boundary conditions and inelastic behaviour. The software is modular, and under cyclic loading the program is controlled to pass through appropriate modules during loading and unloading portions of the cycle. Details of the software are presented in Chapter?2.
A parametric study using this software, varying geometric and material properties of the joint, is presented in Chapter?3. The study regarding the effect of the modular ratio between pin and plate materials revealed that when the modular ratio of the pin exceeds three times that of the plate, joint behaviour closely approaches that of a rigid?pin lug. The investigation of different interference levels also indicated the existence of an optimum interference value for a given joint geometry and material combination.
Damage?tolerance design concepts require analysis of components in the presence of cracks. Accordingly, lug joints with cracks were analysed using elastic contact stress analysis. The stress?intensity factor (SIF) at the crack tip was evaluated and found to vary non?linearly with applied load. During this analysis, it was observed that the radial contact stress distribution exhibits a pronounced peak at the crack mouth. This feature was explained through a continuum elastic analysis at the crack mouth, demonstrating the existence of stress concentration at this location.
Finally, Chapter?5 presents materially non?linear contact stress analysis of cracked lug joints under cyclic loading. A path?independent crack?tip parameter ?Tp was evaluated to correlate numerical predictions with experimental results. An excellent correlation was obtained between experimentally measured fatigue crack?growth (FCG) rates and ?Tp. Using a relationship between low?cycle fatigue (LCF) and fatigue crack?growth properties, FCG rates were estimated and were found to agree well with experimental data. Estimation of fatigue crack?initiation (FCI) life of the joint was also carried out, thereby completing a methodology for estimating the total fatigue life of a lug joint.
The results obtained from these analyses are expected to be highly valuable for the design of structural components in aerospace and other engineering applications. The software developed is extensive and capable of handling complex non?linear joint behaviour. | |
