Structural Evaluation of Defects in Bolt Hole Threads

Abstract

Threaded bolted joints are a critical fastening method in aerospace engineering, used to connect structural components under demanding mechanical and thermal conditions. In many of the assemblies manufactured at GKN Aerospace, bolts are installed directly into tapped holes machined into large, high-value components — without the use of nuts. This thesis focuses on a component called the Intermediate Case (IMC), produced from cast titanium alloy Ti-6Al-4V (Ti-64). The bolts are fabricated from Inconel 718 (IN-718), with a thread specification of .3125-24 UNJF-3A (Unified National Joint Fine). Because the threaded holes are integral to the parent components, any damage or defect renders replacement extremely difficult and costly. Thread defects can arise at various stages of a component's lifecycle, including manufacturing, installation, maintenance, and handling. Currently, GKN Aerospace evaluates such defects using a conservative approach in which the defect is assumed to behave as a pre-existing crack. Crack propagation and fatigue life analyses are subsequently performed using linear elastic fracture mechanics. While this method ensures structural safety, it frequently leads to the rejection of components that may still be structurally serviceable, resulting in unnecessary material waste, increased costs, and extended maintenance downtime. This thesis aims to develop a more realistic and less conservative methodology for evaluating defects in bolt hole threads. The approach involves explicitly incorporating defect geometry into finite element models rather than relying on simplified stress concentration factors and crack assumptions. Numerical simulations are carried out using software tools including Siemens NX and ANSYS. The study encompasses a systematic classification of defects, followed by targeted simulation and evaluation for each defect. The findings are intended to support more informed maintenance decision-making at GKN Aerospace, helping to distinguish between components that require removal from service and those that can safely continue in operation. The outcomes of this work are expected to contribute to improved safety assessment practices while reducing unnecessary conservatism in defect evaluation, thereby minimising operational costs and material waste in aerospace maintenance.