Numerical investigation of stress concentration in preloaded, galvanized threaded fasteners of large diameter
K.M. Manoj (TU Delft - Civil Engineering & Geosciences)
Milan Veljkovic – Mentor (TU Delft - Steel & Composite Structures)
Alessandro Cabboi – Graduation committee member (TU Delft - Mechanics and Physics of Structures)
I. Shakeri – Graduation committee member (TU Delft - Steel & Composite Structures)
H. M. Slot – Graduation committee member (TNO)
J.D. Rodenburg – Graduation committee member (TNO)
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Abstract
The accelerated development of renewable wind energy sources and their large-scale adoption involves an increasing global size of offshore wind turbines, leading to larger structures and connections. This requires the improvement of assessment methods used for these large connections as well as their optimization which ensures low cost and increased structural integrity of these critical components.
As a part of the Bolt and Beautiful project, this research focuses on threaded fasteners of large diameter (M36 and above) which are hot-dip galvanized for corrosion protection, and preloaded for use in ring flange connections. The objective of this thesis is to numerically investigate the local stress state at the critical engaged thread roots, in the presence of galvanized zinc layer and nominal preload. This broad objective is divided into measurable parts: 1) to understand the effect of galvanization, 2) to compare the local stress state in fasteners of increasing diameter, and 3) to quantify the cracking parameters in these fasteners using the fracture mechanics approach.
Based on the numerical research findings, the following conclusions were drawn. Firstly, the presence of inter-metallic layers formed due to hot-dip galvanization has different properties which causes the
initiation of brittle cracks inside the galvanization layer even under nominal preloads. Initiation of these microcracks, which may not be visible on the exterior, is detrimental to the lifetime of fasteners under external loading, and a fracture mechanics approach is utilized to quantify them. Secondly, the comparison between increasing diameters of fasteners suggest that the local stress state greatly depend on the geometry of the threads and the fastener. This geometric size effect is signified by larger stress peaks, peak stresses closer to the root boundary, reduced redistribution of stresses as diameter and therefore sharpness of the notch-like thread root increases. These findings also call for modifications to geometric parameters which allow for a consistent stress behavior between different diameters. Thirdly, the fracture mechanics approach evaluated that micro-cracks in galvanized zinc layer has a greater effect on larger diameter threaded fasteners than smaller ones. This is quantified by a larger crack driving force, signified by the J-integral, which suggests a larger amount of energy released per unit crack extension.
The findings highlight that larger diameter preloaded fasteners are more prone to a reduced lifetime, and even more due to the effect of hot-dip galvanization, by numerically ascertaining the mechanism using a comparative analysis and fracture mechanics approach. This is consistent with the reduced endurance limits reported through experimental investigations for hot-dip galvanized threaded fasteners with increasing diameter.