Analysis of Lug KTS

Pinned or bolted lug connections are a commonly used, and often fatigue-critical, feature in mechanical structures. They typically form a single loadpath whose failure can be catastrophic, but despite this, the detailed distributions of local stresses are not easily found in published literature.

This Jesmond Engineering research project was undertaken to assess the hole-bore stress distributions in typical lug (bracket with a hole for bolt or pin connectors) geometries with varying width and hole diameter ratios to provide engineering data on how stresses are distributed and help in efficient and accurate design.

Understanding how stress is distributed within a mechanical system is a critical aspect of the design process as it provides grounds to assess how a structure will behave during certain operating conditions.

Stress concentrations are a major cause of fatigue failures. They occur at changes of section and at load inputs where local stresses are raised compared with a known far-field value by a ratio known as the stress concentration factor, K_T. An accurate assessment of K_T is vital in performing fatigue life evaluation.

Jesmond Engineering started the research project by assessing the hole-bore stress distributions in typical lug geometries loaded at various different angles. Six lug geometries with varying Width/Hole Diameter ratios were assessed and the results were plotted.

Contour plots showing concentrated stresses were produced. As expected, these show two distinct regions of elevated tensile stress in positions of the hole around 90 degrees from the direction of the applied load. These regions show where the maximum stresses are located, with the exact magnitudes and positions depending on the particular geometry of each model.

A summary of the analysis was produced as charts which demonstrate how the maximum stress concentration values vary with the angle of the applied load.

The output from the research project undertaken by Jesmond Engineering helps to understand stress concentration factors (K_T) for lugs which are commonly used on aircraft structures to transmit loads between parts through a bolt or a pin connection and therefore are susceptible to fatigue failure. This greater understanding helps to inform better designs and reduce the chance of lug failures which can be catastrophic.