Understanding Threaded Fasteners and Rivets

Components of Threaded Fasteners

• Threaded fasteners consist of a bolt, nut, and sometimes washers. The bolt is the main load-bearing element, featuring a head and shank with threaded portions.

• The nut contains interlocking threads that mate with the bolt's threads. Washers are stiff plates that distribute loads over larger areas to prevent damage.

Advantages of Threaded Fasteners

• They can be made from various materials, allowing designers to choose suitable options for different applications (e.g., tension or shear).

• Capable of generating significant clamping forces, which is crucial for tension-type joints and sealing applications to prevent fluid leakage.

• Their removability makes them ideal for maintenance tasks where disassembly is necessary.

Disadvantages of Threaded Fasteners

• While removable, they can loosen during operation due to loading and vibration; hence wire-locking is often used in aircraft applications.

• Wire locking increases costs compared to standard rivets. Accurate machining of threads in bolts and nuts is required for proper fit.

Introduction to Rivets

• Rivets differ from threaded fasteners as they do not have threads; instead, they create permanent connections through plastic deformation during installation.

• They are cost-effective since they require no additional machining steps. A modern aircraft may contain millions of rivets, making this efficiency significant.

Performance Considerations for Rivets

• Although rivets form permanent connections that resist loosening from vibrations, their material options are limited to lower strength and ductile materials.

• This limitation means rivets may not be suitable for all applications due to their generally weaker performance compared to threaded fasteners.

Installation Process of Rivets

• During installation, a rivet is inserted into pre-drilled holes in two sheets. The hole size exceeds the rivet diameter for easier insertion.

• Compressive forces applied cause the rivet to expand elastically then plastically, filling the hole while applying pressure on the inner surfaces.

Benefits of Hole-Filling Behavior

• As compressive forces continue, a deformed head forms on the rivet while also causing plastic deformation around the hole in the sheets.

• After removing installation force, residual compressive stress remains around the hole area due to plasticity effects—this helps mitigate stress concentrations.

Impact on Stress Concentration Factors

Understanding Rivet Mechanics and Stress Concentration

The Role of Rivets in Load Distribution

• The ovalization previously observed is limited by the rivet, which creates contact forces between the rivet and the hole surface, redistributing load effectively. This leads to a reduction in the stress concentration factor to about two on both sides of the hole.

• Even minor discrepancies between fastener diameter and hole size can elevate the stress concentration factor; thus, the ability of rivets to fill holes is crucial for achieving this load distribution benefit.

Impact of Fasteners on Stress Concentration

• The fastener introduces a pin loading into the sheet known as bearing, which also has its own associated stress concentration factor that must be taken into account.

• If a fastener hole remains unfilled, load transfer occurs only on one side of the fastener hole, leading to significant stress concentration within the sheet. Conversely, filling the hole allows for radial compressive loads around it.

Summary of Fastener Types and Their Applications

• Mechanically fastened joints are categorized based on fastener type (e.g., threaded fasteners, rivets, blind fasteners, nails) and primary load transfer methods (e.g., tension and shear).