What causes fatigue in bolts?

One of the most common fastener failure modes is fatigue crack initiation and growth. Inadequate design con- siderations, material problems, insufficient pre- load, loosening, and excessive loads can all contribute to fastener fatigue.

What is bolt fatigue failure?

A bolt fatigue failure involves three stages of damage: 1) initial crack initiation at a thread root, radius or material defect; 2) progressive cyclic fatigue growth; and 3) final sudden failure of the remaining cross section of the bolt.

How do bolted joints fail in fatigue?

Fatigue failure of bolted joints can result from either cracking or self-loosening and can be prevented by appropriate design and fabrication methods. Fatigue design rules for bolted joints are available in a number of Standards.

Why do bolts fail?

Bolt overload is a result of the applied force being sufficiently high, causing the bolt to fail due to overload, which is a direct result of the axial force being applied acting upon the bolted joint, which then causes the bolt to fracture in the threaded area.

What is fatigue strength?

Fatigue strength is the highest stress that a material can withstand for a given number of cycles without breaking. Fatigue strength is affected by environmental factors, such as corrosion. The maximum stress that can be applied for a certain number of cycles without fracture is the fatigue strength.

How do you increase bolt strength?

Improving fatigue resistance

1. Use the highest possible preload.
2. Minimize the bolt to load eccentricity.
3. Use the largest possible contact surfaces.
4. Use the largest possible clamping lengths.
5. In most cases, use a preload higher than the working load.

What are the failures in bolted connection?

The bolt preload is insufficient to resist the applied forces. Thread stripping of the internal or external threads. Fatigue failure of the bolt. Excessive bearing stress under the nut face, bolt head, or within the joint itself.

What happens when a bolt is over torqued?

Typically, an under torqued bolt will deform and be unable to provide as much clamping force as needed. An over torqued bolt will break.

What has an effect on the strength of a bolt?

1. The variation in the dimensions of the thread, (such as major, pitch and minor diameters) has a significant effect on both internal and external threads stripping strength. The tensile force in the fastener acts on the threads and a wedging action generates a radial displacement which reduces thread strength.

What is the main reason for fatigue failure?

Fatigue failure is the formation and propagation of cracks due to a repetitive or cyclic load. Most fatigue failures are caused by cyclic loads significantly below the loads that would result in yielding of the material.