Why Do Bolts Break?

Have you ever broken a bolt? How do you determine why the bolt failed?

One of the most common types of failure is overloading. All bolts have a maximum load that they can bear before they begin to yield, and generally this load is applied in the form of torque. If friction is lower than expected, the bolts may yield before reaching the prescribed torque. When a bolt yields, it will stretch, causing a “necking down” in the threaded area of the clamping zone that is not engaged into the mating threads. Assemblers can usually feel the bolt stretching as it will take many more rotations of the wrench before either breaking or stalling the wrench. If the bolt breaks, you will see an obvious reduction in surface area at the break where the bolt has necked down.

If a bolt breaks after it has been assembled, there a couple of failure modes that should be considered.

How does fatigue failure occur? Fatigue failure happens when the bolts have not been tightened properly, or have loosened up during its service life. If enough force is acting on the loosened joint during use of the product, bending stresses can weaken the fastener, eventually causing it to fail. This can normally be diagnosed by a fastener expert by close examination of the broken fastener and the mating components.

A third, less common type of failure is caused by hydrogen embrittlement. This type of failure is considered a delayed failure and will always happen after assembly. The hydrogen embrittlement time to failure is typically within 48 hours. The break will almost always be directly under the head of the fastener and not in the threads. The head may break off completely, or it may simply crack enough to relieve clamp load, and remain attached. Either way, the joint has failed and is not safe. This type of failure, while not common, almost always occurs in very high strength fasteners, or case hardened fasteners that are electroplated.

Doug Jones
Applications Engineer
djones@bossard.com

Why Do Bolts Break? by Bossard