Delayed fracture. Most common causes

The tension generated during tightening is higher than the nut’s shear strength. There may be different causes for this fault:

• Hydrogen embrittlement: hydrogen released during electrolytic coating processes may remain occluded in the screw. This may be critical in high hardness parts and assemblies under high tensions. Optimize the hardness, the tightening force or baking the parts could reduce this effect. Breaks may happen hours or days after the assembly has been completed.
• Corrosion stress cracking: elements in highly corrosive environments may present cracks due to material loss or to hydrogen induced embrittlement. Breaks usually take place several months after completing the assembly.
• Load fractures: external loads applied to the assembly may elongate the screws beyond their strength limits. Such loads may also separate cover and base parts and transmit tensions and bending stresses that may end up breaking them.