Failure Analysis on threaded stud
The object
A threaded stud is a mechanical component used to attach or connect two or more parts in a mechanical or structural system. It is a component that can be found in any machine, so very common, almost trivial from a mechanical point of view. Yet the failure of a stud can result in serious consequences for the functionality of the machine itself, whether simple or complex, cheap or expensive.
The analyses
Visual Examination
The fracture of the threaded pin is located at a through-hole. The lower portion of the pin is visible in macrograph No. 1.
The fracture surface is oriented orthogonally to the axis of the component and has a dull, grainy appearance and numerous flattened areas, as can be seen in detail in macrograph #2.
The rest of the specimen appears intact with no obvious signs of plastic deformation.
Chemical analysis
The material can be traced to a brass of the CuZn39Pb4 type.
Vickers Hardness Testing
- Imprint Type / Impression: Vickers
- Load / Load: 10 Kg
- Penetrator / Penetrator: diamond pyramid
- Vickers hardness / Vickers hardness: 141 HV
Metallographic examination
Metallographic examination performed on longitudinal sections of the sample, shows the presence of an equiaxial grain structure of copper-zinc solid solution, typical of the alloy under investigation, in which grain-edge precipitate particles are detected (Figure3).
Near the fracture surface, a progressive change in microstructure is observed, changing to a structure consisting of b’ phase and a phase, in which the concentration of precipitates at the grain boundaries also increases. (Micrograph – Figure 4)
The profile of the fracture surface is jagged and irregular.
Fractographic examination
The morphology found on the fracture surface is uniform and consists of equiaxial dimples (micrograph No. 5), with odd particles surfacing on the surface consisting mainly of lead. (Figure 6).
The results
The examined fracture has the typical morphology of a crash fracture of ductile type.
Metallographic examination revealed structural unevenness in the longitudinal direction, with a variation in the ratio of phases and a different content of lead-based precipitates in the different sections, with enrichment at the fracture zone.
Such precipitates, on the one hand, improve the workability of the material while, on the other hand, they reduce its strength properties, which therefore come to be lower precisely at the most stressed section.
In fact, the failure occurred at the cross section of the pin that was less strong due to the presence of the hole.
Conclusions
The threaded pin broke in operation due to overload.
The failure occurred for loads probably lower than design values, due to a reduction in mechanical strength for microstructural reasons.
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