Hydraulic fittings and material internal stress

The hydraulic fittings are sealed using gas threads of the conical type, with the addition of a sealing material on the thread. The material used to make these items is brass that has undergone a hot stamping process which, if not carried out correctly, can lead to serious corrosion cracking problems.
The threaded hydraulic fittings with cylindrical thread are paired with male conical threaded pipes, causing radial stress that is proportional to the torque used to screw them. The crystalline structure of the material must be extremely homogeneous, in order to avoid concentrated stress points which can start the corrosion cracking process. A suitable tightening torque must therefore be applied.
Even the choice of sealant used with the hydraulic fittings can influence this phenomenon, therefore it must be suitable for the type of fitting and for the fluid to be transported.

Among the problems that may be encountered in the use of hydraulic fittings there is the manifestation of subsidence in operation due to the tensions that are generated within the material. The stress study is a central topic of mechanical design as numerous methods have been developed to estimate the stresses that occur during the useful life of a component. The complexity and importance of these aspects make the role of the structural designer central in the success of the products to be made.
 

Making a complete classification of all the types of effort that can be found in exercise is complex, but it is possible to summarize the main tensions:
- Static efforts: these are efforts that have constant intensity and direction and are almost always caused by the application of a load on a component. Normal stresses applied in a direction perpendicular to the reference surface and shear stresses belong to this category. The union of these two main stresses leads to different types of stresses including bending, while more shear stresses lead to torsional stress;
- Fatigue efforts: these are efforts that generally have lower intensities than static ones but which in any case lead to the failure of the components. They are characterized by cyclical trends that can lead, by repeating a number equal to or greater than N times, to the loss of functionality of the components;
- Occasional efforts: they can be caused by numerous agents, among which there is certainly the effort generated by an impact. Therefore, the resilience of the materials also assumes considerable importance;
- Residual stresses: these are stresses that can be found in the material as a result of forming operations, including processing by plastic deformation, especially cold. In particular, they are particularly burdensome when the stress relieving operations are not carried out correctly.
 

The study of materials made it possible to develop central patterns and properties in materials. Among these there are certainly, even if they are not the only ones:
- Yield stresses;

- Breaking effort;

- Impact resistance;

-Fatigue limit (with relative curves).
Furthermore, models have been introduced for comparing the resistance of the material with the stresses encountered during operation, taking into account large safety coefficients that allow to take into account other effects and defects that are not often foreseeable, but which often invalidate the correct behavior of components. In fact, defects, edges and cracks can lead to the intensification of the efforts which are the central topic of Fracture Mechanics, a very important branch of Materials Mechanics.

As introduced in the previous paragraphs, hydraulic fittings can undergo different types of stresses in operation which can lead to damage and breakage of these. There are, however, methods to be able to improve the resistance properties of hydraulic fittings and lengthen their useful life in operation:
- Heat treatments: they are essential to act on the material and improve its mechanical characteristics;
- Coatings: the use of coatings allows you to protect the surface from particularly aggressive environments that can damage the fittings. The varnishing also belongs to this category;
- Surface finishing operations: these allow the introduction of compressive stresses in the material by raising the fatigue limit and the corresponding resistance.
Although these represent only some of the possible methods of action on the fittings to improve their behaviour, it is clear and evident that the preventive study represents the key point of the design. An effective choice of everything concerning these products represents the necessary but not sufficient starting point to avoid problems. Structural and application simulations can help in this phase, allowing you to anticipate any critical issues and deal with them appropriately, also avoiding the safety risks that may arise if the hydraulic fittings of interest should show a malfunction.
 

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