External actions, also referred to as solicitations, are actions, due to any external event, that modify the state of a system. Confining oneself to a purely physical field, stresses are external actions on a solid or structure that modify its initial state, determining a new static or dynamic equilibrium. The stresses on a generic structure, with generic constraints, can be of a different nature:
• Mechanical stresses.
• Displacements in the field or in the field.
• Thermal stresses.
Mechanical stresses are the generalized forces acting on the structure, ie forces or moments, concentrated or distributed. In the static case, or imagining that the structure is loaded in an almost static manner from the initial state to the final state, a static equilibrium is created, with the supportive reactions balancing the forces on the structure. The structure itself will deform and stress will arise in response to the external load. As Hooke stated “tensio sic vis”. A problem dear to the science of construction is that of the Cylinder of Saint Venant, or a schematization of the beam. The mechanical stresses acting on the beam at its ends are classified as:
• Nx axial forces
• Tz Ty cutting forces
• Mz My bending moments
• Torque moments MT
In the case of stresses with forces that vary over time, vibrations can be generated and the phenomenon of mechanical resonance can be reached. In studying the system’s response to non-static mechanical stresses, the inertia forces of the structure must be taken into account to solve the problem of dynamic equilibrium.
As far as the mechanics of bodies are concerned, impulsive stresses are also studied, ie impacts between bodies, of elastic or anelelastic type.
Given the above, the importance of the elastic modulus of the materials making up the system is clear. Different materials under the action of equal forces have different behaviors. In the case of Titanium with which our systems are manufactured, it is not advisable to use elements with different degrees of hardness even if they are connected. In fact, the bar is affected by the stress distribution of the stress states that induce deformations in the pillar elements. (V. Material fatigue)
The displacements consist in the imposition of a geometric variation of the state of the structure, which determines the deformation of the structure itself and therefore the emergence of efforts.
Based on this concept, the preliminary anatomical and physiological analysis of the districts to be implanted is very important.
The anatomical commissures, as they are equipped with micro-movements, must not be excessively constrained. Never use overlapping multi-type bars in incisal areas: The same is true in distal structures in particularly thin and therefore yielding jaws.
On the contrary, in well-represented jaws it is advisable to stiffen the distal sectors by solidifying with two overlapping bars of gunmetal.
At first glance, the use of a large-caliber bar (2mm) appears as a paradox as an element of solidarity and anchorage at the level of the frontal sectors in case of over-denture. In reality these anatomical districts in the event of total edentulism undergo anatomical modifications in accordance with the laws of Biomechanics.
The lack of the occlusal stop constituted by the posterior teeth allows a deformation under the muscular action with fulcrum at the level of the median section. For this reason, especially at the mandibular level, it is possible to highlight a pyramidal zone of more or less abstention thickening according to the extent of the induced deformation which causes the fulcrum to shift towards the distal sectors. The bone thus transformed will have new, much more rigid mechanical characteristics, so that a more rigid constraint becomes justified.
The prediction of the stresses to which a structure will be subjected is a fundamental moment in its design, since it determines the geometric dimensioning and the choice of materials, in order to satisfy the traditional requirements of sturdiness and stiffness in the presence of the load.
In general, in every system we speak of stresses as external actions that modify an initial state of equilibrium to arrive at a new state of equilibrium or a state of non-equilibrium.