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SIMULATION: VIBRATIONS
One can consider a vibration the temporary, periodical variations of different magnitudes. In fact, a mechanical vibration is the movement of a body that oscillates around a point of equilibrium.
The causes of mechanical vibrations are many, but basically they are closely related to mechanisation tolerances, calibration, relative movements between touching surfaces or the balance of rotating or oscillating parts. The phenomena we have just listed almost always produce a movement of the system from its point of stable equilibrium originating a mechanical vibration.
The majority of the vibrations in machinery and structures are undesirable because they increase the strains and for the loss in energy that accompanies them. Furthermore, they are the source of the wearing down of materials, damage due to fatigue as well as of annoying movements and noises.
Simulation by finite elements makes it possible to obtain the intrinsic modes of a structure, i.e. the intrinsic resonance frequencies of the structure with the representation of the deformation of the said frequencies (MODELO MODAL). The knowledge of the intrinsic modes of the structure enables the evaluation of its usage independently of its area of operation. Furthermore, the results of the modal analysis facilitate the definition of structural improvements on contributing a numeric and graphic model of mechanical behaviour.
For the determination of the dynamic behaviour of a mechanical structure 3 different models exist:
- Spatial model: All structures can be modelled spatially by means of a set of mass systems, a shock absorber and an equivalent mould. For this purpose, we define a series of characteristic matrices (matrix of the mass, matrix of the shock absorber, matrix of stiffness) of the system which are under analysis.
- Modal model: given the said matrices, the modal analysis is limited to the resolution of a problem of autovalues.
- Response model: Mathematical expression which, based on the modal model and the shock absorbency value, enables us to obtain the FRF's of a structure, i.e. what its behaviour is in relation to the excitation frequency.
- Fatigue in components:
The application of loads or periodic movements on a component can lead to faults in the load levels that are much lower than would be expected in a monotonic load situation. Predicting life under fatigue of a component can be dealt with using two very different strategies:
- To reproduce and cycle the working conditions in the laboratory until the part fails
- To study the properties of the material under fatigue and estimate its life according to the loads it has to bear
In the study of behaviour under fatigue simulation is a very effective tool given that it enables the determination of the states of tensions and deformations of the components in service under very different load states. These tensions and deformations can combine with the characterisation of the material and the existing theoretical models with the aim of obtaining a life under fatigue estimation of the component. This way, we can greatly reduce the experimental and economic expenses of the first strategy.
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