RICERCA ITALIANA     

Structural durability of mechanical components under random loading

Università degli Studi di Ferrara

Abstract

The research project is based on the design and structural reliability assessment of mechanical components under random loadings.
The project intends to develop both fatigue damage assessment procedures under random variable amplitude loading and the different aspects related to the dynamic structural behaviour of components and mechanical systems subjected to that actions.
This problems can be found in many civil and mechanical engineering applications: as an example, (i) either in vehicles or structures subjected to actions related to atmospheric variability; (ii) in either machines, electrical, electronic or electromechanical devices inside or linked to aforementioned situations.
Traditionally, we adopt a time domain approach in which the external actions are identified through suitable (experimental or simulated) temporal records, on which cycle counting, damage estimation and life prediction are performed.
The project aims to deepen the methodologies developed in the frequency domain and to compare their results to other consolidated, but more expensive, time domain methodologies. In fact, if compared to the classical time domain approach, a frequency domain method is potentially more efficient and rigorous. In this case, external actions are completely identified by their frequency content; structures are analysed in terms of dynamic response and both internal stress and internal strains are defined in the frequency domain. The advantages >>>

Principal Investigator

Roberto TOVO Università degli Studi di FERRARA

Research Objectives

The principal objective of the proposed research project is to develop a method suitable for predicting, during the initial stage of the design process, durability and structural reliability of mechanical components subjected to in service aleatory loadings.
In particular, the project deals with the problem of studying and developing those methodologies which are capable of describing the load histories damaging real components in the frequency domain. Moreover, the problem of the dynamic behaviour of mechanical components will be addressed by using numerical methods. Both the theoretical proposals and the numerical methods will systematically be validated by ad-hoc experimental investigations.

As highlighted in the section named "National or International Scientific Background", it is common practise to use methods addressing the aforementioned problems in the time domain. At present, the use of in frequency domain based approaches is restricted to few peculiar applications, even though it is recognised to be more accurate and rigorous. The main limitation in applying such approaches is that exhaustive theories have not been completely developed yet, so that, there are no sound and rigorous methods capable of predicting the fatigue damage by studying the problem just in the frequency domain. The state of the art shows that validity and accuracy of such approaches have been checked just considering systems having one degree of freedom subjected to >>>

First Results

The first phase is based on the following intermediate results:
- the definition of the types of the PSD's to be used in the research;
- the identification of the mechanical components to be analysed by numerical techniques;
- the identification of the materials and the components to be analysed by experimental tests;
- the effective draft of calculus and experimental capabilities of units involved in.Optimal results expected from phase 2 are the achievement of five tasks of first level already mentioned as the main task of this research project.Concerning phase three, the main expected results are the achievements of the four tasks of the second level already given in the research tasks overview.
Moreover we expect to address also the tasks of level three; but, in this moment, it is not possible to asses if the higher level tasks can be completely full-filled within this project.Hopefully the main scientific results should be already been achieved during phase two and three.
In this last phase the predicted results are the systematic revision of obtained experimental data. Hence we expect to compare all the theoretical and numerical approaches developed with the experimental data obtained during the research.

Timescale

24 months

National and international background

The structural reliability of mechanical as well as structural components depends on their capability of being resistant to in field applied loadings. Among the different physical mechanisms damaging components, fatigue must be mentioned, because it occurs in the presence of cyclic loadings, which are very common in practical applications.
Fatigue failures can also be generated by internal loads which do not vary in a deterministic way, since external actions depend on random phenomena. The most interesting applications are:
- automotive vehicles: for instance, vehicles (having two wheels or more) running on uneven paths are damaged by fatigue loadings which depend on the interaction between the wheels and the roughness of the path;
- naval structure and aircrafts: even in this case, the load history depends on the interaction between vehicle and environment and the applied loadings are random;
- structures subjected to environmental phenomena: for instance, either mechanical structures under wind loading or off-shore structures subjected to sea's action [46];
In these situations, the problem of the reliability involves both the principal structural elements and the different devices within the structures themselves. This makes it evident that this problem must be addressed also for either mechanical or electronic devices situated within the vehicles.
The fatigue assessment in the early stage of the design process involves:
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