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Summary

The task of a designer of a mechanical system is to determine the constructive variables, such as the type and dimensions of the components. Furthermore, it is necessary to analyse the system design in order to optimise it, and to verify if it can fulfil its functions. Normally, a designer uses a deterministic method to analyse a design. In many cases this method is satisfactory. However, in some cases this design method is not adequate, because it results in a system design  that is to expensive or not reliable enough. In these cases a probabilistic approach or risk analysis may be helpful. Risk analysis experts support the designers to guarantee the costs and safety of complex systems. They analyse whether the system can perform its major functions, such as carrying load and executing motion, using a probabilistic model. Both the load carrying capacity of a system as well as the external loads can show a stochastic behaviour. This thesis concentrates on modelling the stochastic behaviour of the system; minor attention is given to the stochastic character of the loads.

The most frequently used risk analysis techniques are: Failure Modes and Effects Analysis (FMEA), Fault Tree Analysis (FTA), and event tree analysis. Fault tree analysis is the most commonly used technique, because the results of the analysis quantify the reliability, making it possible to compare design solutions, and localise the critical spots in a structure. This technique, however, is complex and labour intensive.

Therefore, the designer usually does not execute the fault tree analysis himself, but a risk analysis expert assists him in this task. Risk analysis is executed at the end of the design process, when the lay out of the structure is more or less definite. In this stage of the design process, it is not desirable to introduce major changes in the structure, because they would increase the costs. In this way the results of the reliability analysis scarcely influence the layout and the quality of the design. Changes are restricted to adaptations of details.

Reliability analysis would have a major influence on the design, if it were to be applied during the conceptual design phase. This would result in more reliable and less expensive structures; a structure that is reliable in concept is less expensive than a structure that is not reliable in concept, but was improved in a later phase of the design process.

The introduction of reliability analysis in the conceptual design phase would have consequences on the design process. To achieve this, the designer and risk analyst would have to work closely together. The risk analyst would have to make fault tree analyses for many design solutions, increasing the costs of the design.

Automation can make the analysis less complex, can reduce the time of an analysis, and can prevent errors. Can automation support the reliability analysis in a way that the designer can execute the analysis himself? In this way the designer would not have to depend on the availability of a risk analysis expert. He would not have to wait for the results of the analysis, because he could execute the analysis himself, and immediately decide whether the design should be improved or not. Also, the designer could optimise the reliability of a structure by determining the reliability for a number of concepts. Thus, reliability analysis could be applied in many more cases than before, which should lead to better designs.

To verify this idea, this thesis will answer the following questions:

To answer these questions a design and reliability analysis method was developed, and implemented into software. This software package contains two parts: a component modeller that helps the designer compose a structure, and an analysis program that calculates the reliability of the structure.

The first part, the modeller, enables the designer to think in terms of mechanical components, rather than in terms of reliability analysis. The modeller stores the design as components instead of plain geometry, which makes it possible to automate several types of analysis, such as fault tree analysis. The second part, the analysis program, automatically produces the result of a fault tree analysis.

To automate the fault tree analysis it is necessary to make a more abstract description of the functions of a structure. Chapter 2 demonstrates, that the major functions of a drive system can  be decomposed into a function carry load and a function execute motion.  Thus, an abstract description for only two functions is necessary. It is  possible to describe these two functions with a specially adapted finite  element theory. The equilibrium equations, DT. sigma = f, describe the function carry load, and the continuum equations, epsilon  = D. u , describe the function execute motion.

The analysis program uses these equations for the reliability analysis. Assume that a particular combination of components has failed. It is possible to express this in the finite element equations. When it is not possible to find a permissible stress distribution  that can carry the load, or kinematically permissible displacement field u that realises the desired motion, the combination of failing components is a failure mode. All failure modes are found by trying all combinations. Finally, the probability of failure for all failure modes is calculated.

Conclusion

It appears to be possible to automate reliability analysis. This automated method was implemented into software, that can be integrated into the design process, producing valid results. Though the software supports the designer in such a way, that he can execute the reliability analysis himself, it is still wise to consult a risk analyst for a final judgement of the results.

The automated method analyses the reliability of a structure in two steps: first, it determines the failure modes of the structure, and second it quantifies the probability of occurrence of these modes. This thesis describes two models to quantify the probability of failure: failure of discontinuous processes and failure of continuous processes. The division in continuous and discontinuous processes is not accurate enough, to describe the behaviour of various types of structures. Therefore a subdivision was made: Continuous processes can be divided into rest and action, and discontinuous processes into start and stop. In each of these subprocesses different failure mechanisms take place. Thus, it is necessary to apply different failure data in each subprocess.

The reliability analysis software was imbedded in a design system. Social aspects have a major influence on the success of the introduction for such a system. Software that is developed bottom up will be accepted easier than software that is developed top down. However, it is possible to successfully introduce software, that is developed top down. A factor for success is, that the reliability analysis tool does not change any existing work methods. This makes the introduction of the system easier, and allows the users to accept the reliability analysis software.

The reliability analysis software has not been applied in design practise for a long period of time. Therefore, it is difficult to see whether newly designed structures change, and whether designs are improved due to the application of the reliability analysis software. To conclude this, it is necessary to monitor the application of the software over a longer period of time.


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