**4. Discussion**

This chapter does not propose to conclude which methodology is the best. The static approach is the easiest method to numerically foresee the durability of a model; if the loading history is simple (cyclic with frequency below the first natural mode) or the structure is stiff (non-compliant, like the sub-frame), the static technique can achieve results with a similar quality as the two other dynamic approaches. But, if the loading history is complex (random, like the one utilized in this examination) or the structure is compliant (like the torsion axle), the static approach may not be enough to correctly calculate the durability of the structure, since it does not consider the dynamic effects.

However, during the development of any structure, it is common to find scenarios where it is difficult to determine if a signal is simple enough or a structure is stiff enough to relay only to the static methodology. In this chapter, the structures adopted as examples were previously known how to behave, but this may not be the case in most practical applications, since there is no formula or rule to guarantee the use of a specific approach. A modal analysis of the structure and a deep analysis of the signal may help to point a direction of which methodology to choose, static or dynamic, but still cannot conclude by its own.

When comparing the two dynamic methods, the transient modal superposition approach gave more conservative results in the example presented in this chapter. However, the underlying change in stress to cause the contrast is only 11%, and this could be by statistical scatter in the underlying random process. In this way, it is not conclusive which approach is more appropriate for this sort of structure. It has been reported that the frequency domain approach can be performed with considerably less computational resources and so could be preferred for large models [14].
