The shape of the structure under stress is important in determining how it responds to loading. For example, it is well known that sharp edges within structures, and rivet holes, for example, can act as stress concentrators, in such a fashion that very high stress is felt at that point, even though the stress applied to the structure is quite small.
There are many models which may be used to analyse the effect of specimen geometry. Such models include finite element analysis. However, for the purposes of the models here, it is worth just noting that structural geometry is an important issue in any model; an issue that will not be addressed in depth further.