3.2.1. P2P3

The first set of connections consisted of two connections (P2P3). They corresponded in terms of the dimensions of the end plates and the class of screws. In this case, two sets of input vectors were also analyzed:


In both cases, the prediction was made with respect to the S3 screws from both connections (P2, P3). The obtained results of testing, validation, and prediction are shown in Figure 9. There was a good agreement between the measured axial forces (F2, F5) and the values obtained from the ANNs.

**Figure 9.** The results of force identification: (**a**) P2 learning, P3 testing; (**b**) P3 learning, P2 testing.

It can be seen in Table 4 that lower statistical parameters were observed when data from P3 were used for learning and P2 for training and validation.


**Table 4.** Comparison of the learning, testing, and validation errors: P2P3.
