PSI - Issue 5

4

Piotr Nazarko et al. / Procedia Structural Integrity 5 (2017) 460–467 Nazarko and Ziemianski / Structural Integrity Procedia 00 (2017) 000 – 000

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a)

b)

b)

Fig. 2. Signals received (a) on the bolts head and (b) on its end – comparison for the three selected load levels.

4.2. Single bolt in a static test machine

Single bolt M16 was placed in the static test machine (Fig. 3a). It was decided that loads will be applied in four cycles with increasing force levels up to 25, 50, 75 and 100 kN. The force, in this case, was related with load history read out from the test machine (Fig. 3b). The set of these values was then used as the output vector during the process of ANNs training.

b)

a)

Fig. 3. (a) Single bolt in a static test machine; (b) load history associated with recorded signals of elastic waves.

A total of 602 signals were recorded at intervals of 5 s. Examples of signals obtained at selected load stages were shown in Fig. 4a. Amplitude changes are clearly visible there for the selected signals. Their numbers are related to the load levels in Fig. 3b. All the recorded signals were then processed using PCA. Projection of two first principal components is shown in Fig. 4b. These values, however, do not indicate a simple dependence on load changes that would allow direct prediction of forces. Therefore, principal components were used to train an ANN for the purpose of force identification.