PSI - Issue 5

Piotr Nazarko et al. / Procedia Structural Integrity 5 (2017) 131–138 P.Nazarko et al./ Structural Integrity Procedia 00 (2017) 000 – 000

134 4

a

b

Fig. 2. Equipment used in laboratory tests in case of (a) the GFRP strip; (b) the plate specimens (GFRP/CFRP).

Fig. 3. Exemplary time signals of elastic waves recorded in the undamaged and the damaged GFRP strip.

Principal components calculated for all the patterns were then used as an input vector for training ANN and novelty detection. The results obtained in the form of NI were shown in Fig. 4.

Fig. 4. Novelty index (NI) calculated for the patterns considered in the GFRP strip (after successful training of the ANN).

Introducing certain threshold levels enables not only the classification of damaged and undamaged patterns but also separation of some damage classes. The threshold level for damaged patterns was calculated as follows:

n ∑ i= 1 n

t= 1

NI i +3 σ u ,

(1)

where NI is the novelty index, σ u is a standard deviation calculated for NI i and n is a number of undamaged patterns used for training the diagnostic system. At this stage, when damage is signalled in the specimen investigated, the second level of damage identification may be initiated. In this way, assessment of the damage severity can be achieved by the application of NNs for