PSI - Issue 54

Claudia Barile et al. / Procedia Structural Integrity 54 (2024) 225–232 C. Barile et al. / Structural Integrity Procedia 00 (2023) 000–000

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time, frequency or time-frequency characteristics. To understand the failure characteristics, the ToA of the AE signals are classified with respect to their peak frequencies using a density-based unsupervised clustering technique. First, the neighbourhood clustering density threshold is calculated using the k-nearest neighbour (k-NN) search method. Then using this density threshold the ToA of the AE signals are classified. The results are presented in Figure 6. More details about the density-based unsupervised clustering and the estimation of the clustering threshold are well-documented in the literature [Ester et al. (1996)]. Based on the density of ToA distribution, the AE signals are classified into four clusters (excluding the outliers) with respect to their peak frequencies. Clusters 1 and 2 have very few signals, while most of the signals are classified into Cluster 3, which has between ToA 325 µ s and 420 µ s and peak frequencies above 850 kHz (note that their ToA is greater than 420 µ s in Specimen 3). Typically, brittle fracture in an unsaturated polyester system is associated with crack growth through the surface of the material. The AE signals associated with the crack growth events often have larger peak frequencies. Considering the number of signals present in high-frequency clusters, Cluster 3 and Cluster 4, it is safe to assume that these signals are responsible for the final failure of the test specimens.

Fig. 7. Comparison between AE results and Mechanical Results for a) Specimen 1, b) Specimen 2, and c) Specimen 3

In order to validate this, the classified AE signals are compared with the strain evolution during the tensile loading. The results are presented in Figure 7. In all three specimens, the AE signals in Clusters 1 and 2, which have ToA between 370 µ s and390 µ s and peak frequencies less than 500 kHz can be found throughout the strain evolution stages. These signals could be generated from the local dislocations such as matrix cracking in the vicinity of voids, friction between the matrices, etc. AE signals in Cluster 3, however, begin to appear when the strain reaches around 3000 µϵ in all three specimens. Considering the peak frequency characteristics of these signals and their wide distribution of ToA, it can be assumed that the nonuniform and non-localized stress distribution in the syntactic foams begins around 3000 µϵ , which initiated several local microcracks. The non-localized damage localization might be the reason why the ToA has a larger distribution in these clusters.