# Multiparameter Approach for Damage Propagation Analysis in Fiber-Reinforced Polymer Composites
## 2 Experimental Procedure
### Damage Assessment Using AE Descriptors  
For assessing the damage modes properly, it is essential to map the AE descriptors with the load response of the SLS specimens. The clustered AE data were plotted with respect to the load response of the SLS specimens for detailed analysis. The SLS load response results with I-Frequency and amplitude are presented in Figure 7 and the results with R-Frequency and amplitude are presented in Figure 8.  
In Figure 7a, the amplitude and clustered I-Frequency are plotted with respect to the load response of the SLS 1 specimen. First of all, the load responses had multiple load peaks before the final fracture. In our previous works, the authors classified these peaks as regions of initial rupture and final rupture [21]. For SLS 1, the initial rupture occurred at 7.05 kN and the final rupture occurred at 7.75 kN. From the point of initial rupture to the final rupture (roughly between 1.75 s and 2.75 s, the majority of the I-Frequency components were in cluster 3. Comparing this observation with Figure 5a, it can be identified that these clusters belonged to the category of AE signals with a high amplitude and low I-Frequency (<200 kHz). Similarly, if the results are compared with Figures 6a and 8a, again, the AE signals had a low R-Frequency (<200 kHz). This implies that the signals were highly symmetric since both I-Frequency and R-Frequency were below 200 kHz, and at the same time, the peak amplitude was higher. These types of AE signals normally have interlaminar crack growth as their source. If the same figures are compared for the initial stages of load response, say before 0.5 s, the I-Frequency and R-Frequency were highly asymmetric. The signals during this stage were deemed to be asymmetric in the sense that the I-Frequency and R-Frequency varied significantly in the very few AE signals recorded during that short duration. In our previous reports, the authors indicated that this initial loading stage may represent the sliding of the specimen inside the loading grips [21]. The next important stage was the stage between the initial stage and the initial rupture (between 0.75 s and 1.75 s). In Figure 7a, most of the AE signals had I-frequencies in clusters 1 and 2. A similar observation could be found for R-Frequency in Figure 8a. Owing to the absence of cluster 3 in I-Frequency and R-Frequency, the damage modes in these regions were probably due to the matrix cracking and delamination. It has been indicated by several researchers that in adhesive-bonded components, under static loading conditions, a majority of the load is carried by the adherends [22; 23]. The delamination, however, initiated at the vicinity of the adhesive layer and the adherend and extended through the thickness of the adhesive layer. The region before the initial rupture suffered extensive microcracking in the thick adhesive layer and the delamination was initiated. However, the specimen SLS 1 retained its load-carrying capability until it reached 7.75 kN. During this transition stage, the crack had grew through the thickness, releasing AE signals with a high symmetry between I-Frequency and R-Frequency, resulting in the final fracture. For specimen SLS 2 (Figures 7b and 8b), more than one load peak can be observed. Nonetheless, the major initial rupture occurred at 1.5 s with the load peak of 7.05 kN and the final rupture at 2.25 s at 5.96 kN. It is clear from this observation that the damage modes of SLS 2 were different from those of SLS 1. However, there was an ambiguity in how this damage mode progressed. Comparing these results with the I-Frequency and R-Frequency clusters of Figures 5b and 6b, the cluster 3 signals of I-Frequency and R-Frequency were distributed throughout the loading regions. Between the transition period of the initial and final ruptures, which lasted for only 0.75 s, a significant number of cluster 3 signals were observed. Even though this stage was shorter in duration, these signals still referred to the interlaminar crack growth.