## 3 Results and Discussions
### Acoustic Emission Results: Cumulative Counts and Cumulative Energy  
The damage characteristics or the mode of failure in the adhesive bonded specimens cannot be explained through the mechanical characteristics. Furthermore, predicting the failure at an early stage is also not easy by studying only the mechanical characteristics. It can be explained by the acoustic emission characteristics. The load vs. time curve of the JLS specimens is plotted over the cumulative \begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Specimen**} & \multicolumn{2}{c}{**Peak Load (\(k\)N)**} \\ \cline{2-3}  & **Initial Rupture** & **Final Rupture** \\ \hline JLS 1 & 6.02 & 4.09 \\ JLS 2 & 2.67 & 3.89 \\ JLS 3 & 3.86 & 5.57 \\ Mean & **4.18** & **4.52** \\ Std. Dev. & **1.70** & **0.92** \\ SLS 1 & 7.05 & 7.75 \\ SLS 2 & 7.06 & 5.96 \\ SLS 3 & 5.93 & 7.36 \\ Mean & **6.78** & **7.02** \\ Std. Dev. & **0.75** & **0.94** \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Damage characteristics in JLS and SLS specimens. In Figure 5, it can be observed that whenever there is a load peak in the load vs. time curve in any of the JLS specimens, the cumulative counts and cumulative energy show a steep increase. The initial load drops in all JLS 1, JLS 2 and JLS 3 at 1.1 s, 0.1 s and 0.35 s, respectively, have been associated with the increase in cumulative counts and cumulative energy. Similarly, at the final stages of loading prior to failure 1.8 s, 1.35 s and 2.25 s, respectively, for JLS 1, JLS 2 and JLS 3 are also associated with the increase in cumulative counts and energy. However, there are some instances, particularly around 1.2 s in the JLS 2 specimen, where the cumulative counts and energy increased unexpectedly without any change in the load vs. time curve. This phenomenon could not be explained without utilizing another characterizing technique or a deeper investigation like the frequency content of the acoustic data. Nonetheless, analysing the waveform of those signals in their time-frequency domain can also provide information about whether these signals are representatives of machine or surrounding noises. This is not the only information that can be obtained from the AE results. In JLS 1 specimen, the cumulative energy increases steeply around 1.1 s duration and it remains almost constant until the specimen reaches its final rupture region. Nonetheless, the cumulative counts kept increasing gradually beyond the 1.1 s duration and the slope took a steep increase as the material reaches failure. This means that a large number of counts have been recorded with very low energy from 1.1 s duration to the final failure. In a non-linear material, such as plastics or composite materials, the large counts with low  
Figure 6: Load, cumulative counts, cumulative energy vs. time: (**a**) SLS 1; (**b**) SLS 2; (**c**) SLS 3.  
energy represent the matrix cracking. The ratio between the cumulative energy and the cumulative count varies at different damage modes in a material. When this ratio exhibits a very low value, it can be said that the energy is low with large counts. On that basis, this can be attributed to the matrix cracking. Brittle fractures such as interlaminar crack growth and fiber breakage release large energy acoustic events, which are responsible for the initial and final ruptures. Between the stages of the initial failure and the final failure in the JLS 1 specimen, the material has experienced only matrix cracking. This indicates that shearing has occurred in the adhesive region rather than peeling which attributes to the low-energy acoustic events. This indicates that the JLS 1 specimen has carried more load before failure than the JLS 2 and JLS 3 specimens due to the shearing mode of failure. This could be one of the reasons why the JLS 1 specimen has a higher peak load when compared to JLS 2 and JLS 3. In the cases of JLS 2 and JLS 3, they have similar load vs. time, cumulative counts and energy patterns.