Co-authors​ :

 Kalliopi-Artemi KALTEREMIDOU (BELGIUM), Dimitrios AGGELIS , Lincy PYL (BELGIUM), Danny VAN HEMELRIJCK  

Composite materials are gaining more and more interest during the last years due to their high specific strength and stiffness and their relatively easy manufacturing methods. At the same time, composite laminates are characterized by a flexibility in their design since different stacking sequences can be manufactured by changing the fiber orientation and the sequence of layers. Cross-ply laminates are among the most common laminate configurations used in different engineering applications. The order and the thickness of the individual layers can significantly affect the damage initiation and propagation in these laminates. This, even if not directly depicted on the static mechanical properties, can significantly affect the performance of the material, since different damage phenomena can lead to dissimilar stiffness propagation and interaction of damage modes, especially during more dynamic loading. In order to evaluate the differences in the damage performance between varying cross-ply laminates, three PEKK/carbon fiber configurations, in which the thickness of the 0 and 90 layers was gradually increasing, were tested under static loads with the use of acoustic emission. Major differences in the acoustic signals revealed that the damage sequence was significantly affected by the stacking sequence, even if not directly reflected on the mechanical properties. Certain recommendations for the optimal architecture of the cross-ply laminates could be established. Moreover, by changing the fiber orientation in the off-axis plies of multilayer composites, certain stress states can be developed. Acoustic emission can also in this case be effectively used in order to pinpoint the dominant stress states within the material even at early loading stages. This was showcased in this study by testing epoxy/carbon fiber laminates with different off-axis layers. The most useful acoustic features were identified and the power of acoustic emission for the prediction of the subsequent damage in the tested composites was highlighted. Certain correlations of the acquired signals with the developed biaxiality ratios were obtained, opening the road for acoustic emission to be used for the structural health monitoring of real components where composite materials are involved. These observations were acquired not only for static but for dynamic conditions as well, with certain correlations of the acoustic features to the stiffness degradation and the evolution of the Poisson’s ratio throughout the fatigue life.