17 Fracture Behavior of Unidirectional Composites Analyzed by Acoustic Emissions Technique 125 Fig. 17.5 The load displacement and crack length displacement diagrams Another important aspect to be considered is the possibility to detect the crack initiation in the specimen by observing three different graphs obtained by only analyzing the acoustic emission data. In Fig. 17.6 it is possible to observe the cumulative hits vs time graph, the cumulative energy vs time graph and the derivative hits/time vs time graph of one representative specimen of the 9. In all of diagrams it is possible to identify three different regions representing three different states of material. In the I region, a low level of hits, energy and derivative of hits is accounted. The time range compared with the camera images recorded corresponds to the crack growth inside the insert. Then the crack initiation phase starts and so the slopes of both cumulative hits and energy deeply grow; moreover, the derivative hits/time graph shows a very interesting result since it underlines an intense acoustic emission activity in a limited period (90–110 sec) which includes the instant correspondent to the maximum load used to calculate GIC (101 sec) and the visual observation crack according to ASTM standard [3]. Then the III region is identified, it represents the crack propagation phase characterized by a relevant acoustic emission activity but lower to the previous region. This trend is confirmed by both the cumulative hits and energy graphs in which the third slope of the curve is lower than the II region but higher than the I region. The more accuracy in detecting crack initiation respect to the crack propagation was tested in previous works but only referring to fatigue crack propagation on aluminum [18], steel [19, 20] and titanium [21, 22] specimens. For this reason, this result represents an important breakthrough for the scientific literature since it allows to obtain truthful information on non-homogenous material. This observation confirms the importance of using acoustic emission technique in monitoring delamination fractures since it allows to obtain predictive information about the state of material more accurately than the visual observation used according to ASTM 5528–01. It is important to underline that more visible results are obtained observing the derivative graph of hits respect to the cumulative one, but these are more confidently for the crack initiation phase than for the crack propagation. 17.4 Conclusion In this work, the acoustic emission technique is applied on DCB mode I delamination tests. The approach was used to obtain predictive information about the state of material. Significant results were obtained by manipulating acoustic data and observing three different kind of graphs: cumulative hits/time, cumulative energy/time, derivative hits/time. It results that the last one graph seems to be the more promising representation for predict initiation crack delamination and that the initiation phase, as verified in metallic materials, is better relieved than the propagation stage. Future works have been planned to identify a good method to follow the crack propagation and to evaluate the fracture toughness by using the sentry function as already done for GFRP.
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