Intra-layer crack

Impact failure, sandwich structures, intra-layer crack, inter-layer crack, finite elements and cohesive elements. 

The model experiments of Xu and Rosakis on low-speed impact over sandwich structures were simulated applying cohesive models. The simulation captures qualitatively the main experimental observations. The most relevant correspondence is in the development of the first crack at the interface between the layers, the presence of shear stresses along the interface, which renders the crack shear driven and often inter-sonic, and the transition between interlayer crack growth and intra-layer crack branching. The effects of impact speed and bond shear strength are also investigated and highly satisfactory predictions are obtained. 

The simulations in this paper give failure modes sequences very similar to the actual ones observed in the experiments. The model predicts the formation of shear-dominated inter-layer (or interfacial) cracks that initiate first and that such cracks grow very dynamically, their speeds and shear nature being enhanced by the large wave mismatch between the core and the face sheet. The triggering of the complex mechanism of the intra-layer failure of the core structure is also well reproduced. 

The effects of different interfacial strengths and impact speeds were also investigated. The results show that even small variations in impact speed and bond strength could substantially influence the initiation behavior of de-lamination (location and nucleation time) and lead to substantially different inter-layer crack speed histories and therefore influence the timing sequence and final extent of subsequent intra-layer damage within the sandwich structures. Fig. 11-12, which agrees very well with the observations by Xu and Rosakis [1]. 

Fig. 17.24 TEM micrographs of nylon 6/organoclay/EOR-g-MA (76/4/20) ternary nanocomposite showing (a) submicron and nano-voids which are associated with intra-gallery delamination of some organoclay layers (note that the section is not selectively stained in order to clearly reveal delaminations of clay layers), (b) cavitation of EOR-g-MA particles which preferentially starts from the larger particles as indicated by arrows, and (c) extensive matrix shear yielding at the arrested crack tip which in turn causes the EOR-g-MA particles and delaminated clay layers to collapse within the matrix. A schematic of the arrested crack tip illustrating different locations from where TEM micrographs (a-c) were taken is also shown. Note that the schematic is not to scale (Lim et al. 2010)...

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