Identification of 4D damage precursors in 3D woven composites

The lack of strengthening or toughening mechanisms in the through-thickness or zdirection in two-dimensional (2D) laminated composite structures has been considered a cause for poor mechanical performance in demanding aerospace and automotive applications. To that effect, 3D woven composite systems consisting of z-binders have been introduced to potentially provide additional means for enhanced damage tolerance and reduced mechanical property degradation. To investigate the benefits of these novel composite architectures, this work employs a multimodal nondestructive evaluation and characterization approach consisting of in situ Digital Image Correlation (DIC) and Acoustic Emission (AE) monitoring coupled with ex situ X-ray micro computed tomography (micro-CT) to identify precursors to damage and link them with progressive failure patterns observed throughout the surface and volume. In general, the presence of the z-binder in 3D woven composite specimens, as opposed to their 2D counterpart, is found to be related with increased transverse reinforcement. Furthermore, local 3D surface strain patterns are observed over time (hence a 4D analysis is performed) and are coupled with AE monitoring and micro-CT data. This hybrid nondestructive evaluation setup indicated that a particular 4D mechanism related to the z-binders and consisting of sudden out-of-plane pop-outs is activated during both monotonic and cyclic testing. The activation of this damage precursor was further found to signal the transition of the material to a more advanced damage state. Post-mortem fracture pattern analysis was used to determine specific attributes of the failure patterns, while the micro-CT data confirmed the presence of lateral and transversal cracks, which were intimately related to the formation of this distinct damage precursor.