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This paper presents an experimental demonstration and validation of high-resolution three-dimensional experimental strain measurement using Digital Volume Correlation (DVC) on Carbon-Fibre Reinforced Polymers (CFRPs), via through-thickness strain analysis under a state of pure bending. To permit the application of DVC to displacements and/or strain measurements parallel to the fibre direction in well-aligned unidirectional (UD) materials at high volume fractions, a methodology was developed for the insertion of sparse populations of 400 nm BaTiO3 particles within the matrix to act as displacement trackers (i.e. fiducial markers). For this novel material system, measurement sensitivity and noise are considered, along with the spatial filtering intrinsic to established DVC data processing. In conjunction with Micro-focus Computed Tomography (µCT), the technique was applied to a simple standard specimen subjected to a four-point flexural test, which resulted in a linear strain distribution through the beam thickness. The high-resolution, fibre-level strain distributions (imaged at a voxel resolution of ∼0.64 µm) were compared against the classical beam theory (Euler-Bernoulli) in incrementally decreasing averaging schemes and different sub-set sizes. Different sampling and averaging strategies are reported, showing that DVC outputs can be obtained that are in very good agreement with the analytical solution. A practical lower limit for the spatial resolution of strain is discerned for the present materials and methods. This study demonstrates the effectiveness of DVC in measuring local strains parallel to the fibre direction, with corresponding potential for calibration and validation of micromechanical models predicting various fibre-dominated damage mechanisms.

Modelling composite toughness and what mechanisms are responsible for added toughness has been less tackled within the composites community. With the advances of computational resources and the development of arbitrary cracking models, such as the Augmented Finite Element Method (AFEM), more complex microstructures can now be tackled with multiple interacting cracks. It has been established that Mode I crack propagation in particle-toughened interlayers within a CFRP laminate involve a process zone rather than a distinct crack tip. This involves multiple cracks forming ahead of the main crack that then coalesce and leave behind bridging ligaments that provide traction across the crack flanks. Preliminary idealised 2D AFEM models are presented in this work, that highlight the effects of the relative role of neat resin to ply interface cohesive properties, and the fraction of ‘idealised de-bonds’/discontinuities, in keeping the crack path within the interlayer. 4- dimensional time-resolved Computed Tomography (CT) experiments complement the abstract models, with the chronology of damage events and resultant crack paths being directly identified in different toughened microstructures. Additionally, quantification of the bridging behaviour elucidated micromechanical differences between the systems, with the number of bridging ligaments and the total bridged area being quantified and compared to macro-scale toughness. This work is intended to improve understanding around interlaminar toughness, and lead to the development and validation of physically representative micro-mechanical models

Synchrotron Radiation Computed Tomography (SRCT) allows for non-destructive identification of fracture mechanisms in materials at very high resolutions. In this work, carbon fibre reinforced plastics (CFRPs) were imaged using SRCT to ascertain fracture micro-mechanisms under both quasi-static Mode I and Mode II dominated loading conditions. This, combined with previous work on impacted coupons, provides mechanistic comparison between the different loading conditions on similar material systems. Initial findings have identified particle/matrix debonding, crack bridging and ligamented behaviour as reported previously, but have emphasized micro-cracks and the extent to which particle/matrix debonding occurs ahead of the crack tip under both Mode I and Mode II loading conditions. Such work is intended to support both material development and more accurate structural performance simulation for the toughened materials that are being increasingly used as primary structures in aerospace applications.