Fibre direction strain measurement in a composite ply under pure bending using Digital Volume Correlation and Micro-focus Computed Tomography

Abstract

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, 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 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, 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.

A novel particle-filled carbon-fibre reinforced polymer model composite tailored for the application of digital volume correlation and computed tomography

“…The temperature of the tow spreader and final guide roller were set to 50 C. The fibre sizing was left intact. For a schematic diagram of the manufacturing setup, the reader is referred to Sch€ oberl et al 7 The drum winding process resulted in uniaxial prepreg tape, $0.25 mm thick x $350 mm x $1900 mm, which was cut and laid up to produce the desired [90/0] s cross-ply layup. As indicated in Figure 2, the prepreg stack was cured in an autoclave for 280 minutes at 0.5 MPa (5 bar) and a maximum temperature of 120 C. To minimize void content in the cured material, a vacuum of $0.07 MPa ($0.7 bar) was maintained throughout the autoclave process.…”

“…Representative mCT slices from each material are provided in Appendix 1. To preserve the tensile behaviour of a commercially representative material, whilst still allowing the application of DVC parallel to the fibre direction, 7,8,40 the authors have established a series of considerations:…”

“…Again, the doped material is seen to replicate the key behaviour (cluster formation) of undoped, and more broadly, other carbon-fibre composite systems, validating its use as a model system, with the capability to allow tracking of local strains. 7 Ultimately, within the statistical power of the current sample gauge volumes, it appears that the behaviour of clusters is stochastic, whereby specimen failure is likely to be determined by intrinsic stochastic factors, such as fibre strength and local microstructure, 19 rather than particularly the inclusion of barium titanate. A much larger number of specimens would need to be examined to provide a statistically significant conclusion regarding the relative contribution of the fiducial markers compared to the intrinsic variability.…”

“…[4][5][6] Understanding the fibre fracture process, therefore, becomes essential for achieving a comprehensive interpretation of CFRP tensile failure, alongside the various other forms of composite sub-critical damage. 7,8 A range of predictive models have been developed and adapted to simulate the processes that lead to ultimate failure in fibre fracture-dominated situations. 4 These commonly include the following concepts:…”

“…The simple cylindrical structure and relatively smooth surfaces of the filaments yields little structural variation along the fibre direction, leading to poor image correlation, and correspondingly inaccurate displacement estimates in this direction within a given ply. 40 This paper is the first from a series documenting the application of DVC to displacement and/or strain measurements parallel to the fibre orientation, ultimately aimed at mapping strain fields at fibre break sites 7,8 and other damage critical sites. Here we focus on the material development required to perform such measurements.…”