Deformability of a textile reinforcement modified with nanofibres

Deformability of a textile fabric is studied experimentally using a) friction test, b) out-of-plane compression, and c) bending. These tests reveal that a grafting of the fabric with carbon nano-fibres can significantly deteriorate its deformability. Therefore an optimal CNF mass fraction should be chosen for a particular production case, to obtain a compromise between improved strength and decreased drapability

Strain mapping at the micro-scale in hierarchical polymer composites with aligned carbon nanotube grafted fibers

For the first time, micro-scale digital image correlation (μDIC) is investigated for measurement of strain fields in hierarchical fiber-reinforced composites. The methodology is developed on an exemplary alumina fiber/epoxy composite laminate with aligned carbon nanotubes (A-CNTs) grown on fibers. Utilizing environmental scanning electron microscopy and nano-scale random speckle patterns, sufficient precision is achieved to detect the influence of the A-CNTs on the deformation field around the fibers. Debonded regions at the fiber/matrix interface with openings as small as 35 nm could be detected. μDIC could identify the propagation of the debonded region based on the non-linear increase of the opening. The image correlation uncertainty in the displacement analysis is estimated to be below 5 nm. The experimental results are validated by computational analysis performed on the region of interest. For this, an advanced model with two scales of reinforcement (microscopic fibers and nanotubes) and boundary conditions taken from the experiment is used. As verified by the model, A-CNTs are found to constrain matrix deformation in their longitudinal direction. Keywords: Digital image correlation (DIC); Scanning electron microscopy (SEM); Carbon nanotubes; Mechanical properties; Finite element analysis (FEA

Synchrotron radiation computed tomography for experimental validation of a tensile strength model for unidirectional fibre-reinforced composites

Synchrotron radiation computed tomography has been used to analyse fibre break accumulation in unidirectional composites loaded in tension. The data are compared to model predictions. The model only slightly overestimated the composite failure strain, but predictions of fibre break density were too high, which can be mainly attributed to errors in the Weibull distribution. Both the number and percentage of interacting fibre break clusters were under-predicted by the model. This was attributed to an underestimation of stress concentrations in the model. While the experimental observations revealed mainly co-planar clusters, the model predicted mainly diffuse clusters. The experiments showed that the clusters did grow any further after their formation, while the model predicted a gradual development. Both local and dynamic stress concentrations were hypothesised to be key features for further exploration. The discrepancies identified, inform suggestions for directions advancing the state-of-the-art strength models of UD composites

Hybrid effects in thin ply carbon/glass unidirectional laminates: Accurate experimental determination and prediction

Experimental results are presented which allow the hybrid effect to be evaluated accurately for thin ply carbon/epoxy-glass/epoxy interlayer hybrid composites. It is shown that there is an enhancement in strain at failure of up to 20% for very thin plies, but no significant effect for thicker plies. Hybrid specimens with thick carbon plies can therefore be used to measure the reference carbon/epoxy failure strain. The latter is significantly higher than the strain from all-carbon specimens in which there is an effect due to stress concentrations at the load introduction. Models are presented which illustrate the mechanisms responsible for the hybrid effect due to the constraint on failure at both the fibre and ply level. These results give a good understanding of how variability in the carbon fibre strengths can translate into hybrid effects in composite laminates

Benchmarking of strength models for unidirectional composites under longitudinal tension

Several modelling approaches are available in the literature to predict longitudinal tensile failure of fibre-reinforced polymers. However, a systematic, blind and unbiased comparison between the predictions from the different models and against experimental data has never been performed. This paper presents a benchmarking exercise performed for three different models from the literature: (i) an analytical hierarchical scaling law for composite fibre bundles, (ii) direct numerical simulations of composite fibre bundles, and (iii) a multiscale finite-element simulation method. The results show that there are significant discrepancies between the predictions of the different modelling approaches for fibre-break density evolution, cluster formation and ultimate strength, and that each of the three models presents unique advantages over the others. Blind model predictions are also compared against detailed computed-tomography experiments, showing that our understanding of the micromechanics of longitudinal tensile failure of composites needs to be developed further

Micro-CT-based analysis of fibre-reinforced composites:Applications

The paper presents an overview of cases in which the analysis of the internal structure and mechanical properties of fibre reinforced composites is performed based on the micro-computed X-ray tomography (micro-CT) reconstruction of the composite reinforcement geometry. In all the cases, the analysis relies on structure tensor-based algorithms for quantification of the micro-CT image, implemented in VoxTex software