The influence of carbon ply blocking on tensile stiffness in interlaminar flax–carbon epoxy hybrids is studied through various lamination schemes of similar nominal fiber volume fractions. Finite element mesoscale analysis accounting for distortions induced during compaction in the thinner carbon plies as a result of crimp mismatch was conducted. These distortions are shown to undermine hybrid stiffness, the extent of which is less pronounced when plies are blocked due to fewer hybrid interfaces, denoting that higher stiffness is achievable with blocked carbon plies. Backed by experimental evidences, it is suggested that detailed analysis on hybrid stiffness should treat hybrid interface as an independent variable a priori, and that modeling tools for carbon–flax fiber reinforced composites should consider not only the nonlinearity of flax response but also the waviness of both fabrics.
Natural fibre based composites are garnering attention owing to their optimal trade-off between mechanical properties and environmental sustainability properties. It has been proposed that they could potentially replace synthetic and mineral fibre composites due to their minimized impact on human health and the natural environment. Though several studies have been dedicated to understanding certain mechanical properties like strength and fatigue life, fewer reported studies have focused on their response to impact or shock loads. In the present work, we have performed shock tests using a shock tube on flax/epoxy and flax/polypropylene unidirectional and cross-ply laminated composites. The objectives are, to compare the blast-resistance of polypropylene against epoxy in their use as matrix in flax–reinforced composites, and, secondly to assess the performance of cross-ply over unidirectional fiber orientation. The present results showed that the cross-ply samples retained their structural integrity at peak pressures that were sufficient to break unidirectional samples, indicating that cross-ply samples are superior candidates for applications where shock loading needs to be factored in. Furthermore, we also qualitatively assessed the failure modes predominant in each of the studied orientations.
Structural adhesive joints were subjected to high loading rates in mode I and their resulting fracture behaviour was studied in detail. Joints were formed between unidirectional carbon-fibre epoxy composites and between aluminium-alloy substrates bonded with a tough, hot-cured, single-part epoxy adhesive. In all the tests described in the present paper the joints failed by a crack propagating cohesively through the centre of the adhesive layer. Double cantilever beam (DCB) and tapered double cantilever beam (TDCB) tests were performed, from quasi-static loading rates up to 15 m/s, and a test rig was developed incorporating high-speed video acquisition for the high-speed tests. A detailed analysis strategy, and associated equations, were developed to account for (i) the types of different fracture behaviour regimes encountered, (ii) the dynamic effects in the test data, and (iii) the contribution of kinetic energy to the energy balance. Using the above analysis strategy and associated equations, increasing the test rate over six decades (from 10-5 to 10-1 m/s) was found to lead to a reduction in the value of the adhesive fracture energy, GIc, by about 40% of its quasi-static value, i.e. from 3500 to about 2200 J/m2. Further, at quasi-static loading rates, the measured adhesive fracture energies were independent of substrate material and test geometry (i.e. DCB or TDCB). However, at faster loading rates, the TDCB tests induced higher crack velocities for a given loading rate compared with the DCB test geometry, and neither the test rate nor the crack velocity were found to be the parameter controlling the variation in GIc with increased test rate. Thus, an isothermal-adiabatic model was developed and it was demonstrated that such a model could unify the DCB and TDCB test results. Indeed, when the GIc values, determined from the analysis strategy and associated equations proposed, were plotted as a function of 1Ntime, where the time, ti, was defined to be from the onset of loading the material to that required for the initiation of crack growth, the results collapsed onto a single master curve, in agreement with the isothermal-adiabatic model.
Poly(l-lactide) cellulose nanocrystals-filled nanocomposites were fabricated by blending of cellulose nanocrystals-g-rubber-g-poly(d-lactide) (CNC-rD-PDLA) and commercial PLLA, in which CNC-g-rubber was synthesized by ring opening polymerization (ROP) of d-lactide and a ε-caprolactone mixture to obtain CNC-P(CL-DLA), followed by further polymerization of d-lactide to obtain CNC-rD-PDLA. X-ray diffraction (XRD), nuclear magnetic resonance (NMR), and solubility tests confirmed successful grafting of the rubber segment and the PDLA segment onto CNC. Stereocomplexation between CNC-rD-PDLA nanofillers and PLLA matrix was confirmed by FT-IR, XRD, and differential scanning calorimetry (DSC) characterization. The PLLA/CNC-rD-PDLA nanocomposites exhibited greatly improved tensile toughness. With 2.5% CNC-rD-PDLA loading, strain at break of PLLA/CNC-rD-PDLA was increased 20-fold, and the composite shows potential to replace poly(ethylene terephthalate). SEM and small-angle X-ray scattering (SAXS) investigations revealed that fibrillation and crazing during deformation of PLLA/CNC-rD-PDLA nanocomposites were the major toughening mechanisms in this system. The highly biodegradable and tough cellulose nanocrystals-filled PLLA nanocomposites could tremendously widen the range of industrial applications of PLA.
Fibers derived from plants are sustainable and environmentally friendly. Their mechanical properties, however, are generally much lower than synthetic fibers, such as carbon. This has been a leading cause for their limited usage in many engineering applications. For natural fibres to find its way into semi-structural applications, significant improvements in mechanical properties are necessary. Towards this end, we investigate the performance enhancement achievable through hybridization with high strength and high stiffness carbon fibres in this work. Results showed that with just 14% of carbon fibers in flax-carbon epoxy hybrid, flexural stiffness can be increased by up to 5.5 times, and strength 2.7 times. This demonstrates the potential of natural-synthetic hybridization even at low volume fraction of synthetic fibers. The effects of stacking sequences on performance under bending were also investigated and the reliability of using Classical Lamination Theory (CLT) to predict flexural modulus of hybrid carbon-flax was assessed.
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