Natural materials such as wood exhibit high mechanical properties through cellulose structured at multiple length scales and embedded in a matrix of similar chemical structure. These hierarchical ...materials have inspired the design of lightweight composites composed of naturally occurring polymers. However, the close proximity of melt and decomposition temperature remain a challenge. In this work, cellulose propionate (CP) is modified to reduce its glass transition temperature and melt viscosity, allowing its use as a matrix in a natural fibre-reinforced composite. Through better impregnation, the modified CP matrix composites showed an increase in stiffness and strength of ∼10% and 20%, respectively, in comparison to unmodified CP matrix composites. The impact properties also increased by up to 28%, showing that modified CP is a credible matrix for realising sustainable all-cellulose natural fibre composites with high stiffness, strength and toughness.
The effect of ply thickness on the notch sensitivity and bearing properties on carbon fibre reinforced polymer composites and their hybrid laminates with steel foils were studied. Laminates with ply ...thicknesses of 0.3 mm and 0.03 mm comprising of CFRP and hybrid laminates were manufactured and characterized using tension, open hole tension and double lap bearing tests. A 25% ply substitution was found to double the bearing load with extensive plastic deformation in the joint while maintaining high stress and maintaining constant cross-sectional thickness in the laminate. With a good agreement between the finite element predicted values and failure behaviour, the damage initiation and progression behaviour could be observed experimentally. We numerically captured (i) rapid failure of 0° plies in the thin ply CFRP hybrid and (ii) continuous delamination with significant plastic deformation for the thick ply CFRP hybrid. The numerical results significantly reduce future experimental work when designing hybrid laminates and could allow the laminate lay-up to be tailored for load cases. Both the experiments and numerical models underline the distinct size effects occurring with respect to the ply thicknesses when hybridising a very ductile metal with a brittle yet strong composite material.
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•Thick and thin-ply CFRP laminates were mechanically characterized with and without steel foil reinforcement.•Local steel foil reinforcement around holes can overcome the notch sensitivity of thin-ply composites.•The onset (0.5%) and 2% bearing strength of the thick-ply CFRP were more than doubled with the addition of steel foils.•Onset bearing strength of the hybrid CFRP are also higher than the ultimate strength of the pure CFRP.•Ply thicknesses may be tuned to generate stable damage propagation in the hybrid configuration.
Composite materials based on carbon fiber (CF) are prone to failure at the fiber-matrix interface upon compression or stress transverse to the fiber axis. The direct growth of carbon nanotubes on CF ...constitutes a novel approach to enhance the mechanical properties of the interface. However, the challenge is that, during the growth, tensile properties of the fiber are altered due to the diffusion effect of iron nanoparticles used in the process, leading to CF surface defect formation. In this work, we deliver and discuss an analysis methodology on ptychographic X-ray computed tomography (PXCT) images in order to assess the iron nanoparticle abundance within CFs. PXCT provides 50 nm - resolved 3D electron density maps of the CFs. We evidence the protective effect of an ultrathin alumina film against iron infiltration into CF during the CNT growth. This method potentially allows to evaluate the efficiency of other diffusion-minimizing approaches. The conclusions of the PXCT examination are validated by energy-dispersive X-ray spectroscopy and scanning transmission electron microscopy carried out on thin sample slices cut with a focused ion beam. The results provide a new insight into the mechanical performance of CFs and therefore constitute valuable knowledge for the development of hierarchical composites.
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To evaluate compatibility between a substrate and a thermoplastic polymer, the established methodology is to estimate their surface composition in terms of surface energy components, ...utilizing the results of contact angle measurements of probe liquids onto substrate and polymer surfaces at room temperature. Using this methodology, polymer surfaces are studied in solid state, however, during spreading of polymers on a substrate, polymers are in molten state and at high temperature, having different surface energies and more complex polymer/substrate interactions due to polymer chain mobility.
This paper presents a model study with practical relevance to predict polymer/substrate compatibility including contact angle measurements at high temperature directly performed between molten thermoplastics; polypropylene (PP), polyvinylidene fluoride (PVDF) and maleic anhydride-grafted polypropylene (MAPP), on smooth glass fibres and plates. The values of total surface energy of thermoplastics at high temperature (260 °C) are down to 57% of that measured at room temperature, which has a strong influence on the wetting prediction. Surface energies of both the polymer and the substrate were found not to be the only factor controlling the wetting behaviour of molten polymers and the level of adhesion with the substrate, but also some intrinsic characteristics of the polymer melt play a role. We also observed that the wetting behaviour of molten MAPP is affected by the maleic anhydride (MA) content, demonstrating dramatically different results to room temperature measurements, which is suspected to be due to the formation of covalent bonds of MA groups with the glass surface enhancing the interface strength beyond the shear strength of MAPP.
The increasing use of carbon fibre reinforced polymer (CFRP) composites in aviation and automotive industries has led to the adoption of automated production methods such as pultrusion or resin ...transfer moulding (RTM) for cost reduction in the production lightweight structures. These processes however, offer limited freedom to locally reinforce structures. This paper describes an approach to utilise a basic geometry for several similar parts and add local reinforcement patches only in regions of load introduction or high local stress. The approach offers the benefit of being able to combine automated production methods with unprecedented design freedom. The specific bearing performance for three different local reinforcement using (1) add-on CFRP patches, (2) surface mounted steel foils and (3) steel foil interleaving in replacement of 90° plies with foils of the same thickness as the CFRP plies (0.125 mm) is compared by double lap bearing tests. The bearing strength improves with the addition of patches, for surface mounted steel foils, more so as CFRP co-cured patches, and most as an interleaved configuration. Quasi ductile failure of the bearing joints was maintained due to additional plasticity of the steel foils, producing a joint that fails safely while enhancing the bearing strength. When examining the hybrid laminates, all samples buckled and failed in bearing compression/shear. Brooming was evident on the compressive side of the hole where the bolt indented the laminate. Indentation led to shear kink bands along the washer supported region and appear as large compression/shear damage above the washer confined region of the laminate. When normalised by weight, the three approaches show similar bearing performance. However, each approach has specific advantages with regards to processing, electrolytic potential, or absolute bearing strength, depending on the design of the load introduction.
The damping behaviour of continuous carbon fibre and flax fibre reinforced polymer (CFRP and FFRP) composites was studied by comparing angle-ply laminates. Using logarithmic decrement measurements, ...dynamic mechanical analysis and vibration beam measurements, the damping was described as the specific damping capacity ψ in order to compare data using the different methods.
Our results show approximately 2–3 times better damping of FFRP compared to CFRP at low frequency and low strain. We show that the damping of both materials increases with increasing angle-ply orientation below 300Hz. While the matrix and interface seems to contribute mainly to damping at lower frequencies, the fibre shows an increasing contribution with ψ = 64.4% for unidirectional FFRP at 1259Hz in the 5th mode of vibration, without a notable change in the elastic modulus. This work demonstrates that the FFRP may be simultaneously stiff and efficient at damping.
Ultrasonic welding is a promising joining technique for thermoplastic composite parts. On the way towards upscaling and industrialisation of this technology, it is crucial to understanding how it is ...affected by manufacturing tolerances. The objective of the research presented in this paper was to investigate the influence that misaligned adherends have on static ultrasonic welding of thermoplastic composites. Different angles were created by changing the clamping configuration. The results showed that increasing the angle between adherends decreases the power consumed and increases the process time while decreasing the weld uniformity and increasing the risk of overheating. These effects were associated to the impact of the different clamping configurations on the cyclic strain in the energy director and adherends during the welding process. Decreasing the top adherends bending stiffness by increasing the clamping distance was found to significantly reduce the adverse effects of adherend misalignment on weld quality.
This paper evaluates the potential of changing the welding force and the compliance of the energy director (ED) to reduce the effects caused by misaligned adherends, which were: increased ...through-thickness heating, reduced size of welded area and increased heating time. In the methodology that was followed, we welded adherends misaligned by approximately 4.5° in different scenarios: with higher welding force; with increased ED compliance by the use of a thicker ED and; with increased ED compliance by the use of a discontinuous ED. The most significant reduction of the effects caused by misaligned adherends was obtained when combining the use of both increased welding force and discontinuous ED. Such improvement derives from the imposed parallelism caused by the use of a higher welding force and from a more efficient concentration of heat generation at the weld line that occurs when a discontinuous ED is used.
The paper presents a new model for interlaminar normal stress distribution in moderately thick and singly curved laminates. It requires in-plane strain information which must also map the kinematical ...effects caused by the curved laminate shape. The number of free parameters of the exact solutions for each layer is reduced and determined by a finite-element procedure which is also explained. The results of the mechanical model are compared with FEM results for unidirectional and cross-ply laminates. The new model can be developed for element-level post processing in a FEM program with simple finite structural elements.
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