Positive size and scale effects of all-cellulose composite laminates

Negative size effects are commonly reported for advanced composite materials where the strength of the material decreases with increasing volume of the test specimen. In this work, the effect of increasing specimen volume on the mechanical properties of all-cellulose composites is examined by varying the laminate thickness. A positive size effect is observed in all-cellulose composite laminates as demonstrated by a 32.8% increase in tensile strength as the laminate thickness is increased by 7 times. The damage evolution in all-cellulose composite laminates was examined as a function of the tensile strain. Enhanced damage tolerance concomitant with increasing specimen volume is associated with damage accumulation due to transverse cracking and strain delocalisation. A transition from low-strain failure to tough and high-strain failure is observed as the laminate thickness is increased. Simultaneously, scale effects lead to an increase in the void content and cellulose crystallinity at the core, with increasing laminate thickness.     

Biodegradability of all-cellulose composite laminates

The high mechanical properties of single-polymer composites based on degradable non-derivatised cellulose, aka all-cellulose composites, have recently captured the attention of researchers. All-cellulose composites possess the intriguing combination of high strength and biodegradability. However, the biodegradation behaviour of all-cellulose composites has so far not been reported. In this work, soil burial experiments were carried out to compare the biodegradation behaviour of all-cellulose composites with conventional biocomposites in order to investigate the end-of-life disposal of this relatively new class of bio-based composite materials. All-cellulose composites are characterised by exceptional biodegradability with mass losses of up to 73% following a soil burial time of 70 days. An investigation of the mechanisms of biodegradation of all-cellulose composites is undertaken for the first time.     

Binderless all-cellulose fibreboard from microfibrillated lignocellulosic natural fibres

Fully biobased all-cellulose fibreboards are produced without the use of additional bonding agents such as polymer resins or binders as in the case of e.g. medium density fibreboard (MDF) or natural fibre reinforced plastic (NFRP). These materials make use of the self-binding capability of cellulose, exploiting the enhanced hydrogen bonded network present in micro- and nanofibrillated cellulose, resulting in good mechanical performance. After the optimisation of refinement, drying and hot-pressing conditions, binder-free panels from microfibrillated flax fibres with excellent mechanical properties of around 17 GPa and 120 MPa for flexural modulus and strength, and relatively low water sorption are achieved, making these materials competitive with conventional cellulose based composite materials. The work shows the potential of creating all-cellulose engineering materials using only the intrinsic bonding capacity of microfibrillated lignocellulose, potentially leading to environmentally friendly panel board materials, which are entirely based on renewable resources, recyclable and biodegradable.     

Mechanical characterization of hierarchical carbon fiber/nanofiber composite laminates

Susceptibility to matrix driven failure is one of the major weaknesses of continuous-fiber composites. In this study, helical-ribbon carbon nanofibers (CNF) were dispersed in the matrix phase of a continuous carbon fiber-reinforced composite. Along with an unreinforced control, the resulting hierarchical composites were tested to failure in several modes of quasi-static testing designed to assess matrix-dominated mechanical properties and fracture characteristics. Results indicated CNF addition offered simultaneous increases in tensile stiffness, strength and toughness while also enhancing both compressive and flexural strengths. Short-beam strength testing resulted in no apparent improvement while the fracture energy required for the onset of mode I interlaminar delamination was enhanced by 35%. Extrinsic toughening mechanisms, e.g., intralaminar fiber bridging and trans-ply cracking, significantly affected steady-state crack propagation values. Scanning electron microscopy of delaminated fracture surfaces revealed improved primary fiber–matrix adhesion and indications of CNF-induced matrix toughening.     

Compression properties of z-pinned composite laminates

The effect of z-pinning on the in-plane compression properties and failure mechanisms of polymer laminates is experimentally studied in this paper. The reduction to the compression modulus, strength and fatigue performance of carbon/epoxy laminates with increasing volume content and diameter of pins is determined. The elastic modulus decreases at a quasi-linear rate with increasing pin content and pin diameter. Softening is caused by fiber waviness around the pins and reduced fiber volume content due to volumetric swelling of the laminate from the pins. A simple model is presented for calculating the compression modulus of pinned laminates that considers the softening effects of fiber waviness and fiber dilution. The compression strength and fatigue life also decrease with increasing volume content and diameter of the pins. The strength and fatigue properties are reduced by fiber kinking caused by fiber waviness around the pins and the reduced fiber content caused by swelling. The deterioration to the compression properties is also dependent on the fiber lay-up pattern of the laminate, with the magnitude of the loss in properties increasing with the percentage of 0° (load bearing) fibers in the laminate. The paper gives suggestions for minimizing the loss to the compression properties to laminates due to pinning.     

Review of z-pinned composite laminates

This paper reviews published research into polymer composite laminates reinforced in the through-thickness direction with z-pins. Research into the manufacture, microstructure, delamination resistance, damage tolerance, joint strength and mechanical properties of z-pinned composites is described. Benefits of reinforcing composites with z-pins are assessed, including improvements to the delamination toughness, impact damage resistance, post-impact damage tolerance and through-thickness properties. Improvements to the failure strength of bonded and bearing joints due to z-pinning are also examined. The paper also reviews research into the adverse effects of z-pins on the in-plane mechanical properties, which includes reduced elastic modulus, strength and fatigue performance. Mechanisms responsible for the reduction to the in-plane properties are discussed, and techniques to minimise the adverse effect of z-pins are described. The benefits and drawbacks of z-pinning on the interlaminar toughness, damage tolerance and in-plane mechanical properties are compared against other common types of through-thickness reinforcement for composites, such as 3D weaving and stitching. Gaps in our understanding and unresolved research problems with z-pinned composites are identified to provide a road map for future research into these materials.     

Fatigue response of APC-2 composite laminates at elevated temperatures

The response of thermoplastic AS-4/PEEK composite laminates of two lay-ups, such as cross-ply and quasi-isotropy, subjected to tension–tension (T–T) fatigue loading at elevated temperatures was investigated. It is found that the ultimate strength of cross-ply laminate is higher than that of quasi-isotropic laminate at various temperatures, so does the fatigue strength. However, the slope of normalized stress vs. cycles curves in the quasi-isotropic laminates is higher than that of the cross-ply laminates at elevated temperatures. Finally, the simple semi-empirical predictive models in statistical analysis and multiple regressions are proposed and provided for design and application purposes.     

Interlaminar and intralaminar reinforcement of composite laminates with aligned carbon nanotubes

Three-dimensional reinforcement of woven advanced polymer–matrix composites using aligned carbon nanotubes (CNTs) is explored experimentally and theoretically. Radially-aligned CNTs grown in situ on the surface of fibers in a woven cloth provide significant three-dimensional reinforcement, as measured by Mode I interlaminar fracture testing and tension-bearing experiments. Aligned CNTs bridge the ply interfaces giving enhancement in both initiation and steady-state toughness, improving the already tough system by 76% in steady state (more than 1.5 kJ/m² increase). CNT pull-out on the crack faces is the observed toughening mechanism, and an analytical model is correlated to the experimental fracture data. In the plane of the laminate, aligned CNTs enhance the tension-bearing response with increases of: 19% in bearing stiffness, 9% in critical strength, and 5% in ultimate strength accompanied by a clear change in failure mode from shear-out failure (matrix dominated) without CNTs to tensile fracture (fiber dominated) with CNTs.     

A review: Starch-based composite foams

The large quantities of the petroleum-based foam materials used have raised concern due to their negative effects on the environment, predominantly single-use articles in packaging applications. Thus, considerable efforts have been put forth to develop environmentally friendly alternatives and, in particular, starch foams. Many techniques including extrusion, hot-mold baking/compression, microwave heating, freeze-drying/solvent exchange, and supercritical fluid extrusion can be used to produce starch foams with different cellular structures and properties. Starch by itself is, however, rather weak and water sensitive. To improve microstructure, mechanical and thermal properties, moldability, water resistance, lightness and other properties of starch-based foams, many approaches, e.g., chemical modification of starches, blending with various biodegradable polymers, incorporation of natural fibers, and addition of nanofillers, have been attempted and are intensively reviewed in this article.     

In situ processing of cellulose nanocomposites

Nanocellulose has gained attention in recent times due to their light weight, high strength, stiffness, biodegradability and renewability. Natural fibres have been used as reinforcement in composites for past many years, but the use of nanocellulose as reinforcement in composites is relatively new. The main challenges of preparing nanocellulose based composites include (i) generation of nanocellulose from natural resources, (ii) production in larger scale, (iii) enhancing compatibility with hydrophobic polymers, and (iv) achieving uniform dispersion in polymer matrices. These challenges have encouraged researchers to innovate efficient processes and techniques to utilise the maximum benefit of such green nanoscopic materials. In situ fabrication of cellulose nanocomposites is one such technique of achieving uniform nanocellulose dispersion in polymer matrices and obtaining a stronger filler/matrix interface. This review summarises the recent progress in the field of in situ processing of cellulose nanocomposites.     

Influence of activated drinking-water treatment waste on binary cement-based composite behavior: Characterization and properties

Drinking water treatment plants regularly dispose of large volumes of industrial sludge in landfill sites, which often has negative environmental consequences. The calcination products of these kaolinite-based sludges have properties that could make them appropriate supplementary cementing materials in the production of blended binary cements.This research analyses the pozzolanic and thermodynamic properties of a Venezuelan drinking water sludge activated at 600 °C for 2 h and its behavior in blended cement matrices prepared with 15% Activated Waste (AW) and 85% Ordinary Portland Cement (OPC). Our results show that this activated drinking water sludge presents high pozzolanic properties, mainly during the first 24 h of reaction. The XRD, SEM/EDX and thermodynamic studies confirm the formation of C₂ASH₈, C–S–H gels and C₄AH₁₃ as the hydration products from the pozzolanic reaction. The binary mixture of 15% AW/85% OPC complied with the physical and mechanical specifications contained in current European cement standards.     

Characterization of polyamide 6/carbon nanotube composites prepared by melt mixing-effect of matrix molecular weight and structure

Effects of molecular weight and structure of polyamide 6 (PA6) on morphology and properties of PA6/MWCNT prepared by melt mixing were investigated. Microscopic analysis showed fine dispersion of MWCNT within low viscosity PA6s due to domination of melt infiltration into MWCNT agglomerate at low viscosity matrices with linear structure. Rheological data indicated good interfacial interaction with no percolation of MWCNT up to 2 wt% loading. DSC thermograms showed nucleating role of MWCNT on crystallization of PA6s with marginal effect on crystallinity. Experimental data supported with micromechanical model showed limited improvement on mechanical properties, but it was closely consistent with degree of dispersion of MWCNT.     

A new specimen geometry to determine the through-thickness tensile strength of composite laminates

The tensile strength in thickness direction is one of the dimensioning parameters for composite load introductions, which are exposed to complex three-dimensional stress states, like e.g. composite lugs. In the present paper a simple test setup which introduces the load into the specimen by a form fit was chosen to determine the through-thickness tensile strength of quasi-isotropic carbon/epoxy laminates. By means of detailed finite element analyses a new quadrilaterally waisted specimen geometry was developed and validated by mechanical testing. The influence of the manufacturing process on the location of failure was investigated and recommendations for future tests are made. Compared to alternative state of the art methods the proposed test method leads to higher accuracy and reproducibility of the determined through-thickness tensile strength.     

A novel process to improve yield and mechanical performance of bamboo fiber reinforced composite via mechanical treatments

The aims of the present study are to produce bamboo fiber reinforced composite (BFRC) with high yield and to investigate the mechanical properties of BFRC comparing with those of commercial bamboo scrimber (BS) and laminated bamboo lumber (LBL). A novel process was developed for production of BFRC using oriented bamboo fiber mat (OBFM) made by a pilot machine. The yield and the mechanical properties of BFRC were investigated and analyzed in comparing with those of raw bamboo and other bamboo-based composites. The results show that the novel process produces 92.54% yield of OBFM due to without any chemical and special removing of inner and outer layer of bamboo during processing. In addition, all the mechanical properties and the variability of BFRC were significantly enhanced comparing with those of raw bamboo and other bamboo-based composites.     

Evaluation of the interfacial properties of polypropylene composite laminates,reinforced with paper sheets

Laminates, composed of different papers and polypropylene (PP), were fabricated by a manual stacking and hot pressing. The laminates were characterized by mechanical testing and the results were compared to glass fiber reinforced PP. Furthermore, a detailed evaluation of the interfacial properties and the paper structures was carried out by means of data modeling via rule of mixtures (ROM), as well as electron microscope (SEM) analysis. For investigating the influence of the laminate’s composition on the water adsorption behavior, water diffusion coefficients were determined. As a result, laminates with a tensile modulus up to 6 GPa and a tensile strength of 80 MPa were obtained. The property changes of the papers upon processing were successfully modeled, revealing a significant increase of the paper’s mechanical properties after fiber embedding. In general, the obtained results indicate a high potential of paper as a suitable reinforcement material for low to middle strained applications.     

Facile preparation and characterization of free-standing stiff carbon-based composite films with excellent performance

Novel free-standing stiff all carbon films based on multi-walled carbon nanotube (MWNT)/glassy carbon (GC) with excellent performance were fabricated. MWNTs, as excellent reinforcing materials, were successfully dispersed in polyimide (PI) matrix by in situ polymerization. The resultant MWNT/PI nanocomoposite films were used as precursors and underwent carbonization process. As a result, all carbon constituted MWNT/GC composite films were obtained. Mechanical results showed the maximum 3-point bending strength and modulus reached 575.5 MPa and 7.7 GPa respectively, improved by 54% and 78% compared to those of neat GC films. This method is simple, and the free-standing composite films can be prepared in large scales, which hold great potential in many applications.     

Embedded flaws for crack path control in composite laminates

An experimental investigation was conducted on using small flaws purposefully introduced into composite laminates to control growth of interlaminar cracks and through-thickness crack branching. Mode I crack growth specimens were used to study branching through 0°, 90° and 45° plies. The results showed that crack growth through 0° plies could be promoted by a ply gap, but this was not as controllable as combining a ply gap with a pre-crack to create a “crack branch flaw”. Crack branching through 45° plies could be controlled using crack branch flaws, and also promoted controllably using ply gaps. Crack branching through 90° plies was seen without any flaws, but was better controlled with embedded delaminations. Using these outcomes, crack branching through two quasi-isotropic laminates was demonstrated. The results have application to improved damage tolerance and fracture toughness, by taking advantage of high toughness crack growth mechanisms.     

Multifunctional graphene nanoplatelets/cellulose nanocrystals composite paper

We demonstrate a water-based method to fabricate strong, electrically and thermally conductive hybrid thin films (papers) made from the combination of graphene nanoplatelets (GnP) and cellulose nanocrystals (CNC). Unpressed and hot-pressed GnP papers containing CNC ranging from 0 wt% to 25 wt% were prepared. It is found that the GnP is well aligned within the hybrid paper, and a higher degree of alignment is induced by the hot-pressing process. The mechanical properties of the resulting papers increased with increasing content of CNC. The hot-pressed 25 wt% CNC hybrid paper showed the best mechanical properties among all the papers studied and improved the tensile strength by 33% and the modulus by 57% compared to neat GnP paper. Both the highest in-plane and though-plane thermal conductivity of 41 W/m K and 1.2 W/m K were measured respectively for the hot-pressed 15 wt% CNC hybrid paper. The electrical conductivity decreased continuously with increasing content of CNC but the thin film was still conductive at the highest CNC content in this study. The low-cost, environmental-friendly, thermally and electrically conductive flexible GnP/CNC hybrid papers have a set of properties making them suitable for many potential applications.     

Stress distributions in bluntly-notched ceramic composite laminates

We present a methodology for determining stress distributions ahead of blunt notches in plates of fiber-reinforced ceramic–matrix composites subject to uniaxial tensile loading, accounting for the effects of inelastic straining due to matrix cracking. The methodology is based on linear transformations of the corresponding elastic distributions. The transformations are derived from adaptations of Neuber’s law for stress concentrations in inelastic materials. Comparisons are made with results computed by finite element analysis using an idealized (bilinear) form of the Genin–Hutchinson constitutive law for ceramic composite laminates. Effects of notch size and shape as well as the post-cracking tangent modulus are examined. The comparisons show that, for realistic composite properties, the analytical solutions are remarkably accurate in their prediction of stress concentrations and stress distributions, even in cases of large-scale and net-section inelasticity. Preliminary assessments also demonstrate the utility of the solution method in predicting the fields under multiaxial stressing conditions.     

Improving the through-thickness thermal and electrical conductivity of carbon fibre/epoxy laminates by exploiting synergy between graphene and silver nano-inclusions

Fibre-reinforced polymer composites typically feature low functional (e.g., electric and thermal conductivity) and structural (e.g. mechanical strength and fracture toughness) properties in the laminate’s thickness direction. In the event of lightning strikes, overheating, and impact by foreign objects, composite laminates may suffer wide spread structural damage. This research explores the synergistic physical interaction between two-dimensional nanostructured (graphene nano-platelets) and, zero- or one-dimensional conductive fillers (silver nanoparticles or silver nanowires, respectively) when both are dispersed in fibre–polymer laminates. The results reveal a synergistic improvement in the through-thickness thermal conductivity that is more than the additive improvements by each constituent. Specifically, the simultaneous inclusion of graphene nano-platelets and silver nanoparticles/nanowires at a combined loading of 1 vol% resulted in approximately 40% enhancement in the through-thickness thermal conductivity while the inclusion of graphene nano-platelets alone at the same loading resulted only in 9% improvement. Similarly, the through-thickness electrical conductivity of carbon fibre/epoxy laminates incorporating graphene nano-platelets together with silver nanoparticles/nanowires was notably higher (⩾70%) than can be achieved by graphene nano-platelets alone (∼55%). These results demonstrate that the presence of nano-reinforcements exhibiting varied phonon transport and electron transfer pathways, and geometric aspect ratios promote synergistic physical interactions. Small improvements were found in the mechanical properties, including tensile, flexural or compressive properties of the carbon fibre-reinforced laminates, due to the relatively low concentrations of the nano-fillers.