Effect of thermal cycling and open-hole size on mechanical properties of polymer matrix composites

This paper investigates the effects of thermal cycling on mechanical degradation of polymer matrix composites (PMCs). Un-notched and open-hole specimens are tested using developed thermal cycling apparatus and tensile test machine. In addition, the hole-size effect of open-hole tension glass/epoxy composite laminates is investigated. The tensile strength, mass loss and surface degradation of the specimens were obtained during 250 cycles. Experimental results showed that the holes diameter is the main parameter to control the thermal cycling effects on open hole structure. Also, it is found that laminates with smaller holes have higher tensile strength variation than those with larger holes. The results showed that increment of the hole diameter and number of cycles decreases the tensile strength.     

The influence of pre-treatment of Spartium junceum L. fibres on the structure and mechanical properties of PLA biocomposites

Different chemical pre-treatments of Spartium junceum L. fibres using alkali (NaOH), nanoclay (MMT) and Citric acid (CA) with the aim of producing biodegradable composite material are discussed. As environmental requirements in processing technologies have been higher in recent years, the Polylactic acid (PLA) is used in this research as a matrix, due to its renewability, biodegradability and biocompatibility. Biocomposites are prepared by reinforcing PLA with randomly oriented, short Spartium junceum L. fibres in order to increase material strength. The effects of different pre-treatments of Spartium junceum L. fibres on the mechanical properties of final biocomposite material are examined. Fibre tenacity is studied using Vibroscop and Vibrodyn devices. Tensile strength of biocomposite material was measured on the universal electromechanical testing machine Instron 5584. The results indicate that biocomposites reinforced with fibres modified with MMT and CA show upgraded mechanical properties of the final composite material in comparison with the composite materials reinforced with referenced (nontreated) fibres. Infrared spectra of tested fibres and biocomposites were determined with Fourier transform infrared spectroscopy using Attenuated total reflection (FT-IR ATR) sampling technique and the influence of fibre modifications on the fibre/polymer interfacial bonding was investigated. The interface of Spartium/PLA composites was observed with scanning electron microscope (SEM) and it was clearly visible that biocomposites reinforced with fibres modified by MMT and CA showed better interaction of fibres and matrix.     

Basalt fibre-reinforced high density polyethylene composite development using the twin screw extrusion process

Offshore renewable energy can lead the way towards sustainable energy harvesting and support the achievement of the CO2 reduction target by 2030. To achieve this goal it is necessary to decrease the manufacturing and deployment cost of the offshore devices. This paper focusses on the mechanical, chemical and microstructural assessment of a novel high density polyethylene (HDPE) reinforced with short basalt fibres for potential application as a hull material for wave energy devices. The choice of short fibres ensures the new composite can utilise existing low cost manufacturing methods for HDPE structures. In particular this study compares the properties of material with a recycled HDPE matrix with the properties of a material using a virgin HDPE matrix. The mechanical properties achieved by the novel composites exceed an improvement of ~300% in the properties of the monolithic polymer hence indicating the potential of this material, both for recycled and virgin HDPE. Furthermore, exploration in detail of the interaction fibres/matrix indicated the dynamic reaction between coupling agent and polymeric matrix showing the formation of molecular bonding perpendicular to the fibres, hence enhancing a 3D network that further increases the reinforcement abilities of the fibres.     

The effects of cauliflower-like short carbon fibers on the mechanical properties of rigid polyurethane matrix composites

Stress concentration and weak interfacial strength affect the mechanical properties of short carbon fibers (CFs) reinforced polymer composites. In this work, the cauliflower-like short carbon fibers (CCFs) were prepared and the point was to illuminate the effects of fiber morphology on the mechanical properties of the CCFs/rigid polyurethane (RPU) composites. The results indicated that the surface structure of CCFs could increase the surface roughness of the fibers and the contact area between fibers and matrix, thereby promoting the formation of irregular interface. Compared with pure RPU and initial CFs/RPU composites, the strength and toughness of CCFs/RPU composites were simultaneously improved. The satisfactory performance was attributed to the special fibers structure, which played an anchoring role and consumed more energy during crack propagation.     

Melt flow behavior of polypropylene composites filled with multi-walled carbon nanotubes during extrusion

Polypropylene (PP) composites filled with multi-walled carbon nanotubes (MWCNTs) were prepared using a twin-screw extruder. The melt flow properties of the composites were measured with a capillary rheometer in a temperature range from 180 to 230 °C and at various apparent shear rates varying from 100 to 4000 s⁻¹. The results showed that the melt shear stress increased almost linearly while the melt shear viscosity decreased almost linearly with increasing shear rates in a bi-logarithmic coordinate system. The melt shear flow followed the power law relationship and the dependence of the melt shear viscosity on temperature obeyed the Arrhenius equation. The relationship between the melt shear viscosity and the MWCNT weight fraction was roughly linear under the investigated range of temperature or shear rate.     

Mechanical and biodegradation performance of short natural fibre polyhydroxybutyrate composites

The work outlined in this paper describes the evaluation of polyhydroxybutyrate (PHB) based natural fibre composites via an extrusion – injection moulding process. Virgin PHB was compounded with two different naturally occuring plant fibres, hemp and jute, and a third, regenerated cellulose fibre, lyocell. Composite materials containing 10–30 wt% of each type of fibre were obtained by twin screw extrusion and the resultant material was injection moulded to produce tensile samples suitable for mechanical characterisation. Mechanical properties were determined using tensile, impact and flexural testing. Melt flow index and water absorption studies were also carried out on the biocomposite materials, and Fourier transform infrared spectroscopy was used to examine the bonding between the polymer and each fibre type. The rate of biodegradation was also observed by placing composite samples in compost and measuring weight loss weekly. The biocomposites produced using this method were shown to have increased rates of biodegradation whilst exhibiting significantly improved flexural properties.     

Requirements of amount of carbon nanotubes for damage detection in large polymer composite structures

Due to their very high electrical conductivity, the addition of carbon nanotubes (CNTs) into polymers such as epoxies makes these materials conductive. This conductivity has been utilized to provide damage sensing in composite structures. Usually, the amount of CNTs needs to be more than the percolation threshold to assure electrical conductivity. The percolation threshold is usually determined using small samples. For large samples, the amount of CNTs needs to be higher to take into account some non-uniformity of the dispersion. More CNTs would provide better conductivity. One normally expects that more CNTs would also provide better damage detection. However, it was found that this is not the case. Certainly, the amount of CNTs needs to be more than a certain lower limit to assure conductivity throughout the large structures. Once this condition is met, adding more CNTs would reduce the sensitivity for damage detection. The sensitivity of damage detection can be measured by the change in electrical resistance (due to the occurrence of damage) between grid points that are attached on the surface of the composite structure. Higher sensitivity in damage detection would enable coarse grids (larger distance between grid points). Coarse grid points would mean lower number of grid points, less space, less wiring and less weight. This paper describes this phenomenon in detail. It provides models that simulate the conductivity configurations. It also introduces a new term call “Aggregately Conductive Materials” to distinguish the particular conductive characteristics of materials that are made conductive by the addition of nano-particles.     

Crystallization properties and thermal stability of polypropylene composites filled with wollastonite

The influence of wollastonite (CaSiO₃) content on the crystallization properties and thermal stability of polypropylene (PP) composites was investigated. The results showed that the crystallization temperature, crystallization end temperature and crystallization temperature interval, as well as the degree of crystallinity of the composites, were higher than those of the unfilled PP resin, while the crystallization onset temperature was little changed from that of the unfilled PP resin. The increase of degree of crystallinity for the composites could be attributed to the heterogeneous nucleation of the CaSiO₃ in the PP matrix. The thermal stability increased with increasing filler weight fraction (ϕ_f); the thermal decomposition rate decreased nonlinearly with increasingϕ_f. Finally, the dispersion of the filler particles in the matrix was observed, and the mechanisms of thermal stability and crystallizing behavior were discussed.     

Enhanced thermal conductivity of epoxy–matrix composites with hybrid fillers

The results of thermal conductivity study of epoxy–matrix composites filled with different type of powders are reported. Boron nitride and aluminum nitride micro‐powders with different size distribution and surface modification were used. A representative set of samples has been prepared with different contents of the fillers. The microstructure was investigated by SEM observations. Thermal conductivity measurements have been performed at room temperature and for selected samples it was also measured as a function of temperature from 300 K down to liquid helium temperatures. The most spectacular enhancement of the thermal conductivity was obtained for composites filled with hybrid fillers of boron nitride–silica and aluminum nitride–silica. In the case of sample with 31 vol.% of boron nitride–silica hybrid filler it amounts to 114% and for the sample with 45 vol.% of hybrid filler by 65% as compared with the reference composite with silica filler. However, in the case of small aluminum nitride grains application, large interfacial areas were introduced, promoting creation of thermal resistance barriers and causing phonon scattering more effective. As a result, no thermal conductivity improvement was obtained. Different characters of temperature dependencies are observed for hybrid filler composites which allowed identifying the component filler of the dominant contribution to the thermal conductivity in each case. The data show a good agreement with predictions of Agari‐Uno model, indicating the importance of conductive paths forming effect already at low filler contents. Copyright © 2014 John Wiley & Sons, Ltd.     

Effects of heat treatment on electrical conductivity of HDPE/CB composites

The influence of heat treatment on the electrical conductive behavior of carbon black (CB) filled high density polyethylene (HDPE) composites was investigated. The results showed that the effects of annealing temperature on the resistivity and the PTC intensity of the HDPE/CB composites were significant; the resistivity and the PTC intensity of the composites varied with increasing number of thermal cycles; while the variation became small after the third thermal cycle. Furthermore, the variation of the resistivity was 1.7 times higher than that of the composites without annealing, and the variation of the PTC intensity of the composites was 0.22, which were smaller than those of the specimens without heat treatment. A suitable annealing heat treatment could reduce the resistivity and enhance the PTC intensity of the composites; it was also helpful to improve the stability of the properties of the composites and the repeatability of the PTC effect.     

Effect of gamma radiation on the performance of jute fabrics-reinforced polypropylene composites

Jute fabrics-reinforced polypropylene (PP) composites (50% fiber) were prepared by compression molding. Composites were fabricated with non-irradiated jute fabrics/non-irradiated PP (C-0), non-irradiated jute fabrics/irradiated PP (C-1), irradiated jute fabrics/non-irradiated PP (C-2) and irradiated jute fabrics/irradiated PP (C-3). It was found that C-3 composite performed the best mechanical properties over other composites. Total radiation dose varied from 250–1000 krad and composites made of using 500 krad showed the best results. The optimized values (C-3 composites) for tensile strength (TS), bending strength (BS) and impact strength (IS) were found to be 63 MPa, 73 MPa and 2.93 kJ/m², respectively.     

Improving piezoelectric performance of lead-free polymer composites with high aspect ratio BaTiO3 nanowires

Flexible and lead-free piezoelectric nanocomposites were synthesized with BaTiO₃ nanowires (filler) and poly(vinylidene fluoride) (PVDF) (matrix), and the piezoelectric performances of the composites were systematically studied by varying the aspect ratio (AR) and volume fraction of the nanowire and poling time. BaTiO₃ nanowires with AR of 18 were synthesized and incorporated into PVDF to improve the piezoelectric performance of the composites. It was found that high AR significantly increased the dielectric constant up to 64, which is over 800% improvement compared to those from the composites containing spheroid shape BaTiO₃ nanoparticles. In addition, the dielectric constant and piezoelectric coefficient were also enhanced by increasing the concentration of BaTiO₃ nanowires. The piezoelectric coefficient with 50-vol% BaTiO₃ nanowires embedded in PVDF displayed 61 pC/N, which is much higher than nanocomposites with spheroid shape BaTiO₃ nanoparticles as well as comparable to, if not better, other nanoparticle-filled polymer composites. Our results suggest that it is possible to fabricate nanocomposites with proper mechanical and piezoelectric properties by utilizing proper AR fillers.     

Tensile and flexural properties of polypropylene composites filled with highly effective flame retardant magnesium hydroxide

The reinforcing effects of highly effective flame retardant magnesium hydroxide (FMX) content on the tensile and flexural properties of filled polypropylene (PP) composites were investigated within the FMX weight fraction range from 5 to 60 wt%. It was found that the Young's modulus and flexural modulus increased approximately linearly while the tensile yield strength and tensile fracture strength decreased slightly with increasing the FMX weight fraction. When the FMX weight fraction was lower than 20%, the tensile elongation at break decreased considerably, and then decreased slightly; the flexural strength increased when the FMX weight fraction was lower than 30%, and then decreased slightly. The tensile properties increased with increasing rate of tension. Moreover, the tensile yield strength of the composites was estimated using an equation proposed in previous work, and good agreement was shown between the predicted and the measured data.     

Thermal and volumetric property analysis of polymer networks and composites using elevated temperature digital image correlation

Digital Image Correlation (DIC), which exploits non-contact advantages and full-field analysis, provides more data in-situ that are not possible with traditional techniques. In this work, elevated temperature digital image correlation techniques were applied to a glassy polymer network via thermal expansion and contraction experiments to study volumetric behavior during the curing process. A glassy epoxy network was tested in both cured and under-cured states and heated to the ultimate cure temperature. Matrix volume changes due to both thermal expansion and cure shrinkage were quantified. Concurrently, the thermal expansion of aerospace-grade composite laminates was also observed in matrix and fiber-dominant directions. Additionally, the strain-free temperature of a non-symmetric composite laminate was identified through thermal compensation of process-induced curvatures. Finally, laminate dimension changes were related to the strain-free temperature as means to probe process-induced strains within composite laminates. Thermal properties of the neat matrix and composite laminates were compared to traditional techniques, validating the benefit of elevated temperature digital image correlation for composite matrix qualification.     

Rheological properties of wood polymer composites at high shear rates

This paper presents the rheological properties of wood-polymer composites (WPC) with a polypropylene (PP) matrix in the corrected shear rate range from approx. 20 s⁻¹ to 150 000 s⁻¹. Tests were conducted using a capillary rheometer and a rheological head of the author's construction, for which the working element is a thermoplastic injection moulding machine. The constructed tool was found to be very useful, especially for the determination of the processing characteristics of WPC composites containing a large particle-size filler. It was observed that the rheological properties of wood-polymer composites in the shear rate range of up to several thousand s⁻¹ significantly depended on the filler content of the polymer matrix; at the same time, at higher shear rate, a clear decrease in the effect of the wood filler content on the viscosity of the composites and on the flow behaviour, as described by the power law, took place.     

A state-of-the-art review on particulate wood polymer composites: Processing,properties and applications

In the present decade, the demands for recyclable, environmentally friendly and low-cost with good strength composites materials have been significantly increased. In this context, the particulate wood polymer composites have attracted the researchers owing to their eco-friendliness, low-cost as they are prepared using waste wood particles, and good mechanical and physical properties. These composites were prepared by filling the waste wood particles into the polymers using different fabrication methods such as extrusion, hand layup, compression moulding, injection moulding and additive manufacturing (3D printing). A good number of research works have been reported on the testing and characterization of wood composites for the various applications so far. This fact motivated to prepare a state-of-the-art review on the recent developments in processing, characterization, and applications of wood composites. This paper presents a discussion on the chemical structure and properties of different types of wood species. The mechanical, thermal and water absorption behaviour of thermosets, thermoplastics and biopolymers based wood composites have also been discussed. Further, characterization of the nano biocomposites prepared using nanocellulose/nanoparticles of wood are also presented. The outcomes of the present review provide a good understanding of wood composites that will encourage the researchers for further research works & developments of novel wood composites for the advanced applications.     

Non-linear material characterization and numerical modeling of cross-ply basalt/epoxy laminate under low velocity impact

The low velocity impact behavior of basalt/epoxy composites, seen as an eco-friendly replacement of glass-epoxy composites, has not been studied systematically so far. Here, the elastic elasto-plastic properties, strengths, intralaminar and interlaminar fracture energies were determined. The intralaminar energies were determined using compact tension and compression tests. The elasto-plastic properties needed in the plastic potential were determined using off-axis test. These properties are used in Finite Element (FE) code with an elasto-plastic damage model developed earlier to simulate the impact response of cross-ply laminates basalt/epoxy laminates. Low velocity impact (LVI) experiments at 10 J, 20 J and 30 J are performed on these composites. The FE simulation is successful in capturing force, energy, deflection histories and damage zones showing a close match to the experiments. A comparison of impact force history and damage area (ultrasonic C-scan) of basalt-epoxy laminates with glass epoxy laminates having same volume fraction shows nearly similar peak forces but the major axis of the ellipsoidal damage zone was bigger in glass/epoxy laminates.     

Thermo-oxidation behaviour of composite materials at high temperatures: A review of research activities carried out within the COMEDI program

The present paper presents a review of the main activities carried out within the context of the COMEDI research program, a joint collaboration involving three research teams focusing on the thermo-oxidation behaviour of composite materials at high temperatures.The scientific aim of the COMEDI research program was to better identify the link between the physical mechanisms involved in thermo-oxidation phenomena: oxygen reaction-diffusion, chemical shrinkage strain/stress, degradation at different scales and to provide tools for predicting the thermo-oxidation behaviour of composite materials under thermo-oxidative environments including damage onset.This aim was accomplished by investigating experimentally the thermo-oxidation behaviour of pure resin samples - both industrial and “model” materials - and by interpreting the results by a coupled reaction-diffusion-mechanics multiphysics model.A dedicated numerical model tool has been developed and implemented into the ABAQUS^® finite element commercial software. This tool was employed to simulate the thermo-oxidative behaviour of a fibre-matrix microscopic representative composite cell.Finally, the model predictions for the composite have been validated by comparing the experimental and the simulated local matrix shrinkage displacements and the mass loss of composite specimens.     

Effect of local microstructure on the indentation induced damage of a fiber reinforced composite

Multicycle grid nanoindentation tests, combined with high resolution Scanning Electron Microscopy (SEM) and Scanning Probe Microscopy (SPM) observations, were applied on a commercial Carbon Fiber (CF) reinforced epoxy matrix composite in order to study the induced damage mechanisms with respect to: (a) the orientation of the CFs relative to the surface and (b) the CF packing density. Normal to the surface CFs showed a multiple cracking pattern, those forming 45° showed distinct cracking, while CFs parallel to the surface did not suffer cracking. CF detachment from the epoxy matrix was observed in all cases. Pop-in type discontinuities were observed only in the samples where cracking ensued, as revealed through SEM and SPM observations. The load to induce CF cracking increased with increase of the matrix pocket area. Elastic modulus, hardness and significance of elastic deformation as an indentation energy absorbing mechanism, were reduced right after pop-in.