Poly(lactic acid)/pulp fiber composites: The effect of fiber surface modification and hydrothermal aging on viscoelastic and strength properties

Poly(lactic acid) (PLA)/kraft pulp fiber (30 wt%) composites were prepared with and without a coupling agent (epoxidized linseed oil, ELO, 1.5 wt%) by injection molding. The non-annealed composite samples, along with lean PLA, were exposed to two hydro-thermal conditions: cyclic 50% RH/90% RH at 23 and 50°C, both up to 42 days. The aging effects were observed by size exclusion chromatography, differential scanning calorimetry, dynamic and tensile mechanical analysis, and fracture surface imaging. ELO temporarily accelerated the material's internal transition from viscous to an increasingly elastic response during the aging at 50°C. ELO also slowed down the tensile strength reduction of the composites at 50°C. These observations were explained with the hydrophobic ELO molecules' coupling and plasticizing effects at fiber/matrix interfaces. No effects were observed at 23°C.     

Demonstration of pseudo-ductility in unidirectional hybrid composites made of discontinuous carbon/epoxy and continuous glass/epoxy plies

A new, partially discontinuous architecture is proposed to improve the mechanical performance of pseudo-ductile, unidirectional (UD) interlayer carbon/glass hybrid composites. The concept was successfully demonstrated in different laminates with high strength and high modulus carbon and S-glass epoxy UD prepregs. The novel hybrid architecture provided pseudo-ductile tensile stress–strain responses with a linear initial part followed by a wide plateau and a second linear part, all connected by smooth transitions. The best hybrid configuration showed 60% improvement in modulus compared to pure glass, 860 MPa plateau stress and 2% pseudo-ductile strain. The initial modulus, the plateau stress and the overall tensile stress–strain response of each specimen configuration were predicted accurately.     

Elastic behaviour and failure mechanism in epoxy syntactic foams: The effect of glass microballoon volume fractions

A representative elementary volume (REV) in epoxy syntactic foams was generated to incorporate randomly distributed glass microballoons that followed a log-normal size distribution. Finite element modelling of the REV foam was developed and experimentally validated to investigate the elastic behaviour and failure mechanism in the foams with different microballoon volume fractions (V). The localised stresses concentrate in various zones within the foam, and can cause the vertical splitting fracture of microballoons and the micro-crack formation in the matrix. Dependent on the microballoon volume fraction, micro-cracks can propagate to join adjacent micro-cracks and voids left by fractured microballoons, and finally develop into a macro-crack either in the preferred longitudinal (for low V) or diagonal (for high V) directions. This is consistent with the macroscopic observations of the fracture process in the foam specimens. It was also found that elastic characteristics of the foam vary with microballoon volume fractions.     

Matrix influence on the piezoelectric properties of piezoelectric ceramic/polymer composite exhibiting particle alignment

This study was addressed to the influence of an electric field strength applied at fabrication process and matrix properties, such as the dielectric constant and the Young's modulus, on “pseudo‐1‐3 piezoelectric ceramic/polymer composite” in order to further enhance the piezoelectricity of that. The pseudo‐1‐3 piezoelectric ceramic/polymer composite consists of linearly ordered piezoelectric ceramic particles in polymer material. Silicone gel, silicone rubber, urethane rubber, and poly‐methyl‐methacrylate, which exhibit different dielectric constants and Young's modulus, were used as matrices to evaluate the matrix influence. The piezoelectricity of the pseudo‐1‐3 piezoelectric ceramic/polymer composite was evaluated using the piezoelectric strain constant d₃₃. The d₃₃ is one of the indices of the piezoelectric properties for piezoelectric materials. As a result, it was confirmed that d₃₃ of the pseudo‐1‐3 piezoelectric ceramic/polymer composite increased with the increase of the electric filed strength applied at fabrication process, though, it reached a constant value at a certain strength value. Further it was confirmed that dielectric constant of the matrix had a small influence on d₃₃ of the pseudo‐1‐3 piezoelectric ceramic/polymer composite, however, in case of matrix of lower Young's modulus, d₃₃ was increase. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41817.     

Self-cleaning engineered cementitious composites

Engineered cementitious composite (ECC) is a strain hardening cementitious composite with extreme tensile ductility of several percent. Few emerging applications of ECC, including lightweight building façade and pavement, make self-cleaning a desirable functionality to be added into the material. This study aims to impart photocatalytic properties into ECC for engaging self-cleaning. Influence of TiO₂ content on mechanical properties, cleaning efficiency, surface wettability, and dirt pick-up resistance of white ECC was studied. It shows that the inclusion of TiO₂ in ECC engages photocatalysis, facilitates the decomposition of RhB, and enhances photo-induced hydrophilicity significantly. As a result, TiO₂-ECC possesses self-cleaning with higher dirt pick-up resistance than normal ECC. However, TiO₂ photocatalysis may adversely affect the flexural strength and ductility of ECC due to weakened fiber/matrix interface bond after UV/sunlight irradiation.     

Microstructure evolution during fabrication and microstructure–property relationships in vapour-grown carbon nanofibre-reinforced aluminium matrix composites fabricated via powder metallurgy

Microstructure evolution of vapour-grown carbon nanofibre (VGCF)-reinforced aluminium matrix composites during fabrication and microstructure–property relationships of these materials were studied. Composites were fabricated using powder metallurgy, i.e. by mixing VGCFs and aluminium powder via ball-milling followed by sintering or hot extrusion. The mixing condition was selected to achieve small powder particle size and homogeneously dispersed VGCFs. Aluminium grains and VGCFs were elongated along the longitudinal direction of aluminium particles in the mixed powder. Detailed observation of the aluminium grains in the composites found grain size and morphology dominated by recrystallization. Apparently, grain growth was inhibited by VGCFs. Theoretical models considering strength increment due to grain refinement resulting from VGCF addition, load bearing of VGCFs, thermal mismatch of VGCFs and aluminium and Orowan effect were developed. Theoretical values coincided well with hardness, yield strength, and ultimate tensile strength of the composites, and thus the models could precisely explain the microstructure–property relationships.     

A developed model to assume the interphase properties in a ternary polymer nanocomposite reinforced with two nanofillers

In the present study, a model is developed for prediction of tensile modulus and interphase properties in ternary polymer nanocomposites containing two nanofillers. In this regard, Ji model which assumes the interphase in particulate filled nanocomposites is developed for ternary systems in which, the arrangement of two nanofillers in parallel and series orders, the role of interphase properties and the dispersion quality of nanoparticles are taken into account by a simple approach. The calculations by the developed model are compared with the experimental results obtained for polypropylene (PP)/montmorillonite nanoclay (MMT)/CaCO₃ ternary nanocomposite. It is found that the absence of interphase causes much dissimilarity between experimental and theoretical data. However, a good agreement is obtained between the experimental data and the predicted results by the developed model for ternary polymer nanocomposites.     

Static stress analysis of carbon nano-tube reinforced composite (CNTRC) cylinder under non-axisymmetric thermo-mechanical loads and uniform electro-magnetic fields

Static stresses analysis of carbon nano-tube reinforced composite (CNTRC) cylinder made of poly-vinylidene fluoride (PVDF) is investigated in this study. Non-axisymmetric thermo-mechanical loads are applied on cylinder in presence of uniform longitudinal magnetic field and radial electric field. The surrounded elastic medium is modeled by Pasternak foundation because of its advantages to the Winkler type. Distribution of radial, circumferential and effective stresses, temperature field and electric displacements in CNTRC cylinder are determined based on Mori–Tanaka theory. The detailed parametric study is conducted, focusing on the remarkable effects of magnetic field intensity, elastic medium, angle orientation and volume fraction of carbon nano-tubes (CNTs) on distribution of effective stress. Results demonstrated that fatigue life of CNTRC cylinder will be significantly dependent on magnetic intensity, angle orientation and volume fraction of CNTs. Results of this research can be used for optimum design of thick-walled cylinders under multi-physical fields.     

Model for prediction of simultaneous time-dependent damage evolution in multiple plies of multidirectional polymer composite laminates and its influence on creep

A model to predict time-dependent evolution of simultaneous transverse cracking developed in multiple plies during creep loading and its effects on creep of multidirectional polymer matrix composite laminates is presented. The stress states in the intact regions of the plies are determined using the lamination theory during an incremental change in time. The stored elastic energy, determined using this stress state, is compared with a critical stored elastic energy value for damage to determine if a ply would fracture after the increment. If fracture is predicted, variational analysis is used to determine the perturbation in ply stresses due to cracking. This procedure is repeated to determine the crack evolution and creep strain. Model predictions compared well with experimental results for a [±θ_m/90_n]_s laminate.     

Development of a biocomposite based on green epoxy polymer and natural cellulose fabric (bark cloth) for automotive instrument panel applications

Natural fiber reinforced composites have attracted interest due to their numerous advantages such as biodegradability, dermal non-toxicity and with promising mechanical strength. The desire to mitigate climate change due to greenhouse gas emissions, biodegradable resins are explored as the best forms of polymers for composites apart from their synthetic counterparts which are non-renewable. In this study biodegradable bark cloth reinforced green epoxy composites are developed with view of application to automotive instrument panels. The optimum curing temperature of green epoxy was shown to be 120 °C. The static properties showed a tensile strength of 33 MPa and flexural strength of 207 MPa. The dynamic mechanical properties, frequency sweep showed excellent fiber-matrix bonding of the alkali treated fabric with the green epoxy polymer with glass transition temperature in the range of 160 °C–180 °C. Treatment of the fabric with alkali positively influenced the mechanical properties of the fabric reinforced biocomposites.