Procedural influences on compression and injection moulded cellulose fibre-reinforced polylactide (PLA) composites: Influence of fibre loading,fibre length,fibre orientation and voids

The influence of fibre loading (20, 30, 40 mass%), fibre fineness, and the processing procedure (compression moulding – CM and injection moulding – IM) on the tensile and impact strength of lyocell/PLA composites was examined. The results revealed a significantly higher tensile and impact strength for CM composites compared to IM composites. An increase in strength up to a fibre loading of 40% was determined for CM composites, while for IM composites the highest values were measured at a fibre loading of 30%. Composites were investigated for their void content, fibre orientation, fibre length and process-induced fibre damage. A better fibre/matrix adhesion and compaction of IM composites was found while fibre orientation as well as mechanical properties of extracted fibres show no significant differences between CM and IM composites. The different mechanical characteristics of CM and IM samples are attributed predominantly to the fibre aspect ratio and the distribution of voids.     

Effect of grafting cellulose acetate and methylmethacrylate as compatibilizer onto NBR/SBR blends

Compatibilizer is used for improving of processability, interfacial interaction and mechanical properties of polymer blends. In this study acrylonitrile butadiene rubber (NBR) and styrene-butadiene rubber (SBR) blends were compatibilized by a graft copolymer of acrylonitrile butadiene rubber (NBR) grafted with cellulose acetate (CA) i.e. (NBR-g-CA) and acrylonitrile butadiene rubber (NBR) grafted with methylmethacrylate i.e. (NBR-g-MMA). Compatibilizers were prepared by gamma radiation induced grafting of NBR with cellulose acetate (CA) and methylmethacrylate (MMA) were added with different ratios to NBR/SBR (50/50) blend. The compatibilized blends were evaluated by rheometric characteristics, physico-mechanical properties, swelling behavior, scanning electron microscope (SEM) and thermal analysis. The results showed that, the blends with graft copolymer effect greatly on the rheological characteristics [optimum cure time (Tc₉₀), scorch time (Ts₂), and the cure rate index (CRI)]. The physico-mechanical properties of the investigated blends were enhanced by the incorporation of these graft copolymers, while the resistance to swelling in toluene became higher. SEM photographs confirm that, these compatibilizers improve the interfacial adhesion between NBR/SBR (50/50) blend which induce compatibilization in the immiscible blends. The efficiency of the compatibilizer was also evaluated by studying the thermogravimetric analysis.     

Durability assessment via residual strength and viscoelastic observations on filled rubber compounds

Because rubber compounds are widely used under cyclic loading conditions, such as tire applications, their fatigue behaviour has been studied for a long time. Two main stages are commonly considered in studying rubber fatigue: crack nucleation and crack growth. Not many studies exist on the residual strength of rubber subsequent to fatigue loading. In this study, the residual strength method is used to model fatigue behaviour of carbon black‐filled and silica‐filled styrene‐butadiene rubber (SBR) compounds. Samples are subjected to repeated fatigue loading (ie, nonzero mean stress) using different strain amplitudes and then subjected to uniaxial constant crosshead rate, relaxation, and creep tests to assess their residual strength and viscoelastic behaviours respectively. The residual strength results are compared with typical S‐N curves. Initial relaxation rates, initial creep rates, asymptotic relaxation values, and secondary creep rates are plotted as functions of fatigue cycle number to understand the viscoelastic behaviours of carbon black and silica‐filled SBR compounds as affected by fatigue processes.     

Estimation of the tensile strength of UHPFRC layers based on non-destructive assessment of the fibre content and orientation

In the present study we propose a procedure for estimating the tensile strength of thin ultra-high performance fibre-reinforced cement-based composite (UHPFRC) layers, which eliminates the need of extracting cores or samples from the structure. This procedure relies on a non-destructive testing (NDT) method based on the ferromagnetic properties of the steel fibres for estimating the parameters of the underlying physical model, namely, the fibre content and the fibre orientation factor, and on laboratory tensile tests for estimating the equivalent rigid-plastic fibre-to-matrix bond strength. An experimental program was developed for establishing the relation between the NDT measurements and the orientation parameters determined from image analysis. Following the proposed procedure, the tensile strength of 36 specimens with varying fibre content and fibre orientation distributions is estimated based on the magnetic measurements and compared to experimental results. The good correlation that is found demonstrates the significance of the proposed NDT method in the implementation of quality control procedures of thin UHPFRC elements/layers.     

Fracture mechanisms and failure analysis of carbon fibre/toughened epoxy composites subjected to compressive loading

This study investigates the failure mechanisms of unidirectional (UD) HTS40/977-2 toughened resin composites subjected to longitudinal compressive loading. A possible sequence of failure initiation and propagation was proposed based on SEM and optical microscopy observations of failed specimens. The micrographs revealed that the misaligned fibres failed in two points upon reaching maximum micro-bending deformation and two planes of fracture were created to form a kink band. Therefore, fibre microbuckling and fibre kinking models were implemented to predict the compressive strength of UD HTS40/977-2 composite laminate. The analysis identified several parameters that were responsible for the microbuckling and kinking failure mechanisms. The effects of these parameters on the compressive strength of the UD HTS40/977-2 composite systems were discussed. The predicted compressive strength using a newly developed combined modes model showed a very good agreement to the measured value.     

Influence of fibre length and concentration on the properties of glass fibre-reinforced polypropylene: Part 3. Strength and strain at failure

In this report we present the results from the third part of a study on the influence of fibre length (0.1–50 mm) and concentration (3–60% w/w) on the properties of glass fibre-reinforced polypropylene laminates. These laminates were prepared in the laboratory using a wet deposition method and compared with samples prepared on a commercial melt impregnation GMT line. We found that laminate tensile strength increased linearly with fibre concentration up to 60% w/w. Laminate strength was also found to increase with increasing fibre length. At high values of fibre length (> 3–6 mm) the strength reached a plateau level which was directly dependent on fibre content. The matrix molecular weight appeared to have little direct influence on the level of laminate strength. However, the glass fibre sizing compatibility was found to have a strong effect on the tensile strength of both laboratory made wet deposited laminates and commercially prepared GMTs. The tensile strength of the GMT samples also showed a clear correlation with the measured fibre strength. A modified version of the Kelly-Tyson model gave calculated values of laminate strength which correlated well with the experimental data. We propose that the tensile strength of these laminates is governed by the properties of the fibres which have an orientation close to parallel with the loading direction.     

Strength and failure modes in resistance welded thermoplastic composite joints: Effect of fibre–matrix adhesion and fibre orientation

The strength and failure modes of resistance welded thermoplastic composites were investigated. Special attention was paid to the effect of basic characteristics of the adherends such as fibre–matrix adhesion and fibre orientation. 8HS woven GF/PEI composites were resistance welded. Intralaminar failure was found to be the major failure mechanism for the well welded joints, consisting of either fibre–matrix debonding or laminate tearing. An improved fibre–matrix adhesion was found to result in significantly higher lap shear strength. Besides, the main apparent orientation of the fibres on the welding surfaces was found to have an effect on the strength of the joints.     

Low-velocity impact response of non-woven hemp fibre reinforced unsaturated polyester composites: Influence of impactor geometry and impact velocity

In this study, the influence of varying impactor geometries on the impact damage characteristics of hemp fibre reinforced unsaturated polyester composites were subjected to a low-velocity impact loading using an instrumented falling weight impact test setup. The three varying tup geometries: hemispherical, 30° and 90°, at four different impact velocity levels: 2.52 m/s, 2.71 m/s, 2.89 m/s and 2.97 m/s were assessed. The experimental results to investigate the influence of impactor geometry suggest that HFRUP composites were able to withstand higher loads when tested with hemispherical impactor and also absorbed more energy than that for 90° and 30° shaped tup geometry. The post impact damage patterns and failure mechanisms of impacted samples were further characterised by ultrasonic (UT) inspection. Impact induced damage characterised by scanning electron microscope (SEM) suggests that damage induced by the impact included a typical failure mechanisms showing matrix cracking, fibre breakage and fibre pullout. As the impact velocity increases the damage to back face of the laminate increased for laminates tested with a hemispherical impactor while it decreased to certain extent for laminates tested with 90° and 30° impactor geometries.     

Preparation, characterisation and processing of carbon fibre/polyamide-12 composites for selective laser sintering

Aiming at developing carbon fibre/polyamide-12 (CF/PA) composite powders for manufacturing high-performance components by selective laser sintering (SLS), the preparation, characteristics and sintering process of the composite powders and mechanical properties of sintered components were studied. Surfaces of the carbon fibres were treated by the oxidation modification and coated with polyamide-12 through the dissolution-precipitation process to provide good interfacial adhesion and homogenous dispersion within the polyamide-12 matrix. The particle size and micro-morphology analyses show that the CF/PA composite powders with 30 wt%, 40 wt% and 50 wt% carbon fibres present the suitable powder sizes and format for SLS. The incorporation of carbon fibres into the polyamide-12 matrix decreases the initial melting temperature and consequently lowers the SLS part bed temperatures, implying lower energy requirement and less thermal degradation in the sintering process. The CF/PA composites also represent higher thermal stability than the pure polyamide-12. The CF/PA sintered components with 30 wt%, 40 wt% and 50 wt% carbon fibres exhibit the greatly enhanced flexural strengths by 44.5%, 83.3%, 114%, and the flexural modulus by 93.4%, 129.4%, 243.4%, respectively, as compared with the pure polyamide-12 sintered parts. Fractured surface analysis shows that the carbon fibres are encapsulated and bonded well with the polyamide matrix. The complex SLS parts with the thinnest wall of 0.6 mm, the density of 1.09 ± 0.02 g/cm³ and the relatively density of 94.13 ± 1.72% were manufactured using the CF/PA composite powder with 30 wt% carbon fibres. This study demonstrates that the CF/PA composite powders prepared by the surface treatment and dissolution-precipitation method represent suitable interfacial adhesion, filler dispersion, particle sizes and sintering behaviours for SLS and enable the manufacture of complex components with high performance.     

Investigation of structure,mechanical and tribological properties of short carbon fiber reinforced UHMWPE-matrix composites

Structural, mechanical and tribological properties of composite materials based on ultra-high molecular weight polyethylene reinforced with carbon fibers were investigated. The effect of surface modification of carbon fibers on the interaction at the fiber–matrix interface in UHMWPE based composites was studied. It was found that the thermal oxidation of carbon fibers by air oxygen at 500 °C can significantly enhance the interfacial interaction between the polymer matrix and carbon fibers. This allowed us to form composite materials with improved mechanical and tribological properties.     

Liquid crystalline elastomer actuators with dynamic covalent bonding: Synthesis,alignment, reprogrammability,and self-healing

Liquid crystalline elastomers (LCEs) have demonstrated tremendous potential in applications such as soft robotics, biomedical materials, electronics, sensors, and biomimetic systems. The physical properties of LCEs are controlled by the degree of crosslinking, nature of the mesogens, and mesogen orientation in the LCE network structure. A wide range of dynamic covalent bonds (DCBs) capable of dynamic bond exchange reactions (DBERs) have been introduced into LCE structures to obtain intelligent materials in recent decades. In this review article, we discuss the molecular constitution, macrostructure, morphing mechanism, recent advances in LCEs with dynamic covalent bonds, the influence of DCBs on self-healing, reprogramming and reprocessing properties of LCE actuators, and challenges and opportunities in incorporating dynamic chemistry in the field of LCE actuators.     

Development of aligned-hemp yarn-reinforced green composites with soy protein resin: Effect of pH on mechanical and interfacial properties

Unidirectional hemp yarn-reinforced green composites were fabricated with soy protein concentrate (SPC) resin processed at various pH values. To preserve the yarn alignment during the fabrication of green composites, hemp yarn was wound onto a metal frame with slight tension and precured SPC resin was applied to the yarns. Effects of pH values on the tensile properties of the SPC resin and hemp yarn/SPC resin interfacial shear strength (IFSS) were investigated. Increasing pH of the SPC resin from 7 to 12 decreased its fracture stress and Young’s modulus from 13.1 MPa and 357.5 MPa to 8.1 MPa and 156.2 MPa, respectively. At the same time fracture strain and moisture content increased from 31.5% and 15.65% to 53·4% and 19.30%, respectively, indicating resin plasticization. However, hemp yarn/SPC resin IFSS increased from 17.7 MPa at pH 7 up to 28.0 MPa at pH 10, after which it decreased. The fracture toughness of the composites increased up to pH of 10 but further increase in pH reduced the toughness. SEM photomicrographs showed fracture surfaces of hemp yarn-reinforced green composites that indicated better resin/fiber interaction at pH of 10 than 7 or 12.     

Dynamic mechanical and thermo-gravimetric analysis of Sansevieria cylindrica/polyester composite: Effect of fiber length,fiber loading and chemical treatment

The Sansevieria cylindrica (SC) fiber reinforced polyester matrix composites (SCFRPCs) were fabricated using compression molding machine. The influences of fiber length, fiber loading and chemical treatments of SCFRPCs over the mechanical and thermal stability were analyzed at different temperatures. The dynamic characteristics such as storage, loss modulus and damping were significantly influenced by the increase in fiber length and fiber loading but not in a geometric progression. Among various chemical treatments, the potassium permanganate treated SCFRPCs show the maximum increase in storage and loss modulus values. This result concluded that in addition to the reinforcing element (fiber length and wt% of fiber) the interfacial bonding between the fiber and the matrix plays a vital role in restricting the molecular mobility which was apparent from the storage modulus values. Efficient stress transfer at the interface is necessary to produce better dynamic properties rather than having more interfacial region. The change in morphology of cleaned and roughened SC fiber and the degree of interfacial adhesion between the fiber and matrix were studied using scanning electron microscope (SEM). The weight loss of SCFRPCs were also studied under varying temperatures with the help of thermo-gravimetric analysis (TGA).     

Mechanical properties of graphene and graphene-based nanocomposites

In this present review, the current status of the intrinsic mechanical properties of the graphene-family of materials along with the preparation and properties of bulk graphene-based nanocomposites is thoroughly examined. The usefulness of Raman spectroscopy for the characterization and study of the mechanical properties of graphene flakes and their composites is clearly exhibited. Furthermore, the preparation strategies of bulk graphene-based nanocomposites are discussed and the mechanical properties of nanocomposites reported in the literature are analysed. In particular, through the analyse of several hundred literature papers on graphene composites, we have found a unique correlation between the filler modulus, derived from the rule of mixtures, and the composite matrix. This correlation is found to hold true across a wide range of polymer matrices and thus suggests that the common assumption that the filler modulus is independent of the matric is incorrect, explaining the apparent under performance of graphene in some systems. The presence of graphene even at very low loadings can provide significant reinforcement to the final material, while the parameters that affect the nanocomposite strongly are thoroughly reviewed. Finally, the potential applications and future perspectives are discussed with regard to scale up capabilities and possible developments of graphene-based nanocomposite materials.