Silane coupling agents used for natural fiber/polymer composites: A review

Natural fiber reinforced polymer composites (NFPCs) provide the customers with more alternatives in the material market due to their unique advantages. Poor fiber–matrix interfacial adhesion may, however, negatively affect the physical and mechanical properties of the resulting composites due to the surface incompatibility between hydrophilic natural fibers and non-polar polymers (thermoplastics and thermosets). A variety of silanes (mostly trialkoxysilanes) have been applied as coupling agents in the NFPCs to promote interfacial adhesion and improve the properties of composites. This paper reviews the recent progress in using silane coupling agents for NFPCs, summarizes the effective silane structures from the silane family, clarifies the interaction mechanisms between natural fibers and polymer matrices, and presents the effects of silane treatments on the mechanical and outdoor performance of the resulting composites.     

Quantitative non-destructive characterization of damage in graphite-epoxy composites

We investigated the effect of controlled damage on the residual strength, failure mechanism and acoustic emission (AE) activity of graphite epoxy composites. The controlled damage was introduced by indenting unidirectional tensile specimens using a hardness machine with a diamond indentor. The specimens were loaded parallel to the fiber direction. The damage progression mechanisms operating upon subsequent loading and the failure modes were found to be different for sound and indented specimens. The difference in mechanical behavior affected also the AE, as displayed by the statistical moments of the peak amplitude distribution. Specifically, the variation of moment values with stress level were found to be different for the two groups. This difference can be used to sort sound from defective material during loading. In addition, this approach can in future be used as a means to ratify the soundness of structures, if a reliable database can be generated.     

Coupled thermal–mechanical modeling of carbon fibers reinforced polymer composites for damage detection

An analytical solution is derived to describe the correlation between the thermal resistance change and fiber damage evolution in unidirectional composites under loading conditions. A key parameter, thermal characteristic length, is obtained, which represents the sensitivity of the thermal property change to mechanical damage. A coupled thermal–mechanical model is also developed to predict the failure stress, internal fiber breakage and thermal resistance change as a function of applied strain. The results show that the number of fiber breaks is proportional to thermal resistance change during loading while the thermal resistance change increases exponentially with increasing the applied strain. The analytical solution is in good agreement with the numerical results. Finite element models are developed to verify the coupled thermal–mechanical models. The present study shows that longitudinal thermal resistance change is sensitive to damage in a predictable manner and can be used together to improve the reliability of damage assessment during loading of carbon fiber reinforced polymer.     

A micromechanics approach for damage modeling of polymer matrix composites

A new model for damage evolution in polymer matrix composites is presented. The model is based on a combination of two constituent-level models and an interphase model. This approach reduces the number of empirical parameters since the two constituent-level models are formulated for isotropic materials, namely fiber and matrix. Decomposition of the state variables down to the micro-scale is accomplished by micromechanics. Phenomenological damage evolution models are then postulated for each constituent. Determination of material parameters is made from available experimental data. The required experimental data can be obtained with standard tests. Comparison between model predictions and additional experimental data is presented.     

A review of particulate reinforcement theories for polymer composites

A concerted survey is presented of the existing theories for predicting the strength and modulus of particulate-filled polymeric composites. The macroscopic behaviour of particulate composites is affected by the size, shape, and the distribution of the inclusions. The interfacial adhesion between the matrix and inclusion is also important. The limitation of theoretical models in describing these parameters and expressing the experimental data on the macroscopic behaviour is demonstrated.     

In situ assessment of structural timber using non-destructive techniques

This paper summarizes the test recommendations for selected non-destructive testing (NDT) techniques as developed by members of the RILEM Technical Committee AST 215 “In situ assessment of structural timber”. The recommendations cover visual inspection, moisture content determination, species identification, digital radioscopy, and ground penetrating radar. The paper includes a matrix of common NDT to assess structural timber. The discussion of each technique is intended to provide users with sufficient information to understand the theoretical basis, typical equipment set up, and basic capabilities and limitations.     

A review of processing techniques for graphene-reinforced metal matrix composites

In the quest to enhance reinforcement efficiency of graphene in metal matrices, various processing techniques have been devised over recent years. As the advancement in this field nowhere seems to slow down, the processing aspects of graphene-reinforced metal matrix composites are becoming more relevant than ever. In that premise, there lies an imminent need for a critical assessment of existing fabrication routes and their ability to extend a solution for the primary challenges of agglomeration, dispersion, interfacial interaction and structural integrity of graphene in metal matrix composites. This review presents a brief yet a meaningful insight to the processing techniques for graphene-reinforced metal matrix composites, while highlighting the key findings from individual studies, thereby expressing the primitive challenges and strengthens of these techniques. A critical evaluation of state of the art is presented alongside an inclusive review of improvement in mechanical, thermal, and electrical properties of composites fabricated by various processing routes. In the consideration of reviewed literature, it is established that a comprehensive processing strategy with a potential to simultaneously address all of the key processing challenges of graphene, is yet to nurture. Conclusively, future road map and a potential solution encompassing hybrid processing strategies, is opined.     

Time-dependent damage in carbon fibre-reinforced polymer composites

Time-dependent damage (matrix cracks) evolution in AS4/3501-6 cross-ply laminates was studied using constant strain rate and constant stress tests. First ply failure stress and strain as well as the matrix crack density at a given stress level were found to be strongly dependent on strain rate. Matrix crack density increased with creep time at a constant stress level. The compliance and creep rate of the laminate increased in the presence of these cracks. These results emphasize the importance of the knowledge of time-dependent damage evolution in a lamina/laminate of a polymer composite for reliable prediction of creep and creep rupture.     

A review of the effect of stitching on the in-plane mechanical properties of fibre-reinforced polymer composites

This paper reviews over fifty studies into the effect of through-the-thickness stitching on the in-plane mechanical properties of fibre-reinforced polymer composites. Reviewed are the in-plane tensile, compressive, flexure, interlaminar shear, creep, fracture and fatigue properties, although little work has been undertaken on the last three properties. When comparing studies it is apparent that many contradictions exist: some studies reveal that stitching does not affect or may improve slightly the in-plane properties while others find that the properties are degraded. In reviewing these studies it is demonstrated that predicting the influence of stitching on the in-plane properties is difficult because it is governed by a variety of factors, including the type of composite (eg. type of fibre, resin, lay-up configuration), the stitching conditions (eg. type of thread, stitch pattern, stitch density, stitch tension, thread diameter), and the loading condition. The implications of these findings for the use of stitching in lightweight engineering structures are discussed.     

Post-fire mechanical properties of marine polymer composites

Changes to the tensile and flexure properties of marine-grade glass-reinforced polyester, vinyl ester and resole phenolic composites after exposure to radiant heat are investigated. The properties were determined at room temperature after the composites had been exposed to heat fluxes of 25–100 kW/m² for 325 s or to a heat flux of 50 kW/m² for increasing times up to 1800 s. The stiffness and failure load of all three composites decreased rapidly with increasing heat flux or time due mainly to the thermal degradation of the resin matrix. The post-fire tension and flexure properties of the resole phenolic composite were similar to the properties of the other composites, despite its superior fire resistance. Models are presented for determining the post-fire mechanical properties of fire-damaged composites, and are used to estimate the reductions in failure load of composite ship materials caused by fire.     

Thermoelastic assessment of damage growth in composites

This paper examines, experimentally and numerically, the use of thermal emission measurements as a means of assessing severity of damage and monitoring damage growth in composite materials. In contrast to most traditional methods the thermal emission profile reflects the interaction of load, geometry, material and damage in a non-destructive fashion. It represents a possible method for the scaling of test data, obtained from coupon tests, to tests on full scale structures.     

A review of shape memory polymer composites and blends

Shape memory polymers (SMPs) are a kind of very important smart polymers. In order to improve the properties or obtain new functions of SMPs, SMP composites and blends are prepared. We thoroughly examine the research in SMP composites and blends achieved by numerous research groups around the world. The preparation of SMPs composites and blends is mainly for five aims: (1) to improve shape recovery stress and mechanical properties; (2) to decrease shape recovery induction time by increasing thermal conductivity; (3) to create new polymer/polymer blends with shape-memory effect (SME); (4) to tune switch temperature, mechanical properties, and biomedical properties of SMPs; (5) to fabricate shape memory materials sensitive to electricity, magnetic, light and moisture. The trend of SMP composite development is discussed. SMP composites and blends exhibit novel properties that are different from the conventional SMPs and thus can be utilized in various applications.     

Synergy in hybrid polymer/nanocarbon composites. A review

The aim of this review article is to report the most recent developments in the understanding of and beliefs about the properties of polymer hybrid composites that are reinforced with various combinations of nanometer-sized carbon and mineral fillers. The discussions are primarily focused on an analysis and comparison of the electrical, thermal, and mechanical properties. It is shown that the introduction of a mixed (hybrid) system of filler nanoparticles into polymer matrices enhances the macro- and microproperties of the composites as a result of the synergistic interactions between the fillers and the simultaneous creation of a unique filler network in the polymer. The synergy of various types of carbon nanofillers and combinations of nanocarbon materials with inorganic fillers manifests itself as modifications of most of the properties of hybrid polymer composites relative to the properties of a polymer system containing a single filler. The reinforcing effect is related to the structure and particle geometry of the hybrid fillers, the interactions between the fillers, the concentrations and the processing methods.The existence of synergy between different types of carbon nanofillers, as well as with mineral fillers, shows great potential and could significantly increase applications of carbon-based nanomaterials.     

A short review on basalt fiber reinforced polymer composites

A recent increase in the use of ecofriendly, natural fibers as reinforcement for the fabrication of lightweight, low cost polymer composites can be seen globally. One such material of interest currently being extensively used is basalt fiber, which is cost-effective and offers exceptional properties over glass fibers. The prominent advantages of these composites include high specific mechano-physico-chemical properties, biodegradability, and non-abrasive qualities to name a few. This article presents a short review on basalt fibers used as a reinforcement material for composites and discusses them as an alternative to the use of glass fibers. The paper also discusses the basics of basalt chemistry and its classification. Apart from this, an attempt to showcase the increasing trend in research publications and activity in the area of basalt fibers is also covered. Further sections discuss the improvement in mechanical, thermal and chemical resistant properties achieved for applications in specific industries.     

Artificial damage techniques for low velocity impact in carbon fibre composites

Materials and techniques used in artificially induced damage are evaluated and compared. The damage induced by low velocity impacts is characterised using penetrant enhanced radiography and deply techniques. This information is used to construct specimens with simulated damage that will facilitate the study of damage growth. Specimens manufactured match quite closely the properties of specimens which contain actual damage.     

Single polymer composites: a review

Increasing concern for the environment has stimulated interest in the research and development of single polymer composite (SPC) materials. These materials are an emerging class of composite materials with mechanical properties comparable to heterogeneous composites. The SPCs are fully recyclable and have specific economic and environmental benefits. The small melting temperature difference between the fiber and the matrix poses a great challenge in the fabrication of SPCs. This article gives an overview of developments in SPCs relating to the materials and fabrication methods used.     

Tribological performance of polymer composites used in electrical engineering applications

Sliding wear performance of 20% mica-filled polyamide 6 (PA6 + 20% mica) and 20% short glass fibre-reinforced polysulphone (PSU + 20 GFR) polymer composites used in electrical applications were investigated using a pin-on-disc wear test apparatus. Two different disc materials were used in this study. These are AISI 316 L stainless steel and 30% glass fibre-reinforced polyphenylenesulphide (PPS + 30%GFR) polymer composite. Wear test was carried out at 10, 20 and 30 N applied load values and 0·5 m/s sliding speed and at ambient temperature and humidity. Different combinations of rubbing surfaces were examined and friction coefficient and specific wear rate values were obtained and compared. For two material combinations used in this investigation, the coefficient of friction shows insignificant sensitivity to applied load values and large sensitivity to material combinations. For specific wear rate, PA6 + 20% mica composite has shown insensitivity to change in load, speed and materials combination while PSU + 20% glass fibre composite has shown high sensitivity to the change in load and material combinations. The friction coefficient of PA6 + 20% mica and PSU + 20 glass fibre rubbing against the AISI 4140 steel disc is between 0·35 and 0·40. In rubbing against PPS + 30% glass fibre their values were between 0·25 and 0·30. Specific wear rate for PA6 + 20% mica and PSU + 20% glass fibre composites are in the order of 10^???13 to 10^???14 m²/N. Finally, the wear mechanisms are a combination of adhesive and abrasive wear processes. In terms of application, especially in electrical systems, a substantial contribution was provided to extend switch life. Thus, besides robustness, this also ensured safety for the system and the users against undesirable situations.     

A non-destructive X-ray microtomography approach for measuring fibre length in short-fibre composites

An improved method based on X-ray microtomography is developed for estimating fibre length distribution of short-fibre composite materials. In particular, a new method is proposed for correcting the biasing effects caused by the finite sample size as defined by the limited field of view of the tomographic devices. The method is first tested for computer generated fibre data and then applied in analyzing the fibre length distribution in three different types of wood fibre reinforced composite materials. The results were compared with those obtained by an independent method based on manual registration of fibres in images from a light microscope. The method can be applied in quality control and in verifying the effects of processing parameters on the fibre length and on the relevant mechanical properties of short fibre composite materials, e.g. stiffness, strength and fracture toughness.     

Thermo-mechanical damage modeling of polymer matrix sandwich composites in fire

The objective of this research is to develop a modeling and simulation approach for predicting the thermo-mechanical damage of composite materials subjected to fire environments. A 3D thermal damage model is developed for glass-reinforced polymer composite materials subject to high temperature and radiative environments. Homogenization methods are used to formulate the damaged material in terms of fiber, resin and char. The thermal damage model is implemented in Abaqus via an overlaid element approach. The solution of the mechanical response uses the existing functions in Abaqus for large-displacement analysis. Composite sandwich panels with balsa core are examined. Reasonable agreement in temperature is obtained between predictions and available experimental data. For the sandwich panels, delamination failure is predicted at the sandwich interface – consistent with the experiments. Comparisons of time-to-failure of the sandwich panel show the predictions are reasonable.