The influence of coastal weathering in tropical countries is a concern in terms of applications of wood-plastic composites (WPCs). Therefore, developing the WPCs into hybrid composites for increasing the resistance to the coastal climate needs further investigation. The current work studies the effects of different coastal climates (Gulf of Thailand and Andaman Sea), exposure times, and latex sludge contents on the properties of rubberwood-latex sludge flour reinforced with polypropylene hybrid composites. The hybrid composites were manufactured with a twin-screw extruder for mixing and a compression molding machine for forming. The results revealed that the hybrid composites weathered for 12 months significantly (α = 0.05) decreased the modulus of rupture, modulus of elasticity, screw withdrawal strength, and hardness with a maximum reduction of 218.6%, 207.4%, 84.2%, and 11.4%, respectively. However, adding the latex sludge flour of about 25 wt% increased crystallinity degree and thermal stability as compared with the WPCs filling 50 wt% rubberwood flour. The hybrid composites weathered under the Andaman Sea exhibited less loss of all the mechanical properties than that weathered under the Gulf of Thailand. It is therefore suggested that the hybrid composites added to the latex sludge waste have the potential to be used to produce the engineering products that were applied under the coastal climates.

Due to several advantages that Fiber Reinforced Polymer (FRP) composites offer in comparison with other strengthening techniques for steel structures, they have attracted considerable interest over the past three decades. However, the efficiency of FRP-strengthening for structures was found to be highly dependent on the quality and durability of the adhesive bonding between FRP and steel surface. This paper presents a state-of-the art review on the bond degradation in FRP-steel joints under different environmental conditions. Topics reviewed in this paper comprise the effect of different environmental conditions and their combination on bond durability, synergic effect of combining environmental condition with mechanical load, and the recent developments in the numerical simulation of aged bond. The paper concludes with the research gaps and uncertainties for further investigations.

Abstract not available

Eco-friendly surface treatment of natural fibers using sodium acetate (CH3COONa) affects the mechanical properties of the developed composites in many ways. In present study, geometrically different kenaf fiber mats (bidirectional (BC), unidirectional (UD) and randomly oriented (RO) were treated at different concentration (10, 15 and 20 percentage w/w) of sodium acetate aqueous solution for varying time (24, 48 and 72 hr.) at room temperature. PLA (Poly-Lactic Acid) was used for the fabrication of treated fiber reinforced bio-degradable composites. The influence of above parameters on mechanical properties were studied. Response surface methodology (RSM) module face centered central composite design was employed for the development of regression models. The relationship between chemical treatment parameters and mechanical responses were predicted by quadratic model. In this study, predicted model was developed for two numerical factors (chemical concentration (CC) and treatment time (TT)) and one categorical factor (type of mat (TOM)). Tensile strength (TS), flexural strength (FS) and impact strength (IS) are considered as response variables. The statistical analysis showed that chemical concentration, treatment time and kenaf mat type have individually and interactively influenced the response of experiments. Chemical concentration was found to be the most influencing factor among all for the changes in mechanical properties. Optimization of input variables was done based on predicted model within bounded reason of responses.

Owing to the excellent mechanical performance, the metal foam sandwich structures have potential applications in the design of protective equipment, which can improve the impact resistance of engineering structures. In this paper, the nonlinear finite element model is employed to investigate the dynamic behaviors of metal foam sandwich beam (MFSB), and the accuracy of the finite element method is verified by repeated impact tests. In addition, the effects of impact location and face thickness on the dynamic behavior of the MFSB are examined. Results show that, as the distance from the impact position to the middle span of the beam increases, the deflections of face sheets decrease gradually, and the loading and unloading stiffness increases. The thickness of front face sheet affects the depth of indentation, while the total thickness of the front and back face sheets decides the overall deformation, loading and unloading stiffness. Overall, as the impact number increases, the influences of impact location and face thickness on the dynamic behavior of MFSB enhances gradually.

In this work, we discuss the development of a bio-based polypropylene composite by using recycled polypropylene (rPP) and almond shell with hull as green phases. In the first step, a blend of rPP and virgin PP (vPP) was manufactured and evaluated. The melt flow index (MFI) of the blend was higher (∼20 g/10 min) as compared to the neat rPP (∼3 g/10 min). With the introduction of polyethylene-octene elastomer (POE), the impact properties improved dramatically from 29.5 J/m to 203 J/m while the MFI remained close to ∼20 g/10 min which is ideal for injection molding applications. A finely ground almond shell powder was introduced to produce the composites at 20 wt.% filler content. The particle-matrix interface was clearly improved by the addition of 3% maleic anhydride grafted polypropylene (MA-g-PP), as observed in the SEM images. The good particle-matrix interfacial adhesion also improved the tensile and impact strengths by 17% and 15%, respectively. Results obtained in these experiments prove that recycled PP-almond shell reinforced composites show promise for the preparation of alternative new green materials to those manufactured with 100% non-upcycled materials.

Carbon fiber composite adhesives joints provide higher specific strength and fatigue life than traditional joints. However, carbon fiber composites and adhesives are polymeric materials that may lose their integrity in hygrothermal environment. Predicting the environmental degradation behavior of composite joints is challenging because of limited understanding of the complex failure behavior. Moreover, the experiments are time-consuming and therefore it is essential to use accelerated aging methods such as the open faced method. However, the challenge is how to incorporate the accelerated fracture test data into the fracture prediction of real joints. To address this, a methodology is presented for predicting mode II cohesive law parameters of degraded CFRP epoxy adhesive joints using a direct method, and an accelerated aging test. End notched flexure (ENF) specimens made with CFRP laminate and epoxy adhesive were used to study the mode II fracture. Accelerated and uniform aging was achieved using open-faced specimens aged at 40 °C and 82% relative humidity (RH). Fracture testing of aged open-faced specimens along with crack-tip images were used to determine the degradation in trapezoidal traction-separation law (TSL) with aging. These variations in trapezoidal TSL with aging were used to develop a 3D finite element (FE) model of a closed ENF specimen that captures the non-uniform degradation across the specimen width. The TSL parameters degraded significantly at the specimen edges compared to width center for the closed ENF specimens. Validation of the FE model with aged closed ENF joint showed good agreement.

Carbon fiber reinforced polymer (CFRP) has been in high demand over the last few decades, especially in the aircraft manufacturing industry, due to their superior properties. However, drilling of CFRP materials is more complicated than common metal due to their exceptional anisotropic and non-homogenous features. This unique microstructure often leads to drilling-induced defects such as delamination, thermal damage and burr formation which compromises the structural integrity. Therefore, a good understanding of this composite material is required to realize its full potential. This review paper summarizes the knowledge available in literature on drilling of CFRP and their outcomes are compared comprehensively in a simple way. It covers key aspects such as drilling mechanism, cutting parameters and tool geometries for conventional drilling. Also, drilling-induced damages and potential solutions for those problems are addressed and covered in this paper

In automated carbon fiber-reinforced plastic (CFRP) lamination technologies, such as AFP, the design of the path for accurate tape application is challenging. Therefore, a molding technology that allows the use of variable tape widths and thicknesses, which are fixed in previous methods, is currently being developed. Fiber orientation and layer thickness should be optimized in a 2D plane. However, for structures with free-form surfaces, such as propellers, three-dimensional (3D) manufacturing design becomes necessary. A method is proposed here for optimizing the fiber orientation and layer thickness of a prepreg tape for a 3D propeller model with a curved surface structure. This method enables curve boundary-agnostic tape lines, by projecting curves created on a two-dimensional plane onto a curved surface. Multi-objective optimization with displacement and weight as objective functions reduced the displacement by 20.0% while maintaining strength. These results are likely to be informative for weight reduction and manufacturing design of CFRP structural components.

Fatigue and impact resistance are essential performance indicators in structural biocomposites. Integrating multilayer and oxygen-rich graphene oxide (GO) crystals as a fibre surface modification or reinforcing agent in polymer matrix systems have been shown to enhance the interfacial strength and toughness of natural fibre composites. However, the state-of-the-art literature on the GO-modification of composites has focused mainly on their microscale and quasi-static mechanical performance. Here, the fatigue testing results showed that surface modification of flax fibres with GO reduces the slope of the S-N curve by 17% and promotes fibre pull-outs upon failure. Based on the in-situ impact damage analysis, the GO-modification delayed the impact damage initiation and prolonged the stable damage progression phase. The impact perforation energy was similar for modified and unmodified specimens. At kinetic energies below the perforation limit, the GO-modification suppressed the extent of fibre failure and endowed flax-epoxy specimens with better damping performance.

Naval ships, military aircraft and land defence platforms are mostly constructed using steel, aluminium alloy, carbon fibre composite and/or glass fibre composite materials. Defence platforms are at risk from shock wave loads generated by explosive events, and therefore it is imperative that the construction materials are resistant to blast-induced deformation and damage. A comparative assessment is presented into the dynamic deformation and damage of metals and composites representative of naval construction materials when subjected to explosive air blasts. Flat plates of equal thickness (4 mm) or plates of similar areal density (in the range 6.2–9.2 kg/m2) made of the four materials were subjected to explosive blasts. Experimental blast testing and finite element (FE) modelling revealed that when the plate thickness was the same (4 mm) then the steel experienced less deformation (by ∼50–60%) and plasticity than the aluminium alloy due to its higher mechanical properties. The steel and aluminium plates were more resistance to blast-induced deformation (by up to ∼260–320% and ∼130–145% respectively) than the composite materials of the same thickness (4 mm). However, when the materials are compared on similar areal density, which is critical for lightweight design, the blast performance of the composite materials was similar to aluminium alloy and superior to steel. Deformation of the steel was up to 50% higher than the other construction materials, with the percentage increase rising with blast impulse.

Accurate and reliable modeling techniques are required to properly understand and predict manufacturing processes and the quality of the final product. The Automated Fiber Placement (AFP) process in its entirety is commonly difficult to predict due to the complex and interconnected phases of the manufacturing lifecycle. Currently, modeling within AFP utilizes physics-based models (PBM) to understand the design of a structure and its translation to a manufacturing plan. Although this is an excellent choice for simpler problems, adding in multi-scale or transient properties can render PBM incapable of detecting minute variations and hidden patterns. Data-driven models (DDM) can be employed to understand and utilize manufacturing and inspection related data which presents a clear and successful option in complex cases that are not easily represented by PBM. This paper outlines a systemic review of the use of PBMs and DDMs in AFP along with methods for their combination into hybrid models (HM). The review concludes with identifying gaps in current modeling techniques and demonstrating the efforts being undertaken to further advance modeling efforts for AFP manufacturing.

This work explored the importance of quantitative observation through imaging methods of optical and electron microscopies on the mechanical properties of particulate polymeric composites. Egg shell powder (ESP) reinforced polypropylene carbonate (PPC) polymeric composites with different filler weight percentage (wt.%) from 1 to 5 wt.% were considered. A cost-effective Image Analysis Software (IAS) was developed to extract black particles from the original optical images. During this process, the optimal image can be reproduced based on its originality by controlling the threshold values from 0.1 to 0.6 in real time situation. Using one-dimensional (1D) Gaussian distribution analysis, the authentication of the particle distribution data was studied and linked to the tensile strength of the composites. The mean value of the particle area collected from the left and right side of the scattered curves has a significant effect on the tensile strength of the composites. The proposed model was validated by comparing the predicted statistical results with the measured tensile strength for different wt.% of ESP composites. From the results obtained, a close agreement of 99% accuracy was observed between the experimental results and the proposed model for the tensile strength of the composites. The innovative study provides more practical and quantitative knowledge on improved particulate polymeric composites, in addition to the detection of failure processes through optical/electron microscopic examination of images. Evidently, the proposed cost effective, accurate and less stressful model can be employed by several composite-based industries to correlate the tensile strengths of particulate polymeric composites with their morphological properties.

This work unveils the linear aeroelastic flutter attributes of functionally graded triply periodic minimal surface (FG-TPMS) beams. The Euler-Bernoulli theory including neutral axis shift effect is used to model the FG-TPMS beams. The functional grading is achieved by varying the wall thickness of unit cells according to power-law form. Analysis is carried out for four TPMS patterns, mainly gyroid, primitive, diamond and IWP, under various boundary conditions. Using Hamilton's principle, governing differential equations are derived whose solutions are obtained numerically using the Ritz method. The mode shapes at various values of aerodynamic pressure have also been evaluated. It can be concluded that the type of pattern, boundary conditions, relative cell density, neutral axis shift effect and gradient index plays a crucial role in the prediction of flutter instability.

Virgin and recycled poly(lactic acid) (PLA) based nanocomposite materials were obtained and subjected to microstructural, thermal and mechanical analysis in view of fabricating efficient microwave absorbers. PLA was first exposed to artificial accelerated aging, next was mechanically recycled through grinding followed by reprocessing using melt extrusion and compression molding, resulting in recycled PLA samples (rPLA). Addition of organically modified montmorillonite (OMMT) as nanoclay was performed in a second melt extrusion process in order to obtain virgin and recycled PLA-OMMT nanocomposites. The impact of recycling process and presence of OMMT nanoclay in the host PLA matrix has been studied by FTIR, TGA and DSC, while the mechanical performance has been investigated by micro-hardness test. The dielectric properties were measured in the 26–40 GHz frequency range using a Vector Network Analyzer to assess the performance of virgin and recycled PLA and OMMT-PLA material as microwave absorbers. The FTIR results show that the recycling process generated more C = O groups in the polymer. These polar groups tend to orient themselves in the direction of the applied field and increase the dielectric constant (ε'). Measured electromagnetic absorption index revealed that rPLA-4OMMT with a thickness of 400 µm is able to absorb 20.3% on average of the spectrum with a peak of 36%, while 200 µm-thick films of rPLA-4 wt.% OMMT has a mean absorption index of 14.5%. The overall results show that mechanically recycled polymer can replace virgin polymer in this kind of applications.

Plant fibers are increasingly used in fabricating polymer composite components useful in the automotive, construction, and aerospace industries. This surge in the usage of plant fibers in different industries is owing to the improved understanding of the toxicity of synthetic fibers. It is essential to point out that “Humans need earth, not earth needs humans” therefore policymakers and researchers are working on replacing traditional materials with green materials. Plant fibers are green materials with many advantages over synthetic materials, such as easy processing, reduction of CO2 emissions, biodegradable, recyclable, good thermomechanical properties, and better compatibility with human health. Therefore, plant fibers are extensively used as a modifier for polymers. The drawbacks of plant fibers are the presence of OH groups in their basic structure and the presence of amorphous components. Both these drawbacks can be reduced by chemically treating the fibers. Further coupling agents can be used to increase the compatibility between the fiber and polymer. It is reported that incorporating fibers (non-continuous or continuous), and fiber mats as a reinforcement for polymers improve the mechanical, thermal resistance, thermal conductivity, and surface properties. Accelerated aging studies also reported favourable results for the use of plant fiber-based composites for long-term outdoor applications. However, plant fibers have lower strength and are hydrophilic compared to synthetic fibers, more research is required to overcome fully these drawbacks. This review examines and discusses the fundamentals of plant fiber, its processing, drawbacks, recent research trends, composites properties, prospects, and potential applications.

The orientation of cellulose microfibrils within plant fibres is one of the main factors influencing their mechanical properties. As plant fibres are more and more used as reinforcement for agro-composites, their mechanical properties have a strong influence on the final composite properties. It is, therefore, of interest to obtain reliable information about the microfibril angle (MFA) to better support the choice of fibres depending on the product requirements. In the present study, the reliability and specificities of three non-destructive methods that allow analysis on the same fibre glued on a holder; X-ray diffraction (XRD), second harmonic generation (SHG) and transmission ellipsometry (TE) microscopy; are investigated. Three types of plant fibres, with both low (nettle), and high (cotton, sisal) MFA values, are compared and their geometry and biochemical composition are characterised. The results obtained on the same fibre confirm that MFA analysis remains tedious and that despite their limitations, the methods are complementary depending on the information requested. Indeed, SHG is recommended for direct, qualitative and plane-selective mapping of heterogeneities in macrofibril orientations at various depths. However, reliable quantitative results with SHG depend on the initial image quality and could benefit from further image processing refinement. On the contrary, XRD and TE measure MFAs over the entire fibre thickness and provide variations along the fibres if a sufficient optical/spatial resolution is reached. Regarding the characterization of intrinsic defects in plant fibres, both SHG and TE suffer from uncertainties induced by the disorganization of the microfibril network and the lack of symmetry between the front and back fibre walls. Finally, all techniques prove to be dependant on the initial fibre alignment and geometry (i.e. twisting, double fibre configuration or form factor) which vary along the fibre length and should be carefully taken into account.

Graphene oxide addition to epoxy still presents challenges related to attaining optimized dispersion in scale-up production. Herein, the synthesis of high-quality GO by exfoliation in ethanol enabled the spontaneous phase transfer of GO from solvent to epoxy resin to produce a masterbatch with 1.5 wt.% GO content. Nanocomposites with 0.25 wt.%, 0.50 wt.% and 0.75 wt.% nanofiller contents were obtained by masterbatch dilution in epoxy resin. The results reveal the presence of a partial cross-linked network in these nanocomposites associated with the presence of GO, even without the use of a hardener. Thermogravimetric analysis showed the influence of GO on the oxidative decomposition mechanism of epoxy resin, whose behavior was assigned to strong interactions in the GO-epoxy interphase. Furthermore, the rheological behavior changes demonstrate the high quality of the GO dispersion in the resin obtained in this work. The nanocomposites showed high viscosity increases over time and pseudoplastic behavior, which was completely different from the neat epoxy resin. Aged nanocomposites without hardener present a first chemically cross-linked layer of epoxy to GO (∼ 10% for nanocomposite at 0.50 wt.% GO content) and a supramolecular coating interacting and entangled with these structures (> 40% of epoxy chains for EP-GO50).

In this paper we execute a complex test campaign to develop a novel methodology for the Remaining Useful Life (RUL) estimation of complex multi-stiffened composite aeronautical panels utilizing Machine Learning models trained with Structural Health Monitoring (SHM) data from hierarchically simpler elements, i.e., single-stiffened panels. Distributed Fiber Optical sensors (DFOS) are employed to monitor the panels’ behavior undergoing variable amplitude compression-compression fatigue after multiple impacts. A data processing methodology is first applied to the DFOS data, to both alleviate the effect of the variable loading conditions on the monitored strain and ease the computational burden. In this upscaling endeavor, an advanced strain-based Health Indicator (HI) based on Genetic algorithms, created and validated on the single-stiffened panel data, is utilized as the prognostic feature for the RUL estimations of the multi-stiffened panels. The HI displays favorable characteristics in terms of monotonicity and prognosability which are highly desirable for more accurate RUL estimations. For the prognostic task, standard machine learning models are trained using the historical degradation data of the single-stiffened panels and a similarity analysis is performed to enhance the accuracy when predicting the RUL of the multi-stiffened panels. Despite the increased structural complexity of the multi-stiffened panels, we demonstrate that the RUL is able to be predicted with reasonable accuracy. The present work paves the road for upscaling and applying prognostic methodologies to more complex structures beyond simple coupons or generic elements.

A new approach that utilizes Thermoelastic Stress Analysis (TSA) is proposed to investigate fatigue-induced material degradation in laminated fiber-reinforced polymer composites (FRP). The proposed model accounts for non-adiabatic conditions, the effects of the material temperature on the material properties, and the effects of stiffness material degradation due to damage. Experimental data from the literature is used to validate the part of the model that simulates the heat transfer, which results in a non-adiabatic contribution to the thermoelastic response. Specimens made from E-glass FRP representative of those used in wind turbine blade manufacture are used in the study, which make a challenging proposition for TSA. The evolution of tunneling cracks caused by cyclic loading causes stiffness degradation and changes in the thermoelastic response. The added features of the proposed model are shown to be necessary to interpret the thermoelastic response. The model improves correspondence with experimental data compared to previous TSA methods. Hence a generalized framework is proposed for incorporating the mechanisms that affect the thermoelastic response as materials degrade due to fatigue loading.