The buckling and nonlinear postbuckling analysis of toroidal shell segments with auxetic-core layer and Graphene-reinforced polymer coatings under torsional loads is reported in the present research. The functionally graded Graphene-reinforced polymer coatings are considered with three distribution laws, and the auxetic cores are designed in the honeycomb lattice forms. An auxetic homogenization technique is applied to establish the stiffness terms of the auxetic core. The combination of nonlinear von Karman Donnell shell theory and the Stain and McElman approximate is applied to formulate the nonlinear equilibrium equations of shells with the shallow longitudinal curvature considering the two-parameter foundation model. The deflection of shells is assumed to be a three-term form corresponding to the pre-buckling, linear and nonlinear postbuckling behaviors, and the Galerkin method is employed for three terms of defection. The torsion-deflection and torsion-twist angle postbuckling behaviors can be obtained in explicit forms. The numerical examinations validate the large effects of honeycomb lattice auxetic core, the functionally graded Graphene-reinforced polymer coatings, the parameters of shell’s geometric and foundation on the nonlinear buckling behaviors of shells.

Experimental and numerical studies have been carried out to predict the behavior of composite laminates under low-velocity impact loads. Numerical models were built based on shell elements in order to simulate it efficiently. A strain-based failure theory was employed to predict the damaged area of laminates using a user-defined subroutine. Drop-weight impact tests were performed for verifying the analysis results of the low-velocity impact. The damaged areas were inspected using ultrasonic C-scans, and three types of thick composite laminates were examined. The impactor velocity was measured at time before and after contact with the target. Finally, the proposed numerical model with the failure theory was verified in that it well predicted the impact damage of the three composite laminates chosen for this study.

In this study, high-purity cellulose nanofibrils (CNFs) were environmentally friendly extracted from natural bamboo. Physical, chemical, and mechanical treatments were comprehensively investigated, and then ensembled to effectively and efficiently remove lignin and hemicellulose with low energy from raw bamboo fibers, which combine microwave liquefaction, hydrogen peroxide-acetic acid delignification, alkaline treatment, and along with acid hydrolysis. This was able to allow for the extraction of CNFs. Compositional analysis, Fourier-transform infrared spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and thermogravimetric analysis were performed to investigate and confirm the successful removal of undesirable lignin and hemicellulose. The resultant cellulose fibers were then subjected to a mechanical treatment to obtain the cellulose nanofibrils. It was found that the obtained CNFs have an average diameter of 13 nm with a cellulose purity of 94.52%. Without any further surface modification, the incorporation of bamboo-based CNFs into natural rubber (NR) exhibits significant improvement in their mechanical properties including modulus (~66.67%), tensile strength (~120.83%), toughness (~139.34%), and elongation at break (~31.41%). Interestingly, there is no compromise between tensile strength and elongation at break for the nanocomposites. DMA results also showed that both storage and loss moduli were dramatically enhanced over those of neat NR in their glassy region, which is typically unusual. This study can show, if optimized for material extraction, the great promise to the potential use of natural materials to develop environmentally friendly composites with multi-functionality in a variety of real-world engineering structural applications.

In this work, a three-phase constitutive model describing the magneto-thermo-mechanical behaviors of porous Ferromagnetic Shape Memory Alloys (FSMAs) is established through a combination of micromechanical and thermodynamic theories. The kinetic equation is obtained in accordance with the thermodynamic driving force caused by the reduction of Gibb’s free energy of porous FSMAs. The thermodynamic driving force in combination with the corresponding resistance is used to establish the balance equation for calculating the volume fractions of martensitic variants under the action of different given magnetic fields, stresses and temperatures. Good agreement between the theoretical prediction of dense FSMA models and published experimental data is observed. The mechanical behaviors of porous FSMAs under magneto-thermo-mechanical coupling with different porosities and initial conditions are then discussed, which will provide a reliable theoretical basis for the future research of these functional materials as sensors or actuators.

Aramid nanofibers have recently been used as novel reinforcing materials to overcome the limitations of epoxy resin because of their superior strength and toughness. Until now, aramid nanofibers were hybridized into the epoxy resin by adding water into an aramid nanofiber solution to obtain free-standing powders. Unfortunately, such a water-based method for inducing the gelation hinders high-level dispersion due to the re-agglomeration of the nanofibers by protonation. This study presents a novel technique utilizing a solvent-exchange method to maximize the dispersion degree of aramid nanofibers, thereby increasing the mechanical properties of epoxy nanocomposites. The mechanical properties of aramid nanofiber-reinforced epoxy nanocomposites by the solvent-exchange method were improved significantly (3.4 GPa in Young’s modulus and 90.0 MPa in tensile strength – improvements of 28.8% and 27.7%, respectively) although a high viscosity epoxy resin (bisphenol-A type) was considered. Therefore, the suggested solvent-exchange method is an efficient manufacturing process to produce highly dispersed aramid nanofiber/epoxy nanocomposites.

Components made of fibre-reinforced polymers (FRP) may suffer from various types of damages, such as cracks or delamination. In order to monitor the structures’ material condition, non destructive testing (NDT) methods have emerged – among them, the use of ultrasonic waves. Inter alia, research focused on Lamb waves (LW), which offer promising conditions for NDT processes. However, prior implementation in a specific production environment, practitioners are faced with many open questions such as excitation frequencies to be used or which mode to select for defect identification. Therefore, the study aimed at demonstrating how a finite element analysis (FEA) can assist the experimental design of an LW driven NDT process. For that, a numerical model was developed, capable of computing LW propagations in an exemplary component: Stringer-stiffened C-FRP panels. The study was divided into two parts: In this part, all information regarding the model building is presented and principal functionality demonstrated. In the second part, the FEA is validated and then used to perform parameter studies, highlighting important conditions for practical application. The results of the present paper showed that stiffeners provoke varying interactions with LW modes – effects that increase wave field complexity but do not limit defect identification within or behind the stringer.

Glass fiber reinforced polymer (GFRP) composites are widely used in aviation, aerospace, sports and special vehicles because of their light weight, high wear resistance, good fatigue resistance and low thermal expansion coefficient. However, the anisotropy and low interlaminar strength easily lead to defects such as wire drawing, delamination and tearing during drilling process, which seriously restricts the development of GFRP composites. In this study, the effects of drilling tool material and geometrical parameters on the machining quality of GFRP composites in ultrasonic-assisted drilling are revealed through single-factor experiment. Unidirectional laminated GFRP epoxy plates of 80 × 50 × 5 mm are used in the experiment, and the thrust in the machining process is collected by a dynamometer. An industrial camera is used to record the exit morphology of the hole, and then MATLAB is used for image processing to convert it into a greyscale map. Finally, the size of the delamination factor Fd is obtained according to the equation. The experiment results indicate that using the carbide drilling bit with a diameter of 4 mm and a point angle of 120° is beneficial to reduce the thrust and suppress the machining defects.

Low fracture toughness is the Achilles heel of many polymers for practical application, particularly for long-term and special applications such as medical implants and critical part design. The toughening of polymers is a complicated process, and the toughening mechanisms in many composite systems have not yet been fully understood. Machine learning (ML) has emerged as a promising tool for modeling various complex systems in the field of materials science. In this respect, ML appears to be an effective, affordable, and accurate technique in the modeling of the toughening process. This review article provides an overview of the recent ML-based approach for analyzing toughening effects in rigid particulate-filled polymer composites. The results summarized here would contribute to further possibilities for researchers to explore emerging trends for designing toughing and high-performance polymers.

An applied electrochemical method in the form of a chronocoulometry technique is proposed for measuring Fick’s diffusion constant of a thin protective coating layer over the conductive base materials. Using this method, the capacitance is monitored for a capacitor formed using the base material, an electrolyte such as aqueous NaCl, and an insulating coating layer in between. The capacitance increases when the distance between the base material and the electrolyte decreases in the diffusion of the electrolyte through the insulating coating layer. Therefore, the change in the capacitance provides a diffusion rate related to Fick’s diffusion constant. Conventional gravimetric methods require a long measurement time to reach saturation, and thin coating layers of minimal weight increment through water diffusion were demonstrated to be inadequate for the methods. However, the new electrochemical method is appropriate for thin coating layers and requires a limited duration after the onset of diffusion. Experimental for 1-mm thick PA6 films provided Fick’s diffusion constants of safer side with the new method of 15h by reflecting the heterogeneous properties of the surface, while the conventional gravimetric method in 360h provided smaller or danger-side values.

Thermoplastic composite materials are typically used as feedstock filaments in material extrusion-based 3D printing. Understanding the mechanism of stress transfer at the filler/matrix interface in a composite filament helps in improving its properties. In this work, ABS/reduced graphene oxide (rGO) composite has been prepared by the solution mixing method, and 3D printable filaments are made by single screw extrusion. At filler loading of 0.2 wt. %, tensile strength of the composite filament is increased by ~40 % compared to the neat ABS filament. To understand the mechanism of stress transfer , a multi-scale mathematical model is presented. At the microscale, interface stress transfer is studied by using two-dimensional finite element (FE) analysis. The filler dimensions required for FE analysis are estimated using XRD and Raman analysis of rGO. In the present case, the filament making process yields filler dimensions comparable to the critical length, hence providing better stress transfer. The macroscopic strength of nanocomposite is estimated by using rule of mixtures. The better interface stress transfer and extrusion-induced filler orientation increases the filament strength. T.

Burst pressure is used as a safety design limit for a pressure vessel. This research aimed at finding burst pressure of three models of cylindrical pressure vessels, i.e. a) composite overwrapped pressure vessel (COPV) with an aluminum liner, b) aluminum pressure vessel with the same thickness as the liner of COPV, and c) aluminum pressure vessel with the same thickness as the total thickness of COPV. Non-linear finite element simulations,were used as a numerical tool to find stress, strain, and deformation of the cylinder. It was found that the aluminum material used as an aluminum liner of COPV has a higher burst pressure up to 55 MPa, while burst pressure of the other two aluminum cylinders only reaches 11.6 and 17.6 MPa. These results show good correlations with the theoretical calculation, which is calculated by using Faupel formulae and the Barlow Equation. In order to investigate the effect of using the CFRP layer on the severity of the crack defect in the aluminum liner, fracture analysis was held to find the stress intensity factor (SIF) along the crack front of a semielliptical crack. It was found that using CFRP layers, the stress intensity factor and energy release rate are reduced to 64.84% and 87.09%, respectively.

Recently, there has been significant interest in the development of high-performance electroactive ionic actuators that possess biocompatible, biodegradable, and bio-friendly characteristics for use in human-attachable electronics. However, research on biopolymer-based soft actuators has been relatively limited due to challenges such as the low ionic conductivity of biopolymers, difficulty in processing to achieve desired functionalities, and improper mechanical stiffness. Therefore, in this study, an electrospun silk membrane coated with sulfonated chitosan, in which membrane exhibits stable mechanical and electrochemical properties due to its interdigitated structures, is utilized to realize high-performance ionic biopolymer actuators. In particular, the introduction of sulfonated chitosan-ionic liquid on electrospun silk lowers the mechanical stiffness and increases the ionic conductivity, resulting in the provision of ion pathways through the interdigitated silk-chitosan domain. On bare electrospun silk membrane, the ionic electrospun silk-sulfonated chitosan actuators with graphene-PEDOT:PSS electrodes show a 165% increase in bending performance compared to those with only PEDOT:PSS electrodes. The developed biopolymer actuator, which shows substantial improvement in actuation performance, can be a promising candidate for use in skin-attachable and biomedical devices.

Lamb wave propagation must be understood comprehensively for simply evaluating delamination during ultrasonic testing. However, the difference between wave propagation, visualized using laser Doppler vibrometer and pulsed-laser scanners, has not been sufficiently investigated, and knowledge of optimal conditions for evaluating delamination is limited. Thus, in this study, the mode conversion and multiple reflections of Lamb waves propagating in a delaminated cross-ply laminate were visualized using different laser scanners, delamination depths, and wave incident angles. Delamination was characterized using maximum-amplitude map postprocessing under specific conditions. Further numerical analysis revealed that owing to multiple reflections of the antisymmetric mode in incident and mode-converted waves, standing waves were generated in the delaminated sublaminate. Dispersion curve and flexural stiffness calculations confirmed the conditions required for high-amplitude standing waves, thereby providing guidelines for simply and reliably evaluating delamination during inspections.

Carbon fiber/epoxy tow prepreg is an intermediate material for filament winding, consisting of fibers pre-impregnated with a matrix resin in a specific ratio. This study predicted the long-term viscoelastic properties of tow prepreg using DMA and TTSP. Typically, DMA involves applying a wound or laminated flat specimen, but a strand specimen was attempted to predict the characteristics of tow prepreg. The long-term viscoelastic properties were storage modulus, creep compliance, and relaxation modulus. Three test modes were performed in DMA to obtain each result. Smoothly overlapped master curves were obtained in all results. From the master curves, the viscoelastic properties of the material were predicted for 1 year, 10 years, and more extended periods. For 10 years, it was estimated that the storage modulus, creep compliance, and relaxation modulus changed by 7%, 132%, and 48%, respectively. When tests are conducted, considering future design variables, such as temperature, humidity, and out time, the results can be utilized to predict the long-term performance of tow prepreg and optimize composite manufacturing processes.

The mechanical property of the thin-ply composite are improved, which has more uniform microstructure and fewer manufacturing defects compared with the standard ply composite. Based on the micromechanics theory, an RVE analysis method is proposed considering the interlaminar resin region in this paper. The influence of the interlaminar resin region on the RVE model of thin-ply composites is studied. The initiation and propagation of cracks are discussed in the thin-ply laminate under transverse tensile, and the in-situ transverse strength of 90° ply is predicted with different thicknesses. The results show that considering the interlaminar resin region has a great influence on the in-situ strength prediction results. Reducing the ply thickness can change the fracture mechanism and improve the transverse in-situ strength of the material.

In this study, supercritical carbon dioxide (scCO2) was used to form a network structure of expanded graphite (EG) and multi-walled carbon nanotube (MWCNT) to achieve high thermal conductivity. Continuous processes, such as supercritical drying and rapid expansion of supercritical solutions were applied to pristine graphite and MWCNT. Polyamide 6 (PA6) nanocomposites with scCO2-treated EG and MWCNT were prepared by twin-screw extruder-based melt compounding. In the PA6 nanocomposites, the EG sheets were homogenously dispersed, while the MWCNTs were located between the EG sheets, thereby acting as a bridge for the EG. Subsequently, the 35 wt.% imbedding in PA6 nanocomposite had an effective heat transfer pathway and high thermal conductivity (2.12 W m−1 K−1). It was observed that supercritical fluid processing is a facile and effective strategy to improve the thermal conductivity of polymer nanocomposites.

This paper presents a progressive fatigue damage model to predict the damage progress and the fatigue life of composite laminates under cyclic loading. First, the maximum stress criterion was applied in the fiber direction for fiber failure (FF), and Puck’s failure criteria were employed in the matrix direction in which the fracture plane is defined to determine the inter-fiber fracture (IFF). Next, material degradation rules consisting of strength and stiffness degradation were derived, and different degradation rules according to the presence of failure and failure mode were utilized for each material. The proposed model was implemented into the UMAT subroutine of ABAQUS for the finite element (FE) analysis with tension–tension cyclic loading. Finally, the progressive fatigue damage model was validated with flat-bar specimens with various lay-ups ([0]8, [90]8, [30]16, [02/902]s, [0/902]s, [0/904]s) and compared with the experimental data of static and fatigue tests. The fatigue life prediction was also conducted on the pin-loaded quasi-isotropic (QI) laminates. The simulation results showed a good agreement with the experimental data and the ability to capture the damage progress of composite laminates during their lifetime.

A facile, safe, and scalable method for fabricating electrically conductive-reduced graphene oxide (rGO) e-fabric using a plasma-assisted coating process has been demonstrated. A wearable sensor based on rGO e-fabric was fabricated and used to detect and distinguish deadly chemical warfare agents (CWAs). The rGO e-fabric-based CWA sensor exhibited consistent responses upon repeated exposure to chemicals, including dimethyl methylphosphonate (DMMP) and soman (GD), which is a nerve agent. A difference in responses of the sensor toward GD and DMMP was observed. Theoretical studies further confirmed the potential of the as-prepared device as an e-fabric sensor for the detection of nerve agents. This novel functional e-fabric holds promise for future applications of wearable chemical agent sensors in smart personal protective suits.

Conventional micromechanical experiments have difficulties in measuring the normal strength of the fiber/matrix interphase. In this paper, molecular dynamics simulations were used to construct the interphase models from molecular scale and investigate the normal and tangential strength of the fiber/matrix interphase. Furthermore, a multiscale framework for composites considering interphase based on molecular dynamics and finite element method was constructed and validated by transverse fiber bundle tensile (TFBT) experiments used to indirectly measure the interfacial normal strength in the literature. The results showed that the interfacial normal and tangential strengths were 100.9 MPa and 51.9 MPa, respectively. The prediction error of the TFBT specimens by the newly constructed multiscale framework was less than 2%, and the failure mode was the same as that observed in the experiments. The good agreement between the experimental and simulation results indicates that the method can provide a way to study the structural analysis of fiber-reinforced composites.

To quantify carbon fiber-reinforced plastic (CFRP) fatigue, herein, we investigate the relationship between fatigue and an epoxy resin used in CFRPs. Generally, fatigue is related to the entropy, which comprises the mechanical entropy, calculated from the dissipated energy and temperature, and thermal entropy, calculated from the relationship between specific heat capacity and temperature. According to previous studies, mechanical entropy generation and thermal entropy generation are equal. Herein, 100 cyclic loading tests are conducted on epoxy resin specimens consisting of 4,4’-DDS and bisphenol a diglycidyl ether. The dissipated energy is determined based on stress – strain curves, and mechanical entropy generation is quantified. An equation for the relationship between the specific heat capacity and temperature is developed based on the Debye model, and the increase in specific heat capacity is calculated for equal mechanical and thermal entropy generations. Generally, differential scanning calorimetry is used for specific heat capacity measurements; however, because these measurements are performed by cutting the specimen, a nondestructive measurement method is required. In this study, the specific heat capacity is measured using lock-in thermography (LIT), and the measured and estimated values are comparable. Thus, fatigue can be estimated by quantifying the thermophysical properties, and the lock-in thermography method is a suitable thermophysical property measurement method for this application.