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The occurrence of peak-split or distortion in the reflected light of fiber Bragg grating (FBG) sensors with metallic coatings embedded in composites is inevitable during the curing process, regardless of protection layers. In this study, we present a comprehensive methodology to numerically predict the reflected spectrum of metallic-coated FBG sensors, considering the process-induced residual stress in carbon fiber/epoxy composites. The finite element analysis was utilized to simulate the residual stress, which primarily arises from mechanical, thermal, and chemical cure mechanisms of the composites, including the thermosetting resin. Subsequently, the reflected spectra were calculated using the coupled mode theory. Contrary to common expectations, our findings indicate that the coating thickness has minimal influence on the reflected spectrum, while the residual stress and embedding position significantly impact it. By employing this proposed methodology, the number of experimental trials can be reduced, enabling the development of robust structural and state monitoring systems for composites using metallic-coated FBG sensors. [ABSTRACT FROM AUTHOR]

A novel thermo-chemical–mechanical analysis of the pultrusion process is presented. A process simulation is performed for an industrially pultruded rectangular hollow profile containing both unidirectional (UD) roving and continuous filament mat (CFM) layers. The reinforcements are impregnated with a commercial polyester resin mixture (Atlac 382). The reactivity of the resin is obtained from gel tests performed by the pultruder. The cure kinetics parameters are estimated from a fitting procedure against the measured temperature. The cure hardening instantaneous linear elastic (CHILE) model is adopted for the evolution of the resin elastic modulus using the temperature-dependent elastic response provided by the resin supplier. The numerical model predictions for the warpage trend at the end of the process are found to agree well with the warpage observed in the real pultruded products. In addition, the calculated warpage magnitude is found to be in the measured range of warpage magnitude for the manufactured part. [ABSTRACT FROM AUTHOR]

Thermal curing is a common practice to manufacture high temperature thermosetting matrix composites, in order to improve the mechanical properties of the final product. The cycle design i.e. the definition and optimization of the temperature–time curve is a key issue for a competitive production. In this framework, a suitable model describing the composite temperature and degree of cure variations versus the imposed thermal cycle is highly desirable. An effective procedure, based on the coupling of a finite element thermochemical model of the process and an artificial neural network, is herein proposed and tested. Obtained outcomes highlight the remarkable capabilities of the implemented procedure in terms of reliability of temperature and degree of cure predictions. [ABSTRACT FROM AUTHOR]

Abstract: This study investigated the dual effect of freeze–thaw cycles with sodium sulfate solution on the performance of non-air-entrained Engineering Cementitious Composites (ECCs) with high volumes of slag. ECC specimens containing three different levels of slag content as a replacement for cement (55%, 69% and 81% by weight of total cementitious material) were exposed to aggressive sodium sulfate solution under freezing–thawing cycles. The load–deflection response associated with ultimate mid-span deflection and flexural strength/stiffness was determined, along with crack development behavior. For comparison purposes, the freezing–thawing resistance (in water) of control ECC specimens was also evaluated. Modified point count method air-void parameters, compressive strength, porosity, water absorption and sorptivity tests were also conducted on the virgin ECC specimens (those not exposed to freezing–thawing cycles in water or aggressive sodium sulfate solution). The test results for the virgin specimens revealed that increased slag content (S/PC) improved the ductility, hardened air content, water absorption, porosity and sorptivity of ECC, while marginally decreasing the compressive and flexural strengths. Freeze–thaw cycles in water or sodium sulfate solution showed that the ductility of ECC specimens decreased remarkably, irrespective of slag content and applied freezing–thawing process. Reduction in mass loss was at minimal levels and no significant behavior change was monitored between the specimens undergoing freeze–thaw cycling in water and the aggressive sodium sulfate solution. Moreover, the decrease in flexural stiffness was more evident than the reduction of the flexural strength for all ECC mixtures. [Copyright &y& Elsevier]

Abstract: In the present study the control volume based finite difference (CV/FD) method is utilized to perform thermo-chemical simulation of the pultrusion process of a composite rod. Preliminary, the model is applied for a simple setup without die and heaters and the results match well with those obtained experimentally in the literature. In order to study the effects of the thermal contact resistance (TCR), which can also be expressed by the heat transfer coefficient (HTC), on the pultrusion process, a cylindrical die block and heaters are added to the original problem domain. The significance of using the TCR in the numerical model is investigated by comparing constant and variable TCR (i.e. position dependent) at the interface. Results show that the use of a variable TCR is more reliable than the use of a constant TCR for simulation of the process. [Copyright &y& Elsevier]

Abstract: The aim of this work is the study of a mixed method used for the joining of aluminum alloys with glass reinforced polymer’s substrates (in the next GFRP). In particular, the technique of self-piercing riveting (in the next SPR) was applied on a co-cured joint in order to evaluate the influence of the time of inserting the rivet on the mechanical behavior of the mixed joints. Three different joints were realized: adhesive by co-curing technique, mechanical by self piercing riveting (in the next SPR) and a mixed one in which the joining techniques (i.e. adhesive and mechanical) were combined. In particular, to determine the optimum time to insert the rivet, three different times from the beginning of the curing reaction of the resin (i.e. after 2h, 5h and 24h from the moment of mixing resin with its hardener, respectively) were chosen to insert the rivets in the co-cured adhesive joints. Moreover, for each condition, the oil pressure of the riveting system needed to optimize the performances of the joints was investigated. To determinate the times of riveting to be investigated, the curing process of the epoxy resin was previously analyzed by means of a differential scanning calorimetry (DSC) and a stress controlled rheometer. All the realized samples (i.e. five for each configuration) were characterized by single lap joint tests. The experimental results, supported by the ANOVA test, prove that the time at which the rivet is inserted is a critical parameter as it greatly influences the performances of the joints. In particular the mixed joints show higher resistance than the adhesive ones only if the rivet is inserted when the resin is in its fluid state (i.e. after 2h) with the oil pressure equal to 180bar or if the rivet is inserted when the resin is in its solid state (i.e. after 24h), with the oil pressure equal to 280bar. Vice versa if the rivet is inserted when the resin is in its rubber state (after 5h), the resistance of the mixed joints is lower than the adhesive ones regardless of the value of the oil pressure. [Copyright &y& Elsevier]

Abstract: Thermal behavior of some diglycidyl ether of bisphenol A/p-aminobenzoic acid systems with calcium carbonate as a filler were studied. The kinetics parameters of the curing reactions were estimated using the variable peak exotherm method of Kissinger and Ozawa. The most probable kinetic model and the kinetic parameters of the degradation process were also estimated. The energies of crosslinking reactions are situated in the range of 46–57kJmol−1 for the first exotherm and between 68 and 72kJmol−1 for the second exotherm. The cured resins have a good thermal stability and the activation energies of degradation reactions are situated in the range of 178–218kJmol−1. [Copyright &y& Elsevier]

Abstract: Polyether imides (PEI)/silica nanocomposites, prepared by sol–gel process, were used to modify the epoxy resin (ER), and the effect of silica particles on reaction-induced phase separation and mechanical properties of these systems were investigated. SEM images of the fracture surface of ER/PEI/silica composites showed an interesting morphology transformation with the increase of silica particle content. SEM–EDX results indicated that silica particles once formed in the PEI gradually migrated and concentrated in epoxy-rich region during the phase separation because of the better affinity between silica particles and epoxy resin. FTIR measurement and rheological test confirmed that the silica particles make the polymerization reaction of epoxy faster and the dynamic DSC results demonstrated that the activation energy of these systems decreased with the increase of the silica particles. Mechanical measurements approved that the introducing of PEI/silica nanocomposites into the epoxy could lead to great improvement of the impact strength and storage module. [Copyright &y& Elsevier]

Abstract: The development of smart composites capable of self-repair on aeronautical structures is still at the planning stage owing to complex issues to overcome. A very important issue to solve concerns the components’ stability of the proposed composites which are compromised at the cure temperatures necessary for good performance of the composite. In this work we analyzed the possibility to apply Hoveyda–Grubbs’ second generation catalyst (HG2) to develop self-healing systems. Our experimental results have shown critical issues in the use of epoxy precursors in conjunction with Hoveyda–Grubbs’ II metathesis catalyst. However, an appropriate curing cycle of the self-healing mixture permits to overcome the critical issues making possible high temperatures for the curing process without deactivating self-repair activity. [ABSTRACT FROM AUTHOR]

For manufacturing of aerospace composites, several parts may be processed simultaneously using convective heating in an autoclave. Due to uncertainties including tool placement, convective Boundary Conditions (BCs) vary in each run. As a result, temperature histories in some of the parts may not conform to process specifications due to under-curing or over-heating. Thermochemical analysis using Finite Element (FE) simulations are typically conducted prior to fabrication based on assumed range of BCs. This, however, introduces unnecessary constraints on the design. To monitor the process, thermocouples (TCs) are placed under tools near critical locations. The TC data may be used to back-calculate BCs using trial-and-error FE analysis. However, since the inverse heat transfer problem is ill-posed, many solutions are obtained for given TC data. In this study, a novel machine learning (ML) framework is presented capable of optimizing air temperature cycle in real-time based on TC data from multiple parts, for active control of manufacturing. The framework consists of two recurrent Neural Networks (NN) for inverse modeling of the ill-posed curing problem at the speed of 300 simulations/second, and a classification NN for multi-objective optimization of the air temperature at the speed of 35,000 simulations/second. A virtual demonstration of the framework for process optimization of three composite parts with data from three TCs is presented. [ABSTRACT FROM AUTHOR]

One-component epoxy adhesives, in which the epoxy resin and curing agent are premixed, have many commercial advantages including reduced working time and stable performance. However, these adhesives suffer from a short shelf life even at room temperature. Here, core-shell structured curing agents were prepared via a dry particle coating (DPC) process that improved the storage stability of one-component epoxy adhesives. The DPC process is a simple, economic, and solvent-free method to fabricate core-shell structured materials using mechanical forces. Graphene nanoplatelets (GNPs) were used as encapsulating materials due to their high thermal conductivity and large surface areas. With the GNP-encapsulated curing agents, a one-component epoxy adhesive displayed significantly enhanced storage stability while maintaining its fast curing behavior. In particular, the pot life of the adhesive increased to 60 days, which is over two-times longer than that of a pristine epoxy adhesive. The curing temperature increased by up to 5.6 °C, which is a smaller increase than that observed when non-thermally conductive materials were used for encapsulation. Furthermore, the GNPs provided reinforcement in the cured epoxy adhesive, thereby improving the lap shear strength by 20–30%. • A core-shell latent curing agent was prepared via a dry particle coating process. • The dry particle coating process is a simple, solvent-free, and economic method. • GNP shell layers enhanced storage stability while kept the fast curing kinetics. • Lap shear strength was enhanced due to better dispersion of the GNPs in epoxy resin. [ABSTRACT FROM AUTHOR]