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The main objective of this study is to investigate the effect of curing temperature on the mechanical properties of hemp fiber reinforced polymer (HFRP) composites. In this study, hemp fiber was considered as a reinforcement and epoxy resin with hardener as a matrix. The mercerization process was used to remove contaminants from the fiber surface. The overall volumetric fraction used was 40% fiber and 60% matrix. One composite sample was cured at open temperature (27 ± 3 °C) for 15 days and others at elevated temperatures such as 80, 120 and 160 °C for various durations such as 1, 2, and 3 h (h). Different mechanical tests were performed under ASTM standards to examine the mechanical performance of HFRP composites. The tensile and compressive properties such as Young's modulus, tensile and compressive strengths are higher in the specimens cured at 120 °C for 3-h. The flexural strength and stiffness are maximum in the composites cured at 80 °C for 3-h. The maximum interlaminar shear strength representing the composite's resistance to delamination is observed at 120 °C for 3-h curing condition. The Fourier transform infrared (FTIR) study identifies that the chemical treatment has reduced the impurities present on the fiber surface. Scanning electron microscopic (SEM) studies indicate better fiber/matrix adhesion in specimens cured at elevated temperatures than those cured at open temperature. Finally, this study emphasizes that curing at elevated temperature (120 °C for 3-h) improves the mechanical performance of the NFRP composites. [ABSTRACT FROM AUTHOR]

Pultrusion is a high-volume manufacturing process for Fibre-Reinforced Polymer (FRP) composites. It requires careful tuning and optimisation of process parameters to obtain the maximum production rate. The present work focuses on the correlation between the set die temperatures of 80 °C, 100 °C, 120 °C, and 140 °C and the resin cure state at constant pull speeds. Lab-scale oven trials were conducted to understand the thermal behaviour of the resin system and to provide a temperature range for the pultrusion trials. Dielectric Analysis (DEA) was used during pultrusion trials to monitor the effect of die temperature on the cure progression. The DEA results showed that, by increasing die temperature, the exothermic peak shifts closer towards the die entry. Moreover, the degree of cure for samples processed at 140 °C was 97.7%, in comparison to 86.2% for those cured at 100 °C. The rate of conversion and the degree of cure correspond directly to the set die temperatures of the pultrusion trials, contributing to understanding the effect of die temperature on cure progression. Mechanical and thermal material properties were measured. Samples cured at 120 °C showed the highest mechanical performance, exceeding those cured at 140 °C, linked to the generation of higher internal stress due to the higher rate of conversion. This work can be used as a guide for pultruded composite sections, to understand the cure behaviour of resin systems under various applied temperatures and the impact of the die temperature conditions on thermal and mechanical properties. [ABSTRACT FROM AUTHOR]

Current research intended the fabrication of highly durable conducting rubber products of low cost and wide applicability in the field of electronics. Chlorinated natural rubber (Cl-NR) reinforced with copper alumina (Cu-Al2O3) nanocomposites were fabricated by a solvent-free and economically viable industrial compounding technique with special attention to structural, morphological, rheometric curing behaviour, flame retardancy, electrical conductivity, dielectric, thermal and mechanical properties. FT-IR spectra confirmed the presence of Cu-Al2O3 in Cl-NR. The XRD investigation showed the crystalline peaks of Cu-Al2O3 in Cl-NR nanocomposites. The SEM and TEM results indicated that the nanoparticles were uniformly dispersed into the chlorinated rubber in the nano regime. The DSC results indicated that the addition of nanoparticles into the rubber matrix increased the glass transition temperature of the composites. The conductive metal oxide particles in the rubber accelerate the vulcanization process, which could be more beneficial in the industrial sector. The AC conductivity, dielectric properties, tensile strength, modulus, tear strength, heat build-up and hardness of nanocomposites were greatly increased, whereas the elongation at break, abrasion loss and resilience were decreased with the addition of nanoparticles to the chlorinated natural rubber. These increased mechanical properties of rubber nanocomposites with improved processability, controlled morphology and electrical properties are important parameters in the designing of flexible flame retardant electronic devices, electromagnetic induction shielding and conducting adhesive materials. [ABSTRACT FROM AUTHOR]

Digital light processing is a vital additive manufacturing technology used for manufacturing ceramic parts. The particle size distribution of ceramic suspensions significantly affects the cure behaviour and mechanical properties of ceramics. In this study, the cure behaviour and mechanical properties of Si 3 N 4 ceramics with a bimodal particle size distribution were studied. The results indicated that the suspension with coarse particles had a higher cure depth for a lower absorbance but poor mechanical properties. The bending strength of the samples with the optimal ratio (coarse:fine particles = 3:7) reached a maximum of 728.7 ± 10.33 MPa, which is 16.5% higher than that of the samples prepared using only fine particles. [ABSTRACT FROM AUTHOR]

Present innovation demonstrates the fabrication, characterisation, thermal, flame retardancy, electrical conductivity, vulcanization and mechanical properties of zinc ferrite (ZnFe2O4) nanofiller inserted chlorinated natural rubber (Cl–NR) prepared by an industrial compounding technique. The interaction between ZnFe2O4 and Cl–NR was characterised by FT–IR. The uniform dispersion of nanofiller in the rubber matrix was confirmed by SEM and TEM analysis. XRD showed the structural changes of composite with the presence of crystalline peaks of ZnFe2O4 in chlorinated rubber. The flame retardancy and glass transition temperature of the rubber composites were significantly enhanced with the filler loading in chlorinated rubber was evident from LOI and DSC analysis, respectively. The vulcanization time of rubber nanocomposite was greatly reduced by the use of zinc ferrite that is an important factor for reducing the cost of the preparation of rubber goods. The tensile strength, tear resistance, modulus, hardness and heat build-up of nanocomposites were significantly increased with the addition of zinc ferrite whereas the abrasion loss, resilience and elongation at break were decreased. The electrical conductivity and dielectric properties of rubber nanocomposite were also investigated in various frequencies. The percolating network formed by ZnFe2O4 in Cl–NR results in the high electrical conductivity and dielectric behaviour of the fabricated composites. The higher mechanical properties, glass transition temperature and the electrical conductivity of fabricated composites were beneficial in designing lightweight and highly durable flexible electronic devices based on chlorinated natural rubber. [ABSTRACT FROM AUTHOR]

This paper describes a novel method developed for the optimization of composite components against distortion caused by cure-induced residual stresses. A novel ply stack alteration algorithm is described, which is coupled to a parametrized CAD/FE model used for optimization. Elastic strain energy in 1D spring elements, used to constrain the structure during analysis, serves as an objective function incorporating aspects of global/local part stiffness in predicted distortion. Design variables such as the number and stacking sequence of plies, and geometric parameters of the part are used. The optimization problem is solved using commercial software combined with Python scripts. The method is exemplified with a case study of a stiffened panel subjected to buckling loads. Results are presented, and the effectiveness of the method to reduce the effects of cure-induced distortion is discussed. [ABSTRACT FROM AUTHOR]

Thermoset polymer composites have increased in use across multiple industries, with recent applications consisting of high-complexity and large-scale parts. As applications expand, the emphasis on accurate process-monitoring techniques has increased, with a variety of in situ cure-monitoring sensors being investigated by various research teams. To date, a wide range of data analysis techniques have been used to correlate data collected from thermocouple, dielectric, ultrasonic, and fibre-optic sensors to information on the material cure state. The methods used in existing publications have not been explicitly differentiated between, nor have they been directly compared. This paper provides a critical review of the different data collection and cure state correlation methods for these sensor types. The review includes details of the relevant sensor configurations and governing equations, material combinations, data verification techniques, identified potential research gaps, and areas of improvement. A wide range of both qualitative and quantitative analysis methods are discussed for each sensing technology. Critical analysis is provided on the capability and limitations of these methods to directly identify cure state information for the materials under investigation. This paper aims to provide the reader with sufficient background on available analysis techniques to assist in selecting the most appropriate method for the application. [ABSTRACT FROM AUTHOR]

Metal oxide nanofillers are a special type of additives in rubber composites that impart special qualities such as thermal resistance, tensile strength, electrical conductivity and heat capacity to rubbers. The heat conductivity of metal oxide through the rubber matrix reduces the vulcanization time of the rubber nanocomposites. Hence, the present work focused on the enhancement of these properties in natural rubber (NR) using zinc ferrite (ZnFe2O4) nanoparticles were prepared by a simple two-roll mill mixing technique. Structure, morphology, crystalline nature, cure characteristics, swelling, thermal and mechanical properties of the NR composites containing ZnFe2O4 were analysed in detail. FTIR and UV analysis proved the interaction of zinc ferrite with the macromolecular chain of NR. The XRD patterns of composite films revealed a decrease in amorphousness of NR with well-dispersed crystalline peaks of nanoparticles in the polymer. SEM images evidenced the morphological changes caused by dispersing zinc ferrite in the NR matrix. TEM analysis showed the uniform attachment of nanoparticles in the polymer. Glass transition temperature obtained from DSC was improved with the addition of zinc ferrite. The results from TGA showed that the presence of ZnFe2O4 in the polymer matrix greatly increases the thermal stability of NR. The metal oxide nanofiller significantly reduced the cure and scorch time of the NR composites. The mechanical properties of rubber nanocomposites showed that the addition of ZnFe2O4 improved their modulus, tensile strength, hardness, abrasion resistance and heat build-up, whereas the elongation at break and resilience decreases. The permeation and diffusion of nanocomposites were observed to be diminished with the size of penetrating solvents and also with the loading of filler. Overall, zinc ferrite nanoparticles could be used as a potential filler for improving the processability, mechanical strength, thermal stability and solvent resistance of natural rubber. [ABSTRACT FROM AUTHOR]

• A comprehensive kinetics model crossing curing and post-curing stages was developed. • Maximum DOC was introduced to a thermoset adhesive with low-temperature curing. • Relationships among the kinetic parameters, DOC, temperature, and time were examined. • The post-curing kinetics equation was determined by a normalizing method. The properties of adhesives cured at low temperatures are often significantly affected by post-curing effects. This paper introduces a method to model both the curing and post-curing kinetics for thermoset adhesives. The total and residual enthalpies and the glass-transition temperature (T g) of the adhesive were measured via differential scanning calorimetry. The obtained degree of cure (DOC) and T g were then fitted to determine their functional relationship. Low-temperature curing experiments were conducted, and an n th-order curing kinetics equation was developed through the fitting of data. Post-curing experiments were performed on samples with varying initial DOCs, and the resulting post-curing data were normalized and fitted using an n th-order model. The relationships between the kinetic parameters and the initial DOC, post-curing temperature and time were analyzed, and a comprehensive kinetics model crossing the curing and post-curing stages was successfully established. Model validation was carried out, and the results demonstrated a good predictive capability. [Display omitted] [ABSTRACT FROM AUTHOR]

A multi-objective optimization method which takes the multi-physics coupling characteristic into account is proposed to determine the cure cycle profile for polymer-matrix composites. First, a numerical model which considers the effects of heat transfer, cure kinetics, resin flow-compaction process has been developed to predict the temperature and degree of curing. The simulation results agree well with the experimental measurements from the previous publication to validate the practicability of the FE model. A surrogate model based on the Surface Response Method is built to make the solution feasible according to the entire calculation time. The surrogate model was integrated into the optimization framework to optimize cure cycle profile using NSGA-II algorithm. The results show that the duration of the cure time and the maximum gradient of temperature are about 44.8% and 34% shorter than in the typical cure profile, respectively. It is also shown that the multi-physics coupling characteristic should be considered in the optimization process for thick composite component. [ABSTRACT FROM AUTHOR]

The geometric stability of structural parts is a critical issue in the aeronautical industry. However, autoclave curing of primary structural composite parts may cause significant distortions and divergences between the mould nominal geometries and the final shapes of the parts. To be able to anticipate such distortions, a robust simulation tool is needed, which can be implemented only if the phenomena involved are properly understood and characterized. The thermo-kinetic behaviour of the M21EV/IMA prepreg is fully characterized in this paper. Thermal strains and chemical shrinkages are measured using Thermo-Mechanical Analysis during the cure and the experimental method developed allows the thermo-chemical strains to be obtained even during the early stages of the cure. An experimental setup is developed to measure the thermo-mechanical behaviour of the material during its cure. Thanks to these measurements, a new constitutive mechanical model, inspired from the CHILE model, is defined. These data are then used as inputs for an FEA simulation of the entire curing process. Finally, the model is validated using the cure degree, glass transition and temperature monitoring, and post-cure distortion measurements. [ABSTRACT FROM AUTHOR]

Thermochemical properties are needed to develop process models and define suitable cure cycles to convert thermosetting polymers into rigid glassy materials. Uncertainty surrounding the suitability of thermal analysis techniques and semi-empirical models developed for conventional composite materials has been raised for the new class of particle interleaf materials. This paper describes kinetics, conductivity, heat capacity and glass transition temperature measurements of HexPly® M21 particle interleaf material. Thermal models describing conventional, non-particle epoxy systems were fit to the data and validated through a thick-section cure. Results from curing experiments agree with heat transfer simulation predictions, indicating that established thermal analysis techniques and models can describe polymerisation and evolving material properties during processing of a material representing the class of interleaf toughened systems. A sensitivity study showed time savings up to about 20%, and associated energy-efficiency-productivity benefits can be achieved by using cure simulation for particle interleaf materials. [ABSTRACT FROM AUTHOR]

Abstract Although metals are the most commonly used tooling materials to cure composites, they do not provide optimal results in a microwave environment. Following a selection process based on the properties of the materials, an alternative tooling material in carbon fibre reinforced plastic (CFRP) was successfully utilised to cure CFRP panels in laboratory and industrial microwaves. The conductive carbon fibres in the tool facilitated the fast heat transfer across the part. Other tooling materials including a glass fibre cyanate ester prepreg and tooling board were trialled, although the latter exhibited damage during cure. These advantages demonstrate that the CFRP tool is a compatible material that can be used when microwave curing composites. [ABSTRACT FROM AUTHOR]

The sequential heat release (SHR) taking place in dual‐curing systems can facilitate thermal management and control of conversion and temperature gradients during processing of thick composite parts, hence reducing the appearance of internal stresses that compromise the quality of processed parts. This concept is demonstrated in this work by means of numerical simulation of conversion and temperature profiles during processing of an off‐stoichiometric thiol–epoxy dual‐curable system. The simulated processing scenario is the curing stage during resin transfer moulding processing (i.e. after injection or infusion), assuming one‐dimensional heat transfer across the thickness of the composite part. The kinetics of both polymerization stages of the dual‐curing system and thermophysical properties needed for the simulations have been determined using thermal analysis techniques and suitable phenomenological models. The simulations show that SHR makes it possible to reach a stable and uniform intermediate material after completion of the first polymerization process, and enables a better control of the subsequent crosslinking taking place during the second polymerization process due to the lower remaining exothermicity. A simple optimization of curing cycles for composite parts of different thickness has been performed on the basis of quality–time criteria, producing results that are very close to the Pareto‐optimal front obtained by genetic algorithm optimization procedures. © 2018 Society of Chemical Industry Dual‐curing systems with sequential heat release (SHR) and tailorable network build‐up facilitate the obtaining of uniform conversion and temperature profiles during composite processing. [ABSTRACT FROM AUTHOR]

Abstract The development of a novel hot-stage microscopy technique to measure the level of cure shrinkage that occurs in an epoxy thermoset microdroplet with different epoxy-to-hardener ratios is presented. The equipment setup, sample preparation, and experimental procedure are described in detail. A comparable method to characterise cure shrinkage, a modified rheometry technique, is also reviewed. Shrinkage measurements using the hot-stage microscopy method are shown to characterise the full range of shrinkage that occurs both before and after the resin gel point, hence producing values greater than those found previously in the literature. However, when used in conjunction with the gel point values for the different ratios, measured using rheometry, the technique produces results for shrinkage post-gelation that concur well with literature values. The modified rheometry technique showed potential for measuring the level of cure shrinkage with a varying cure temperature profile, with more work required to perfect the method in defining the cross-over point for sample loading for different epoxy-to-hardener ratios. Highlights • A novel technique for measuring the cure shrinkage of an epoxy droplet is proposed. • The level of cure shrinkage that occurred was found to increase with the R value. • Hot-stage technique found to measure all possible shrinkage that would occur. • Gel point found to decrease with increases in R value. • Results were comparable to those in literature after accounting for gel point. [ABSTRACT FROM AUTHOR]

This study investigated the feasibility of using an electron beam (EB) process to cure chemically impregnated wood products. Maple wood planks were impregnated with the low-viscosity resins 1,6 hexanediol dimethacrylate (HDDA) and trimethylolpropane trimethacrylate (TMPTA). The addition of nanoparticles into the formulation was also studied. The impregnated wood was then cured by EB irradiation. The EB curing method utilizes highly energetic electrons at a controlled energy level to polymerize and cross-link the polymeric materials. The thermal analysis results of differential scanning calorimetry (DSC) confirmed that the curing of chemically impregnated wood by electron beam radiation was validated. Polymerization exotherms were observed for the neat acrylate resin and formulations of acrylate/nanoparticles impregnated maple samples. No polymerization exothermal peaks were observed for both EB-cured impregnated maple and control maple samples, confirming that EB irradiation can serve as an efficient curing method to polymerize acrylate-impregnated wood products. The surface hardness of the EB-cured impregnated maple wood was improved up to 200%.

The thermochemical and the thermomechanical phenomena that happen during the cure process of a composite material laminate are responsible for the rise of residual stresses and, consequently, for the deformation at the end of the manufacturing process. In this work, the influence of some parameters, such as the laminate thickness, the stacking sequence and the mold radius, on the spring-in angle of a U-shaped laminate was studied exploring a full factorial plan through numerical simulations. Moreover, the influence of the laminate shape on the deformation was investigated. First of all, a numerical model appropriate for cure simulation was introduced and its suitability to simulate the deformation behavior was demonstrated. Using a simulation model instead of running experimental test was important for analyzing the parameters influence without reduce the full factorial plan to a fractional one and without wasting a lot of time and material. As a result, only the stacking sequence influenced the spring-in value, while the effect of the tool radius and laminate thickness was minimal. Finally, a comparison between U-shaped and L-shaped laminates evidenced that the spring-in value was independent of part shape, while this parameter affected the flange warpage. [ABSTRACT FROM AUTHOR]

Process-induced distortions in composite structures lead to problems with dimensional control, which can be a major concern in the manufacturing. This paper focuses on the prediction of the warpages and curvatures in unsymmetrical laminate plates using closed-form solution and finite element method. A new characterization method that the shape of the deformed composite specimen is conducted in virtue of the coordinate measuring machine through contact method is adopted to experimentally measure the process-induced distortions in unsymmetrical laminate plates. The comparison of numerical and analytical prediction with experimental results for warpages in [0 6 /90 6 ] and [0 9 /90 3 ] unsymmetrical laminates indicates that shear layer between tool and part has a significant effect on the process-induced distortions, and PTFE release film and release agent between tool and part cannot eliminate effect of the tool-part interaction on the process-induced distortions in unsymmetrical laminate plates. Moreover, the comparison of analytically and numerically predicted development of curvature with experimental results presented by previous literature for unsymmetrical laminates further reveals the effect of tool-part interaction on the process-induced distortions and the accuracy of closed-form solution. [ABSTRACT FROM AUTHOR]

Multi-zonal, electrically heated moulds for composite processing offer the potential of a direct heat introduction with low thermal lag and high energy efficiency. However, appropriate thermal dimensioning of these tools requires the consideration of the thermal response of the tool itself as well as the thermal and cure behaviour of the part, which is to date mostly estimated based on experience. To realize the full potential of this tool class, a numerical method is presented to determine a sound partitioning of the designated heating area utilizing 3D finite element cure simulation. Further, a cure simulation model of an application case is set up and validated. The capability of the numerical method to significantly increase the temperature accuracy and the degree of cure homogeneity are demonstrated in an evaluation of the numerically improved application case. Finally, the effect of the tool material on the zone allocations and temperature accuracy is studied. [ABSTRACT FROM AUTHOR]

Using a DGEBA/TETA epoxy filled with silver microflakes, we sought to investigate if the onset of conductivity would approximately obey the power-law as cure progresses, as reported in the open literature. We monitored the resistivity change as a function of cure and vitrification behaviour, using a set of concurrent experiments; in-situ four-wire electrical resistance measurements in a newly-developed probe-mold device, and differential scanning calorimetry. It was evident that the electrical conductivity evolved very differently depending on heating conditions, the composite glass transition temperature, and filler content. Periods were observed during cure where electrical conductivity was disrupted, attributed to stresses produced by vitrification. This produced as much as 1500-fold increases in final developed electrical resistance depending on heating conditions and composite glass transition properties, for the same conductive filler content. This discovery has far ranging implications on the industry practice of applying multi-step cure schedules that feature vitrification during cure. [ABSTRACT FROM AUTHOR]

During the cure process of CFRP laminates, some geometrical unconformities can arise, due to several thermomechanical and thermochemical phenomena that make residual stress rise. Among these unconformities, one of the most studied is the spring-in, that is the deviation of the flange-to-flange angle from the design value. In this work a numerical model suitable to determine the spring-in value was developed and verified. The proposed model considered both the thermo-chemical and thermo-mechanical phenomena that happen during the cure process. Then, the numerical model was used for calculating the spring-in angle of lots of different laminates in order to evaluate the influence of the thickness, the corner radius and the layup sequence on the laminate deformation. For evaluating the influence of the above mentioned parameter on the spring-in a full factorial plan was designed, and the FEM analysis allowed a saving of time, energy and material. It was found that only the layup sequence influenced the spring-in. Finally, a preliminary analysis stated the possibility to extend the proposed model also to other geometries, such as U-shaped laminates. [ABSTRACT FROM AUTHOR]

A new analytical solution considering three dimensional effects on spring-in of curved composite parts has been proposed based on modification of the model presented by Wisnom et al. [28]. In the new analytical solution the composite part is assumed in generalized plane strain condition between gelation and vitrification rather than in plane stress condition as proposed by Wisnom et al. [28]. The material properties are assumed to present a step change in the vitrification point with constant values in each state. Consequently the new solution not only considers the effect of the cure process generated strain in the rubbery state in the through-thickness direction but also the strain in the length direction for prediction of the cure-induced spring-in of curved composites parts. The proposed new analytical solution is verified by the good agreement between the experimentally measured spring-in angels presented by previous studies and analytically predicted results presented by author for both the unidirectional and cross-ply C-sections composite parts. Further investigation by numerical analysis also provides a favourable comparison with experimental findings and analytical results. [ABSTRACT FROM AUTHOR]

In the future, materials will need to be biobased and produced sustainably without compromising mechanical properties. To date, in many cases, the advantages of the bio-origin of the raw material are overridden by the environmental impact of the process. In the present study, we have developed a novel composite material based on woven hemp fabric which reinforce a thermoset polymer produced from birch bark, a low-value forestry byproduct. Results show that this fully biobased composite has specific stiffness and strength equivalent to those of flax fibre-reinforced petroleum-based epoxy composites and slightly lower than glass fibre-reinforced petroleum-based epoxy composites. The sustainability of the material was also evaluated by life-cycle assessment from cradle to gate and showed significantly superior performance with respect to the potential global warming impact than commercial benchmark materials. Furthermore, toxicology studies showed no endocrine disruptive activities. This is an important proof of concept study demonstrating that biobased structural materials can be produced sustainably. [Display omitted] [ABSTRACT FROM AUTHOR]

During the curing process, the exothermic cross-linking reaction inside epoxy resin-based composites causes overheating and, eventually, a thermal degradation of the matrix. Thick sectioned epoxy composites require an optimized curing cycle in order to minimize the temperature raise inside the composite. However, the recommended cycle for thin laminates is often wrongly used also for thick sectioned composites, resulting in an overheating. In this work, the mechanical behaviour of laminates having different thicknesses was investigated. Specimens were cured according to a curing cycle optimal for thin laminates which causes overheating in thicker laminates. The curing temperature histories at several positions inside the laminates were monitored and recorded and, following a new methodological approach, a series of comparative interlaminar shear tests was performed. Experimental results show a decrease of the interlaminar shear strength in thicker laminates. A simple model to account for the detrimental effect of the exothermic peak on the interlaminar shear strength is proposed. [ABSTRACT FROM AUTHOR]

In this paper, the corner spring-in angles of AS4/8552 L-shaped composite profiles with different thicknesses are predicted using path-dependent constitutive law with the consideration of material properties variation due to phase change during curing. The prediction accuracy mainly depends on the properties in the rubbery and glassy states obtained by homogenization method rather than experimental measurements. Both analytical and finite element (FE) homogenization methods are applied to predict the overall properties of AS4/8552 composite. The effect of fiber volume fraction on the properties is investigated for both rubbery and glassy states using both methods. And the predicted results are compared with experimental measurements for the glassy state. Good agreement is achieved between the predicted results and available experimental data, showing the reliability of the homogenization method. Furthermore, the corner spring-in angles of L-shaped composite profiles are measured experimentally and the reliability of path-dependent constitutive law is validated as well as the properties prediction by FE homogenization method. [ABSTRACT FROM AUTHOR]

The paper investigates the effect of essential manufacturing parameters on the surface quality of uncoated carbon fibre-reinforced composites used as car body panels with visible surfaces (Class A properties). A series of carbon fibre-reinforced composites laminates were prepared by the resin transfer moulding technique varying the fibre volume content (30 to 60 %), reinforcement material (woven fabrics vs. unidirectional fibre reinforcements), curing temperatures (40℃ to 120℃), additives (SiO2 nanoparticles as matrix fillers) and using a surface finish applied as an in-mould coating. Laminate surfaces were characterised by roughness analysis (white-light interferometry) and wave-scan measurement to quantify the influence of the different manufacturing parameters on the surface quality. Especially, the used resins were intensively characterised concerning thermal properties and total resin shrinkage. These results correlate very well with the performed analysis of surface roughness. It is found that the fibre print through effect is significantly reduced by realising low total resin shrinkage and an even distribution of resin and fibres at the surface. Thus, using of unidirectional fibre reinforcement (no weft or sewing threads; very fine filaments), low curing temperatures (slow curing processes) and an in-mould coating are most successful for reduction of fibre print through effect and getting surface similar to Class A properties. In addition, the surface quality is quite positively affected by the application of nanoparticles and also strongly controlled by roughness of tooling. [ABSTRACT FROM AUTHOR]

The present study addresses the development of new biomass-based furan resin/glass fibre composites manufactured by the double-vacuum-bag technique using a two-stage cure cycle to allow removal of water from the resin. The volumetric composition and mechanical properties of the composites are measured and analysed with focus on the porosity content. The so-called matrix correlated porosity factor is determined to be 0.096, meaning that the furan matrix itself contains 8.8% porosity. In the optimal case of no matrix porosity, stiffness of the composites compares well with the stiffness of conventional thermosetting/glass composites, but with lower strength. The findings of the present study show that a more efficient water removal during manufacturing, a lower porosity content and a less brittle stress–strain behaviour of the furan matrix are to be addressed to further improve the properties of the composites. [ABSTRACT FROM AUTHOR]

Geometric unconformities may arise due to residual stresses during the manufacturing process of polymer matrix composite materials. Compaction of the laminate, due to resin squeeze, is another phenomenon that occurs and it influences the mechanical behaviour of the laminate itself. In the past, models for spring-in or for compaction were developed separately, whereas in the present work a new kind of analysis is developed to take into account both residual stresses and compaction that take place during cure simultaneously. This analysis is composed by two different steps: in the former the compaction is predicted, in the latter the cure state and the residual stress condition are determined. Finally, the relationship between different degrees of compaction and the corresponding values of spring-in is studied, and it was found that a higher value of thickness reduction produces the smallest residual stresses for the composite structures with angled cross sections. [ABSTRACT FROM AUTHOR]

This paper investigates the effect of polyamide 6 (PA 6) nanofibres and microfibres, as well as the effect of moisture present in these structures, on the curing kinetics of a diglycidyl ether of bisphenol A (DGEBA)-methylenedianiline (MDA) system. Modulated temperature differential scanning calorimetry measurements show that the initial reaction rate follows a linearly increasing trend as a function of the moisture content present in the nanofibrous structures. Compared to PA 6 microfibrous structures, incorporating PA 6 nanofibrous structures exposed to the same humidity resulted in a higher initial reaction rate, which is in agreement with the higher water absorption of the nanofibrous structure, measured with differential vapour sorption. Overall, the nanofibres themselves affect the curing characteristics, and the moisture present in the structures enhances this effect. [ABSTRACT FROM AUTHOR]

Determination of gel point is important for a modelling assessment of residual stresses developed during curing of composite materials. Residual stresses in a composite structure may have a detrimental effect on its mechanical performance and compromise its integrity. In this article, the evolution in bending stiffness of a glass/epoxy composite material during an isothermal curing process is examined to identify different material stages and behaviour. Differential scanning calorimetry and dynamic mechanical analysis are used to analyse the material behaviour. Gelation is identified as a clear onset in bending stiffness, and vitrification is seen as a decrease in the bending stiffness rate. Often gel point predictions for composite materials are based on neat matrix measurements. However, the results presented in this article demonstrate that the gel point is affected by the presence of the fibre reinforcement. [ABSTRACT FROM PUBLISHER]

Abstract: Epoxidized soybean oil (ESO) containing octadecyl trimethyl ammonium functionalized montmorillonite (OMMT, 1–5wt.%) were thermally cured using methylhexahydropthalic anhydride in the presence of 2-ethyl-4-methylimidazole catalyst. The mechanical properties of ESO nanocomposites were studied through the tensile and fracture toughness tests. The thermal properties were characterized by means of dynamic mechanical analyzer, thermogravimetric analyzer and differential scanning calorimeter. The addition of OMMT into ESO significantly improved the modulus, tensile strength, glass transition temperature, crosslink density, gel content and thermal stability of ESO/OMMT bio-thermoset. These are mainly attributed to the well-exfoliated OMMT and the co-catalytic effect of octadecyl trimethyl ammonium. [Copyright &y& Elsevier]

Three different organo-modified clays have been incorporated by sonication into a high performance epoxy resin before the cross-linking reaction. The X-ray analysis indicated that, depending on the organoclay type, partially exfoliated and partially intercalated composites have been obtained. As shown by the DSC analysis, the clay addition seems to interact with the cross-linking reaction. The incorporation of organoclay into epoxy increased free volume and micro-voids in the samples. Sorption of water in the composite samples resulted higher than that of the pristine resin, whereas the diffusion coefficient is significantly lower. The lower value of diffusion makes the permeability at ambient conditions lower than the pristine resin. The elastic modulus of the composite sample results higher than that of the pristine resin, especially in the temperature region around the glass transition. The presence of organoclay in epoxy matrix decreased the glass transition temperature, whether the nanocomposites were in a dry or wet condition. [ABSTRACT FROM AUTHOR]

Various aspects related to the durability of Fibre Reinforced Polymer (FRP) composites used in the rehabilitation, repair and strengthening of existing concrete structures were studied as a function of cure behaviour. The performance of these FRP-composites containing thermoset matrix and implemented under civil engineering conditions, is unstable and their properties will change over time due to their partial cure. The present work attempts to raise awareness about the characteristic changes of FRP-composites materials immediately after their ambient cure, while emphasizing the implication of such phenomena on the durability. Both glass transition temperature Tg and mechanical characterization tests were performed to identify the effects of curing on thermorheological and mechanical stability of the composites immediately after the implementation. The experimental results indicate that overall composite performance was sensitive to the cure conditions. The implication of this sensitivity on the durability of FRP-composites was addressed and clearly identified through accelerated and natural exposure tests which reflected the civil engineering environment. [ABSTRACT FROM AUTHOR]

A certified aerospace resin (RTM 6) normally utilised for resin transfer moulding is considered for vacuum infusion. The resin was subjected to simulated vacuum infusion conditions by using a specialised thermogravimetric analysis that enables control of pressure as well as temperature. By varying conditions, it was possible to investigate the expected occurrence of volatile loses during infusion that could cause mechanical or cosmetic defects in a part. With particular reference to RTM6, it was determined that full vacuum could be used for infusion provided that the temperature was kept below ∼130 °C. Higher temperatures could be used, but the applied vacuum should be significantly reduced. Of note is that the manufacturers datasheet recommends processing parameters that could result in volatile loss. As such, the pressure enhanced TGA method may be considered more widely for providing processing conditions supplemental to the manufacturers recommendation for any liquid resin used under vacuum conditions. • Thermogravimetric assessment of RTM-6 resin with pressure and temperature control. • Mass loss begins to occur at > 130 °C, independent of pressure. • Cessation of mass loss appears to correlate with gel point. • Pressure and temperature region predicted to avoid mass loss. [ABSTRACT FROM AUTHOR]

Differential scanning calorimetry is used for the analysis and quantitative evaluation of the reaction between diglycidyl ether bisphenol-A and 2,2'-dimethyl-4,4'-methylenebis (cyclohexylamine) according to changes in concentrations of reactants and products. First, this concerned determining the variation in enthalpy of reaction over time for different curing temperatures, plotting of glass transition temperatures versus time, calculation of a polynomial for glass transition temperature against fractional conversion, and the production of plots of fractional conversions versus time. Secondly, the fractional conversion data were used for the kinetic analysis. The line of best fit to these data points revealed an essentially logarithmic relationship that was used to evaluate the reaction rate, dx/dt corresponding to the conversion x, then plot the changes in concentrations (in moles) of all the reactants and products involved in the reaction versus time. This produced a quantitative prediction of the reaction over the curing period. [ABSTRACT FROM AUTHOR]

Abstract: The isothermal cure of a dicyanate ester monomer has been investigated by differential scanning calorimetry (DSC) in the presence of different quantities of a catalyst system formed by nonylphenol (NP) and cobalt (II) acetylacetonate (Co(AcAc)2). Two sets of experiences were studied. Firstly, the NP composition was varied from 0 to 10 per hundred resin (phr) and secondly, the Co(AcAc)2 loadings were changed at 2 phr of NP. It has been observed that the cyanate conversion increases significantly after the first addition of the metal catalyst and, also, at the same time the thermal stability improves. Moreover, the kinetic data have been fitted with a second-order equation respect to the cyanate conversion in the kinetically conversion regime. Also, both the activation energies and the kinetic order respect to the NP and the Co(AcAc)2 have been determined. [Copyright &y& Elsevier]

An optical fibre sensor (OFS) for the online monitoring of the polymerization reaction is described in this paper. The sensor, based on Fourier transform infrared (FT-IR) and differential scanning calorimetry (DSC) analysis, was used to study the reaction between diglycidyl ether bisphenol-A (DGEBA) and 2,2′-dimethyl-4,4′-methylenebis(cyclohexylamine) (DMMB-CHA). Changes in concentrations of reactants and products were monitored over the curing period at different temperatures by the FT-IR technique, and from DSC runs the variation in the enthalpy of reaction over time for different curing temperatures was determined. Plots of fractional conversions versus time from the two methods were produced and compared. An attempt to deduce glass transition temperatures from the fractional conversion curves produced by FT-IR is also described. This involved the use of the fitting polynomial calculated from the DSC results. As a result, a sensor designed for embedding into a system curing at elevated temperature was constructed and tested. [ABSTRACT FROM AUTHOR]

The tool-part interaction was experimentally carried out using FBG sensors adhesively bonded to the thin tool in a tool-part assembly, and corresponding interfacial properties parameters were theoretically quantified according to the acquired data and shear-lag theory. Subsequently, these derived parameters were adjusted and fed into FE model of C-shaped composite structure cured on the aluminium tool to numerically predict its spring-in, in company with experimental validation. Meanwhile, two comparative simulations neglecting tangential friction and tool expansion were provided to clarify the significance of tool-part interaction. The results from the assembly show that the sticking condition mainly exists until interfacial shear stress reaches a critical value at a certain time in the cool-down stage, after which the sliding friction dominates. The good agreement of spring-in between experiment and simulation provides a favourable support for the numerical simulation scheme, and comparative simulations reveal that tangential friction contributes to spring-in but the tool expansion alleviates it. [ABSTRACT FROM AUTHOR]

When cured in a microwave, flat thin composite panels can experience even heat distribution throughout the laminate. However, as load and geometric complexity increase, the electromagnetic field and resulting heat distribution is altered, making it difficult to cure the composite homogeneously. Materials that absorb and/or reflect incident electromagnetic radiation have the potential to influence how the field behaves, and therefore to tailor and improve the uniformity of heat distribution. In this study, an absorber was applied to a composite with non-uniform geometry to increase heating in the location which had previously been the coldest position, transforming it into the hottest. Although this result overshot the desired outcome of temperature uniformity, it shows the potential of absorbing materials to radically change the temperature distribution, demonstrating that with better regulation of the absorbing effect, a uniform temperature distribution is possible even in non-uniform composite geometries. [ABSTRACT FROM AUTHOR]

Analytical solutions for predicting spring-in angles of U-shaped composite parts generating in rubbery and glassy states are proposed. In the newly developed analytical solutions, the process-induced distortions of web and flange are mathematically converted to spring-in angles within the scope of small deformation theory, and then corresponding analytical expressions of web, corner, flange and total spring-in are separately derived. These new solutions can quantitatively give the correlation of web, corner, flange and total spring-in with material properties-related and geometrical configuration-dependent parameters, and evaluate the spring-in angles of multi-angle ply composite parts just using lamina properties. The reliability of newly proposed analytical solutions is checked by the comparison between the analytically calculated and experimentally measured spring-in angles for unidirectional and quasi-isotropic U-shaped composite parts. Meanwhile, numerical simulations on the U-shaped composite parts with various geometries and layups also provide a favourable agreement with analytical results. • Analytical solutions for spring-in of U-shaped composite parts are proposed. • New solutions can quantify effect of properties and geometry on spring-in angle. • New solutions can evaluate the spring-in angles just via lamina properties. • The reliability of solutions is experimentally and numerically validated. [ABSTRACT FROM AUTHOR]

Aim This study tested the hypothesis that the cure behaviour (depth of cure and polymerization contraction) of light-activated pattern materials was no worse than that of light-activated composite resins, allowing them to be handled in a similar fashion. Methodology Depth of cure was measured by a penetrometer method. Results Cure depths were comparable to those of composite resins, ranging from 3.52mm (Lumin-X paste) to 6.76mm (Visioform) after visible lightactivation for 30s. There were significant differences in the depth of cure of the three materials tested (P0.001). Polymerization contraction was assessed by a minimal load transducer method. Values ranged from 0.45% (Lumin-X paste) to 1.89% (Visioform), lower than that of composite resins. There were significant differences in the polymerization contraction values for each of the three materials (P0.001). Conclusions It was concluded that light-activated pattern materials cure in a manner comparable to composite resins, and may be built up incrementally in a similar fashion. [ABSTRACT FROM AUTHOR]

The aim of the present study has been to evaluate the effect of hybridization of montmorillonite (MMT) and multi-walled carbon nanotubes (MWCNT) on the thermal and viscoelastic properties of shape memory epoxy polymer (SMEP) nanocomposites. In this study, ultra-sonication was utilized to disperse 1%, 3%, and 5% MMT in combination with 0.5%, 1%, and 1.5% MWCNT into the epoxy system. The fabricated SMEP hybrid nanocomposites were characterized via differential scanning calorimetry, dynamic mechanical analysis, and thermogravimetric analysis. The storage modulus (E'), loss modulus (E"), tan δ, decomposition temperature, and decomposition rate, varied upon the addition of the fillers. Tan δ indicated a reduction of glass transition temperature (Tg) for all the hybrid SMEP nanocomposites. 3% MMT/1% MWCNT displayed best overall performance compared to other hybrid filler concentrations and indicated a better mechanical property compared to neat SMEP. These findings open a way to develop novel high-performance composites for various potential applications, such as morphing structures and actuators, as well as biomedical devices. [ABSTRACT FROM AUTHOR]

This paper aims to understand formation mechanism of the process-induced distortions in the L-shaped composite parts by analytical solutions. To this end, an analytical solution for the total spring-in of L-shaped composite parts is derived by defining the total spring-in as the integrated process-induced distortions in the flange and corner components in combination with shear-lag theory. The new solution, together with previously proposed analytical solution for the corner spring-in of L-shaped parts, not only quantifies the correlation of total spring-in with material and geometrical properties, but also calculates the specific contribution of flange and corner to the total spring-in. The close consistency and trend between analytical predictions and experimental data on the spring-in of L-shaped composites parts show the validity of new analytical solution. Further comparison between numerical simulations and analytical predictions for total spring-in of L-shaped composite parts with various configurations also gives favourable validation for the newly proposed analytical solution. [ABSTRACT FROM AUTHOR]

• The modeling of the curing kinetics of the epoxy resin was performed. • A phenomenological kinetic model has been proposed. • Diffusion effects were considered for kinetic modeling. • The estimation of kinetic parameters was performed. • The kinetic model was incorporated to simulate the pultrusion process. Pultrusion is a continuous process for manufacturing polymer composite with uniform cross-sectional profiles. In this process the pulling speed and die temperature are the main process variables that can be used to improve the chemical and mechanical properties of the pultruded polymer composite. A critical processing step in reactive polymer composites that involves thermoset resins is the curing reaction that starts from monomers/oligomers, which forms a three-dimensional cross-linked network. While empirical kinetic models for the prediction of the degree of cure are easy to handle, they are limited in terms of providing a complete understanding of the system, due to the absence of knowledge regarding the full kinetic of the functional groups. In this regard, the use of phenomenological models, based on material balances of functional groups involved in the curing reaction, is a noteworthy strategy. In this work two kinetic models were tested to simulate the pultrusion process: (i) empirical model and (ii) phenomenological model. Diffusional limitations on the cure kinetics were coupled into both models. The kinetic parameters of both models were estimated from differential scanning calorimetry (DSC) experiments of an epoxy resin derived from an unmodified liquid diglycidyl ether of Bisphenol A (DGEBA resin) in a mixture with an Anhydride Curing Agent and an Accelerator like DMP-30 (2,4,6-tris(dimethylaminomethyl) phenol). The results revealed that the kinetic models could be reasonably adjusted to the experimental curing behavior, allowing to obtain accurate values for different curing rates. The kinetic models were then implemented into the pultrusion model, by the use of the FE software, ANSYS-17.2. According to the results of ultruded thermal and curing profiles of pultruded parts, it is shown that the kinetic models are suitable for predicting the thermal and curing behavior of the pultrusion process. [ABSTRACT FROM AUTHOR]