Uneven Curing Research Articles

The impact of various curing methods on the curing process of polymer-fiber composites analyzed using analytical approach

Ti/Gf/Cf represents a novel form of hybrid fiber metal laminates (HFMLs) that successfully blend the benefits of titanium alloy, glass fiber reinforced plastic (GFRP), and carbon fiber reinforced plastic (CFRP). These materials have garnered attention in the field of engineering due to their exceptional levels of stiffness, strength, and shear resistance. The quality of HFMLs during the curing process was significantly impacted by the chosen curing method. Moreover, the use of multiple types of fibers complicated the optimization of curing parameters in HFMLs. A comprehensive study was conducted using a multi-physics field coupling finite element method to investigate the impact of various curing methods on the development of temperature distribution, curing degree, and curing stress in HFMLs during the curing process. By integrating thermal transmission equations, curing kinetics, and viscoelastic mechanical models, a detailed curing simulation model was established. Increasing the cooling rate from 1 °C to 5 °C was shown to raise the maximum curing stress by 4.7 MPa. Additionally, higher curing temperatures were found to induce internal curing stress due to variations in thermal properties. Insufficient insulation times were observed to exacerbate non-uniform temperature distribution and curing degree fields in HFMLs, potentially leading to increased internal stress. To mitigate the generation of large residual stress, it is recommended to lower the heating and cooling rates, reduce curing temperatures, and extend the insulation time appropriately during the curing process of HFMLs. The primary goal of this research was to accurately predict the evolution of curing parameters in HFMLs under different curing regimes, which were impacted by the chosen curing method. The results of the study demonstrated that rapid changes in temperature during heating and cooling stages can lead to uneven curing, resulting in the accumulation of residual stress within the laminates.

Analysis on the ultraviolet curing process of resin‐based glass fiber composites based on finite element mesoscale model

AbstractThe resin curing process triggered by ultraviolet radiation possesses numerous advantages, including rapid speed, low cost, and minimal environmental impact. Due to the hindrance of light propagation by the medium, uneven curing or incomplete curing at the bottom may occur frequently, particularly when the material thickness is excessive. To address these issues and delve into the regularities of the light‐curing process, this study employed a modeling approach that separately represents fiber bundles and the resin matrix to construct a mesoscopic model of the resin‐based glass fiber‐reinforced composite material glass fiber reinforced polymer (GFRP). Based on the models of light propagation, light‐curing kinetics, and heat conduction, a multiphysics field decoupling model is established to simulate the light‐curing process of GFRP. Using this model, the temporal and spatial variations of the internal light field, degree of curing field, and temperature field within GFRP during the curing process are simulated. The mechanistic reasons for these variations are analyzed. Finally, in conjunction with the regular analysis based on the macroscopic model, the accuracy of the model and the obtained regularities are validated through experiments. This provides an effective analytical method for the study of light‐curing in resin‐based fiber‐reinforced composite materials.

Non-Destructive Characterization of Cured-in-Place Pipe Defects.

Sewage and water networks are crucial infrastructures of modern urban society. The uninterrupted functionality of these networks is paramount, necessitating regular maintenance and rehabilitation. In densely populated urban areas, trenchless methods, particularly those employing cured-in-place pipe technology, have emerged as the most cost-efficient approach for network rehabilitation. Common diagnostic methods for assessing pipe conditions, whether original or retrofitted with-cured-in-place pipes, typically include camera examination or laser scans, and are limited in material characterization. This study introduces three innovative methods for characterizing critical aspects of pipe conditions. The impact-echo method, ground-penetrating radar, and impedance spectroscopy address the challenges posed by polymer liners and offer enhanced accuracy in defect detection. These methods enable the characterization of delamination, identification of caverns behind cured-in-place pipes, and evaluation of overall pipe health. A machine learning algorithm using deep learning on images acquired from impact-echo signals using continuous wavelet transformation is presented to characterize defects. The aim is to compare traditional machine learning and deep learning methods to characterize selected pipe defects. The measurement conducted with ground-penetrating radar is depicted, employing a heuristic algorithm to estimate caverns behind the tested polymer composites. This study also presents results obtained through impedance spectroscopy, employed to characterize the delamination of polymer liners caused by uneven curing. A comparative analysis of these methods is conducted, assessing the accuracy by comparing the known positions of defects with their predicted characteristics based on laboratory measurements.

Active control of cure-induced distortion for composite parts using multi-zoned self-resistance electric heating method

High dimensional accuracy of the carbon fiber reinforced polymer (CFRP) composite parts has been a long-term challenge for manufacturers in the aerospace industry. Traditionally, the uneven curing temperature could aggravate the cure-induced distortion (CID) and should be avoided as much as possible. But even under an ideally uniform temperature field, the CID of curved parts still cannot be eliminated because of the anisotropic shrinkage nature of the CFRP laminate. In this work, a novel CID control method based on the multi-zoned heating methodology is proposed, which actively applies an optimized non-uniform temperature field in different zones of the part to counteract the CID influenced by geometric structures. The counteracting principle of the proposed method lies in that using the actively formed through-thickness temperature gradients in different zones generates numerous small local thermal deformations, and their combination can significantly reduce the overall CID of the CFRP part. A co-optimization framework combined with the numerical model and genetic algorithm is established to rapidly compute the optimized multi-zoned temperature field for the CFRP parts with arbitrary complex structures. A good agreement between the numerical and the previous experimental results shows the effectiveness of the numerical model, and the multi-zoned temperature field optimization case of a scaled air-intake CFRP part demonstrates the performance of the co-optimization framework. Finally, by using the multi-zoned self-resistance electrical heating method, the CID reduction of the proposed method is experimentally validated on a complex CFRP part, where the average displacement reduces by >60 % compared to that of the traditional oven process.

Evaluation and Comparison of Fracture Toughness of Glass Fibre Reinforce Composites

The study of the behaviour of a broken structure or component under service conditions is known as fracture mechanics. Impurities, uneven curing, holes and notches are all sources of fractures. Cracks are common local discontinuities in materials caused by a variety of factors. Such discontinuities cause the structure's rigidity and consequently load bearing capability to deteriorate. It is known that for the crack to propagate, the stress in the locale of the crack tip should reach the critical value. Once stress level is critical, the crack propagates and leads to failure of the structure. A segment of the crack is divided into three modes namely Mode I (Opening mode), Mode II (sliding mode), Mode III (tearing mode). The current study presents a computational and experimental study on fractography and notch sensitivity evaluation in glass-fiber-reinforced-laminate under quasi-static load. For both numerical and experimental damage assessments, three volume fractions of glass and resin plies (50/50, 60/40, and 70/30) have been used. The fracture toughness investigation was carried out in accordance with ASTM standards, utilising a universal testing equipment. The numerical study is conducted out using the J-integral approach. The fracture toughness increases with resin content and is determined by the ductility of the plastic zone surrounding the crack tip. Within an acceptable range, the numerical findings are equivalent to the experimental values. When compared to the other modes, Mode 1 is the fatal. The mode I fracture toughness of several materials is evaluated experimentally and compared in this study.

Basic conceptual scheme of the threshing and separating device of the flax fiber combing machine

Harvesting of flax for fiber and seed production is carried out depending on the specific conditions by different technologies: direct, separate and factory. A technical and technological study of separate harvesting of flax fiber by improving the basic conceptual scheme of the picker-combing machine is presented. The traditional technologies of harvesting flax fiber and technical means that ensure the implementation of technological operations related to the separate technology of harvesting are considered. Disadvantages of the technical means used in the separate harvesting technology of flax fiber are revealed. They consist in varying degrees of impact of threshing and separating combing bodies of harvesting machines of both rake and rotor type on the stems, which causes uneven curing along their length and leads to the deterioration of fiber quality, reduction of the number of extra valuable long fiber and losses of the seed part of the crop. A conceptual scheme of the basic threshing and separating device of a roll-fulling type for separate technology of flax fiber harvesting by improving the picker- combing machine is proposed. The proposed device variant provides simultaneous threshing of flax fiber strips and creates favorable conditions for quality curing of straw to the state of flax straw simultaneously along the entire stalk length by its uniform conditioning in the gaps between the rubberized rollers. The combing header equipped with a threshing and separating device of the roll-fulling type allows to separate the seeds from the uncombed flax strip regardless of the location of the seed part relative to the movement direction of the harvesting machine. Versatility, simplicity and wide range of application of the threshing and separating device of the roll-fulling type are reasonable to use when designing machines for harvesting flax fiber for direct, separate and factory harvesting technologies.

Multi-Objective Optimisation of Curing Cycle of Thick Aramid Fibre/Epoxy Composite Laminates.

Aramid fibre-reinforced epoxy composites (AF/EP) are promising materials in the aerospace, transportation, and civil fields owing to their high strength, high modulus, and light weight. Thick composite laminates are gradually being applied to large composite structures such as wind turbine blades. During curing, temperature overheating is a common problem in thick composites, which leads to matrix degradation, thermal residual stresses, and uneven curing. This paper proposes a signal-to-noise ratio (SNR) method to optimise the curing cycle of thick AF/EP laminates and reduce the overheating temperature. During curing, the temperature and strain evolution in a thick AF/EP laminate were monitored using fibre Bragg grating sensors. The effects of the curing factors on the overheating temperature of the thick AF/EP laminate were evaluated using the Taguchi method and predicted via the SNR method and analysis of variance. The results indicate that the dwelling temperature is the main factor affecting the overheating temperature. The optimal curing cycle involves an overheating temperature of 192.72 °C, which constitutes an error of 2.58% compared to the SNR method predictions. Additionally, in comparison to the initial curing cycle, the overshoot temperature in the optimised curing cycle was reduced by 58.48 °C, representing a reduction ratio of 23.28%.

Adjusting the accuracy of PEGDA-GelMA vascular network by dark pigments via digital light processing printing.

Vascularization is one of the most important factors greatly influencing scaffold regeneration. In this study, a precise network of hollow vessels was printed by digital light processing (DLP) with poly(ethylene glycol) diacrylate (PEGDA)/gelatin-methacryloyl (GelMA), and dark pigmentation absorbers were added to ensure printing accuracy. First, the compound bio-inks of the PEGDA-GelMA hydrogel were prepared for direct vascular printing, and a high-precision DLP system was established. Second, the printing effects of three dark absorbers, namely, nigrosin, brilliant black, and brilliant blue, on the x-, y-, and z-axes were studied. By printing models with different densities, it was determined that 0.2% nigrosin, 0.1% brilliant black, and 0.3% brilliant blue had better effects on the x- and y-axes accuracy, and the absorbance of the absorbers played a decisive role in adjusting the accuracy. Additionally, to solve the problem of uneven curing on the upper and lower surfaces caused by the addition of an absorber with high absorbance, a model of the difference in curing width between the upper and lower surfaces of a unit-layer slice based on high-absorbance absorbers was established, and the reference value for the slice thickness was calculated. Third, the biological and mechanical properties of the bio-inks were verified with scanning electron microscopy and Fourier transform infrared, and by tensile, swelling, degradation, and cytotoxicity tests on different concentrations of PEGDA-GelMA hydrogel and absorbers. The results showed that 30% PEGDA-7% GelMA/0.1% brilliant black was the optimal preparation to print a hollow vascular network. The error of the printing tube wall and cavity was between 1% and 3%, which demonstrates the high precision of the method. Human umbilical vein endothelial cells were planted in the lumen, and the survival rate achieved 107% on the seventh day, demonstrating the good biocompatibility of the composite hydrogel.

Regulation of Mechanical Properties of Advanced PCM Products with the Aid of Catalysts

The article considers the problem of the effect of uneven curing caused by the temperature gradient across the thickness of the material on the anisotropy of the strength properties of polymer composite materials. The effect of catalysts on the curing of the epoxy binder EDT-69N, used for the manufacture of multilayer polymer composite materials, was studied. According to dielectric spectrometry, the accelerating effect of the selected compounds on the curing process of the EDT-69N epoxy binder during fiberglass molding has been proved. The possibility of controlling the curing process using catalysts to reduce the influence of the temperature gradient on the anisotropy of the strength properties of the matrix in the manufacture of polymer composite materials is shown.

Regulation of Mechanical Properties of Advanced PCM Products with the Aid of Catalysts

The article considers the problem of the effect of uneven curing caused by the temperature gradient across the thickness of the material on the anisotropy of the strength properties of polymer composite materials. The effect of catalysts on the curing of the epoxy binder EDT-69N, used for the manufacture of multilayer polymer composite materials, was studied. According to dielectric spectrometry, the accelerating effect of the selected compounds on the curing process of the EDT-69N epoxy binder during fiberglass molding has been proved. The possibility of controlling the curing process using catalysts to reduce the influence of the temperature gradient on the anisotropy of the strength properties of the matrix in the manufacture of polymer composite materials is shown.

The use of inhibited layers to improve mechanical properties of thermoset composite materials

The present paper discusses the research data on strength properties of polymer composite materials using fiberglass reinforced plastic (FRP) as an example. During the process of fiberglass reinforced plastic curing with the use of hot molding, a thermal gradient inevitably appears throughout the material thickness. This phenomenon results in uneven curing of prepreg layers. Internal stress consequently occurs in the material. It is proposed to control the kinetics of binder curing throughout the material thickness by introducing an inhibitor into the various prepreg layers. Multichannel dielectric analysis was used to study the curing kinetics of fiberglass reinforced plastic. It has been found that the time spread for gelation is 8.6 min in the 15-layer sample. The study of strength properties of the obtained samples demonstrates an increase in flexural strength of PCM-modified samples by 12.7% – 17.7% and a reduction in their anisotropy.

Optimizing curing process of graphene oxide/waterborne epoxy blends by curing kinetics simulation considering the coupling of heat conduction and curing reaction

Curing kinetics of the waterborne epoxy emulsion (EPWE)/Graphene oxide (GO) system was investigated by non-isothermal differential scanning calorimetry (DSC), which shows an autocatalytic behavior and can be well predicted by model fitting kinetic method. By combining with the kinetics simulation and the conventional extrapolation method, an novel three-stage curing process with a high initial cure temperature and a cooling stage was proposed to inhibit the thermal shock inside materials and the uneven curing reaction during the curing process of the materials. The experimental results indicate that compared to the conventional two-stage curing process, the new curing procedure containing a cooling section significantly increased the bending strength by 14.9%, elastic modulus by 7.3% and Tg by 8 K of the cured GO/EPWE material, and the total curing time clearly decreased.

Boundary Control of Embedded Heaters for Uniform Bondline Temperatures During Composite Joining

This paper investigates an out-of-autoclave (OoA), embedded-resistive heating method to precisely control the bondline temperature when curing high strength adhesives for joining composite adherends. A challenge with OoA methods is that nonuniform heat loss, e.g., due to substructures that act as local heat sinks, can lead to nonuniform temperatures in the bondline, which in turn, can result in uneven curing, residual stresses, and potentially weak joints. The main contribution of this work is to apply a voltage pattern at the boundary of the embedded heater to control the distribution of the electrical power at the interior bondline, and thereby reduce temperature variations. Additionally, this work devises an empirical model (that can be applied when material parameters and models are not readily available) to predict the desired power generation, and to design the embedded heater and voltage pattern that minimizes the bondline temperature variation. The technique is demonstrated experimentally for bonding a single-lap joint, and the maximum temperature variation in the bond area was reduced by five times from 31.6 °C to 6.0 °C.

Development of a predictive model for choosing cure agents for elastomer blends

ABSTRACTIn elastomer blends with either polarity differences or unbalanced double bond concentration, cure agents have a preferential partition to one of the phases, with a tendency of having overcure in one phase and undercure in the other. To achieve proper vulcanization and improved performance in the final product, it is essential to have a correct balance between solubility and miscibility of each of the various cure agents in the elastomer blend composite. Cure agents with varying polarities and various reactivities have different crosslinking densities in the rubber compound. A homogeneous crosslinking density leads to superior mechanical properties. The method presented in this work includes a new tool based on a genetic optimization algorithm for assessing the partitioning of cure agents in different elastomers and their blends. The quantitative data allowed for a series of analyses of the solubility of the cure agents in the elastomer mixtures of different phases and was validated by correlation with their physico–chemical and mechanical properties of the resulting blend. This is an important tool for planning a cure system in rubber blends and to avoid incorrect partitioning of cure agents and consequent uneven curing of the final compound. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 41929.

Irradiation System Design of Rotating Painting Ink Curing Based on UV LED

The effects of painting ink’s color on UV absorptivity and UV-light wavelength on curing depth in the process of rotating curing are analysed. Aiming at radial uneven curing, the scheme is proposed that using the UV LED with wavelengths of 365nm and 395nm to form a fan-shaped array on cylindrical surface. The curing system can achieve UV irradiance be enhanced gradually along the radial, and the technique problem of uneven curing is solved. The simulation result shows that the curing system meets design requirements on Tracepro.

Experimental Study of the Packaging Failure for Optical Fiber Arrays

Optical fiber array is a key component in the assembly of planar optical waveguide devices and enable greater channel counts than previously. However, assembling optical fiber array has been a challenge due to the stringent performance and reliability requirements posed by optical fiber communication applications. This paper investigates the causes of the failures involving the adhesive including: surface contamination, air entrapment, uneven curing, curing shrinkage; and thermal expansion coefficient mismatch.

Uneven curing induced interfacial delamination of UV adhesive-bonded fiber array in V-groove for photonic packaging

The common approach to attaching a large number of fibers to a guided-wave device is to fabricate a linear array using V-grooves. UV-curable adhesives are used to attach the fiber in V-groove for the fabrication of fiber array. Interfacial delaminations at the adhesive fiber interfaces are common after the reliability test due to uneven curing of the UV adhesive. This paper investigated the causes of uneven curing that gave rise to delamination. An analytical ray tracing technique was used to estimate the variation in light intensity or shadowing during the UV curing of adhesive. These effects were found to be very severe when the refractive index difference between the adhesive and the cladding materials is very large. Based on this study, it can be concluded that the delamination problem can be minimized by selecting a UV-curable adhesive having the same refractive index of the cladding material. It is also recommended to light illuminate from the topside for fiber packaging

Delamination Problems of UV-Cured Adhesive Bonded Optical Fiber in V-Groove for Photonic Packaging

In this letter, we investigated the origin of the interfacial delamination due to uneven curing of ultraviolet (UV) adhesive in the fixing process of a fiber on a V-groove. The analytical ray tracing technique is used to estimate the UV intensity at different locations along the adhesive-fiber interface. Consequently, the unexposed or shaded region in the adhesive layer is determined, through which the interfacial delamination as a result of the reliability test is predicted.

Analysis of thin-walled structures by finite strip and finite layer methods

The finite strip and finite layer methods are powerful tools for the analysis of thin-walled structures. In this paper, the finite strip method is applied to study the behavior of cold-formed steel beams including webs with longitudinal stiffeners. Comparisons are made with AISI specifications and published data. The finite layer method is used to investigate the buckling behavior of sandwich panels with thin facings and rigid foam cores. Effects of variable core stiffnesses (due to uneven curing, etc.) on the buckling strength are quantified and presented.

Uneven curing in epoxy resins

Uneven curing in epoxy resins