In this article, a novel method of measuring resin flow front under vacuum condition is presented. The in situ monitoring system with metal hollow probe based on gas flow balance can be used in resin film infusion (RFI) process, where resin film is used and transverse flow is dominated along thickness direction of fiber preform. The diameter of the probe was chosen to increase the measuring accuracy, and the reliability of the method was evaluated by comparison of visualization experiment. Experimental results demonstrate that the method is suitable for monitoring resin flow in RFI process with and without autoclave, and can obtain the information about resin filling time, nonuniform flow front, and the permeability of fiber preform. Furthermore, by means of the established monitoring system, the influences of pressure and lay-up sequence of carbon fiber fabric on epoxy resin flow during RFI process were investigated. In addition, resin flow pattern with changing viscosity of epoxy resin was studied. POLYM. COMPOS., 35:681–690, 2014. © 2013 Society of Plastics Engineers

Phenolic foams reinforced with pristine and functionalized multiwalled carbon nanotubes (MWCNTs) were fabricated to develop fire-resistant materials with improved mechanical properties. The influences of the contents of carboxyl multi-walled carbon nanotubes (MWCNTs-COOH) and of MWCNTs types on the compressive properties of the composite foams were investigated. The microstructure and detailed failure behavior of MWCNTs/phenolic composite foams were studied using scanning electron microscopy (SEM) and in situ quasistatic compression inside SEM, respectively. In addition, thermal performances were evaluated by thermogravimetric analysis (TGA) and vertical burning method. It is found that as heterogeneous nucleation agents, MWCNTs increase cell density and decrease cell size of the produced foams, and that as reinforcements located in cell walls, MWCNTs impart high strength and stiffness to brittle foams. Moreover, MWCNTs reinforced foams have higher thermal stability than raw foams and exhibit similar excellent resistance to flame, confirming the effectiveness of MWCNTs as stabilizers. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 1479–1488, 2013

Carbon fiber-reinforced epoxy composites, with incorporated carboxylic multiwall carbon nanotubes (CNTs), were prepared using vacuum-assisted resin infusion (VARI) molding, and the in-plane and out-of-plane properties, including mode-I (G_Ic) and mode-II (G_IIc) interlaminar fracture toughness, interlaminar shear strength (ILSS), tensile, and flexural properties were measured. A novel spraying technique, which sprays a kind of epoxy resin E20 with high viscosity after spraying the CNTs, was adopted to deposit the CNTs on the surface of carbon fiber fabric. The E20 was used to anchor CNTs on the fabric surface, avoiding that the deposited CNTs were removed by the infusing resin during VARI process. The spraying processing, including spraying amount and spraying sequence, was optimized based on the distribution of CNTs on the fibers. After that, three composite specimen groups were fabricated using different carbon fiber fabrics, including as-received, CNT-deposited with E20, and CNT-deposited without E20. The effects of CNTs on the processing quality and mechanical properties of carbon fiber-reinforced polymer composites were studied. The experimental results show that all studied laminates have uniform thickness with designed values and no obvious defects form inside the laminates. Compared with the composite without CNTs, depositing CNTs with E20 increases by 24% in the average propagation G_Ic, by 11% in the propagation G_IIc and by 12% in the ILSS, while it preserves the in-plane mechanical properties, However, depositing CNTs without E20 reduces interlaminar fracture toughness. These phenomena are attributed to the differences in the distribution of CNTs and the fiber/matrix interfacial bonding for different spraying processing. POLYM. COMPOS., 2013. © 2012 Society of Plastics Engineers

A new cocured process called coresin film infusion (co-RFI) process, which combines RFI process and prepreg/autoclave process, was introduced and four kinds of commercial carbon fiber prepreg material systems and a kind of resin film were applied to fabricate co-RFI laminates. The compatibility between the resin film and the prepreg matrix and the application of co-RFI process were investigated based on the resin flowability, glass transition temperature of cured resin, processing quality of laminate, and variation in resin modulus on cocured interphase region measured by nanoindentation. Furthermore, mode I (G_IC), mode II (G_IIC) delamination fracture toughness, and flexural strength and modulus were measured to evaluate the mechanical properties of cocured laminates with different prepreg materials. The experimental results show that thickness and fiber volume fraction of co-RFI laminates with the four kinds of prepreg materials are similar to those of prepreg laminates and RFI laminate with acceptable differences. In addition, there are no obvious defects in co-RFI laminates. Moreover, the reduced modulus of resin at cocured interface and glass transition temperature values of the mixed resin reflect good compatibility between prepreg matrix resin and RFI resin. The G_IC, G_IIC values, and flexural performances of cocured laminates lie between and even exceed those of prepreg laminates and RFI laminates, indicating no weakening effect in the cocured interface. Therefore, the co-RFI process is believed to effectively fabricate composite with low cost and it can be applied using various prepreg systems. POLYM. COMPOS., 34:2008–2018, 2013. © 2013 Society of Plastics Engineers

The changes in interfacial fracture energy of three kinds of commercially sized carbon fiber (CF)/epoxy resin composites in the range from ambient temperature to 130°C were investigated using the single-fiber fragmentation test to evaluate the heat resistance of the interphase. The effects of CF sizing on the interfacial bonding property were studied using desized CF/epoxy resin composites. Thermogravimetric analysis and differential scanning calorimetry of the combination of sizing and matrix were employed to investigate the role of sizing on the variations in the fiber/matrix interfacial property under elevated temperature. The interfacial fracture energy values of all the studied CF composites were found to decrease quickly during the initial stage of temperature rise and drop gradually at higher temperature. At elevated temperature, the desized CF composites had higher heat resistance than the corresponding sized fiber composites. The differences in the interfacial heat resistance among the three kinds of CF composites and the difference in the interfacial thermal stability between the sized and the desized fiber composites were related to different glass transition temperatures of the interphases. The interaction between sizing and the matrix and the chain motion of the crosslink structure of the interphase has been suggested to determine the interfacial heat resistance. POLYM. COMPOS., 2012. © 2012 Society of Plastics Engineers

The toughness of the interphase between carbon fibers and epoxy resin was characterized by the interfacial fracture energy, which was derived from the modified Wagner-Nairn-Detassis (WND) model, considering the moisture swelling stress. The characterization was used to evaluate the changes of interphase bonding before and after water aging, including boiling water and 70°C water immersion. The effects of the water aging on the parameters in WND model were analyzed. The mechanism of interphase degradation under water aging was interpreted considering the change of interphase thickness, which was measured using the dynamic nanomechanical mapping method. It is shown that the single-fiber fragmentation test with the proposed energy-based model can quantify the degradation of interphase toughness after water aging. For the studied system, the interphase thickness has a close relationship with the interfacial fracture energy, indicating that the swelling of the resin matrix and interphase results in an increase in interphase thickness and a decrease in interphase bonding property. POLYM. COMPOS., 2012. © 2012 Society of Plastics Engineers

Resin pressure is one of the most important parameters in manufacturing composites during autoclave process. It not only greatly influences resin flow behavior, but also has effects on void formation and elimination. Online monitoring resin pressure can provide an important guidance for the optimization of the processing parameters and the control of the quality of composites. In this study, a resin pressure online measuring system for autoclave process was established based on the principle of pressure transfer in liquid, and the size of the measuring probe of the system was optimized to increase the accuracy of measured resin pressure. The results indicate that the accuracy and the dynamic response of the system can meet the requirements of resin pressure measurement during autoclave process. Furthermore, by means of this proposed resin pressure measuring system and the measurements of compaction properties of the fabric stacks, the resin pressures inside carbon fiber fabric/epoxy resin and glass fiber fabric/epoxy resin prepreg stacks during autoclave process were investigated, especially for the zero-bleeding process which is prevailing for aircraft composite structures. It is demonstrated that during zero-bleeding process, the resin pressures, which conform to the spring and piston model, uniformly distribute along through-thickness and in-plane directions. In addition, the resin pressure profile is significantly influenced by the fiber volume fraction of the prepregs, indicating that fiber content of prepreg should be optimized for achieving free defects and uniform fiber distribution. POLYM. COMPOS., 2011. © 2010 Society of Plastics Engineers

For resin matrix composites, voids are common defects that can seriously deteriorate the properties of the composite parts. Thus, the elimination of voids is a crucial element in controlling the manufacturing process of composite parts. This article focuses on void formation originating from hygroscopic water for resin matrix composite laminates prepared with hot pressing process. The Kardos void formation model was developed to analyze the critical resin pressure for the initiation of voids, and the influencing factors were investigated experimentally to validate the modified model. It is found that resin pressure and gel temperature are the two key parameters to control void defects and that entrapped air in prepreg stacks must be considered in the void formation model. Furthermore, a simple method was established to measure the relationship between porosity and the processing parameters, and the void formation conditions of the resin and the prepreg stack were also studied. The theoretically predicted void formation conditions and the experimental results were compatible for the studied cases. These results are valuable for eliminating void defects, optimizing processing parameters, and enhancing the performance of composite parts. POLYM. COMPOS., 31:1562–1571, 2010. © 2009 Society of Plastics Engineers

The ability to predict the viscosity of thermoset resin is important to understand the manufacturing process of composites and optimize the processing parameters. During resin or prepreg storage course, the cure reaction may happen and the degree of cure increases gradually. The storage aging effect reduces the fluidity of resin, and hence alters the processability of resin. In this article, the rheological properties of an epoxy resin and a bismaleimide resin used in composite autoclave process were measured and a viscosity model was established, which can predict the viscosity progression during cure for different aging degree of resin. Moreover, a computer simulation method was used to study the effects of aging degree on the composite consolidation and the processing operations. It is found that the viscosity model of aged resin can be obtained by modified dual Arrhenius model of fresh resin with the dynamic rheological measurement. The resin aging strongly alters the flowability, so influences composite consolidation. According to the simulated results, the processing parameters need to be adjusted to achieve cured composites with appropriate fiber content. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers