Non-destructive damage sensing and cure monitoring of carbon fiber/epoxyacrylate composites with UV and thermal curing using electro-micromechanical techniques

Interfacial properties and cure monitoring of single-carbon fiber/thermosetting composites by thermal and different ultraviolet (UV) curing processes were investigated using electro-micromechanical test and electrical resistance measurement. During curing process the residual stress was monitored in terms of the electrical resistance and then they were compared to various curing conditions. In thermal cure the tensile strength and modulus of epoxyacrylate matrix were higher than those of UV cure, whereas the failure strain was lower. Interfacial shear strength (IFSS) increased gradually with elapsing UV exposing time and then saturated. For thermal cure the IFSS was significantly higher than that of UV cure, and cure shrinkage was observed due to matrix shrinkage and residual stress due to the difference in thermal expansion coefficient (TEC). The difference in electrical resistance, ΔR during thermal curing was larger than that of UV cure. In thermal cure apparent modulus indicating embedding matrix modulus and interfacial adhesion was highest and reaching time up to same stress was shortest. Thermal cure showed the strong durability against the IFSS deterioration after boiling test compared to UV cure.     

Composite cure simulation scheme fully integrating internal strain measurement

This study proposes a new approach to determine key material parameters for stress/strain calculation of curing composite laminates and validate the simulation. Specifically, fiber Bragg grating (FBG) strain sensors are embedded in a composite laminate and the two key parameters for simulation, composite shrinkage strain and stiffness change during curing, are simultaneously determined from in-situ measurements by the embedded sensors. Furthermore, the simulation is validated using internal strain change during curing. This paper begins by presenting an overview of the proposed simulation scheme and by comparing it with previous approaches to highlight its advantages. Material parameter determination using a shear-lag effect at the edge of the embedded sensors is then described and the practical procedure to obtain the key parameters is demonstrated using a carbon/epoxy laminate. Finally, cure simulation is conducted for validation. Further extension to more general cases including thermoplastic composites is also discussed.     

Optimization of infrared radiation cure process parameters for glass fiber reinforced polymer composites

Elevated temperature post curing is one of the most critical step in the processing of polymer composites. It ensures that the complete cross-linking takes place to produce the targeted properties of composites. In this work infrared radiation (IR) post curing process for glass fiber reinforced polymer composite laminates is studied as an alternative to conventional thermal cure. Distance from the IR source, curing schedule and volume of the composite were selected as the IR cure parameters for optimization. Design of experiments (DOE) approach was adopted for conducting the experiments. Tensile strength and flexural strength of the composite laminate were the responses measured to select the final cure parameters. Analysis of variance (ANOVA), surface plots and contour plots clearly demonstrate that the distance from the IR source and volume of the composite contribute nearly 70% to the response functions. This establishes that polymer composites cured using IR technique can achieve the same properties using only 25% of the total time compared to that of conventional thermal curing.     

In situ assessment of carbon nanotube flow and filtration monitoring through glass fabric using electrical resistance measurement

Filtration of nanofillers into porous fabric media is still an issue during the preparation of advanced fiber-reinforced composites. The assessment of resin/multiwall carbon nanotube (MWCNT) flow, MWCNT filtration, and the cure monitoring of glass fiber/carbon nanotube-polyester composites by means of the measurement of the electrical resistance was introduced. The vacuum-assisted resin transfer molding technique was used. The electrical resistances measured over the span of a composite were qualitatively correlated with MWCNT flow and the degree of MWCNT filtration. It was found that while the complexity of the fabrics could likely introduce preferential deposition of MWCNTs, their filtration is mainly affected by their dispersion state in the resin suspension. Relationships among critical parameters such as the lengths and diameters of MWCNTs, the inter- and intra-tow dimensions of glass fabrics, the dispersion level of MWCNTs, and the viscosity of nanocomposite samples are discussed and correlated to the filtration, cure, and flow phenomena. We showed that our method can also serve as an early warning to obviate defects in the resulting composite.     

In situ measurements of through-the-thickness strains during processing of AS4/8552 composite

A novel approach has been developed to measure in situ the through-the-thickness strains during processing of epoxy composites. The technique captures the thermal expansion during the heating stages, the laminate consolidation throughout the cure process, and also the cure shrinkage. The tests were performed on 45 mm × 45 mm × 4 mm samples, for both unidirectional and cross-ply laminates. A ply pull-out technique was used to determine the position of the gel point of the AS4–8552 composite system for various curing temperatures. The relationship between the chemical shrinkage and the degree of cure was deduced from a cure kinetics model. Finally the relative contributions of cure shrinkage, thermal expansion and consolidation to the through-thickness strain were distinguished.     

The development of a dielectric system for the on-line cure monitoring of the resin transfer moulding process

The aerospace industry has identified the need for an on-line cure monitoring system for the resin transfer moulding (RTM) process which can determine the through-thickness cure state of a composite, without affecting the integrity of the finished component. Several techniques have been extensively investigated but dielectric analysis (DEA) appears to offer the greatest potential. The parallel plate sensor configuration is appropriate for through-thickness measurements. Using a laboratory dielectric instrument, dielectric properties in fibre (conductive and non-conductive) reinforced composite samples have been measured during a simulated RTM cure cycle. Particular parameters derived from dielectric measurements have been shown to be useful in terms of monitoring and optimising the RTM cure cycle. These parameters can be used to identify the key stages in the curing process and to estimate the values of the resin properties at these stages. Correlation of key dielectric events with other thermal data has been shown. Sensors currently are being developed with a view to incorporation into the RTM mould. The ultimate aim of this work is the development of an on-line cure monitoring system for the RTM process in collaboration with Bombardier Shorts.     

Damage detection of glass fiber reinforced composites using embedded PVA–carbon nanotube (CNT) fibers

Polyvinyl alcohol–carbon nanotube (PVA–CNT) fibers were embedded in glass fiber reinforced plastic composites and used as strain sensors for damage monitoring of the composite. Sensing of the structural integrity of the composite was made by the in situ measurement of the electrical resistance of the embedded PVA–CNT fiber during the mechanical tests. The multi-functional materials were tested in tensile progressive damage accumulation (PDA) tests. These tests aimed to seek the electrical response of untreated and pre-stretched PVA–CNT fibers with known level of progressively induced damage to the composite. The advantages and disadvantages of each PVA–CNT fiber used as a sensor are analyzed; the electrical resistance readings of the PVA–CNT fibers were correlated with known parameters that express the induced damage of the composite.     

Nondestructive sensing evaluation of surface modified single-carbon fiber reinforced epoxy composites by electrical resistivity measurement

Nondestructive sensing of a single-carbon fiber reinforced epoxy composites was evaluated by the measurement of electrical resistivity under reversible cyclic loading. For the strain–stress sensing, the strain up to the maximum load of a bare carbon fiber itself is larger than that of carbon fiber composite. As curing temperature increased, apparent modulus up to the maximum load increased and the elapsed time became shorter. Higher residual stress might contribute to the improved interfacial adhesion. The strain up to the maximum load at low temperature was larger than that at higher temperature. The strain of electrodeposition (ED) treated carbon fiber was smaller than that of the untreated carbon fiber composite until the maximum load reached. This could be due to higher apparent modulus of composite based on the improved interfacial shear strength (IFSS). Since the electrical resistivity was responded well quantitatively with various parameters, such as matrix modulus, the fiber surface modification, the electrical resistivity measurement can be a feasible method of nondestructive sensing evaluation for conductive fiber reinforced composites inherently.     

Rapidly cured epoxy/anhydride composites: Effect of residual stress on laminate shear strength

The drive towards rapid cure thermosetting composites requires a better understanding of the residual stresses that develop during curing. This study investigates the impact of residual stresses on the interlaminar shear strength of resin-infused epoxy/anhydride carbon-fibre laminates. The magnitude of the residual stress was varied by changing the initial injection cure temperature between 75 °C and 145 °C. The corresponding cycle times and the final glass transition temperature of the resin were also measured. The experimentally measured chemical shrinkage and thermal expansion properties of the resin after vitrification were used as inputs to a finite element analysis to calculate the peak residual stresses in the composite. An increase in the initial cure temperature from 85 to 135 °C resulted in an increase of 25% in the residual stress, which led to an experimentally measured reduction in the composite’s short beam shear strength of approximately 16% (8 MPa), in good agreement with model prediction.     

Interfacial evaluation and self-sensing on residual stress and microfailure of toughened carbon fiber/epoxy-amine terminated (AT)-polyetherimide (PEI) composites

Interfacial evaluation and self-sensing of tensile loading/subsequent unloading and microfailure detection of the carbon fiber/epoxy-amine terminated (AT)-polyetherimide (PEI) composites were investigated using micromechanical test and electrical resistance measurement with an aid of acoustic emission (AE). As AT-PEI content increased, both fracture toughness of epoxy-AT-PEI matrix and interfacial shear strength (IFSS) increased due to the optimized matrix modulus for energy absorption. With increasing curing temperature and time, the IFSS increased and then decreased. During curing process, the change in electrical resistance, ΔR increased gradually with adding AT-PEI contents because of different thermal and curing shrinkage of epoxy matrices. Moisture adsorption under durability test could cause to the change in matrix modulus and thus resulted in the change in electrical resistivity correspondently. Under changeable cyclic loading/subsequent unloading, apparent modulus and electrical resistivity during curing process were consistent well with the fracture toughness of epoxy modified with AT-PEI. In compressive test, the electrical resistivity decreased gradually initially and then increased rapidly during subsequent progress of microfailure including fiber fracture showing the buckling pattern.     

Material characterization of a polyester resin system for the pultrusion process

In the present work, the chemo-rheology of an industrial “orthophthalic” polyester system specifically prepared for a pultrusion process is characterized. The curing behaviour is first characterized using the differential scanning calorimetry (DSC). Isothermal and dynamic scans are performed to develop a cure kinetics model which accurately predicts the cure rate evolutions and describes the curing behaviour of the resin over a wide range of different processing conditions. The viscosity of the resin is subsequently obtained from rheological experiments using a rheometer. Based on this, a resin viscosity model as a function of temperature and degree of cure is developed and predicts the measured viscosity correctly. The evolution of the storage and loss moduli are also measured as a function of time using the rheometer which provides an information about the curing as well as the gelation. The temperature- and cure-dependent elastic modulus of the resin system is determined using a dynamic mechanical analyzer (DMA) in tension mode. A cure hardening and thermal softening model is developed and a least squares non-linear regression analysis is performed. The variation in elastic modulus with temperature and phase transition is captured for a fully cured resin sample.     

Optimization of composite patch repair processes with?the?use of genetic algorithms

The aim of this contribution is the optimization of some parameters of the composite patch repair technique (CPR). This technique is mainly used by the aircraft industry, as it offers high reliability, short repair times and reduced cost in compare to other methods, such as the riveted joints. CPR consists of adhesively bonding thin composite patches over cracked or corroded areas with heat supply. As the polymer-matrix composite patch is heated, it cures and toughens. Proper curing insures structural reliability of the repair. Short duration curing cycles are of great importance for the aircraft availability. With the use of Genetic Algorithms, we design minimum time curing cycles. The optimization is subjected to the following constraints: (1) Maximum allowed temperature in order to avoid residual stresses, (2) Minimum temperature in order to initiate the cure reaction, (3) Sufficient degree of cure at the end of the process and (4) Maximum heat generation rate that can be achieved by the device. Our design vector contains the duration of the plateau stage of the cure cycle and the characteristic thermal profile. The degree of cure is estimated with the use of the Kamal cure rate model for thermosetting polymers. For the numerical time integration of the cure rate equation, a second order, implicit Runge-Kutta scheme is employed.     

Anisotropic Dielectric Properties of Carbon Fiber Reinforced Polymer Composites during Microwave Curing

Microwave cuing technology is a promising alternative to conventional autoclave curing technology in high efficient and energy saving processing of polymer composites. Dielectric properties of composites are key parameters related to the energy conversion efficiency during the microwave curing process. However, existing methods of dielectric measurement cannot be applied to the microwave curing process. This paper presented an offline test method to solve this problem. Firstly, a kinetics model of the polymer composites under microwave curing was established based on differential scanning calorimetry to describe the whole curing process. Then several specially designed samples of different feature cure degrees were prepared and used to reflect the dielectric properties of the composite during microwave curing. It was demonstrated to be a feasible plan for both test accuracy and efficiency through extensive experimental research. Based on this method, the anisotropic complex permittivity of a carbon fiber/epoxy composite during microwave curing was accurately determined. Statistical results indicated that both the dielectric constant and dielectric loss of the composite increased at the initial curing stage, peaked at the maximum reaction rate point and decreased finally during the microwave curing process. Corresponding mechanism has also been systematically investigated in this work.     

双酚A型环氧树脂/催化型固化剂体系的粘度模型

研究了催化型固化剂用量对双酚A型环氧树脂体系粘度变化的影响规律。根据对树脂固化特性和等温粘度曲线的分析,建立了树脂体系的工程粘度模型。该模型能够有效地预测体系的粘度-时间-温度关系,反映了固化剂用量对固化过程中体系粘度变化的影响规律,为复合材料成型工艺模拟分析及工艺参数的准确制定奠定了基础。      

Evolution of cure,mechanical properties,and residual stress during electron beam curing of a polymer composite

Evolution of cure, mechanical properties, and residual stress during E-beam (Electron Beam) processing was studied to evaluate the influence of process parameters – dose and dose per pass – using an epoxy reinforced with IM7 carbon fibers. The composite prepreg was also cured thermally to various cure levels and compared with the E-beam cured composite. Cure evolution changed substantially with irradiation condition; lower dose/pass and wider scanning width of the beam (for the same dose/pass) resulted in rapid curing. For a given degree of cure, the longitudinal (E₁₁) and transverse (E₂₂) moduli of the E-beam cured composite varied with dose/pass and were less than that of the thermally cured composite. Transverse strength and failure strain of the composite cured at 20 kGy/pass were higher than that of composites cured thermally and at other dose/pass conditions. E-beam curing resulted in lower residual stresses than thermal curing and lower dose per pass resulted in lower cure-induced residual stress than higher dose/pass.     

Curing of Thick Thermoset Composite Laminates: Multiphysics Modeling and Experiments

Fiber reinforced polymer composites are used in high-performance aerospace applications as they are resistant to fatigue, corrosion free and possess high specific strength. The mechanical properties of these composite components depend on the degree of cure and residual stresses developed during the curing process. While these parameters are difficult to determine experimentally in large and complex parts, they can be simulated using numerical models in a cost-effective manner. These simulations can be used to develop cure cycles and change processing parameters to obtain high-quality parts. In the current work, a numerical model was built in Comsol MultiPhysics to simulate the cure behavior of a carbon/epoxy prepreg system (IM7/Cycom 5320–1). A thermal spike was observed in thick laminates when the recommended cure cycle was used. The cure cycle was modified to reduce the thermal spike and maintain the degree of cure at the laminate center. A parametric study was performed to evaluate the effect of air flow in the oven, post cure cycles and cure temperatures on the thermal spike and the resultant degree of cure in the laminate.     

复合材料固化相关黏弹性性能演化及残余应力分析

通过对复合材料固化度和温度相关黏弹性本构方程的分析,定义一个能综合反映固化度和温度等对复合材料黏弹性性能影响的无量纲参数De_m。当参数De_m都大于10~2时,复合材料基体处于流动状态;当参数De_m都小于10~(-2)时,复合材料为弹性状态;仅当部分参数De_m小于10~2而大于10~(-2)时,复合材料处于黏弹性状态。以AS4纤维/3501-6树脂复合材料为例,基于对其参数De_m在典型固化工艺过程中的演化,研究该复合材料黏弹性性能的发展过程,发现基于参数De_m分析得到的凝胶点时间与实验结果一致。根据复合材料黏弹性性能对残余应力发展的影响,将复合材料残余应力计算分为流动阶段和黏弹性阶段,并建立了相应的状态相关黏弹性本构模型。最后通过与原始模型预测结果的比较验证了提出的本构模型,表明本文提出的计算方法与原始黏弹性本构模型计算结果一致,但大大降低了计算所需的时间和存储空间。     

In situ monitoring of the strain evolution and curing reaction of composite laminates to reduce the thermal residual stress using FBG sensor and dielectrometry

Generally, a large, thermal residual stress is generated during the curing process for composite laminates due to differences in the coefficients of thermal expansion of the respective layers. The thermal residual stress during fabrication greatly decreases the fatigue life and dimensional accuracy of the composite structures. In the present study, through a fiber bragg grating (FBG) sensor and dielectrometry in an autoclave, the strain evolution and curing reaction in composite laminates with a stacking sequence of [0₅/90₅]_S were monitored simultaneously during a conventional cure cycle and a modified cure cycle to reduce the thermal residual stress. From the study, it was verified that about 50% of the thermal residual stress during fabrication could be reduced in a composite laminate by adjusting the cure cycle; this improved the static strength and fatigue life by 16% and up to 614%, respectively, for a peak ratio of 0.9.     

Consolidation of polymer-derived SiC matrix composites: processing and microstructure

SiC fiber reinforced SiC matrix composite (SiC/SiC composite) has been developed by polymer impregnation and pyrolysis (PIP) method, which consists of impregnation, curing, consolidation, and re-impregnation and pyrolysis. As a prospective approach to fabricate a high performance composite, consolidation conditions, such as curing temperature to make a green body, pressure and heating rate during consolidation, were systematically controlled for effective consolidation. Because of its advantage in controlling physical characteristic, polyvinylsilane (PVS) that is liquid thermosetting organo-silicic compound was utilized as the matrix precursor. Based on the pyrolytic behavior of PVS, effects of the process conditions on microstructure of the consolidated bodies were accurately characterized. To relate those microstructure with mechanical property, flexural tests were performed for the composites after multiple PIP processing. Consequently, process conditions to make a high performance composite could be appeared. Structural conditions to be optimized for further improvement in mechanical and environmental properties were also discussed.     

Enhancement of dielectric and electrical properties in BT/SiC/PVDF three-phase composite through microstructure tailoring

A polymer composite with high dielectric permittivity was prepared by embedding silicon carbide (SiC) whisker with an average diameter of 500 nm–1 μm in poly(vinylidene fluoride) (PVDF). However, the high dielectric loss and electrical conductivity of the two-phase composite prohibits its potential applications. Barium titanate (BT) particles with average diameter of 100 nm and 1 μm were incorporated as a third phase to fabricate a three-phase composite. The morphology structure, dielectric and electrical properties before and after the addition of BT particles were investigated. The three-phase composite exhibits largely suppressed dielectric loss and electrical conductivity without sacrificing the high dielectric permittivity, which was extremely hard to be realized for two-phase composite. It is also found that the nano-size BT is more favorable in achieving high dielectric permittivity than the micro-size BT, where their dielectric loss and electrical conductivity are similar. Furthermore, electric modulus analysis confirms the largely suppressed electron conduction process which results in the enhanced dielectric and electrical properties in three-phase composite.