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.     

Neural-fuzzy optimization of thick composites curing process

This article addresses the optimization of curing process for thick composite laminates. The proposed methodology aims at the evaluation of the thermal cycle promoting a desired evolution of the degree of cure inside the material. At the same time, temperature overshooting as well as excessive temperature and cure degree gradient through the thickness of the material are prevented. The developed approach is based on the integrated application of artificial neural networks and a fuzzy logic controller. The neural networks promptly predict the behavior of composite material during curing process, while the fuzzy logic controller continuously and opportunely adjusts the proper variations on the imposed thermal cycle. The results highlighted the efficiency of the method in comparison with the cure profiles dictated by the material suppliers. For thick laminates, a reduction of 35% of cure time and improvements of approximately 10% of temperature overshooting was obtained compared to conventional curing cycles. The method was validated by experimental tests.     

Reduction of residual stresses in thick-walled composite cylinders by smart cure cycle with cooling and reheating

The nozzle parts of solid rocket motors must endure both the internal pressure generated by high temperature exhaust gas and the mechanical load generated by steering operation. Therefore, the nozzle parts of solid rocket motors are fabricated with thick carbon fiber phenolic resin composites. When the thick-walled phenolic composite cylinder is cooled down from the curing temperature of about 155 °C to the room temperature, thermal residual stresses are created due to the anisotropic thermal deformation of the composite structure.

In this paper, a smart cure method with cooling and reheating was developed to reduce residual stresses in thick-wound composite cylinders made of carbon phenolic woven composite. The optimal cure cycle was obtained to reduce the residual stresses without increasing processing time and applied to fabrication of the thick-walled composite cylinder. From the residual stresses measured by the radial-cut-cylinder-bending method, it was found that the residual stresses were reduced 30% by using the smart cure method.     

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.     

Experimental study of tool–part interaction during autoclave processing of thermoset polymer composite structures

Autoclave manufacturing of thermoset polymer matrix composite structures with high dimensional fidelity requires a good understanding of various parameters affecting process-induced warpage and application of this knowledge to minimize the warpage through appropriate process control. One important contributor is the interaction between a composite part and the tool on which the part is laid and cured. This experimental study quantified the tool–part interaction by measuring the static and dynamic frictional coefficients as a function of process time, using a friction test fixture specially designed to simulate the autoclave environment. Temperature ramp rate was varied to understand the effect of autoclave cure cycle on the friction coefficients. Measured friction coefficients were maximum at the start of the cure cycle and varied as a function of degree of cure (α) and ramp rate owing to change in the tool–part interface, cure shrinkage, resin/composite properties, residual stress, and mode of interface failure.     

Cure cycle design for composite materials using computer simulation and optimisation tools

In the following we present a computer simulation tool coupled with a numerical optimisation method that have been developed for use in the optimal design of the cure cycle of the production of thermoset- matrix composite parts. Their use permits the fast and straightforward optimal design of the cure cycles and leads to improved quality, reduced production times and reduced scrap. For the simulation of the cure cycle a one-dimensional non-linear transient model has been developed and coupled with the Evolution Strategy, of the Genetic Algorithms family, for the optimal design of the cure cycle of thick composite parts taking into account various realistic restrictions and targets of the production. The effectiveness and the usefulness of the proposed approach are demonstrated for several target performances together with a comparison between optimisation methods for the optimal tuning of the cure cycle.     

OPTIMIZED CURING OF THICK SECTION COMPOSITE LAMINATES

Conventional autoclave curing cycles for thermosetting composite laminates are generally derived from trial-and-error experimentation. Cure cycles are readily available for thin composite laminates. However, developing cure cycles for thick section laminates is time consuming and hence, costly. In addition, one cannot be sure that a selected cycle will be optimum if it is based on a conventional method. This paper shows that through numerical simulation and some minimal experimentation, optimized cure cycles can be developed for thick laminates. The heating cycles for thick laminates can be established by an iterative numerical method. Autoclave processing with conventional and optimal curing cycles for 12.5-mm-thick laminates was performed. The mechanical properties of the products were determined and shown to be comparable, with significant time saving realized in using the optimized cycle.     

PC-Based Monitoring and Control System for Microwave Curing of Polymer Composites

Microwave, curing is increasingly being considered as an economically viable process in the production of polymer composite parts. Nevertheless, the improper control of microwave energy for industrial production, especially for the processing of thick laminates, can lead to quality problems such as the formation of voids, non-uniform heating, and over curing. In this paper, the design and development of Cure-Control for the monitoring and control of the microwave curing of polymer composite components is discussed. In particular, this paper discusses some of the quality and production issues, as well as the control of the process parameters such as pressure, temperature and the rate of power intensity. Preliminary experiments confirm the viability of the Cure-Control system in the monitoring and controlling the curing of polymer composite components. The work has the potential of being commercialised on a large-scale, on-line basis.     

Effect of the smart cure cycle on the performance of the co-cured aluminum/composite hybrid shaft

In this work, a smart curing method for the co-cured aluminum/composite hybrid shaft which can reduce the thermal residual stresses generated during co-curing bonding operation between the composite layer and the aluminum tube was applied. In order to reduce the thermal residual stresses generated during co-cure bonding stages due to the difference of coefficients of thermal expansions (CTE) of the composite and the aluminum tube, a smart cure cycle composed of cooling and reheating cycles was applied. The heating and cooling operations were realized using a pan type heater and water cooling system. The thermo-mechanical properties of the high modulus carbon epoxy composite were measured by a DSC (differential scanning calorimetry) and rheometer to obtain an optimal time to apply the cooling operation. Curvature experiment of the co-cure bonded steel/composite strip was performed to investigate the effect of cure cycle on generation of the thermal residual stress. Also, the thermal residual stresses of the aluminum/composite hybrid shaft were measured using strain gauges with respect to cure cycles.

Finally, torsional fatigue test and vibration test of the aluminum/composite hybrid shaft were performed, and it has been found that this method might be used effectively in manufacturing of the co-cured aluminum/composite hybrid propeller shaft to improve the dynamic torque characteristics.     

Study on the curing process for carbon/epoxy composites to reduce thermal residual stress

In this work, a cure monitoring system using dielectrometry and a fiber Bragg grating (FBG) sensor, was devised to measure the dissipation factor and thermal residual stress of carbon fiber-reinforced epoxy composite materials. Three rapid-cooling points, which were based on the cure initiation point, were chosen as test variables to investigate the effect of cure cycle on process-induced internal strain. The internal strains generated in the composite specimens were measured using embedded FBG sensors. Three-point bending tests were conducted to investigate the effect of thermal residual stress on the flexural strength of the composite specimens.     

Cure behaviour of visible light activated dental composites

In this paper, the non-isothermal cure behaviour of a dental composite, activated by visible light, is described using a heat transfer model that, coupled with a reaction kinetic expression, is able to predict the temperature and the degree of reaction in the composite. The temperature and the degree of reaction profiles inside the composite are calculated, as a function of the cure time, taking into account the system geometry, the thermal diffusivity of the composite, and the resin reaction rate. Material properties, boundary and initial conditions and the kinetic behaviour are the input data of the heat transfer model. Once the degree of reaction is known, the glass transition temperature profiles across the thickness of the composite are calculated. Experimentally measured glass transition temperatures are used for the evaluation of an extinction coefficient capable of accounting for the effects of the light absorption through the thickness on the polymerization kinetics. Finally, the effects of the non-isothermal cure conditions on the application of these materials in dental restorations are discussed.     

Chemical and thermal shrinkage in thermosetting prepreg

A methodology for predicting residual cure deformation and stresses in composite laminates during cure is proposed. The technique employs an unbalanced cross-ply strip denoted as a “bi-lamina” strip to measure the in situ development of chemical and thermal shrinkage deformation during a specified thermal cycle. The constitutive model of the composite material was developed based on self-consistent micro-mechanical homogenization with variable resin thermo-mechanical material properties during the cure cycle. The resin properties were determined as a function of cure and temperature using different experimental techniques, including differential scanning calorimetry, digital image correlation, rheometry and dynamic mechanical analysis. The predicted bending deflection profiles of the strip agreed closely with experimental observations. The proposed methodology can be used to validate the material model of the resin and composite during the cure cycle.     

Study on the Effect of Cure Cycle on the Process Induced Deformation of Cap Shaped Stiffened Composite Panels

Cap-shaped stiffened composite panels offer many excellent properties such as low density, high strength, high stiffness to weight ratio, and design flexibility. During their manufacturing processes, however, thermo-curing inherently produces the undesired residual stresses and cure deformations, which limits the applications of composite structures in a certain degree. In order to reduce the cure deformation, in this paper, the effect of cure cycle (curing temperature, curing pressure, cooling rate) on the process-induced deformation of cap-shaped stiffened composite panels was presented. A simple mathematical model based on the curing dynamics was established to predict the deformation of the cap-shaped stiffened composite panels. The deformation calculated by the mathematical model and experimental studies were compared, and an Error Correction Model was established. The Error Correction Model showed a good agreement with the experimental results.     

自动铺放工艺制备罐外固化复合材料的力学行为

针对碳纤维增强树脂基复合材料IM7/CYCOM5230-1罐外固化预浸料,研究了自动铺放（AFP）罐外固化（OOA）预浸料的制备过程并优化了铺放工艺参数,采用热分析手段研究了CYCOM5230-1树脂固化动力学及黏度特性,在此基础上开发了一种短时固化工艺,并评价了基于此工艺制备的OOA复合材料力学性能。结果表明,AFP铺放过程中预浸料间缝隙会影响OOA复合材料的成型质量,采用铺放压力为180 N、加热温度为50℃、铺放速率为0.20 m/s的铺放参数,可获得表面平整、成型质量优异的复合材料样件。热分析结果表明,罐外固化CYCOM5230-1树脂室温黏度大,满足OOA工艺中真空压实排气需求。短时固化工艺可达到与典型固化工艺相同水平固化度,提升了固化效率,且制备的复合材料可以达到59%的纤维体积分数及低于0.5%的孔隙率,其力学性能与典型固化工艺制备的复合材料相当,并且能够达到热压罐复合材料的水平。     

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.     

复合材料固化过程的智能化监控及其智能生产系统

复合材料由于其优良的性能而拥有广泛的应用领域,但其成型工艺复杂,成本高使其应用受到限制。针对成型过程开发智能化技术是提高生产效率和产品质量的有效手段,其中包括建立数学模型,以专家系统、神经网络或分步预测优化的模型系统为中心的在线监控系统。本文作者针对这些智能化工艺技术进行了总结分类和比较,对智能成型工艺系统进行了阐述,包括其功能、结构和辅助部件。分析了关于复合材料智能化生产技术的发展方向和社会效益。     

树脂基复合材料曲面结构件固化变形数值模拟

为了研究树脂基复合材料曲面结构件的固化变形过程,首先分析了碳纤维增强树脂基复合材料在固化过程中密度、模量、热膨胀系数、比热容及热传导系数等材料物性的变化,并将这些变化引入到数值模拟当中。接着,针对复合材料复杂曲面结构件,提出了利用定常流动的流线方程构建曲线坐标系的新方法。然后,根据建立的曲线坐标系,运用有限元法计算了某轻型飞机机翼上蒙皮板在固化过程中内部温度、固化度和内应力的分布情况以及材料物性随固化度的变化情况。最后,计算了由于内部温度场和固化度场的不均匀、热膨胀系数的各向异性和固化引起的树脂体积收缩而导致的结构变形。结果表明:引入材料物性变化使固化过程的数值模拟更加合理、模拟结果更加精确,利用定常流动的流线方程构建的曲线坐标系适用于复合材料曲面结构件的有限元分析。所得结论对研究树脂基复合材料的固化变形过程和各向异性复合材料复杂曲面构件的三维实体建模均具有指导意义。     

Smart cure cycles for the adhesive joint of composite structures at cryogenic temperatures

Adhesive joints are employed for composite structures used at the cryogenic temperatures such as LNG (liquefied natural gas) insulating tanks and satellite structures. The strength of the adhesive joints at the cryogenic temperatures is influenced by the property variation of adhesive and the thermal residual stress generated due to the large temperature difference (ΔT) from the adhesive bonding process to the operating temperature. Therefore, in this work, the strength and thermal residual stress of the epoxy adhesive at cryogenic temperatures were measured with respect to cure cycle. Also, the cure cycles composed of gradual heating, rapid cooling and reheating steps were applied to the adhesive joints to reduce the thermal residual stress in the adhesive joints with short curing time. Finally, a smart cure method was developed to improve the adhesive joint strength and to reduce the cure time for the composite sandwich structures at cryogenic temperatures.     

Inverse algorithm for optimal processing of composite materials

During thermoset composite materials processing, the chemical reaction is highly exothermic and because of the low thermal conductivity of the material, significant temperature and state of cure gradients can be generated in thick parts. This creates non-uniform stresses that provoke defects. We propose to control the transformation by monitoring the temperature of the mold walls. A general inverse analysis based on the conjugate gradient method of minimization associated to the adjoint equations is used. After having detailed the method, we propose two examples. The first one presents an optimal cycle to obtain uniform conversion at the end of the curing of an epoxy/glass-fiber composite. The second example is concerned with the control of the temperature variations during the curing of a polyester/glass-fiber composite. The method is experimentally validated and proves to be very powerful and flexible.     

Glass transition and viscoelastic behaviour of partially cured composites

For this study, two tests were conducted in order to investigate the cure monitoring of composite parts utilizing XMTM-49 (carbon/epoxy composite) as the specimen. The first test involved the use of a dynamic mechanical thermal analyzer to investigate the nature of the loss modulus while the second test incorporated a differential scanning calorimeter to evaluate the degree of cure of the composite.

From the results of the research, it was found that the loss modulus is an extremely sensitive cure monitoring indicator for composites beyond 70% cured. This is a significant finding since traditional ultrasonic procedures could only be effective in monitoring the cure of composite structures when the degree of cure reaches approximately 70% but decreases when the cure reaches 80% or more. Therefore, it is recommended that future developments should focus attention on a non-contact technique for measuring loss modulus for cure monitoring.