Papers by Keyword: Cure Monitoring

Abstract: Ultrasonic testing is applied to monitor the curing process of GFRP laminates used for rehabilitation of aged sewerage pipes. Instead of conventional method in which through-thickness ultrasonic wave is used, wheel type probes are adopted to transmit and receive in-plane Lamb wave. It is well known that wave velocity depends on the stiffness of the propagating medium, so that it could be used as cure index of resin. However, because of the relatively low signal-to-noise ratio of detected signals, determination of wave arrival time with zero-crossing detecting method leads to nonnegligible error. In order to improve the precision of measurement, AIC (Akaike Information Criterion) is introduced and the arrival time of Lamb wave is monitored in curing process. Two types of waveform, pulse and burst waves are examined to confirm the effect of waveform on the accuracy of measurement. It is found that the arrival time of Lamb wave decreases with the increase of cure degree, and approach asymptotically to constant value during post cure process. Also, AIC analysis with pulse wave is found to be more reliable to monitor the early stage of resin cure.

Abstract: The article discusses the results of experiments on the further development of the method for monitoring the state of the small samples of resin and polymers during curing. The applied method is based on a change in the structure of oscillating transducers leading to variations in the form and/or mode of oscillation. The thin layer of epoxy resin was placed between two piezoelectric transducers in the form of piezoceramic plates. Curing epoxy resin forms a bimorph and its characteristics change along with variations in viscosity or, after the gel point, stiffness. It is possible to establish the level of epoxy resin curing by monitoring changes in the resonance parameters of bimorph elements. The main purpose of cure monitoring of small samples is to develop a new method for evaluating the parameters of resin both before and after the gel point thus taking into consideration that the use of rheological data measured with reference to small samples will be applied for designing or interpreting bulk-flow processes where epoxy may be considered a continuum.

Abstract: Optimizing the curing temperature to reducing the process costs may influence the properties of the material and bring quality assurance aspects. To solving the problem, real-time strain monitoring of the curing process has become more and more important and urgent. Fiber Bragg grating sensors were successfully used in this work to monitoring the residual strain building-up and the gelling phenomena of an RTM epoxy resin. The results shows chemical shrinkage strain during isothermal curing is bigger for higher curing temperature, but the average linear shrinkage ratio during cooling is smaller.

Abstract: Chemical cure shrinkage of polymer matrix is a significant source of residual strain formation in composite products during curing. In this paper, fiber Bragg grating (FBG) sensors were used to monitor the free strain development caused by chemical shrinkage and compared with results of differential scanning calorimetry and rheological analysis. The results showed that the gel point could be clearly identified by FBG sensors and compressive residual strain of around 800με was caused by isothermal curing reaction after gelation. The relationship between the chemical shrinkage ratio and conversion degree was found to be nonlinear.

Abstract: Cure-induced strain is produced inevitably during the fabrication of the composite. To measure the strain, undamaged methods such as using fiber Bragg grating(FBG) sensor are employed. In this paper, nine unidirectional carbon fiber-reinforced polymer(CFRP) laminates are autoclaved produced, with FBGs embedded in different layers through the thickness (0-layer, 5-layer, 10-layer and 13-layer). The experiment measures the difference of the FBGs’ Bragg wavelengths before and after the cure which is linearly relevant to the cure-induced strain, to explore the distribution of strains through the thickness. The experimental results indicate a certain strain in neural plane of approximately 370με under the designed size of the laminates. The results also show that the cure-induced strains in different layers through the thickness are less than 1000με even including all errors, however they do not display distinct regular in thickness direction. Moreover, through the FBG sensors and the thermocouples, the cure process with the strain and temperature variations is understood well. The result verifies that the cure-induced strain is mainly generated at the end of the cure when the temperature cools down.

Abstract: Resin transfer molding (RTM) process is getting popular for fabrications of complicated commercial products made from Glass Fiber Reinforced Polymer (GFRP) at low cost. Cure monitoring is indispensable for RTM process. In the present study, polyester resin is adopted for matrix resin of the GFRP components. The polyester is usually adopted as matrix resin of the low-cost GFRP products. Existing methods for the cure monitoring are, however, expensive for the GFRP products. The polyester resin usually changes its optical property during curing. This enables us to monitor the degree of cure by means of measurements of luminance change of the transmitted light. Since the electrical circuit for measuring luminance change is not expensive, this system utilizes the luminance change for monitoring cure is not expensive system. In the present study, the sensing system employs a LED as a light source and plastic optical fibers as light paths. A photodiode is adopted as a light power sensor. This low-cost cure-monitoring system is applied to monitoring of degree of cure of polyester resin. Degree of cure is measured by means of commercially available dielectric sensors, and results are compared with the results of luminance change. The effectiveness of the method is confirmed experimentally here.

Abstract: Because of their good multiplexing capabilities, fiber Bragg grating (FBG) sensors are being studied more actively than any of other fiber optic sensors. The application fields of FBG sensors have been mainly focused on composite materials through embedding rather than through surface attachments. However, there are many limitations on the embedding of FBG sensors into composite materials because of the birefringence that is induced when FBG sensors are not embedded parallel to the reinforcing fibers. This study investigates the fabrication of FBG sensors that have various grating lengths, good multiplexing capabilities, better stability from birefringence, and ease in production. The signal characteristics of FBG sensors are also verified through the cure monitoring of two composite laminates.

Abstract: Authors have developed an electric resistance change method for delamination monitoring of carbon/epoxy composite laminates. The method employs reinforcement fibers as sensors; electrodes are co-cured. Co-cured electrodes for delamination monitoring are adopted in the present study as sensors for monitoring the degree of cure. This study proposes a new method using electrical capacitance change for monitoring the degree of cure without using additional sensors. Applying alternating current between electrodes during the cure process provides dielectric properties of carbon/epoxy composites. As with the conventional cure monitoring method using extra dielectric sensors, the degree of cure of composites is monitored by measuring the dielectric constant of composites. The dielectric constant of epoxy resin changes concomitant with change of frequency of applied alternating current (AC). Using dependency of the applied AC frequency of the dielectric constant, the degree of cure is measured directly. The proposed method is applied to single and multi-ply carbon/epoxy composite laminates. The method demonstrates excellent estimations of the degree of cure without additional sensors.

Abstract: Cure monitoring and stress-strain sensing of single-carbon fiber composites were nondestructively evaluated by the measurement of electrical resistance. The difference of electrical resistance before and after curing increased highest when gauge length of the specimen was the smallest. As curing temperature increased, the electrical behavior of steel fiber was different from that of semi-conductive carbon and SiC fibers. Residual stress built in the fiber was the highest at the fiber axis direction. Whereas residual stress built in the matrix was relatively high at the fiber circumference and radius directions. Residual stress calculated from the experiment was consistent with the results from the finite element analysis (FEA). The strain at low curing temperature was larger than that of higher temperature until the load reached maximum value. The apparent modulus of the electrodeposited composites was higher than that of the untreated composites due to the improved interfacial shear strength (IFSS). The electrical resistance was responded quantitatively with stress-strain behavior during the test. Electrical resistance measurement can be feasible nondestructive techniques to evaluate cure monitoring and stress-strain sensing in the conductive fiber composites.