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Abstract Due to the appearance of temperature gradient and non-uniform gelation, fully understanding of residual stresses is even more complex in thick laminates. In this paper, basic mechanisms of cure process were presented firstly. Then, experimental investigation of the non-uniform gelation was carried out together with characterization of necessary material properties. Tailed FBG sets were applied to monitor the cure of thick laminates with temperature gradient. The non-uniform gelation and chemical shrinkage gradient were captured successfully. Lastly, FEM analysis integrating thermal-chemical sub-model and stress-displacement sub-model was carried out to reveal the development of residual stresses. It is demonstrated that the high temperature layer gelled earlier and showed more chemical shrinkage. The chemical strain gradient would introduce compression internal stresses in prior gelled layer and introduces tensile internal stresses in later gelled layer. For the case studied here, transverse residual stress generated by non-uniform chemical shrinkage is small. The strategy used here is promising to estimate the residual stresses for more complex cases further. [ABSTRACT FROM AUTHOR]

Many researchers have performed impact localization using multiplexed fiber Bragg grating (FBG) sensors for a unique estimation of impact position. However, the previous impact localization did not show the methodology to choose the location with the lowest error among candidate impact points, estimated from sensors. We propose an error-outlier-based algorithm with Pugh’s concept selection that can choose the impact point with the lowest error among several candidate impact points. The proposed impact localization algorithm was implemented for 30 arbitrary points on a carbon-fiber-reinforced polymer composite plate with two bonded FBG sensors. The results showed that the proposed algorithm could efficiently localize all impact points. Additionally, it could successfully choose the impact point with lower error among two candidate impact points estimated from the two sensors. The accuracy of selection was 93.33% and the totally estimated averaged-error for the selected sensors by Pugh’s concept selection for all tests was 6.24 mm. Therefore, we conclude that the proposed impact localization algorithm is effective in localizing the impact points with the lowest errors when multiplexed sensors are used, regardless of the type of sensor. This technique will contribute to reducing measurement errors when localizing impact points on composite structures. [ABSTRACT FROM AUTHOR]

Chemical shrinkage and gelation are critical issues in curing process of composite structures since they have significant influence on generation of distortion or residual stress. In this paper, a new in-situ method to capture the gel point and to measure the effective transverse chemical shrinkage was presented. A tailed FBG (Fiber Bragg Grate) set consists of several FBG sensors with various tail lengths was embedded perpendicular to the fiber direction to monitor the curing process. Gel point was determined with the aid of profile turning point of the FBG set and the results matched well with DMA test. The effective transverse chemical shrinkage which occurs between gelation and vitrification was obtained and compared with the one determined by TMA or bi-material strip test. Both results were used to predict the spring-in of C-specimen manufactured on CFRP tube. It turns out that accurate prediction of spring-in can be obtained with the effective transverse chemical shrinkage determined by tailed FBG set. The in-situ monitoring method with tailed FBG set is an alternative and promising technology to capture the gel point and to reveal the mechanism of curing distortion. [ABSTRACT FROM AUTHOR]

Carbon fiber reinforced plastic (CFRP) pipes are key structural elements for observation satellites. Significant radial tensile stress, however, is induced in thick-walled CFRP pipes during curing, leading to premature delamination failure and performance degradation. This study proposes a residual stress-reduction method to develop a thick-walled, crack-free CFRP pipe. We began by addressing the effect of stacking sequence on residual stress using theoretical and numerical analyses. The result led us to a novel stress-reduction method where circumferentially stiff 90° layers are gathered close to the inner surface. Two pipes were then manufactured: asymmetric and symmetric lay-ups. The radial strain development was evaluated using our fiber-optic-based monitoring system to confirm the effectiveness of the method. Finally, a thick-walled pipe was fabricated. No failure was observed during curing, successfully demonstrating a thick-walled, crack-free CFRP pipe. Additionally, a low-temperature test was conducted, and the pipe endured down to −120 °C, demonstrating a 140 °C lower operation temperature than that of a conventional symmetric pipe, which is highly beneficial for space applications. [ABSTRACT FROM AUTHOR]

Abstract: In part I of this two-paper series, a method for the localization of an impact using dynamic strain signals from fiber Bragg grating (FBG) sensors is presented. In this paper, an inverse numerical–experimental method allowing to identify the damage based on experimentally measured eigenfrequency changes is developed and validated. The damage identification is limited to a region in the vicinity of the impact position predicted by the localization method. The eigenfrequency changes are determined experimentally from dynamic strain signals obtained with embedded FBG sensors and the parameters of a homogenized damage model are adjusted to fit the numerical results to the experimental data. [Copyright &y& Elsevier]

Abstract: Fiber reinforced composite materials risk to suffer from subsurface, barely visible, damage induced by transverse relatively low energy impacts. This two-paper series presents a method for the localization of an impact and identification of an eventual damage using dynamic strain signals from fiber Bragg grating (FBG) sensors. In this paper, the localization method allowing to predict the impact position based on interpolation of a reference data set is developed and validated. The data utilized in the method are the arrival times of the asymmetric zero order Lamb waves at the different sensors. A high rate interrogation method based on intensity modulation of the Bragg wavelength shift is used to acquire the FBG signals. The localization method allows to predict the impact position with a good accuracy and therefore the inspection of the laminate can be limited to this region. [Copyright &y& Elsevier]

Abstract: Internal strain measurements in cross-ply carbon-epoxy composite plates under dynamic loads are carried out using embedded FBG sensors. The principle of the FBG interrogation is based on intensity demodulation achieved via a Fabry-Pérot filter. To account for the non-linearity of the filter, the system is calibrated and the amplitude of the strain data is validated. Strains are acquired at a rate of 100kHz with a noise level as low as 2με and used for modal analysis and strain monitoring in low energy impact. The experimental results under impact and modal analysis compare very well with pertinent numerical models and modal analysis obtained from laser vibrometer measurements. [Copyright &y& Elsevier]

The vacuum-assisted resin transfer molding (VaRTM) process has attracted increased attention because of its ability to provide fast production times for large structures while maintaining a high quality. However, the occurrence of process-induced defects during the VaRTM process significantly affects the mechanical properties of the composites, resulting in a decrease in the reliability of the process. Therefore, flow monitoring should be performed during the manufacturing process to eliminate process-induced defects, such as dry spots and unimpregnated areas in the preform. Herein, we report a facile method for monitoring resin flow using a distributed strain sensing method during the VaRTM process. For the distributed strain sensing method, optical frequency domain reflectometry (OFDR) and a long-gauge fiber Bragg grating (FBG) sensor were used simultaneously. To analyze the change in the sensor signal according to the resin impregnation state, the strain along the in-plane direction of the preform during the VaRTM process was investigated through a numerical simulation and compared with the strain change measured by the OFDR system. The strain rate from the FBG sensor showed a positive value in the impregnated region but a negative value in the unimpregnated region, which means that the point at which the reversal of the strain rate occurs is the same as the position of the flow front. Thus, not only the flow front over the entire gauge length but also the approximate position and size of the unimpregnated area were accurately monitored. [ABSTRACT FROM AUTHOR]