This goal of our research was to show that E-glass fiber bundles used for reinforcing composites can be enabled to transmit light in a common resin without any special preparation (without removing the sizing). The power of the transmitted light was measured and the attenuation coefficient, which characterizes the fiber bundle, was determined. Although the attenuation coefficient depends on temperature and the wavelength of the light, it is independent of the power of incident light, the quality of coupling, and the length of the specimen. The refractive index of commercially available transparent resins was measured and it was proved that a resin with a refractive index lower than that of the fiber can be used to make a composite whose fibers are capable of transmitting light. The effects of temperature, compression of the fibers, and the shape of fiber ends on the power of transmitted light were examined. The measurement of emitted light can provide information about the health of the fibers. This can be the basis of a simple health monitoring system in the case of general-purpose composite structures.

No abstract

This article examines the use of fiber Bragg grating sensors for cure monitoring purposes in resin transfer molding processes. Within a resin transfer molding test series a thermoset epoxy-amine resin system was used in combination with a woven flax fiber reinforcement. Particular attention was paid on the location of the optical fiber sensor and its sensitive Bragg grating element inside the mold cavity. Three different installation approaches were tested and the correlation of the corresponding strain response with the actual cure state of the resin system was investigated at 50°C and 70°C isothermal cure temperature, respectively. We could demonstrate that characteristic, conspicuous strain changes are directly related to the sol–gel conversion of the thermoset polymer, which was analyzed considering different approaches for the gel-point detection based on rheological measurements. With the installation of the sensor inside a controllable, capsuled resin volume, we could achieve the most reliable strain response that provides capabilities to give in-situ information of the cure state beyond the gelation point.

In this article we studied the mechanism of wood drying using infrared (IR) heat transfer. Norway spruce (Picea abies (L.) Karst.) samples of 50 mm and 200 mm thickness were exposed to IR radiation, and the temperature and moisture profiles were recorded at the surface and at the core of the samples under controlled experimental conditions. It is proposed that the moisture transport in wood during drying is governed by osmotic effects. Based on such a hypothesis, the temperature stagnation was explained by a lower localized pressure at the core, which reduced the boiling point temperature of water. As moisture is drawn away due to osmosis from the central region, it cannot fill the empty lumens again; therefore, the pressure decreases locally. The evaporation of the internal moisture is brought about by a partial vacuum resulting in the disappearance of the liquid water.

This article reports the results of temperature measurements carried out on 50-mm-thick Norway spruce (Picea abies [L.] Karst) wood samples exposed to infrared (IR) radiation. The varied property with respect to the optimization of the drying technology was the initial moisture content of samples. During the experiments, temperature profiles were registered on the surface and in the core of the samples under controlled technological conditions. Based on our osmotic approach, the variability in the curves was interpreted with respect to the stagnation temperature below the fiber saturation point (FSP). We conclude that the amount of liquid water necessary for osmosis must still be available locally in the core. With decreasing initial average moisture content, the time interval of the osmotic process also decreases. In this context our results support the hypothesis that the presence of free water in the wood tissue is necessary for the osmotic mechanism even if the average moisture content falls below the FSP.