Search

Fibre Bragg Gratings (FBGs) were employed as internal strain sensors within tensile specimens that were machined out of a casted aluminium alloy. Embedded FBGs have the potential to directly measure strains within a casted part for a better understanding of load cases for the part design process. In addition, if the FBGs remain in the part, strains can be measured under real-life conditions and return valuable evidence for health monitoring. The proof of application is provided by the presented tests by comparison of external and internal strain measurements. The step-wise calibration test gives evidence for a linear transition between external straining and internal response by the FBGs and thus a constant conversion factor was obtained. The application in a common tensile test proves a precise strain measurement by the FBGs and their durability until a short time before the specimens failed. These findings form the basis for an application of FBGs within real structural parts and thus the production of intelligent parts in future work. As a conclusion, internal strain measurements using FBGs have the potential to provide additional strain data compared to common methods and hence to improve the design process as well as to monitor casted parts under operating conditions.

Current testing methods are capable of measuring strain near the surface on structural parts, for example by using strain gauges. However, stress peaks often occur within the material and can only be approximated. An alternative strain measurement incorporates fibre-optical strain sensors (Fiber Bragg Gratings, FBG) which are able to determine strains within the material. The principle has already been verified by using embedded FBGs in tensile specimens. The transition area between fibre and aluminium, however, is not yet properly investigated. Therefore, strains in tensile specimens containing FBGs were measured by neutron diffraction in gauge volumes of two different sizes around the Bragg grating. As a result, it is possible to identify and decouple elastic and plastic strains affecting the FBGs and to transfer the findings into a fully descriptive FE-model of the strain transition area.We thus accomplished closing the gap between the external load and internal straining obtained from cast-in FBG and generating valuable information about the mechanisms within the strain transition area.It was found that the porosity within the casting has a significant impact on the stiffness of the tensile specimen, the generation of excess microscopic tensions and thus the formation of permanent plastic strains, which are well recognized by the FBG. The knowledge that FBG as internal strain sensors function just as well as common external strain sensors will now allow for the application of FBG in actual structural parts and measurements under real load conditions. In the future, applications for long-term monitoring of cast parts will also be enabled and are currently under development.

An array of regenerated fiber Bragg gratings (RFBGs) was employed to monitor the temperature distribution during an aluminum cast process. Significant temperature gradients in the cast part were observed during the cooling and solidification period. The direction and velocity of the solidification front was revealed by the temperature data obtained by the RFBGs at different measurement points. A generalized calibration curve was established and applied to the RFBG array, avoiding the need of individual calibration measurements of each sensor element of the array. Temperatures measured using the RFBGs and a thermocouple placed in a reference arm of the mold showed excellent agreements and indicated that no thermal drift of the RFBGs sensor elements occurred during the casting. The RFBG measurements were carried out with only a single fiber, which reduced the amount of lead cables in the melt and hence minimized the sensor-induced disturbances to the casting process. In addition, the limited number of cables enables the RFBG sensors to be deployed in small and complex cast geometries. Multipoint temperature sensors based on RFBG arrays can become important tools to improve the quality of casting processes.

A new approach for the analysis of high pressure die casting geometries concerning form filling is introduced. It enables the reduction of simulation cycles needed in the product development process, as they are highly time and cost intensive. Therefore, we developed an analysis tool, which uses shortest paths from each part of the geometry to the chosen ingate surfaces. This way, and by evaluating the information given by basic filling simulations, we can evaluate the usability of a given geometry for a high pressure die casting process and are able to suggest useful strategies to place ingates and to design a filling system.

Fibre Bragg gratings (FBGs) are utilized to function as internal strain sensors during casting, solidification and cooling of two hypoeutectic Al alloys and technically pure Al. In situ neutron diffraction experiments were conducted simultaneously onsite the STRESS-SPEC instrument at the Research-Neutron Source (MLZ) in Garching. The experimental data correlated with structural finite element simulation and X-ray micro tomography (CT) elucidates the mechanisms of this measurement system. The presented work offers a new practical approach to measure in-situ strains during casting of Al alloys by using FBG-based sensors. The FBG strain results correlate with the Al(311) peak obtained by neutron diffraction which is furthermore used for calibration. The examination of interactions between fibres and surrounding cast Al by finite element simulation and CT of the cast specimens enables the application of the sensors for unequivocal strain measurements in castings as well as the characterization of alloys during solidification.

In this study, the response of regenerated fiber Bragg gratings (RFGBs) to axial forces was investigated in a temperature range from room temperature to 900 °C. For the first time, the transition from pure elastic to viscoelastic behavior around 700 °C of a standard SMF28 optical fiber was measured with an inscribed RFBG. An elastic model with linear temperature dependencies of Young’s modulus and Poisson’s ratio was established, and showed good agreement with the measurements up to temperatures of ∼500 °C. In the temperature range up to 900 °C, the RFBG response could be well described with a simple, single-material approach and a Burgers model that consists of a Kelvin and a Maxwell part. Based on the elastic parameter of the Maxwell part, the temperature-dependent force sensitivity of the RFBG was determined, and it showed a linear decrease in the range from room temperature to ∼500 °C, constant values in the range between ∼500 °C and ∼600 °C, and a strong increase at higher temperatures. While fulfilling the condition to operate in the elastic domain of the silica fiber, the investigations demonstrate that RFBGs can be used as force sensors up to temperatures of ∼600 °C - the range in which temperature-dependent force sensitivities have to be considered. The temperature-dependent parameters of the effective single-material model (elastic and viscoelastic part) are essential to describe the effective mechanical behavior of the optical fiber at high temperatures.

In recent years, the observation of the behaviour of components during the production process and over their life cycle is of increasing importance. Structural health monitoring, for example of carbon composites, is state-of-the-art research. The usage of Fibre Bragg Gratings (FBGs) in this field is of major advantage. Another possible area of application is in foundries. The internal state of melts during the solidification process is of particular interest. By using embedded FBGs, temperature and stress can be monitored during the process. In this work, FBGs were embedded in aluminium alloys in order to observe the occurring strain. Two different FBG positions were chosen in the mould in order to compare its dependence. It was shown that FBGs can withstand the solidification process, although a compression in the range of one percent was measured, which is in agreement with the literature value. Furthermore, different lengths of the gratings were applied, and it was shown that shorter gratings result in more accurate measurements. The obtained results prove that FBGs are applicable as sensors for temperatures up to 740 °C.