In situ stress monitoring and calibration of fiber Bragg Gratings embedded inside aluminum samples at high temperatures

Hot tears are one of many defects in castings and weldings. Especially in weldings, this crack-like defect leads to failure or rejection in production. Nevertheless, castings are also being affected by this phenomenon. Several attempts exist to understand and describe hot tearing, but only a few methods exist to analyze the effect in situ. Fiber Bragg Gratings (FBGs) have been used in castings to measure strain and are now used to inspect hot tearing in situ. With a unique test bench, built of a mirror furnace capable of heating specimens and a tensile testing machine that can pull the prepared specimen simultaneously, it is now possible to calibrate fiber Bragg Gratings inside aluminum samples and analyze the formation of cracks with the optical method of FBGs. This experiment allows a defined application and variation of stress on the aluminum sample made of different alloys, and conducts tests at varying temperatures up to the solidus temperature. To get a valid measurement, it is necessary to calibrate the fibers precisely. This task is not trivial since the Bragg Grating is sensitive to changes in temperature and strain. In this paper, we controlled the furnace to sustain a defined temperature and applied increasing stress steps to the sample. In this way, we can differentiate between the changes caused by the increasing stress and the changes from the rise in temperature. This calibration approach makes it possible to more easily detect hot tearing around the line of solidus for hypo-eutectic and hyper-eutectic alloys with a wide solidification range.