A thermal deformation measurement system, composed of fiber Bragg grating (FBG) sensors for strain measurement and a displacement measuring interferometer (DMI) system for accurate specimen expansion data acquisition, was prepared and installed in a vacuum chamber where the temperature of the test specimen can be controlled to simulate space environments. The DMI system, which consists of two heterodyne interferometers, a laser head, electronics and a thermally stable specimen base made of fused silica, was used to validate the thermal expansions of the specimens measured by the FBG sensors. We measured the average coefficient of thermal expansion (CTE) of an Invar specimen, known as a thermally stable material, using both the FBG sensors and the DMI system in vacuum conditions from 20 • C to 40 • C. The CTE results of the Invar specimen were found to be 1.226 × 10 −6 K −1 and 1.298 × 10 −6 K −1 based on the FBG and DMI measurements, respectively. The present results show that it is possible to precisely measure the thermal deformation of a specimen or structure in space environments using FBG sensors.
The effects of transverse strain on the wavelength shifts of surface-mounted fiber Bragg grating (FBG) sensors are investigated. A new FBG model including the concept of strain transmissibility coefficients along the longitudinal and transverse directions of the sensors is proposed. The finite element method is applied to obtain the strain transmissibility coefficients between the FBG sensor and surface of the base structure. The wavelength shifts of each FBG sensor are calculated with respect to the assumed thermal strain condition of the base structures; the estimated strains by the proposed FBG model are compared with the results from conventional FBG model. The numerical results show that the surface strains of the composite structure can be predicted well using the proposed FBG model, while the conventional FBG model results in a larger error when the transverse strain of the base structure is much larger than longitudinal strain of the base structure, as in composite structures under thermal loadings.
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