Calibration facility for quality certification of surface-attached fiber optic and electrical strain sensors

Strain measurement in structures with the purpose of long-term structural health monitoring must provide reliable information about the structure´s behavior over the whole period of use. The user must be sure that installed sensors are validated and work to the utmost satisfaction. For this purpose, sensor systems are tested using special facilities. Because it is not easy to characterize the strain transfer quality from the host structure into surface-applied strain sensors, a unique testing facility has been developed. Originally developed for fiber Bragg grating based sensors, the KALFOS facility (= calibration of fiber optic sensors) can also be used for electrical strain sensors. Calibration measurements are referenced by unbiased Digital Image Correlation (DIC) and Electronic Speckle Pattern Interferometer (ESPI) methods. The strain transfer behavior can experimentally be analyzed and investigated under combined thermal and mechanical loading conditions and allows revealing weaknesses in commonly used attachment methodologies. The deformation of all members (particularly the coating/substrate - adhesive combination) in the sensing area is physically independently gained and recorded. Results achieved allow precise description of the strain transfer function, validation of the longterm strain sensor characteristics, matching of specific measurement requirements with environmental conditions, and, moreover, the verification of standards for use of strain sensors.