A method implemented at a fault detection and isolation, FDI, system for indicating loss of clamping force at a coupling interface of a pair of flanges (306, 308), the method comprising; transmitting, from the FDI system to a sensor, a first request for a first series of measurements, wherein the first request includes a plurality of excitation frequencies or wavelengths to be applied to the sensor; receiving at the FDI system, a first series of measurements made at the coupling interface, wherein each measurement of the first series of measurements is in response to a respective excitation frequency or wavelength of the plurality of excitation frequencies or wavelengths applied to the sensor; the method characterized by: storing the first series of measurements; generating a first polynomial curve based on the first series of measurements; determining a first plurality of coefficients for the first polynomial curve; transmitting, from the FDI system to the sensor, a second request for a second series of measurements, wherein the second request includes a plurality of excitation frequencies or wavelengths to be applied to the sensor; receiving at the FDI system a second series of measurements made at the coupling interface, wherein each measurement of the second series of measurements is in response to a respective excitation frequency or wavelength of the plurality of excitation frequencies or wavelengths applied to the sensor; storing the second series of measurements; generating a second polynomial curve based on the second series of measurements; determining a second plurality of coefficients for the second polynomial curve; comparing the first plurality of coefficients and the second plurality of coefficients to determine if the first series of measurements deviates from the second series of measurements; and generating an indication of the loss of clamping force at the coupling interface of the pair of flanges if the first series of measurements deviates from the second series of measurements. storing the first series of measurements; generating a first polynomial curve based on the first series of measurements; determining a first plurality of coefficients for the first polynomial curve; transmitting, from the FDI system to the sensor, a second request for a second series of measurements, wherein the second request includes a plurality of excitation frequencies or wavelengths to be applied to the sensor; receiving at the FDI system a second series of measurements made at the coupling interface, wherein each measurement of the second series of measurements is in response to a respective excitation frequency or wavelength of the plurality of excitation frequencies or wavelengths applied to the sensor; storing the second series of measurements; generating a second polynomial curve based on the second series of measurements; determining a second plurality of coefficients for the second polynomial curve; comparing the first plurality of coefficients and the second plurality of coefficients to determine if the first series of measurements deviates from the second series of measurements; and generating an indication of the loss of clamping force at the coupling interface of the pair of flanges if the first series of measurements deviates from the second series of measurements.