A computer-implemented method of estimating fatigue damage in a structure, the method comprising: obtaining fatigue damage data collected using at least one sensor associated with the structure during at least one test operation; obtaining structure use parameter data collected during the at least one test operation; using the obtained fatigue data and the obtained use parameter data to compute coefficients of at least one generic polynomial function that outputs a fatigue damage value based on inputs representing use parameter data, and using the at least one generic polynomial function to output an estimated fatigue damage value based on inputs representing use parameter data collected during use of the structure, the method further comprising: comparing an accuracy of the estimated fatigue damage values with a desired accuracy, and if the accuracy does not meet the desired accuracy then dividing the obtained fatigue data and the obtained use parameter data into two or more sets, and using the obtained fatigue data and the obtained use parameter data of each said set to compute coefficients of a said generic polynomial function associated with that set, the method further comprising: associating use parameter range data with each said set; checking if the obtained use parameter data falls within the use parameter range data of one of the sets, and if the obtained use parameter data does fall within the use parameter range data of one of the sets then using the generic polynomial function of that set to output the estimated fatigue damage value, or if the obtained use parameter data does not fall within the use parameter range data of one of the sets then using an alternative technique to output the estimated fatigue damage value. obtaining fatigue damage data collected using at least one sensor associated with the structure during at least one test operation; obtaining structure use parameter data collected during the at least one test operation; using the obtained fatigue data and the obtained use parameter data to compute coefficients of at least one generic polynomial function that outputs a fatigue damage value based on inputs representing use parameter data, and using the at least one generic polynomial function to output an estimated fatigue damage value based on inputs representing use parameter data collected during use of the structure, comparing an accuracy of the estimated fatigue damage values with a desired accuracy, and if the accuracy does not meet the desired accuracy then dividing the obtained fatigue data and the obtained use parameter data into two or more sets, and using the obtained fatigue data and the obtained use parameter data of each said set to compute coefficients of a said generic polynomial function associated with that set, the method further comprising: associating use parameter range data with each said set; checking if the obtained use parameter data falls within the use parameter range data of one of the sets, and if the obtained use parameter data does fall within the use parameter range data of one of the sets then using the generic polynomial function of that set to output the estimated fatigue damage value, or if the obtained use parameter data does not fall within the use parameter range data of one of the sets then using an alternative technique to output the estimated fatigue damage value. associating use parameter range data with each said set; checking if the obtained use parameter data falls within the use parameter range data of one of the sets, and if the obtained use parameter data does fall within the use parameter range data of one of the sets then using the generic polynomial function of that set to output the estimated fatigue damage value, or if the obtained use parameter data does not fall within the use parameter range data of one of the sets then using an alternative technique to output the estimated fatigue damage value. <EOS>