Generation and use of standardised load spectra and load–time histories

Generally the basic fatigue behaviour of materials and structures is determined adopting constant amplitude loading resulting in various characteristic quantities and relationships such as S–N curves, cyclic stress–strain curves, fatigue crack growth rates, threshold values, etc. While constant amplitude loading is fully described by quite a few parameters (maximum/minimum or range/mean and number of load cycles), most structures experience in-service loading environments with variable amplitudes and mean loads. It is well established that fatigue response may be very sensitive to the specifics of the loading encountered. Therefore, tests with realistic load sequences are often required in order to assess any susceptibility to that kind of phenomenon and to demonstrate the in-service integrity for given materials and structures. For this purpose standardised load–time histories have been developed for 30 years since it has been recognised that the use of standardised load–time histories provides a series of advantages—both for studies of a more generic nature and practical applications. The present paper deals with this topic. Starting from an overview on standardised load–time histories available at present, it details the principles applied for collection and analysis of appropriate load data, assessment of operating profiles and generation of the respective load spectra and sequences. The principles may also serve as guidelines for those cases where new load spectra and load–time histories have to be created, for example, for a given design problem. This as well as some other types of application are reported in a final section based on information collected from the literature.