ESDU 97024:2003

ESDU 97024 introduces two Fortran programs, ESDUpacs A9724 and W9724, for the consistent derivation of an S-N curve from a set of fatigue data. The method on which the programs are based is applicable to data for both metallic and non-metallic materials as well as to data from tests on coupons or specimens representing structural features. The method also takes account of censored data, for example, run-outs, in a rigorous manner. S may be in terms of a maximum, an alternating or an equivalent stress, strain or loading with the life, N, expressed in terms of number of cycles, time or some other convenient unit (for example, repeated program blocks). The cycles may be of either constant or variable amplitude. The derived curve is the median curve, that is, it represents a fifty per cent probability of failure. The Fortran programs perform a non-linear constrained optimisation procedure using a maximum likelihood criterion to derive a curve of the form S = P 1 N q + P 2 N r + P 3 . The values chosen for the unknown parameters in the derived curve are those that maximise the probability of the observed data being obtained. A facility is also provided for the user to specify the values of any of the non-zero parameters that can be estimated from experience. An additional option, which is only available when censored data are involved, is that of specifying the value of the coefficient of variation of S, if it is known. The equation gives a convenient means of estimating S from N, but not vice versa. The program will, however, if required, calculate values of N corresponding to values of S specified by the user. Details of the input data required by the programs are provided, along with example input and output files. ESDUpac A9724, which gives only textual output, is provided in the form of Fortran source code and corresponding executable file. ESDUpac W9724, which includes a user-friendly interface and gives graphical as well as textual output, is provided as an executable file along with comprehensive, context-sensitive, help.

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