TWI Industrial Member Report Summary 718/2000
T R Gurney
There is still considerable doubt about the correct treatment of low stress range in a loading spectrum when calculating cumulative fatigue damage. In the British fatigue design rules (BS 5400, Part 10, and BS 7608) it is assumed that they are, relatively, less damaging than the higher stresses, and this is taken into account by bending the design S-N curve to a shallower slope at 10 7 cycles (and thereby increasing the value of N in the calculation of n/N). However, work both in USA and elsewhere involving long life variable amplitude fatigue tests has suggested that, in fact, the S-N curve continues linearly downwards, with no bend, to lower considerably stress ranges assumed in the British rules. With certain types of loading spectra, particularly those containing large numbers of low stress ranges, this may be of considerable importance. Such spectra could, for example, occur both in the offshore industry ( eg risers), in orthotropic bridge decks and elsewhere.
The primary objective was to try to assess the damaging effect of small stress ranges in a loading spectrum, and particularly those in the region of, and below, the assumed 'knee' in the current design S-N curves. It was hoped that this would elucidate the validity of the current use of bent S-N curves for cumulative damage calculations.