TABLE OF CONTENTS The S-N model is based on the assumption that the number of cycles N to failure is directly correlated to the applied nominal stress range ?S. All other parameters are assumed to play a minor role. The background for these curves is a simple linear regression analysis in the finite life region. The data for the curve are obtained by accelerated constant amplitude tests at relatively high stress levels. Furthermore, the model assumes the existence of a fatigue limit, given as a stress range below which no failure will take place. This fatigue limit is subsequently added as a horizontal cut-off line to the regression line. The final curve is bilinear for a log-log scale between N and ?S. These curves are presented as the conventional S-N curves given in rules and regulations; see Chapter 5. At the end of Chapter 3 we showed how S-N data were analyzed to establish the median and the design S-N curve for the fatigue life in the finite life region. The procedure and calculations are quite straightforward; apart from test specimens that are stopped at a given number of cycles before fatigue fracture has occurred. The...
