TABLE OF CONTENTS Fatigue life predictions have been carried out using the S-N approach for over a century and long before the physics of the fatigue process was properly understood. As we have seen, the model is based on simplified assumptions and statistical analysis of the entire fatigue life. The method does not try to analyze the fatigue process itself. Its only goal is to estimate the time to failure at a given probability level. One may in fact use the S-N curves without being aware of the fact that it is a crack growing to a critical size that is the cause of the fatigue failure. In the fracture mechanics approach the entire crack growth history is modeled, not only from the final fatigue life, but also from a small, initial crack to the final, critical crack leading to fracture. The formulation is based on physics, not just life statistics as is the case with the S-N model. After a crack has initiated, the standard S-N curves are no longer applicable, whereas a fracture mechanics model can describe the probable crack behavior and propagation towards final failure. Because the S-N model cannot deal with the presence of cracks, the fracture mechanics model is an indispensable tool in situations when a crack is detected and sized. Furthermore, a fracture mechanics model gives valuable support when planning scheduled in-service inspection. In such cases we will have to know the possible damage evolution in advance in order to know what crack sizes to...
