Analytical Studies on S-N Curves for Some Steels

Experimental data are often employed to obtain stress amplitude versus number of cycles to failure characteristics of materials (S-N Curve). This activity as a laboratory component of a junior level materials science course explores a novel way to understand the fatigue failure process. The students study fatigue failure in terms of crack nucleation where a small crack grows from a zero length to a threshold crack size that depends on the applied stress level and the threshold fracture toughness (a concept that the students learn in the course). A characteristic size or a distance parameter is used in the formulation which is dependent on the endurance or the fatigue limit of the material as well as the threshold fracture toughness. The student start with a given stress range and calculate the crack nucleation life (in cycles) to find the number of cycles to failure. The input material properties needed for the exercise are the endurance limit and the threshold fracture toughness.