Quantitative measurement of fatigue crack initiation and propagation in 304 stainless steel as related to design and nondestructive evaluation

he initiation and propagation of short fatigue cracks in 304 stainless steel have been examined through replication techniques and through a mechanical vibration resonant frequency technique, which allows the measurement of elastic modulus and internal friction. Center hole and side notch specimens 6.3 mm thick were used for low cycles fatigue tests. The notches localize the crack initiation sites to facilitate the observation of the cracks and also simulate stress risers in real parts. The cracks initiate at several locations at the notch root inside the notches. These cracks then grow and link up along the surface of the notches and propagate to the specimen surface. The cracks are often nonlinear and link up through a bridging feature. Once the crack has reached the specimen surfaces at the edges of the notch, it propagates through the specimen as a through crack. The crack profile at this point has a small amount of curvature, allowing a measurement of the crack length through surface examination.;The stress intensity factor is used as the driving force for the small crack propagation. Justification for the linear elastic fracture mechanics based stress intensity factor is found through examination of the cyclic stress-strain curves. The maximum amount of plastic strain is a factor of five less than the 0.2% plastic strain used to define the yield strength. Therefore, there is no gross yielding. The plastic zone size due to the notch was also calculated and found to be smaller than the crack length in most cases. Finally, the elastic plastic fracture mechanics based ;The elastic modulus increases slightly throughout the fatigue life. This is attributed to plastic deformation, primarily in the grip regions of the fatigue specimen. Once a crack is present, however, the elastic modulus sharply decreases. The point where the slope of the elastic modulus is zero is therefore used as a criterion of crack detection. The detection limit of through cracks using this criterion is 40...