Cyclic R-Curve Measurements for Structural Metallic Alloys

Fractures nucleated from defects and subjected to cyclic loading can experience propagation for a range of stress intensity factor Delta K well below the so-called long crack threshold. This phenomenon is attributed to the development of crack closure mechanisms which may differ from those observed in laboratory tests conducted in accordance with current standards. Cracks originating from material defects require a specific degree of extension to develop the plastic wake, thus achieving a stabilized condition called the long crack threshold. However, in certain materials, this stabilization length can extend up to several millimeters, effectively encompassing a significant portion of the component's fatigue life. Therefore, understanding and quantifying the development of Delta K-th with crack extension is important for implementing a reliable assessment procedure based on the fracture mechanics theory. Herein, R-curve measurements are presented for five distinct structural metallic alloys widely used in various industrial applications. Additionally, the impact of load ratio is investigated, providing a comprehensive analysis of fatigue crack resistance in terms of the R-curve concept. This study underscores the necessity of ruling new experimental techniques to measure and implement the long crack threshold, thereby ensuring the development of a reliable and robust framework for fatigue assessment.