Can we predict fatigue crack growth without the help of Paris?

icaf2023 Tracking Number 127

icaf2023 presentation

Since the introduction of the so-called ‘Paris law’, named after Paul Paris, many engineers and researchers have successfully predicted fatigue crack growth in metallic structures utilizing the Paris curves. After this introduction, various versions of the Paris relation have been proposed and used, of which the NASGRO relation and similarly the Hartman-Schijve relation are famous examples. One can debate, however, whether this practice really constitutes predictions, considering that always first sufficient number of fatigue crack growth tests must be performed through which these phenomenological relations are fitted. Essentially, these predictions constitute interpolations between pre-existing fatigue crack growth data, which are in most cases calibrated and validated with the same test data used to generate the input parameters. To really predict fatigue crack growth, i.e. without pre-existing fatigue crack growth data, requires a better understanding of the physics of fatigue crack growth and the role plasticity has on this phenomenon, are needed. The current body of empirical and phenomenological work does not allow for such physics based description. To demonstrate that truly scientific predictions of fatigue crack growth are possible without using Paris curves, this study developed a physics based description of fatigue crack growth in metallic plates, through developing an energy balance between strain energy dissipating mechanisms. In the end, it is demonstrated that essentially only quasi-static stress-strain curves are required to enable the prediction of fatigue crack growth. This paper explains the essential steps necessary to describe the physics of fatigue loading (application of strain energy or work) and the corresponding dissipation of energy through the formation of fracture surfaces, and plastic volume around the crack tip. In fact, the method allows to generate as result the Paris curve using stress-strain data, rather than using it as input in prediction. It is foreseen that further development of this work will accelerate introductions of new materials and alloys in aerospace, simply because material qualification and structural certification require substantially less testing and analysis.