Validation of the state-space model of fatigue crack growth in ductile alloys under variable-amplitude load via comparison of the crack-opening stress data

A state-space model of fatigue crack growth in ductile alloys under variable-amplitude load was presented by Patankar and Ray (Patankar et al. (1998). International Journal of Fracture, 90, 235-249; Patankar and Ray (2000). Engineering Fracture Mechanics, 66, 136-151). This paper presents improvements to the state-space model, through enhancements in the calculations of the constraint factor. These improvements are similar to the calculation procedure of the constraint factor in FASTRAN-II model, which has been extensively validated (Newman (1981). Methods and Models for Predicting Fatigue Crack Growth under Random Loading, ASTM STP 784, 53-84; Newman (1982). Design of Fatigue and Fracture Resistant Structures, ASTM STP 761, 255-277; Newman (1984). International Journal of Fracture, 24, R131-R135; Newman (1992). NASA Technical Memorandum, 104159). The model predictions are compared to various crack growth data as well as FASTRAN-II predictions. Heather to, the state-space model has only been validated through the crack-length data but this paper presents the validation of the state-space model via comparison of the experimental crack-opening stress and crack-length data, thus involving both the states of the state-space model in experimental validation of the model. The experimental data are also compared to the crack-opening stress in FASTRAN-II predictions. Simulation results validate the modeling method of treating the crack-opening stress as a state variable or internal variable. The state-space model considerably reduces the computational complexity of the fatigue crack growth model under variable-amplitude load.