A Model for Small Fatigue Crack Growth

Abstract

Based on the assumption that normalized Kitagawa-Takahashi diagrams for different materials are the same, a unified model for microstructurally small fatigue crack and physically small fatigue crack growth rates was developed to describe their behaviour under different fatigue stress ranges. The stress-sensitive blocking effect of microstructural barriers to small fatigue crack growth is satisfactorily simulated by the model. Incorporated with the materials fatigue limit and microstructural barrier spacing, this model can be easily used in the prediction of small fatigue crack lifetime. Small fatigue crack growth rates of previous experimental studies in 7075-T6 Aluminium alloy and HT60 steel under different stress ranges are in an envelope between two boundary prediction curves corresponding to the largest and smallest stress ranges applied in the experiments. Problems concerning model accuracy and model application are also discussed in the present paper. NOMENCLATUREA , C, m, n = constants c = half surface crack length dc/dn = fatigue crack growth rate D = spacing between microstructural barrier f= barrier blocking factor in small fatigue crack growth model AK = stress intensity factor range do = fatigue stress range Auf = fatigue limit stress range As, =threshold stress range in small fatigue crack regime

Cracking simulation‐based fatigue life assessment

“…The individual short crack behavior imposes great difficulty in regard to modeling the very early stages of crack growth as individual cracks are susceptible to large changes in growth rate depending on their exact length and position relative to microstructural barriers. Most existing models developed to predict FCG consider a single or isolated crack . This inevitably overestimates the fatigue lifetime of the specimen as the coalescence of interacting in‐plane and out‐of‐plane cracks shortens the time to failure.…”

Cracking simulation‐based cumulative fatigue damage assessment

“…The results of the present work can easily show that the behavior of A C against the number of cycles, N , is similar to that behavior of the total length of the existing cracks against N . On the other hand, the cracking‐based cumulative fatigue damage models existing in the literature consider the length of a single isolated dominant crack 34,38,39,50–54 . This is expected to overestimate the fatigue lifetime because the interaction and coalescence of existing small cracks shorten the time to failure 50–54 .…”

“…Existing cumulative fatigue damage models based on cracking consider a single or isolated dominant crack. This inevitably overestimates the fatigue lifetime of the specimen as the coalescence of interacting in‐plane and out‐of‐plane cracks shortens the time to failure 50–54 . Further, the individual short crack behavior imposes great difficulty in regard to modeling the very early stages of crack growth as individual cracks are susceptible to large changes in growth rate depending on their exact lengths and position relative to microstructural barriers, the features of the random distribution of the grains along with other mechanical and metallurgical effects.…”

Cracking simulation-based cumulative fatigue damage assessment

“…The results of the present work can easily show that the behavior of A C with the number of cycles is similar to that of the total length of the existing cracks. On the other hand, the cracking-based cumulative fatigue damage models existing in the literature consider the length of a single isolated dominant crack 34,36,37,[47][48][49][50][51] . This is expected to over-estimate the fatigue lifetime for the interaction and coalescence of existing small cracks shorten the time to failure 47 -51 .…”