Normalization of fatigue crack growth data in AISI H11 tool steel at room and elevated temperature

摘要

The fatigue crack growth rate of materials is shown to be dependent on the testing conditions like load ratio R and testing temperature. Great interest exists in normalizing this data onto a single curve. In this research, some methods commonly used to normalize the effect of R ratio are tested on Fatigue Crack Growth Rate (FCGR) curves of AISI H11 tool steel. These methods are based on either purely mathematical relations or on the effect of crack closure in variable R ratio tests. A model based on the crack tip opening displacement measurements to normalize the effect of R ratio as well as temperature is also used. This model takes into account the material-hardening coefficient, yield stress, Young's modulus, and the crack tip opening displacement measurements. Crack tip opening displacement measurements have also been directly used to characterize the FCGR. A method is presented to find out crack closure as well as crack tip opening displacement using 2D digital image correlation measurements near the crack tip. At the end, a critical analysis of the four normalization techniques is presented.