Abstract
Two-step PP/CP and CP/PP variable straining tests were conducted for Mod. 9Cr-1Mo Steel at 600°C in air to clarify the small crack growth curve in CP test, where the strain range was kept constant and the strain waveform varied from PP to CP (PP/CP tests) or from CP to PP (CP/PP tests). It was found that in PP/CP tests the sum of life ratios was smaller than unity when the primary cycle ratio n1/N1 was large. On the other hand, in CP/PP tests the sum of life ratios was larger than unity in the large n1/N1 regime. The smaller the straining level, the larger the discrepancy between the experimental result and the linear damage rule prediction.
The small crack growth curve in CP test was determined so that the fatigue properties under two-step variable straining tests could be well explained by the proposed crack growth model based on the strain range partitioning concept. It is suggested that the initial crack length in CP test (a0)pp+cp is equal to the CP type initial crack length (a0)cp when Δεcp/Δεpp≥1 and that (a0)pp+cp is equal to the PP type initial crack length (a0)pp when Δεcp/Δεpp<1.
