Investigation of subsurface fatigue crack growth behavior of D2 tool steel (JIS SKD11) based on a novel measurement method

In our preliminary study of the fatigue properties of D2 tool steel (JIS SKD11), it was found that owing to grinding in the specimen preparation process, high compressive residual stresses were introduced in the surface layers of the fatigue specimens, and as a result, the fatigue crack growth of the specimen was entirely subsurface. As relatively little is known about the subsurface growth of fatigue cracks, this finding gave us the opportunity to learn more about the subsurface fatigue crack growth process. An S–N curve covering fatigue lifetimes from 10³ to 10⁸ cycles was established for R = −1 loading condition. The velocity of fatigue crack growth as a function of the maximum stress intensity factor K_max was determined to be in the range of 10⁻¹¹–10⁻⁷ m/cycle. The Murakami √area method and a new experimental procedure were used to estimate the stress intensity factor and crack growth velocity under the specimen surface, respectively. An analytical equation for the velocity of crack growth as a function of the maximum stress intensity factor K_max was developed, which agrees with the experimental results. The integration of this equation led to estimates of the fatigue lifetime for given stress ranges. These estimates were also in agreement with the experimental results.