##### STP675

### A Critical Evaluation of Mathematical Equations for Fatigue Crack Growth with Special Reference to Ferrite Grain Size and Monotonic Yield Strength Dependence

**Abstract**

In order to select the correct mathematical equation among many. for fatigue crack growth or to develop a new criterion, some critical assessments are carried out by compiling the experimental data on the effect of ferrite grain diameter on fatigue crack growth rate for two low-carbon steels and a 3 weight percent silicon iron. The effect of monotonic yield strength is also studied. Emphasis is on fatigue crack growth in the second region of fatigue crack growth rate versus Δ*K* curve. Results suggest that, if fatigue crack growth rate is expressed as da/dN=B(ΔKsσc)′′ where *σc* is some constant with the dimension of stress and *s* is an appropriate measure of length, then *σc* and *B* are practically independent of monotonic yield strength, but *n* has the trend of a slowly increasing function of the ferrite grain diameter and, thus, indirectly a decreasing function of the monotonic yield strength. Moreover, it is suggested that *n* is also affected with some other microstructural factor.

**Keywords:**

fatigue crack growth, ferrite grain size, monotonic or static yield strength, initial cyclic yield strength, fatigue crack growth constants, plain low-carbon steel, silicon iron, tempered-martensitic steel, eutectoid steel, heat-treated bearing steel

**Author Information:**

Yokobori, T

*Professor, Mechanical Engineering Department II, and director, Research Institute for Strength and Fracture Materials, Tohoku University, Sendai,*

**Paper ID:** STP35910S

**Committee/Subcommittee:** E08.05

**DOI:** 10.1520/STP35910S

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