Short fatigue crack growth behavior in a ferritic-bainitic steel
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I.
INTRODUCTION
THE principal factors controlling the life of the engineering components are the initiation and propagation of cracks; the latter could ultimately lead to failure of the components. The fatigue crack growth rate, da/dN, is primarily controlled by the stress intensity factor range, AK. The graphical presentation of da/dN vs AK is a sigmoidal curve, bounded at its upper extreme by the value of Kc fast crack growth regime when Kma~approaches the fracture toughness (Kc) and the lower extreme by AKth when AK approaches the fatigue crack threshold. The sensitivity of the fatigue crack growth rate to microstructural features, environments, and loading conditions is different dependent upon the region. In the near-threshold region, fatigue crack growth rates are very slow and the influence of microstructure, environment, and load ratio on the rates of crack propagation is very significant,fl,2] In the Paris region, there is a linear relationship between log (da/dN) and log (AK). In this region, the crack growth properties are not so sensitive to microstructure and load ratio. In the fast crack propagation regime, the rate of crack growth is relatively sensitive to microstructure and load ratio. It has been shown that short fatigue cracks do not obey the same propagation laws that can apply to long cracks, ta'4,Sj Short surface crack growth behavior is markedly dependent on the microstructural features. The growth rate exhibits a monotonic deceleration as the crack approaches a grain boundary. The growth rate suddenly increases as slip is initiated in the next grain and as the crack crosses the grain boundary. The oscillating behavior of the crack growth rate fades off as the plastic zone size ahead of the crack tip exceeds the average grain size. t61 At this stage, microstructurally sensitive K. HUSSAIN, Senior Engineer, A. TAUQIR, Principal Engineer, F.H. HASHMI, Director of Metallurgy, and A.Q. KHAN, Project Director, are with Dr. A.Q. Khan Research Laboratories, Rawalpindi, Pakistan. Manuscript submitted July 16, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS A
(stage I) to microstructurally insensitive (stage II) crack growth takes place. For this article, experiments were designed to monitor growth behavior of short fatigue cracks with respect to microstructural variations in a dual-phase steel. A previously developedt71two-stage short fatigue crack growth model was modified to compare the observation with predictions of the model on individual grains. It has been shown that the predicted values are in good agreement with the experimental observations. The model was then used to present the variation in short crack growth behavior with respect to microstructural variations.
II.
MATERIAL AND TEST PROCEDURE
The material used for investigation was a low-alloy steel of nominal composition (in wt pct) 0.12 C, 1.4 Mn, 0.26 Si, and balance iron. The mechanical properties of the material used for investigation are listed in Table I. The fatigue limit (Ao') of the material under reversed axial loading was
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