Scaling Laws for Fatigue Crack Growth of Large Cracks in Steels

Abstract

A set of scaling laws has been developed for describing intermittent as well as continuous fatigue crack growth of large cracks in steels in the power-law regime. The proposed scaling laws are developed on the basis that fatigue crack growth occurs as the result of low-cycle fatigue (LCF) failure of a crack-tip element whose width and height correspond to the dislocation cell size and barrier spacing, respectively. The results show that the effects of microstructure on fatigue crack growth can be described entirely in terms of a dimensionless microstructural parameter, ξ, which is defined in terms of yield stress, fatigue ductility, dislocation cell size, and dislocation barrier spacing. For both discontinuous and continuum crack growth, the crack extension rate,da/dN, scales with ξ and(ΔK/E) m, where ΔK is the stress intensity range, m is the crack growth exponent, andE is Young's modulus. Application of the model to high-strength low-alloy (HSLA) and conventional ferritic, ferritic/pearlitic, and martensitic steels reveals that the lack of a strong microstructural influence on fatigue crack growth in the power-law regime is due to increasing yield stress and fatigue ductility with decreasing dislocation barrier spacing, which leads to a narrow range of ξ values and crack growth rates. Variation ofda/dN data with microstructure in HSLA-80 steels is explained in terms of the proposed model. Other implications of the scaling laws are also presented and discussed in conjunction with several fatigue models in the literature.