AN ENGINEERING MODEL FOR PROPAGATION OF SMALL CRACKS IN FATIGUE

The propagation of small cracks in fatigue has received considerable a ttention over the past decade. Microstructurally and even mechanically small cracks have been shown to consistently exhibit higher crack gro wth rates than predicted using standard threshold and Paris growth law concepts, based on linear elastic fracture mechanics (LEFM) applied t o mechanically long cracks. This has been commonly attributed to sever al factors, including the influence of microstructure, the breakdown o f LEFM parameters for representation of the crack tip field, and the t ransient development of plasticity induced closure towards some steady -state value associated with long crack behavior. Other mechanisms rel ated to microstructural effects such as roughness-induced closure and crack face bridging/interference are also potential contributors. Quan titative attempts to explain the fatigue propagation of small cracks i n terms of plasticity-induced closure, along with adoption of an addit ional component of the driving force (e.g. crack tip opening displacem ent) to reflect the contribution of cyclic plastic strain have met wit h some success in correlation of the so-called ''anomalous'' propagati on behavior, including crack deceleration and acceleration transients. However, these models rely on the adoption of highly idealized assump tions regarding the self-similarity of crack growth, neglect of local anisotropy and heterogeneity associated with microstructure, etc.; in spite of these compromises, they still involve a considerable degree o f complexity. Here, we adopt the viewpoint that multiple, microstructu re interactions and closure effects may simultaneously influence the p ropagation of small cracks; moreover, driving force parameters based o n self-similar crack growth arguments of elastic-plastic fracture mech anics (EPFM) for mechanically long cracks, such as the cyclic J-integr al or crack tip opening displacement, may apply in principle, but not rigorously, as the driving force for small cracks. As an engineering a pproach, we consider a recent extension of the multiaxial microcrack p ropagation model first proposed by McDowell and Berard[1,2] for the gr owth of microstructurally small and mechanically small fatigue cracks[ 3] in multiaxial fatigue. Integrated between initial and final crack l engths, the model is fully consistent with standard strain-life laws o f fatigue crack initiation mechanics under various states of stress, [ 1,2] and therefore bridges the mechanics of classical initiation and L EFM/EPFM to some extent. The existence of a fatigue limit (nonpropagat ing crack limit) is neglected in this particular work. It is shown for uniaxial loading of both 1045 steel and Inconel 718 that the model is able to describe, to first order, the anomalous high propagation rate s of small cracks and convergence with long crack da/dN-Delta K data a s the crack transitions from small to mechanically long scales. The li mits of validity of engineering schemes based on decomposition of tota l fatigue life into ''initiation'' and propagation phases that rely on strainlife and long crack propagation laws are discussed[4]. Moreover , it is shown that the model essentially reflects a closure transient in the context of a cyclic J-integral approach, similar to the EPFM pl asticity-induced closure modelling concepts set forth by Newman[5] and McClung et al. [6] for small cracks in fatigue. However, the present model implicitly reflects multiple forms of crack tip shielding effect s, not just plasticity-induced closure. Finally, the model is shown to provide realistic treatment of cumulative damage in two level loading sequences, as reflected by comparison with the damage curve approach of Manson and Halford[7]. Copyright (C) 1996 Elsevier Science Ltd