Fatigue Crack Closure–Study of Creep and Constraint Effects
by
Wei Sun
Department of Mechanical and Industrial Engineering
Abstract
In 1968, Elber observed that fatigue-crack surfaces come to contact even during tension-tension cyclic loading, and crack surfaces open at an applied tensile load near 50% of the maximum load [1-3]. A plasticity induced, crack closure mechanism was forwarded to explain this phenomenon.
During crack growth, plastic zone develops around the crack tip as the yield strength of the material is exceeded. As the crack grows this material is unloaded, and the plastically “stretched” material causes the crack surfaces come to contact before zero load is reached. Upon further unloading compressive residual stresses develop behind the crack tip. Crack opening stress, Sopen’ (or crack opening load, Popen) is defined as the value of applied stress (or applied load) at which the residual stresses are overcome and when the crack is fully open. Fatigue crack growth occurs during the period when the crack is fully open. Therefore, an effective stress intensity factor range, ΔKeff = Kmax – Kopen′ is used in fatigue crack growth characterization. This replaces the total stress intensity range parameter, ΔK.
In recent years much attention has been focused on the fatigue crack closure phenomenon. This mechanism and the influence of the plastic wake on the local crack tip strain fields provided insight into understanding of fatigue-crack growth of metallic materials. Early studies used the concept of crack closure to explain R-ratio effects and overload effects. The concept of plasticity induced crack closure has been recently used to explain state of stress effects, notch size effects, applied stress (load) level effects and accelerated growth of short cracks [4-8]. The experimental determination of crack closure loads still remains a difficult problem. Analytical or numerical determination of crack closure has advantages as this would allow many variables that effect closure to be determined.
Finite element analysis is a helpful tool in characterizing crack opening and crack closure under complicated loading conditions, complex material behavior and complex geometries. Two-dimensional finite-element analysis of crack growth and closure under plane-stress and plane-strain conditions have been conducted [9-12]. Specialized FEM codes are developed with provisions for crack opening and crack contact. (These features are not available in commercial codes such as ABAQUS or ANSYS). In a previous study, a two dimensional finite element model has been developed by Lalor, Sehitoglu [13] and McClung [14], in which crack extension is allowed through the mesh. The crack is advanced one element each cycle, opening and closing of crack surfaces are monitored. These studies provided additional information on stress-strain fields, plastic zone changes and different material behaviors effects on crack closure.
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