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The goal of the work to be presented is a fracture mechanics methodology for the description of fatigue crack propagation (FCP) in which the mechanical driving force has a unique relation to FCP. A review of FCP literature leads to two basic premises for FCP, namely that a) ΔK indirectly describes the stress and strain excursions in the plastic zone, and b) tensile stresses and strains in the near crack front region are a necessary condition for FCP. It is shown that closure as defined by Elber can only act like some externally applied tensile load. Therefore, closure cannot be the mechanism that transfers the external cyclic loading condition to the mechanical driving force. A model is proposed which includes the closure mechanism, but the transfer from external cyclic loading to the mechanical driving force is solely due to the stresses and strains in the plastic zone ahead of the crack front, as originally proposed by Schijve. The stress intensity factor at which the stresses and strains in the near crack front region switch from compression to tension during the loading part of a cycle has been experimentally determined for various cyclic loading conditions. This level is denoted as KPR, the crack propagation stress intensity factor, and allows the determination of the mechanical driving force, ΔKeff.
fatigue crack propagation, crack closure, residual compressive stresses, ΔK, eff, concept, overload effect, load interaction effects, life prediction
Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright-Patterson Air Force Base, OH
DLR, German Aerospace Center, Institute for Materials Research, Cologne,