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Physical Reasons for a Reduced as Correlation for Fatigue Crack Propagation Pages: 13 Published: Jan 2002
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View License Agreement Source: STP1406-EB Abstract It is generally acknowledged that a reduced , namely , is the best correlation between cyclic mechanical loading conditions and fatigue crack propagation rates. Yet, there are differing opinions among scientists as to the cause for such a reduction, and different concepts are proposed for the transfer of into . In this work, it is demonstrated that the highly constrained compressive deformation around the crack front during the unloading part of a fatigue cycle leads to a high hydrostatic stress field that needs to be overcome during the loading part of the following cycle before a crack can further extend. The stress intensity factor, , describes the loading part of the fatigue cycle, while the unloading part of a cycle follows completely different rules with respect to stresses and strains in the plastic zone. A rigorous separation of these two parts of a fatigue cycle is conducted, leading to a new view at the problem. The crack growth increment of a cycle depends on and the crack propagation stress intensity factor, , set by the previous cycle. itself depends on and the unloading level, (or ) and thus solely on the unloading part of the previous cycle. The crack propagation stress intensity factor, , represents the load level where the hydrostatic stresses in the vicinity of the crack tip are released, thus dividing the loading part of a cycle into an effective and a noneffective part.
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