This study was experimentally carried out through a Fine Dot Grid Strain Measurement using a Fine Dot Image Processing System. The cyclic strain distribution(Δε eq) at the local zone was measured and analyzed, and come to be formulated into Δε eq = Δ A- f(θ r- I. The Δ A here could have been proposed as a parameter which quantitatively had characterized the Δε eq in the local strain field and the Δ A might have been defined as the cyclic strain intensity factor. In this study, the ΔA was applied to estimate fatigue crack growth rate. The character was utilized that value of the Δ A was dependent on kind of material and load level. The fatigue crack growth test was carried out at R=0.I, 0.3 and 0.5 in SNC631-A, SNC631-B, S45C, A12024-T3 and AI7075-T6. The fatigue crack growth rates(da/dN) in all kinds of material and stress ratio was expressed unitedly as a function of the cyclic strain intensity factor (Δ A) and validity of the results was discussed.


The fatigue fracture is a very local phenomenon. It is a prerequisite to examine the fatigue fracture problem of materials to study on the distribution of stress and strain at the fracture process zone near crack tip. This fracture process zone is the region in which the immediate crack extention process such as advanced separations, void growth and coalescence, etc. are taking place. Many studies in this viewpoint have been conducted on basis of the fracture mechanics theories and the finite element methods l - 3), etc. However, those are almost theoretical studies. Works to find a theoretical solution for the state of deformation near crack tip in actual material have been very difficult due to its mathematical complexity.

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