Abstract
The dependency of stress intensity factor range, ΔK, on crack growth rate, da/dN, at the early stage, which initiated from a small notch with notch length of 50μm or 200μm and the notch root radius of about 25μm, was examined using a high strength aluminum alloy. The effect of notch length on crack propagation cycles was discussed by integration of da/dN -ΔK curves. The results are as follows.(1) The da/dN of a high strength aluminum alloy can be expressed by the power law as a function of ΔK. The da/dN-ΔK curves can be separated into three stages.(2) In the stage 1 (20>ΔK>8.7kg/mm3/2), the da/dN is discontinuous due to the formation of non-propagating cracks.(3) There is a well defined minimum apparent stress intensity factor for crack propagation from a notch. For R=0.1, this factor is constant regardless of the length of the notch.(4) The apparent crack length for non-propagating cracks for the smooth specimen is 32μm under R=0.1.(5) When the length of a crack from the notch tips, aP, is almost equal to or larger than the notch length, aN, the number of cycles required for a crack to propagate to aP, NP, increases with aN under constant ΔKap and ρ, where ΔKap is the apparent stress intensity factor range and ρ is the notch root radius.
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