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Fatigue crack initiation in notched members is controlled by local strains at the notch root. A number of approaches have been developed for calculating local notch-tip stresses and strains from nominal stress and notch geometry considerations. One such approach uses the parameter ΔK/ρ1/2, where ΔK is the fracture mechanics stress intensity range and ρ is the notch root radius. The parameter ΔK/ρ1/2 has been shown to correlate with local notch-tip strain and provide a means of normalizing cycles-to-initiation, Ni, data for various notch-tip geometries. Fatigue crack growth rate specimens described in ASTM Test Method for Plane-Strain Fracture Toughness of Metallic Materials (E 399), ASTM Test Method for Measurement of Fatigue Crack Growth Rates (E 647) and elsewhere can be used for fatigue crack initiation testing if they have blunt notches. Data in the form of parameter ΔK/ρ1/2 versus Ni has the same units as the traditional S-N curves. The advantage of having data in the form parameter parameter ΔK/ρ1/2 versus Ni is that information is available on ΔK and ρ . As in all fracture mechanics testing, data obtained on one specimen geometry can be applied to a wide variety of structural geometries. This approach can be used in conjunction with commercially available software for fatigue crack growth rate testing, servohydraulic testing equipment, and modified fracture mechanics specimens to automate fatigue crack initiation testing. The combination of all of these elements represents a new test method. Results are presented for aluminum alloy 7075, where the effects of corrosion pits were studied and for titanium alloy Ti-6A1-4V, where the effects of heat treatment on initiation were studied.
constant amplitude, environment-assisted cracking, fatigue, fatigue crack initiation, fracture mechanics, initiation, metallic materials, stress intensity range
Special Assistant, Materials Science and Component Technology Directorate, Naval Research Laboratory, Washington, DC
Materials Research Engineer, Naval Research Laboratory, Washington, DC