Fatigue tests at zero mean load have been carried out on copper and nickel-chromium alloy steel plate specimens containing two small coplanar edge cracks. For a wide range of crack lengths it was found that whether or not a crack grew depended on the value of ρ3l, where ρ is the semi-range of the alternating stress and l is the edge-crack length. If ρ3l is greater than C a crack will grow, if ρ3l is less than C a crack will remain dormant. C had a value of 0·6 for copper and of 5·5 for the alloy steel when ρ was in ton/in2and l in inches.
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