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Fatigue tests have been carried out on mild steel specimens containing edge cracks of various lengths and the alternating stress just insufficient to cause the cracks to grow obtained. It was found that the relation between this critical alternating stress u and the crack length 1 was given by 031 = constant. By means of this relation it was possible to correlate a wide variety of hitherto unrelated notch fatigue data.Fatigue limit based on stress to initiate crack at notch root. o Plate. 0 Round bar. Fatigue limit based on complete fracture. + Plate.x Round bar. Specimens 0.5 in. outside diameter, notches 0.05 in. deep Test markRemarks rmfllfnum stress to propagate crack Ki crit = . . Below this K, value, the nominal initiation stress is higher than the minimum propagation stress and all cracks must eventually cause complete failure. Above this K, value, the critical propagation stress remains constant and is independent of notch root radius. Hence non-propagating cracks will be formed with all notches that are sufficiently sharp to initiate a crack below about 3 3 % ton/in2 and &5$ ton/in2 (net area) for the 0.2 and 0.05 in. deep notches respectively, and the conventional value of K, can never exceed 4 for the former specimens and 3 for the latter. Although for these two different depths of notch the stress to initiate a crack is governed by the same general law the minimum stress required to propagate the crack increases Val 173 No 35 1959 at MICHIGAN STATE UNIV LIBRARIES on June 9, 2015 pme.sagepub.com Downloaded from * Specimen previously tested at a lower stress. Plain fatigue limit of stress relieved plates A13 tonlinz.

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