FATIGUE FRACTOGRAPHS OF SOME HEAT-TREATED STEELS FOR LOWER da/dn VALUES.

Microfractographic aspects of fatigue fracture were examined in relation to the strength level of material and the microcrack propagation rate, da/dn, with a particular emphasis on the low da/dn range. The materials investigated were heat-treated carbon and low alloy steels characterized by different levels of Vickers hardness (H//v equals 140, 330, 480, and 780). Fatigue tests were conducted under repeated tensile loads. For low da/dn values the fatigue fracture surface exhibited a certain amount of intergranular facets, except for the material of H//v 330. The coverage of intergranular facets was variable depending upon the da/dn level, but was found to take a maximum value when the size of cyclic plastic zone at the crack tip was nearly equal to the size of ferritic grains (steel of H//v 140), or prior austenitic grains (steel of H//v 480). While for ordinary propagation rates the striation spacing was confirmed practically coincident with the da/dn value, except for steel of H//v 780, the spacing was greater than the da/dn value by the order of 2, for very low propagation rates.