Abstract
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 (HV=140, 330, 480, and 780). Fatigue tests were conducted under repeated tensile loads.
For low da/dn values (<10-4mm/cycle), the fatigue fracture surface exhibited a certain amount of intergranular facets, except for the material of HV 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 HV 140), or prior austenitic grains (steel of HV 480).
While for ordinary propagation rates (da/dn)>10-4mm/cycle), the striation spacing was confirmed practically coincident with the da/dn value, except for steel of HV 780, the spacing was greater than the da/dn value by the order of 2, for very low propagation rates (da/dn<10-6mm/cycle). The striation patterns observed in such cases were very irregular for steel of HV 140, but regular, though in small amounts, for steel of HV 780.
