Abstract
The fatigue fracture surfaces of some carbon steels were observed by electron microscope and their distinctive features were made clear, as follows. Experimental method: The so-called “Microfractography” with carbon replica by direct evaporation was applied. The carbon replica was stripped off the specimen surface by electrolysing or by immersing in alcoholic acid solution. Specimens: The specimens were made from steel bars with carbon content 0.3% or less and fractured with a rotating beam-fatigue tester at stress levels ranging from 44.8 to 16.3 kg/mm2. Result obtained: (1) The “river” or the “tangue” pattern which appears commonly on tensile or impact fracture surface was not observed on the fatigue surfaces. (2) Patterns observed on the fatigue fracture surface can be classified into five types, i.e. “parallel”, “cliff”, “cleavage”, “caterpillar” and “spine” patterns. (3) The fine parallel patterns with a pitch of 100∼300 Å were found in specimens fractured at the stress near the fatigue limit, while the coarse ones with intervals of 1000∼3000 Å were produced at higher stress levels, (4) Every pitch of parallel patterns nearly corresponds to the stress repeats of about ten cycles. (5) Both the caterpillar and spine patterns appeared on the specimens fractured at the repeats of the order of 106, and they were found in a line perpendicular to the direction of crack propagation. (6) The “pit” of spine patterns and the “ridge” of caterpillar patterns were regularly arranged. If we take a pit or ridge in a row of these patterns and denote the distance to the kth pit or ridge in the direction of broading intervals, with xk then xk=k2⁄C2, where C is a constant.
