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