Abstract
Smooth specimens of 0.04% C annealed steel were fatigued under completely reversed plane bending. The results of microscopic observation of fatigue crack initiation and early propagation conducted both on the surface and in the interior of the specimen were discussed based on micromechanics. The followings are the summary of the study:
(1) The initiation site of cracks was mainly along the grain boundary on the specimen surface and the path of most cracks changed from along the grain boundary to inside the grain at the depth about 1μm. The shape of small cracks was complicated by the grain boundary and orientation, and the crack length-depth ratio was scattered between 0.3 to 1.5. The average plane of microcracks was plane of the maximum shear stress resolved from the applied stress.
(2) Above the fatigue limit, the relation between the stress amplitude, σa, and the initiation life, Nc, of the crack in the order of the grain size was expressed by the following equation derived from the Tanaka-Mura theory of fatigue crack initiation
σa=148+3.83×104Nc-1/2
where 148MPa is the friction stress. The fracture surface energy used in the theory was 1.4× 105J/m2 for the present material.
(3) Below the fatigue limit, non-propagating microcracks whose length were below the grain size were detected, and the aspect ratio of those cracks was around 0.6. The condition of microcrack growth at the fatigue limit was explained from the criterion of the critical microscopic stress intensity factor at the tip of slip bands emanating from microcracks. The critical value was determined to be 0.74MPa√m.
