Investigation on Fatigue Crack Progress of #beta#' Brass Sheet Concerning Crack Opening Displacement

摘要

The fatigue tests with repeated sinuous tensile load cycle rate, f, at 0.15, 1.5, 2.5 or 15 Hz, and with cycle ratio, R, at 0.11 or 0.02, at the maximum tensile load of 1764N were carried out on hot - rolled #beta#' brass plate. Then the following experimental results are attained. The crack opening displacement range increases with the increase of repeated load cycle rate except that at the rate of 15Hz. The crack opening displacement increases as the cycle ratio becomes lower, due to the increase of load range. By SEM examination, along with the progress of fatigue crack, the fracture pattern changes from the conventional fatigue fracture at initial stage to the coexistent pattern of the former fracture and cleavage fracture at transient stage, and then whole into pure cleavage fracture pattern at final stage. From these results, it is inferred as follows that the faster rate of repeated load cycle restrains the development of plastic zone at the front of crack tip forming instantaneously high level of the concentration of elastic stress at the crack tip that makes the separation of crack tip easily in brittleness. Namely, in this case the increase of mechanical energy acting on specimen is consumed mainly in brittle fracture than in plastic fracture at crack tip. And with the growth of crack in the specimen, the more the stress intensity range or the energy actuates, the more the brittle fracture appears so that promotes the crack growth rate much more and the crack opening displacement range greater.