Papers by Author: Q. Chen

Abstract: Effect of high humidity on growth mechanism of a fatigue crack was investigated for an extruded bar of an age-hardened Al alloy 7075-T6 which had the marked texture of plane (111). Fracture in high humidity occurred by the growth of a shear mode crack under high stress levels, though a crack propagated in a tensile mode under low stress ones, macroscopically. Many voids and slip planes were observed on the fracture surface yielded by the shear mode crack. Especially most of the fracture surface was occupied by voids where the crack was small. The void percentage decreased with increasing in the crack depth. Fracture surface yielded by the shear mode crack was a plane (100). The growth direction of the shear mode crack to the cross section of specimen was about 55° corresponding to the angle composed by this plane and the texture of plane (111). These results suggest that the shear mode crack was related to microstructure, stress and environment. The growth mechanism of the shear mode crack assisted by hydrogen was proposed based on the results of the acceleration of crack growth and the formation of voids in high humidity.

Abstract: Rotating bending fatigue tests were carried out to investigate the influence of grain size on the resistance to crack growth of Ni-base super alloy, Inconel 718, using the materials with grain sizes of about 20, 50,100 and 200 たm, at room temperature. The larger grain size, the smaller crack growth rate, though the static strength was decreased with increase in grain size. The growth rate of a small crack was uniquely determined by the term ja na (Small crack growth law), where ja and a are the stress amplitude and the crack length, respectively, and n is a constant. The resistance to crack growth among materials was evaluated based on the law and showed a good correlation with tensile strength jB, meaning that fatigue life can be predicted by using only j B BB.

Abstract: In order to investigate the effect of frequency on the crack growth behavior, ultrasonic fatigue tests were carried out for an extruded age-hardened Al alloy, 7075-T6, and the results were compared with those in rotating bending fatigue. Fatigue strength in ultrasonic was higher than that in rotating bending. This was mainly caused by the retardation of crack initiation. Growth direction of a crack changed from a tensile mode to a shear one in ultrasonic fatigue, though fracture occurred by the growth of a tensile mode in rotating bending. The growth direction of a shear mode crack was inclined about 55 degrees to the tensile axis. The relation between an applied stress σa and a crack depth at transition of growth direction
T was expressed by a n
T=C, where C and n are constants. These results were discussed from the points of view of the time dependent environmental effect and the texture of material.

Abstract: The objective of this study is to determine very long life fatigue and near threshold fatigue crack growth behaviors of 7075/T6 and 6061/T6 Al-alloys using piezoelectric accelerated fatigue at 19.5KHz. The experimental results show the fatigue failure can occur beyond 107, even 109 cycles, and endurance limits could not be obtained in the Al-alloys until 109 cycles. Fatigue voids are noticed on fatigue fracture in both alloys. By using scanning electron microscopy (SEM), the crack initiation and propagation behaviors have been examined. Fatigue crack growth rates of small cracks in the Al-alloys are found to be greater than those of large cracks at the same stress intensity factor range.