Investigation On Fatigue Behavior Of Materials In Very High Cycle Regime Under Vibratory Loading

Investigation on very high-cycle (over 10~7 cycles) fatigue behavior of materials used in high speed running structural parts has become a new subject in the present years. This thesis is aimed at understanding the fatigue behavior and mechanism of nodular cast iron GS51, D38MSV5S steel, cast aluminum 2-AS5U3G-Y35 and intermetallic compound TiAl based alloy undergoing high frequency, low amplitude very-high cycle.On the basis of analysis and design of components and assembly units in torsion and flexural fatigue vibration, the new ultrasonic torsion and three-point bending fatigue system were developed, besides, a kind of optimum fatigue specimen was proposed and discussed.There are no fatigue limits for the three kinds of metallic alloys in tension-compression fatigue test with two kinds of stress ratio (R=-1 and R=0.1) and in torsion fatigue test for D38MSV5S and 2-AS5U3G-Y35 alloys, except D38MSV5S with R=0.1. There is no evident frequency effect for all kinds of alloys tested with different stress ratio. Most of specimens initiated from subsurface defect in the very high cycle regime. However, for the GS51, fatigue crack initiating mode in the very high regime is depended on competing between surface spherical graphite or interior void. Torsion fatigue crack initiation is always from surface.Self-heating temperature change of specimen during ultrasonic fatigue process is measured by infrared imaging system. The temperature evolution is dependent on the amplitude of cyclic load and loading rates. Temperature evolution reflects the local plastic deformation of alloys, it relates to fatigue life.A fatigue limit more than 10~7 cycles is obtained at R=0.7 for TiAl based alloy; however, there is no fatigue limit for R=0.1 and R=0.5. Stress ratio effects the fatigue behavior. Fatigue life of TiAl based alloy is pronounced determined by fatigue crack initiation, fatigue crack always initiate from specimen surface where the stresses were the highest. In the very high cycle regime, fatigue initiation can also initiate from the subsurface defect. Transgranular crack propagation with many secondary cracks and cleavages is the important characteristic.