§11.1 Fatigue,Creep and Fracture 447 Revs 4 Fig.11.2.Simple sinusoidal (zero mean)stress fatigue curve,"reversed-symmetrical". Stress amplitude Stress range m0 m Meon stress 0 Time (t) Fig.11.3.Fluctuating tension stress cycle producing positive mean stress. The stress-cycle curve is shown in Fig.11.3,and from this diagram it can be seen that: Stress range,or =20a. (11.1) Mean stress,om cmax十cmin (11.2) 2 Alternating stress amplitude,o= Omax -Omin (11.3) 2 If the mean stress is not zero,we sometimes make use of the "stress ratio"R,where R:=Omin (11.4) Omax The most general method of presenting the results of a fatigue test is to plot a graph of the stress amplitude as ordinate against the corresponding number of cycles to failure as311.1 Fatigue, Creep and Fracture 447 Fig. 1 1.2. Simple sinusoidal (zero mean) stress fatigue curve, “reversed-symmetrical”. t I 0 Time (t) Fig. 11.3. Fluctuating tension stress cycle producing positive mean stress. The stress-cycle curve is shown in Fig. 11.3, and from this diagram it can be seen that: Stress range, a, = 2a,. (11.1) amax + amin 2 Mean stress, a,,, = (11.2) (11.3) amax - amin 2 Alternating stress amplitude, a, = If the mean stress is not zero, we sometimes make use of the “stress ratio” R, where (11.4) The most general method of presenting the results of a fatigue test is to plot a graph of the stress amplitude as ordinate against the corresponding number of cycles to failure as amin R, = - amax