Research On Fatigue Failure Of Reverse Vaulted And Notched Thin Metallic Shell Under Uniform Pressure

Appliance of the structure of axially symmetric metallic shell with radiating notch is the reverse rupture disk that is cross-grooved. Its appearance is mostly like an axially symmetric metallic shell. It can accurately burst when the pressure is over its bursting pressure and protects the equipment from destruction. At the same time it does not produce any fragment, so it does not cause a fire when it burst. Having such virtue it is widely used in industry and electricity field. Its stress level is very high when it is working, sometimes just close to its burst pressure. And it is often used in the environment which pressure is fluctuating. In this case, it is possible for the rupture disk to burst when its working pressure is under its bursting pressure. Undoubtedly, this is a hidden danger for round-the-clock production in industries and electricity field. Therefore, it is of great consequence to find out the mechanism of fatigue failure of this kind of structure and to acquire its fatigue curve by experiments. This research has not only an important theoretical significance, but also a practice value.In this paper the generalization of the researches of fatigue failure is introduced. Considering this, by the ways of combination between the macro and micro method and combination between the experimental research and theory analysis, the mechanism and fatigue properties of fatigue failure of this specific structure under cyclic changing pressure is deeply studied.1. In order to gain the stress distribution under certain loading, the experiment of static stress survey of the axially symmetric reverse vaulted and cross-grooved specimen that is coved by a radius of 160mm and which bursting pressure under room temperature is 0.9MPa is carried out. And the result indicates the situation of stress concentration in the vicinity of the notch and the stress gradient in this field is very high. The coefficient of stress concentration is above 2. The value of circumferential stress is also higher than the longitudinal stress.2. Finite element method is used to calculate the value of stress, strain and displacements. The result of calculation indicates the value of circumferential stress is higher than any other stress in the whole body. When the value of pressure is 0.6MPa, the value of circumferential stress is over 202MPa that has been above the material yield stress. This also indicates the rupture disk has cumulation of the fatigue damage while working.3. Fatigue experiments of the specific structure are fulfilled at five stress levels, and fatigue curve is acquired. At the same time in order to get the relationship between the stress and strain dynamic stress surveys at certain point are taken out. The experiment result shows the median fatigue life is 7359 cycles when the pressure amplitude is 0.1-0.87MPa. The evaluation value of fatigue life under 99.9% reliability is only 3835 cycles. Increased with stress amplitude the fatigue life of rupture decreases, and the dispersion increases.4. The macroscopic and microcosmic observations of the fracture of fatigued structures are performed. Taking the characteristic of the stress distribution and the present fatigue failure theory and the relationship between stress and strain into consideration, the mechanism of fatigue failure of the specific structure is developed. The shear stress makes the material crystal glissile dislocation take place and in this situation the fatigue micro crack initiates. The relationship between stress and strain in plastic field is nonlinear, and this induces residual tensile stress in the compression loading unloaded process. The tensile stress makes the macro cracks link to each other. When the length of crack is up to certain value, the structure fails suddenly by the crack's rapid linking up.