| Mechanical properties of metal structure and components and parts of huge mechanical equipments degenerate gradually in the long-term complicated working environment. Once the accident happened, great economic loss and casualties will cause easily. Especially with lifting capacity of the casting crane more and more heavy in recent years, the metal structure of the casting crane becomes more and more huge. Once the metal structure breaks down, hundreds of tons of melting steel under bridge dump. Therefore, it’s important to estimate and analyze reliably the lifetime of metal structure, which can prevent accidents from happening.The processing technic of metal structure of casting crane is complex, and the cost is very high, so it’s impossible to do fatigue experimental research with the metal structure of the casting crane directly. Analyzing fatigue damage behavior of materials can predicate lifetime of metal structure. Q345 B steel is one of the widely used structural steels in engineering, however, the study on the mechanical properties of steel Q345 B is only a few data at normal atmospheric temperature. The mechanical properties are also analyzed by static tensile test at elevated temperature. The fatigue damage behavior of steel Q345 B is very few at elevated temperature. In view of the present situation, this paper aims at Steel Q345 B used in the casting crane, showing the tensile fatigue experiments and crack growth experiments at elevated temperature,researching the fatigue behavior of Steel Q345 B in the different stage. Specific content as follows:Static tension property of steel Q345 B has been tested in different temperatures. The influence of temperature to yield plateau has been analyzed through the stress strain curve in different temperature. Meanwhile, analyzing experimental data is to get elasticity modulus, yield limit and ultimate strength of steel Q345 B with change of temperature which provides proof to the following selection of stress level of fatigue test at elevated temperature.The fatigue experiments of stress control in different stress amplitude and environmental temperature have been done this time, and five different groups of fatigue experiments on stress control in different circular stress condition have been done in every temperature. Analyzing fatigue experimental data is to get the changing rule of strain at different temperatures. When environmental temperature changes, changing rule of strain range are different and doesn’t show a monotonic changing trend with the temperature. When temperature is 320℃, strain range in circular initial period decreases quickly, which appears circular hardening phenomenon, but other strain range of temperature increase,showing circular softening phenomenon when temperature is 300 ℃, 320 ℃, and 350 ℃, average strain curves of different temperature which have stable stage of average strain rate have been analyzed,but test pieces in other temperature break,strain rate is not stable. The experimental results show that the dynamic strain aging can effectively restrain the plastic deformation at 320℃.Function relation of the biggest strain and lifetime of steel Q345 B and function relation of stress life time after modification of average stress can be combined in different temperature through fatigue test data of stress control under different temperature, with stress lifetime m, temperature is fitted to fatigue resistance force, fatigue index and fatigue limit of three parameters stress lifetime model respectively, the three parameter stress age model with temperature changing and the revised age model have been got. Provide a calculation model for steel Q345 B in fatigue life prediction at elevated temperature.On the basis of damage mechanic theory, the ratio of the average strain N? in circular process and average strainf? in material breaking are taken as damage variable. Under doublelogarithmic coordinate axis, undetermined coefficientoD(initial damage parameter)and damage index in the damage model have been established through linearity of combining lg(1-D) and lg(1 /)f-N N. After getting correspondingly damage index in different stress and temperature, the biggest circular stress max? and temperature T have been involved in damage index, which shows that different stress and temperature have an effect on the damage model. Stress lifetime model of temperature variable in chapter three has been taken into the damage degeneration model. Finally, damage degeneration model reflecting comprehensively stress level and temperature variable can be gotten.Combined damage mechanics with the finite element method, crack initial stage lifetime of metal structure of casting crane has been done by analog computation with the valid stress method. This method can effectively improve the computational efficiency in finite element model.With crack growth experiments of steel Q345 B in different temperature, the influence of environmental temperature to crack growth behavior of steel Q345 B has been discussed. The changing rule of environment temperature T to parameter C and m in the formula Paris can be got. The calculation of crack propagation in steel Q345 B at elevated temperature is solved.With the combination of damage mechanics theory and the theory of stress-strength interference, the influence of erosion and aging to the strength degeneration has been neglected, and the change of damage which leads in the increase of valid stress has been considered. Strength has been taken as a random variable and stress has been taken as a random variable with the change of load times. Dynamic reliability curve of casting crane hook crossbeam has been simulated with the change of load times, which combines physical experiment with the method of Sampling Simulation. This method provides a new means for the study of dynamic reliability. |
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