Study On The Creep-Thermal Fatigue Life Prediction

With the development of aerospace, energy and chemical industry, the high temperature equipments are used more and more widely. These equipments are subjected to creep damage during steady operation process and thermal fatigue or thermo-mechanical fatigue damage in the frequent on-off operation or the sudden change of the operating mode, its potential danger is huge, the happened accidents are always catastrophic. Therefore, the problem of safety and reliability of equipments under creep and fatigue interaction is increasingly outstanding with the development of power machinery, chemical industry machinery and astronavigation, the damage evaluation and life prediction of high temperature components are currently important study field.Creep fatigue can be divided into creep thermo-mechanical fatigue and creep-thermal fatigue according to the loading type. Creep thermo-mechanical fatigue exists in power machinery more widely and is paid more attention to, at present, lots of life prediction methods have been put forward. Creep-thermal fatigue exist mainly in engine cylinder cover and heat exchanger pipes of high temperature equipments, and few reports about creep-thermal fatigue test and life prediction methods are studied. In the paper, several problems about creep-thermal fatigue are researched, and the results are as follows:1. The method that the creep-thermal fatigue can be equaled to a thermo-mechanical fatigue of the constant stress amplitude and mean stress is brought forward by analyzing the mechanical mechanism of creep-thermal fatigue interaction. Thus, the thermo-mechanical fatigue test data can be used to predict the creep-thermal fatigue life. There are more mature life prediction methods and test equipments for the thermo-mechanical fatigue, and the test of thermo-mechanical fatigue does not need insulating time, requires simple equipments and is effective. The thermo-mechanical fatigue test of ZL111 is done, the P-S-N curve is presented and the method of the creep-thermal fatigue reliable life prediction is studied.2. The creep thermo-mechanical fatigue crack opens under the cyclic external forces, its opening displacement increases due to creep in the duration of constant temperature, therefore, the creep thermo-mechanical fatigue crack growth can be divided into fatigue damage and creep damage crack growth. It is found that the creep-thermal fatigue crack is different by finite element calculation:it is closure during heating and insulating processes and opens at the end of cooling process, here, the temperature around the crack has been lower than creep temperature, thus, the creep crack control parameter C* is not adapted to creep-thermal fatigue crack, the stress intensity factor and J-integral can be used as control parameters of creep-thermal fatigue crack. The creep-thermal fatigue crack opens during unloading process and is affected by residual compression plastic strain around it, so it is not satisfied with the J-integral path independant calculation condition. An improved J-integral calculation method is put forward, and the method is satisfied with not only creep-thermal fatigue crack but also any unloading crack or the crack in the residual compressive plastic strain field. The crack growth test of ZL111 is done and its crack growth reliable life prediction method is studied.3. By studying thermal fatigue inclined crack, it is found that the thermal fatigue inclined crack is pure II mode in the heating and prophase cooling processes andⅠ,Ⅱmixed mode in the late cooling process. The equivalent stress intensity factor Ke is increased with the increase of the inclination angleβ, thus, the thermal fatigue crack is tending to be dangerous and common tension crack is going to be safe whenβincreases.4. The thermal fatigue crack network usually forms on the surfaces of components suffered thermal fatigue damage. The stress intensity factor of the main crack can reflect the damage extent of components; however, it is difficult to calculate due to the shielding effect between cracks. The rule of shielding effect in thermal fatigue crack network is studied; the stress intensity factor of the main crack can be calculated conveniently, quickly and accurately according to this rule.5. A reliability sensitivity analysis method is presented based on the response surface method. The advantages of this method are as follows:(1) It is available even the ultimate state function is unknown. (2) The ultimate state function, a simple quadric polynomial, is convenient for calculating the variance, partial derivative and reliability sensitivity. (3) The function includes the information of linear, quadratic and crossing quadratic terms, which makes the accuracy of reliability sensitivity greatly improved. (4) Owning to its regularity, programming is achievable. The reliability sensitivity of thermal fatigue crack is calculated with inclination angle, the maximum of heating temperature, the crack shielding residual percent and fracture toughness as the random variables.6. The thermal fatigue test machine used for aluminium alloy square sample is designed and manufactured. It is composed of infrared heating equipment, mixed cooling system, clamping device and control system. Its uses are as follows:(1) comparing the ability of all kinds of materials against thermal fatigue damage; (2) comparing the effect to thermal fatigue life of all kinds of processing procedure;(3) comparing the effect to thermal fatigue life of test temperature and the heating and cooling rate; (4) taking a view of effect to thermal fatigue life of hard coat. Its expected functions have been come true totally.