Study On Corrosion Cumulative Damage Rules And The Calendar Life Of Aircraft Structures

The number of takeoff and landing, the flight hours and the calendar time are the three life indexes. There are approaches, such as normal stress approach, local stress-strain approach, for the traditional fatigue life estimation of aircraft structures under mechanical loading, but there is no approach for the evaluation of the calendar life under mechanical loading and corrosive environment. In the present dissertation the focus is on the investigation of fatigue and corrosion cumulative damage rule, for it is the primary element of the approach for the prediction of calendar life and the design of aircraft structures against fatigue and corrosion. The researches focus on four aspects as following:1. Study on the S-N curves of Aluminum alloy LY12CZ under the constant amplitude loading after the corrosion of different equivalent calendar year. This study consists of three parts. Firstly the equivalently accelerative corrosive environment is determined through the analysis of statistic data of corrosion environment. Secondly the fatigue test is carried out under the constant amplitude cyclic loading for the specimens which are exposed in the corrosive environment for equivalent calendar time of zero year, 3 years, 8 years and 13 years. Four S-N curves with different corrosive time are obtained based on the fatigue experiment. Finally the regression method of S-N curves for experimental data is researched and a weighted least square fit is put forward in which the weigh of a group of test data is inversely proportional to the length of the confidence interval.2. Investigation on the fatigue cumulative damage rules under corrosive environment -by - fatigue loading. The fatigue tests have been done for several combinations of mechanical cyclic loading and corrosive environments to probe the damage accumulation mechanisms. On the based of the understanding of fatigue damage accumulation, two cumulative damage rules are put forward, one is nonlinear cumulative damage rule based on dynamic S-N curves and the other is the competition model considering the retarded or accelerated effect on fatigue crack initiation.3. The aircraft structures are acted by mechanical loading and corrosive environment alternatively in service, and usually the corrosive environment is dominant for the structural life. The new model of assessing the calendar life of aircraft structures isIIbrought forward. In this model.4. The corrosive data-base of navy aircraft is established based on Web technology. The corrosive data can be analyzed statistically and use for the maintenance of aircraft structures.The scientific or technical contributions of the dissertation are as below. The first, the fatigue experiment of LY12CZ aluminum alloy specimens with different time exposed in corrosive environment, and the corresponding S-N curves are obtained. The second, two cumulative damage rules are put forward suitable for the different loading and corrosive modes. The experimental results show that the rules have good capacity of predicting the calendar life of aircraft structures acted by mechanical loading and environment alternatively. The third, a weighted least square fit for the S-N curves regression is brought out, in which the weigh of a group of test data is inversely proportional to the length of the confidence interval. And the last, the approach for the estimation of calendar life is presented.