Load Spectrum Equivalent Approach Andscatter Analysis Of Civil Aircraft

The scatter factor and accelerated fatigue test technology are two important problems involved in full scale fatigue test. Fatigue life variation includes the following two aspects: structural property variability and load spectra scatter. A fatigue reliability evaluation method which taking the structural property variability and load spectra scatter into account is developed, and the primary focus is placed on load spectra scatter. In the existing accelerated test methods which are based on deterministic fatigue-damage evaluation, structural property and fatigue life are represented by their mean value. However, due to various uncertainties, random variables should be used to describe the structural property and fatigue life. It is deficient for these existing methods to reveal the essence. Therefore, accelerated test methods which are based on probability fatigue are studied in this paper.Statistical loads data of civil transport airplanes collected by several laboratories is summarized, and the changing trend is analyzed. Form the 1950 s to the 1980 s, fatigue load reduces significantly. But after the 1980 s, it seems to be stable. According to this trend, the load spectra suggested to be adopted in future models are presented, which also provide the basic data for load spectra scatter analysis.Fatigue life variation is mainly affected by variabilities of load spectra and structural property. For civil aircraft structure, various factors which affect load spectra variability such as flight distance, gust and maneuver loads, load-stress transfer coefficient are quantitatively discribed. The flight distance and load-stress transfer coefficient are discribed by random variables. The gust and maneuver loads are discribed by exceedance curves families. On this basis, P-S-N curves are adopted to describe the structural property, and Miner linear damage rule is used to calculate fatigue life distribution.Taking the randomness of structural property and fatigue life into account, a small-load-omitting criterion which is evaluated by whether the truncated spectrum has the same fatigue life probability distribution as the original spectrum is developed. Probabilistic distance is used to measure the difference between two fatigue life probability distributions. The omission level can be obtained when the probabilistic distance is equal to a tolerance value predefined. According to the distribution density function of stress amplitude and the P-S-N curves of the component, the omission level can be calculated. Fatigue tests with smooth sheet specimens made of LC4 CS Aluminum alloy are conducted to verify this criterion. Five different spectra are obtained by removing small cycles of which amplitudes are less than the specified omission level. It is found that experimental results agree well with theoretical solutions.A load spectrum equivalent approach based on probability fatigue is developed. Probabilistic distance is used to measure the difference between fatigue life probability distributions of the original spectrum and the equivalent spectrum. The equivalent spectrum can be obtained by minimizing the probabilistic distance. In the presented approach, the uncertainties of structural property and fatigue life are taken into account. Given the original load spectrum, the P-S-N curves of the component and equivalent stress level, the equivalent spectrum can be calculated. Test results shows that the presented approach is effective in engineering practice. Compared with the small-load-omitting criterion, test time can be futher reduced.