Fatigue Crack Growth in Airframe Elements under Variable Amplitude Loading

The results of analytical and experimental researches concerning predicting of fatigue crack growth in the operating conditions are presented. Main conditions causing a fatigue crack growth at the complex spectrum of loading are analyzed and divided to two groups. Theory of fatigue crack growth indication and the crack growth indicator (CGI) are developed. The fatigue tests with aluminum alloy flat specimen and CGI at constant and variable amplitude of loading was carried out. It is shown the estimate of mean value of constant C* of CGI both cases closes to that one of flat specimen. There is planned and executed a flight experiment using CGI located on two aircraft An-24 and An-26. The combination of CGI technology with fractography of a fatigue crack surface allows to decreases number of necessary tests, terms of their carrying out and increases accuracy and reliability of results. Results of crack growth in CGI at operational load allowed to evaluate the parameters of generalized Paris-Erdogan law and statistical properties of crack increment per flight. It is shown the Kolmogorov-Smirnov test shows the hypothesis about normal distribution of log increment in flight is significant at the 5% leve