Applications of fractography in understanding fatigue failures in aircraft components

Abstract

Statistics show that fatigue fractures constitute the most prominent mechanism of failures in aircraft structures. In general, the components are designed based on the experimentally generated data obtained on the fatigue lives of material using constant amplitude tests. In spite of following the relevant approaches of design such as safe-life and damage tolerance, the fatigue crack growth behavior cannot be predicted very accurately in majority of aircraft applications unless the load-time data on the component is available from service history. Another important concern in design of aircraft components against fatigue is the occurrence of premature failures that are commonly encountered during service as a result of local stress concentrations arising from presence of metallurgical defects, environment induced damages etc.

Irrespective of the problems mentioned above, important information about the failure process can often be obtained from examination of the fracture surfaces of the prematurely failed components, and these data often provide important inputs to the designer as well as the user for accurate estimation of component life. Since fatigue damage occurs due to cyclic loads, the characteristic features of the beach marks and the striation count within the progressive crack propagation zone of the fracture surfaces created during service failures provide information on the type of loading and also the time of initiation and the propagation of cracks. In this talk, a few examples of fatigue fractures in aircraft components that failed in service and on the test bed will be discussed. The application of fractography in understanding the failure process, and the methodology adopted for estimation of the time of initiation of fatigue crack and its propagation through fractography studies will be highlighted from these examples.