Fatigue Crack Growth (FCG) Studies on Landing Gear (LG) Actuating Cylinder of Fighter Aircraft for Life Extension

摘要

Study on life extension was undertaken for actuating cylinder of main and nose landing gear (MLG and NLG) of fighter aircraft. Activities involved for life extension are load estimation on landing gear (using MIL-A-8862) and actuating cylinder, stress analysis of actuating cylinder, fatigue load spectrum generation (using MIL-A-8866C), theoretical fatigue life estimation, and fatigue testing of actuating cylinders. Fatigue testing was carried out on one specimen each of actuating cylinder of MLG and NLG, for load spectrum of 4000 landings, considering scatter factor of 4. Crack was observed on MLG actuating cylinder eye end, one of the major load-carrying member. Therefore, it was decided to carry out fatigue crack growth analysis of eye end to decide the inspection interval in life extended period. Fatigue crack growth analysis is carried out using Paris equation. Suitable stress intensity factor (SIF) for crack geometry and material constants for Paris equation are selected from available literature. It is concluded that, assuming an initial crack of 0.5 mm in eye end at threaded region, it will become critical after minimum 1500 landings. This was practically seen during fatigue test. Crack length reached during fatigue test was around 7 mm; however, unstable crack growth was not observed. This study recommends the thorough inspection of eye end to be carried out at every 400 landings after initial assigned life in number of landings to ensure absence of cracks.