Multiaxial Stress Mapping and Fatigue Failure Prediction of Aircraft Hydraulic Pipes

Frequent fatigue failures of aircraft hydraulic pipes were caused by a stress concentration site in the form of a circumferential groove at the constrained pipe ends. The grooves were formed as a result of the crimping of a metallic sleeve on the pipes. Internal hydraulic pressure fluctuations during aircraft flight resulted in multiaxial and alternating stresses at the constrained (fractured) end. The complete stress state was found using FEM 'Frame 3D' MATLAB (R) code and analytical stress concentration factors. The calculated stresses not only mapped well with the fractured patterns of a failed pipe but also correlated to the computational stress analysis performed separately in an FEA software. The stress calculations were further utilised for fatigue life estimation of the pipe with and without the presence of circumferential groove. The load spectrum of the pipe was developed using 73 flying hours data of hydraulic fluctuations and converting those into stresses. Various stress amplitudes and load ratios were segregated from the cyclic stresses using the rainflow counting method. A linear constant lifeline diagram and Miner's rule of damage accumulation were used for fatigue life estimation. A significant decrease (from 12,500 to 165 flying hours) in the service life of the pipe was predicted due to the presence of the stress concentration. The predictions matched well with the observed failure trend (life) of the pipe.