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® 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.

飞机液压管的频繁疲劳故障是由受约束的部位（在受约束的管端处呈圆周凹槽的形式）引起的。由于金属套管在管上的压接而形成凹槽。飞机飞行期间的内部液压波动会在受约束的（断裂）端产生多轴应力和交变应力。使用FEM'Frame 3D'MATLAB®代码和分析应力集中系数可以找到完整的应力状态。计算出的应力不仅与故障管道的破裂模式很好地对应，而且与在FEA软件中单独执行的计算应力分析相关。在存在和不存在周向沟槽的情况下，应力计算还用于估算管道的疲劳寿命。使用73个飞行小时的水力波动数据并将其转换为应力来开发管道的载荷谱。使用雨流计数方法将各种应力振幅和载荷比与循环应力隔离开来。疲劳寿命估算使用线性常数生命线图和损伤累积的Miner规则。由于应力集中的存在，预计管道的使用寿命将显着减少（从12,500飞行小时减少到165飞行小时）。这些预测与观察到的管道故障趋势（寿命）非常吻合。疲劳寿命估算使用线性常数生命线图和损伤累积的Miner规则。由于应力集中的存在，预计管道的使用寿命将显着减少（从12,500飞行小时减少到165飞行小时）。这些预测与观察到的管道故障趋势（寿命）非常吻合。疲劳寿命估算使用线性常数生命线图和损伤累积的Miner规则。由于应力集中的存在，预计管道的使用寿命将显着减少（从12,500飞行小时减少到165飞行小时）。这些预测与观察到的管道故障趋势（寿命）非常吻合。