Regularities of fatigue failure typical composite flange

One of the most important tasks in the development of aircraft structures from polymer composite materials is to ensure fatigue strength. It is known that the behaviour under cyclic loading of parts and standard samples is not the same. The problem of transferring the results of standard tests to a full-scale product is especially acute due to the variety of reinforcement schemes, the influence of technological and design factors. This forces us to test full-scale products or structurally similar elements. This work is devoted to the study of the patterns of fatigue failure of a typical element of shell aircraft structures - an L-shaped flange made of laminated carbon fibre. A method of fatigue testing of the critical zone of the flange has been developed. It reproduces the operational conditions of flange loading. The fatigue loading of the sample is carried out by the inertia forces of the load attached to it. The tests were carried out with resonant harmonic vibrations on a vibration stand in a bending shape. The experimental setup ensures constant maintenance of the resonant mode. The experimental setup ensures continuous monitoring of deformation, the resonant frequency of vibrations of the sample and the temperature field on its surface. The characteristic patterns of the development of fatigue damage of the flange are established. The main mechanism of destruction is the appearance and development of delaminations in the flange and in the area of its connection with the shell. Fatigue failure is accompanied by a drop in the resonant frequency of vibrations of the sample, due to a decrease in its rigidity. The typical dependences of the resonant frequency drop on the relative fatigue time reflect an abrupt change in the stiffness of the sample. The established regularities of the fatigue damage accumulation process are confirmed by the analysis of the thermal state of the sample during testing, which changes as a result of its self-heating during cyclic loading. The experimental data can be used in the development of the design of flanges made of CFRP and in the development of models for predicting their fatigue life.