An express estimation of fatigue strength of composite flanges by infrared thermography

When designing structures made of polymer composite materials operating under vibration conditions, special time-consuming and long-term tests have to be carried out in order to obtain characteristics of fracture resistance due to multicycle fatigue. Full-scale parts or typical structural elements are tested in critical cases to take into account the influence of structural and technological factors. The method of IR thermography based on the use of the self-heating effect accompanying the appearance and accumulation of fatigue damage is actively developing to an accelerated assessment of the fatigue strength of materials. The purpose of this work is to develop a technique for rapid estimation of the fatigue limit of flanges made of layered polymer composite materials using the IR thermography method. The study is focused on a sample cut from a full-scale composite shell with a flange. A research technique based on block cyclic loading of samples using an electrodynamic vibration stand was developed. The amplitude of deformations of the sample in each block is maintained constant during loading, the temperature field on its surface is recorded using an IR camera. Two parameters characterizing the self-heating of the sample in each loading block were used to assess the fatigue limit: the heating rate at the beginning of the block and the value of the stabilization temperature at the end of the block. They were determined using thermograms and averaged over the most loaded zone of the sample. The heating rate at the beginning of the block and the stabilization temperature at the end of the block increase sharply when the amplitude of deformation in the loading block exceeds the fatigue limit. The values of the fatigue limit obtained using both of these parameters are consistent with each other and with the results of standard fatigue tests. The developed technique makes it possible to obtain an approximate express estimation of the fatigue limit of typical elements of composite structures under conditions of high-cycle fatigue.