Combination of synchrotron computed tomography and acoustic emission measurements for cyclic loading of fibre-reinforced composites

During incremental load-unload cycles of fibre-reinforced materials, the re-initiation of acoustic emission below the previously reached load is frequently observed. This is known as the Felicity effect and its physical origin has been discussed in the community for several decades. The principal explanations for this effect are the rubbing together of existing crack faces which thus act as tribological acoustic emission sources or the initiation of new damage because of material stress-relaxation during the unloading phase. We conducted combined acoustic emission measurements and high-resolution synchrotron computed tomography experiments on a carbon fibre / epoxy prepreg system. Samples were prepared as double-edge notched tensile specimens with [±45 0 ±45], [0 903 0] and [90 03 90] layups. At a voxel resolution of 1.625μm, we were able to visualize the growth of damage as a function of applied load/unload steps in a sequence of volume images. With the simultaneously acquired acoustic emission signals, this allows identification of the origin of the source volumetrically, as three volumes were taken per load step in situ. These investigations are part of an ongoing measurement campaign and represent a first interpretation towards the physical origin of the early acoustic emission onset in fibre-reinforced materials. This provides a first step to reveal the origin of the Felicity effect in fibrereinforced materials.