An in-depth investigation is carried out to characterise the damage accumulation mechanisms and fatigue growth in single wood pulp fibres, which may be thought of as concentrically-layered, filamentary composite tubes that are approximately: 1-3 mm long, 20-40 $ mu$m in cross-section. The novel experimental methodology principally consists of the in situ apparatuses, on the one hand, which comprise the in-house designed and built single-fibre tensiometer, in conjunction with the confocal laser scanning microscope (CLSM) and, on the other, the computer hardware and periphery. The tensiometer's prime components, the loading jaws, are specifically designed to best emulate the force actions to which wood pulp fibres are subjected while between the discs of mechanical refiners (viz.: cyclic shear, radial compression and tension); and incorporate a mechanical fixation mechanism to ensure proper mounting of the single fibres. Moreover, the CLSM, which functions by scanning a diffraction-limited spot of light relative to the specimen in a raster-type scan, is a powerful tool for obtaining qualitative information on the morphology of fractured surfaces and structural behaviour of the fibres being fatigued, as well as providing accurate visual records of the history of crack propagation. The entire fully-automated set-up is controlled, in real time, via a computer algorithm specifically written for displacement-control fatigue-testing, while making efficient utilisation in terms of execution time, memory allocation, signal conversion and data acquisition.