The spectral transmittance of mixed silver-halide polycrystalline fibres was measured while they were undergoing repeated bending leading to mechanical fatigue. Microscopic mechanical defects were detected through their influence on the optical losses, without interfering with the deformation. Optical and mechanical lifetimes of the fibres were found to depend on the composition and to be larger for highly-mixed compositions. Scanning electron microscopy observations of fatigued fibres revealed a strong influence of the fibre composition on the fatigue damage. This dependence on the composition was explained using a theoretical model of solid-solution strengthening.