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Strain-controlled fatigue tests especially at high strain amplitudes for welded joints demand the strain sensors capable of reaching Â±8000 Â¿Â¿ and of a size of a few millimeters. However, these sensors are not available on the market. Here we report the application of a newly-developed flat-cladding Fiber Bragg grating sensor array in the characterization of cyclic deformation behavior of friction stir welded Al-Zn-Mg aluminum alloy tested at high strain amplitudes. Significant difference in the plastic strain amplitudes was detected via these optical fiber sensors mounted in the nugget zone and across the boundary between thermomechanical-affected zone and heat-affected zone. Initial cyclic softening followed by hardening in the nugget zone was observed as cyclic deformation progressed.