Structural integrity is always an important issue for the service life time of any structure. Therefore, joint of the metal is also a vital point to consider. This thesis describes a new method that uses the Fiber Bragg Grating (FBG) sensor to detect the crack initiation of friction stir welded aluminum (AL) alloy by low cycle fatigue test. Experimental setups and procedures are studied, a proper test configuration is determined and a series of testing performed. The applicability of this new method of stress control fatigue test in monitoring the metal crack initiation is demonstrated through the test results, which shows that FBG sensors are sensitive to capture the onset of the crack. Stress controlled fatigue tests especially at high strain amplitude for Friction Stir Welded (FSW) joints demand the sensors to withstand strain level above ±8000 με  and with the size of few mm, and these sensors are not available in the market. In this work, the new sensors arrays are used in the characterization of cyclic deformation behavior of FSW aluminum alloy, which, in some cases, could sustain up to ±11000με. During the experiment significant difference in plastic strain amplitude (PSA) was observed using these optical fiber sensors mounted in nugget zone (NZ) and across the boundary area between thermo mechanical affected zone (TMZ) and heat affected zone (HZ). For all the samples of Al alloy, with the progress of cyclic deformation the cyclic hardening characteristics of the metal were observed.