On the sensitivity and repeatability of fiber Bragg grating sensors used in strain and material degradation measurement of magnesium alloys under cyclic loads

Abstract

In the present study, for the first time, Fiber Bragg Grating (FBG) sensors are employed to measure the cyclic-strain induced on AZ31B extrusion samples during rotational bending tests (RBTs). The importance of this observation will be highlighted by considering the risk of failures under this circumstance in the modern technologies such as aerospace, while any in situ cyclic strain measurement has not been reported under RBTs. For this purpose, first, the accuracy of strain measurements captured by the FBG sensor in the elastic and plastic regimes is investigated by comparing the FBG measured strain with the data obtained simultaneously by a standard extensometer during uniaxial pull-push tests. The results confirm that the FBG sensor is able to measure the strain up to 1.3 % (plastic region) accurately. Next, the strains of the specimens are measured under different bending moments during RBTs in both the elastic and plastic regimes, hence monitoring the degradation of material during the test. The experiments uncover that the strain amplitude of the samples is decreased as the number of cycles increases in the plastic regime until the sample is stabilized. However, the strain response of the material under a bending moment of 2.5 Nm (in the elastic regime) remains constant during the test. This paper concludes that the measurement of strain amplitude evolution, with respect to the number of cycles, in a rotating bending cyclic test with FBG sensors is feasible with high level of accuracy and repeatability in the cyclic testing in the elastic region.