In the design of mechanical components, the fatigue properties of materials are of particular importance as in practice 80% to 90% of metal failures arise from fatigue [1
]. The fatigue life of components is generally determined from low cyclic fatigue tests at large strain amplitudes and high cyclic fatigue tests at low strain amplitudes. To accurately predict the fatigue life, it is critical to measure the strain in push-pull fatigue tests, especially under the large strain amplitude that induces significant plastic deformation. However, the conventional thin-film strain gauge cannot take large cyclic strain amplitudes; and the extensometer, though capable of taking large strain amplitudes, is relatively large in size, unsuitable for the measurements at boundary of dissimilar materials or for localized strain measurements.
Fiber optic sensors, particularly Fiber Bragg Gratings (FBGs) have become increasingly popular in the last decade due to their wide dynamic range, immunity to electromagnetic interference and their multiplexing capability. FBG sensors, inscribed on an optic fiber of 125 μm in diameter, can be made as short as 2 to 5 mm in length and have been used recently in localized strain measurements, such as on a flip-chip ball grid array that was not accessible for conventional strain gauges [2
]. Despite the success of FBGs in strain sensing and structural health monitoring, there are few reports on their applications in fatigue tests of materials, especially in large cyclic strain amplitude tests. When a FBG fiber is bonded to a substrate with epoxy, the substrate strain is transferred to the fiber sensor by the shear stress of the epoxy. Due to the limited contact area between the circular fiber and flat substrate and the shear modulus of the epoxy, a large substrate strain often causes the fiber to slip, rendering the FBG strain reading to be less than the true substrate strain. Using FBGs inscribed on circular fibers, significant chirp in the reflective peaks of the FBG spectra could occur, as shown in Figure 1
, when the sample was fatigued for 100 cycles at a strain amplitude of ±5,000 με. This problem has severely limited the applications of FBG sensors in mechanical tests involving large strain amplitudes. The FBG chirp or reflective peak split is mainly due to the uneven slippage between the FBG and the substrate at large strain amplitudes. Reference [3
] analyzed in detail the effects of geometric parameters of the adhesive on the substrate strain transfer to FBG, however only at low strain levels. Typically the FBG sensor is used when the strain variation is less than 3,000 με [4
]. For example, surface mounted FBG sensors were subject to a mean strain of 1,100 or 2,250 με in the reliability test of the FBG sensors installed on the carbon fiber reinforced polymer cable used in a suspension bridge in Switzerland [5
]. In order to overcome this limitation, it is essential to develop a new type of surface-mounted FBG sensors and a new bonding process for the measurement of large dynamic strain amplitudes in fatigue tests and in other material tests such as crack-initiation detection and crack closure evaluation.
In this paper, we report the development of a flat-cladding FBG sensor that significantly increases FBG’s strain measurement range. With a new fiber type and bonding process, we demonstrated that the sensor could measure reliably large strain amplitudes of up to ±8,000 με in aluminum alloys. To the best of our knowledge, this is the highest strain amplitude that surface-mounted FBG sensors can withstand. The calibration and the bonding process of the sensor are described in detail and an application of this sensor in fatigue tests of magnesium alloy samples is also presented.