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Sensors 2010, 10, 7674-7680; doi:10.3390/s100807674

ISSN 1424-8220 Article

Flat-Cladding Fiber Bragg Grating Sensors for Large Strain Amplitude Fatigue Tests
Aihen Feng 1, Daolun Chen 1, Cheng Li 2 and Xijia Gu 2,*


Department of Mechanical and Industrial Engineering, Ryerson University, Toronto, Ontario, M5B 2K3, Canada;E-Mails: (A.F.); (D.C.) Department of Electrical and Computer Engineering, Ryerson University, Toronto, Ontario, M5B 2K3, Canada; E-Mail: (C.L.)

* Author to whom correspondence should be addressed; E-Mail:; Tel.: +1-416-979-5000, x4151; Fax: +1-416-979-5280. Received: 16 July 2010; in revised form: 9 August 2010 / Accepted:10 August 2010 / Published: 16 August 2010

Abstract: We have successfully developed a flat-cladding fiber Bragg grating sensor for large cyclic strain amplitude tests of up to ± 8,000 με. The increased contact area between the flat-cladding fiber and substrate, together with the application of a new bonding process, has significantly increased the bonding strength. In the push-pull fatigue testsof an aluminum alloy, the plastic strain amplitudes measured by three optical fiber sensors differ only by 0.43% at a cyclic strain amplitude of ± 7,000 με and 1.9% at a cyclic strain amplitude of ± 8,000 με. We also applied the sensor on an extruded magnesium alloy for evaluating the peculiar asymmetric hysteresis loops. The results obtained were in good agreement with those measured from theextensometer, a further validation of the sensor. Keywords: fiber optic sensor; fiber Bragg grating; fatigue tests

1. Introduction 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 largestrain amplitudes and high cyclic fatigue tests at low strain amplitudes. To accurately predict the fatigue life, it is critical to

Sensors 2010, 10


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 theextensometer, 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 theirmultiplexing 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 ontheir 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 thefiber 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...