Influence of mounting on the hysteresis of polymer fiber Bragg grating strain sensors

Abstract

Fiber Bragg grating sensors recorded in poly(methyl methacrylate) fiber often exhibit hysteresis in the response of Bragg wavelength to strain, particularly when exposed to high levels of strain. We show that, when such a fiber grating sensor is bonded directly to a substrate, the hysteresis is reduced by more than 12 times, compared to the case where the sensor is suspended freely between two supports. © . This is important when fiber sensors are used to monitor structures that are themselves rather compliant, as is the case, for example, with a tapestry [3]. In such situations, the use of silica fiber tends to locally reinforce the structure and reports strain much lower than the true background strain in the material. As a viscoelastic material, the tensile properties of polymers are complicated; they display both hysteresis and a dependence on the timescales involved [4] and the magnitude of applied strain [5]. When a polymer optical fiber Bragg grating (POFBG) sensor is placed for several minutes under sufficient tension to cause a considerable elongation, a significant wavelength shift may remain when the tension is removed, which only gradually relaxes over time [6]. This is on the face of it a serious problem for practical applications of such strain sensors.The application of pretension [7] to the POFBG or the annealing of the fiber [8] has been reported to reduce hysteresis in some situations. However, in this Letter we show that, for many practical applications, the hysteresis will actually be much lower than expected from existing published data. The point is that the experiments that have revealed the presence of hysteresis are usually conducted by fixing the fiber at two points on either side of the sensor, with one of these points capable of being moved using a translation stage. However, when grating sensors are applied to the monitoring of strain in a structure, they would normally either be glued to the structure directly [9] or possibly embedded in it [10].In this work we compare the behavior of two identical sensors, where one is strained between two supports while the other is bonded directly to a cantilever. We show that the latter approach, which is much more characteristic of the way sensors would be applied in practice, displays hysteresis reduced more than 1 order of magnitude from the former.Two identical FBGs were fabricated in few-moded microstructured polymer optical fiber (mPOF) fabricated from poly(methyl methacrylate) (PMMA) and obtained from Kiriama Pty Ltd. Most polymer fibers in use today are based on PMMA, although it should be noted that other materials may have advantages for sensing applications [11]. The core of the fiber is bounded by three rings of 5 μm holes spaced 5 μm apart. The few-moded fiber has a core diameter of 50 μm and an outer diameter of 150 μm. A helium-cadmium laser with a wavelength of 325 nm and a power output of 30 mW was used to inscribe the Bragg gratings in the mPOF. The laser beam was focused vertically downward using a 10 cm focal length cylindric...

3D printed sensing patches with embedded polymer optical fibre Bragg gratings

“…Unexpectedly, strain sensitivity of the patch (excluding strain increase in cycle 1) was 0.38 pm/µε, which was some 3.5 times lower than usual values found in the literature 1,9 . A speculative explanation of this may be that the glue shielded a bigger part of the stress exerted on the POF, but a definite answer would need more investigation.…”

Experimental Study and Analysis of a Polymer Fiber Bragg Grating Embedded in a Composite Material

“…This is carried out by fixing the ends of the polymer fibers on translation stages and by applying a fixed step translation. The elongation of the polymer fiber is limited to less than 1 % to make sure that there are no hysteresis effects [27] and in our experiments within the applied strain range no significant hysteresis was found. The measured Bragg wavelength shift for the applied strain is shown in Fig 2(a).…”

Bandpass transmission filters based on phase shifted fiber Bragg gratings in microstructured polymer optical fibers

“…A residual wavelength difference a) 1UU 11111:»11111111111 9 . of 3.4 nm is observed with respect to the released grating just after fabrication (see Figure 7) due to polymer material hysteresis properties although this effect can be significantly reduced when the FBG is bonded directly to a substrate [41]. …”