Dynamic Strain Sensor Research Articles

Visual Strain Sensors Based on Fabry-Perot Structures for Structural Integrity Monitoring.

Strain sensors that can rapidly and efficiently detect strain distribution and magnitude are crucial for structural health monitoring and human-computer interactions. However, traditional electrical and optical strain sensors make access to structural health information challenging because data conversion is required, and they have intricate, delicate designs. Drawing inspiration from the moisture-responsive coloration of beetle wing sheaths, we propose using Ecoflex as a flexible substrate. This substrate is coated with a Fabry-Perot (F-P) optical structure, comprising a "reflective layer/stretchable interference cavity/reflective layer", creating a dynamic color-changing visual strain sensor. Upon the application of external stress, the flexible interference chamber of the sensor stretches and contracts, prompting a blue-shift in the structural reflection curve and displaying varying colors that correlate with the applied strain. The innovative flexible sensor can be attached to complex-shaped components, enabling the visual detection of structural integrity. This biomimetic visual strain sensor holds significant promise for real-time structural health monitoring applications.

Boosting the piezoelectric coefficients of flexible dynamic strain sensors made of chemically-deposited ZnO nanowires using compensatory Sb doping

The piezoelectric devices made of ZnO nanowires have received a great interest in the past decade as potential nanogenerators and sensors. However, their characteristics are still limited significantly by the screening of the piezoelectric potential generated under mechanical solicitations, originating from the high density of free electrons in ZnO nanowires. In order to tackle that issue, we develop the compensatory Sb doping of ZnO nanowires grown by chemical bath deposition, both in the low- and high-pH regions using Sb glycolate and ammonia as chemical additives. The adsorption process of Sb(III) species on the positively charged surfaces of ZnO nanowires proceeds through attractive electrostatic forces, resulting in the significant incorporation of Sb dopants with an atomic ratio in the range of 0.11–0.45%. The optical properties of Sb-doped ZnO nanowires exhibit additional phonon modes related to Sb dopants in the range of 500–750 cm-1 in the Raman spectra, and some specific characteristics in the nature and intensity of radiative recombination in the cathodoluminescence spectra. Importantly, the integration of Sb-doped ZnO nanowires encapsulated in PMMA and grown on PDMS into flexible piezoelectric dynamic strain sensors is shown to drastically boost the piezoelectric charge and voltage coefficients by a factor of 2.65 and 1.91, following the significant incorporation of Sb dopants. A subsequent thermal annealing under oxygen atmosphere is revealed to further increase the piezoelectric charge and voltage coefficients by an additional factor of 1.38 and 1.49, following the activation of Sb doping and engineering of hydrogen-related defects. Two figures-of-merit are eventually derived from the piezoelectric charge and voltage coefficients and their values for flexible piezoelectric composites made of annealed Sb-doped ZnO nanowires are compared to more conventional piezoelectric materials, showing their high potential for medical devices. The present findings highlight the strategy consisting in simultaneously introducing compensatory acceptors and engineering hydrogen-related defects to reduce the screening effect, representing an additional powerful way to enhance the characteristics of the piezoelectric devices integrating ZnO nanowires.

Real-Time Monitoring for Effects of Vibration and Temperature of Construction Site on Steel Assembly Bracing of Foundation Pit

In this paper, a real-time monitoring system—including vibration acceleration sensors, temperature sensors, and static and dynamic strain sensors—is used to monitor the safety status of a steel assembly bracing in a practical project. It uses 5G wireless networking technology to transmit monitoring data to a cloud server for early warning of abnormal changes and development trends. Real-time monitoring data obtained in a construction site are used as the inputs of the finite element model and the corresponding results of numerical simulation are compared with the results from the real-time monitoring. It can be concluded that: (1) the stress caused by environmental temperature is very significant which can be higher than the initial prestress of the steel assembly bracing; (2) the stress caused by the vertical vibration mainly from construction vehicles is not remarkable, however, the vertical frequency-weighted acceleration of support vibration is relatively large which can affect the sense of safety of engineering technicians on site; (3) the combination of the environmental temperature and vertical vibration does not affect the safety of the steel assembly bracing.

High-Resolution Random Laser Dynamic-Strain Sensor Based on Two-Cavity FBG-FP Interferometer

A high-resolution optic fiber dynamic strain sensor based on a two-cavity fiber Bragg grating Fabry-Perot (FBG-FP) interferometer and random fiber laser (RFL) is demonstrated. The specially designed two-cavity FBG-FP interferometer acts as a sensing element and can effectively suppress the competition-induced multi-spike lasing modes and frequency jitter, which contributes to establish single-wavelength laser and suppress frequency noise of RFL. The frequency noise is reduced from about 12 to 3 Hz/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\surd $ </tex-math></inline-formula> Hz above 4 kHz. An ultrahigh dynamic-strain detecting resolution of 30 femto-strain ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{f}\varepsilon)/\surd $ </tex-math></inline-formula> Hz is achieved. To the best of our knowledge, it is the first time that two-cavity FBG-FP interferometer has been used in sensing field.

Theory and Practice of a Flexible Fiber-Optic Cable in a Horizontal Well Used for Crosswell and Microseismic Hydraulic Fracture Monitoring

Summary A flexible optical fiber cable, either as a wireline or a disposable fiber deployed using a pumped fiber payout shuttle, in a horizontal well, can be used to measure distributed near-static or dynamic strain. These measurements can be used to monitor the hydraulic fracturing treatment of nearby wells. We present a theoretical framework for the understanding of the cable behavior and to compare the theory with field measurements. The theory predicts the conditions under which slippage occurs between the optical fiber cable and the wellbore, if coupling is provided by Coulomb friction. For near-static strain, as used in crosswell strain monitoring, the theory explains the broadening of the strain zone detected with a wireline cable or disposable fiber when slippage occurs. However, for a grease-covered disposable fiber, the theory underestimates the coupling where the grease provides a level of adhesion between the fiber and the wellbore. Understanding the fundamental theory explains measured data and enables confident data interpretation regardless of sensing cable configuration. For the dynamic strain, the theory confirms the generally good response observed using the flexible cables for microseismic monitoring due to the low amplitude of the dynamic strains involved. The low amplitude of the strains means that the strain gradients and inertial forces are also small, such that Coulomb friction is sufficient to provide the needed coupling. An interesting result of the theory is the existence of a resonance condition allowing for large amplitudes to be detected faithfully even if only Coulomb friction is present. This resonance does not depend on signal frequency but on the match between the intrinsic travel speed of a disturbance on the cable and the apparent phase velocity of the seismic signal in the well direction. Most importantly, the theory enables (i) a comparison of different cable types for the near-static and dynamic strain applications, and (ii) better data interpretation and associated decisions. Field examples are provided to show both when the theory is applicable and where the obtained coupling exceeds what is predicted by the theory. The novel aspect of the paper is the first presentation of a theoretical background for the understanding of the performance of flexible cables inside horizontal wells used as static or dynamic strain sensors for the monitoring of hydraulic fracturing jobs.

Ultralow‐Noise Single‐Frequency Fiber Laser and Application in High‐Resolution Fiber‐Optic Dynamic Strain Sensing

A 1.5 μm ultralow‐noise single‐frequency fiber laser (SFFL) is realized based on a delay fiber phase‐locked method and the nonlinear amplification effect of a semiconductor optical amplifier (SOA). Via feedback locking of the phase jitter signal, the phase noise achieves the minimum value of −160 dB rad Hz−1/2 m−1 with a maximum reduction of 50 dB. Meanwhile, SOA reduces the intensity noise by a maximum of 65 dB, making it near the shot‐noise limit for frequencies above 40 kHz. To demonstrate this state‐of‐art SFFL with ultralow‐noise property, a high‐resolution fiber‐optic dynamic strain sensing based on a fiber‐based Michelson interferometer (FMI) is performed. As a result, the strain resolution reaches 26 fε Hz−1/2 in the 30–200 Hz band, which is improved by almost three orders of magnitude than that realized by a free‐running SFFL. To the best of the knowledge, this is the first time to achieve such a high‐resolution fiber‐optic dynamic strain sensor in the low‐frequency range utilizing the FMI system. This high‐resolution fiber‐optic strain sensing has promising applications in geophysics and biophotonics. Moreover, this ultralow‐noise SFFL is competitive for sophisticated applications, such as precise absolute distance measurement, quantum key distribution, and generating squeezed light.

High-Voltage Wave Induced a Unique Structured Percolation Network with a Negative Gauge Factor.

Nanocomposite percolation networks have attracted increasing attention in the field of wearable devices. Generally, the large junction resistance caused by the small contact area in the percolation network is considered as the bottleneck in preparing high-performance electronics. In such electronics, an applied strain will lead to deformation on the fiber junction, subsequently increasing the sheet resistance. However, taking advantage of the dominant role of the contact resistance in the percolation network, the overall resistance of the network can be controlled by skillfully adjusting the contact area. Here, we designed a combined gold-polycaprolactone (Au-PCL) network with a unique buckling net structure. When the thickness of the gold nanolayer is 50 nm, the network shows typical percolation behavior with high transparency (93%), good conductivity (20 Ω/sq), and good ductility. Moreover, the networks show a unique positive relationship between the conductivity and strain due to the variation of contact resistance. By designing different waving angles, the network can be used as a dynamic strain sensor with a tunable gauge factor ranging from -0.8 to -1.8. Overall, these highly stretchable and transparent Au-PCL networks show promising applications in the field of high-performance electronic and optoelectronic nanodevices.

Single-process 3D-printed structures with vibration durability self-awareness

The recent advances in additive manufacturing technology allow the realization of single-process thermoplastic material extrusion (TME) 3D-printed embedded sensors, leading to the easy and inexpensive production of smart structures. While single-process TME dynamic strain sensors have already been researched, vibration durability self-awareness is more than just an additional 3D printed strain sensor and several questions need to be answered. Is the durability self-aware sensors position structure-specific? Is the fatigue life of the sensory element longer than the base structure? Does the fatigue influence the self-awareness capability? Those and several other questions are theoretically and experimentally addressed in this research. Two different fatigue identification methods are researched (i.e. the peak-response and the frequency-drop methods). It was found that the vibration durability self-aware structure printed in a single process is viable and the frequency-drop based method gives reliable fatigue estimation; the fatigue damage was correctly identified even in the case the sensory element was 3D printed in the fatigue zone and already significantly damaged. This research opens up new capabilities for self-aware TME 3D-printed structures. • 3D printed durability self-aware structures are introduced. • Sensing element location and geometry is researched for durability and identification influence. • Two self-awareness identification methods are researched (amplitude and frequency based). • Frequency based self-awareness is reliable even when damage is already relatively high.

Large dynamic-range fiber Bragg grating sensor system for acoustic emission detection.

A distributed feedback (DFB) fiber laser and fiber Bragg gratings (FBGs) are configured to demodulate the wavelength shifts of FBG dynamic strain sensors. The FBG sensors act as sensing units to detect the dynamic strain and the demodulators while the DFB fiber laser only acts as a narrow-linewidth light source. As the reflective spectrum of the FBG sensor changes due to dynamic strains, the output is subsequently converted into a corresponding intensity change and detected directly by a photodetector. The 0.2 nm linewidth FBG sensor can detect the impact signal with a frequency of up to 300 kHz with a maximum of 29.17 µɛ, which is comparable with the detecting result of the piezoelectric transducer sensor. Moreover, the directional response of the FBG sensor is maximized when the direction of acoustic wave propagation is parallel to the optical fiber. The relation between the sensitivity and the FBG spectrum linewidth is presented, and the detectable strain range versus different FBG linewidths is also discussed.

3D torsional experimental strain modal analysis for structural health monitoring using piezoelectric sensors

Modal analysis is a dynamic response-based experimental structural identification procedure widely used for condition assessment and structural health monitoring (SHM). Piezoelectric sensors are smart material-based dynamic strain sensors being progressively implemented in vibration-based monitoring techniques. However, these have been so far limited to preliminary investigations on 1D and 2D structures with regard to modal analysis. Strain modal analysis using piezoelectric sensors has so far utilized only lower bending modes for frequency and mode shape-based SHM. This paper aims to evaluate the piezoelectric sensors for modal response measurement, modal parameter identification, and structural health monitoring of 3D structures, especially using torsional modes. It reports a comparative experimental study between the piezoelectric sensors and the accelerometers for dynamic response measurement, modal parameter extraction and damage identification and localization. Experimental modal analysis using impact hammer-based excitation is performed on two lab-sized specimens under different boundary conditions. Displacement mode shapes from both the sensors are used to determine the location and the severity of the damage. Torsional and bending modes are studied distinctly for sensitivity towards damage monitoring. Modal assurance criteria (MAC), coordinated MAC (CoMAC), and modal strain energy (MSE) are adopted as damage indicators. Results of the study show that the piezoelectric sensors are equivalent to accelerometers for response measurements and modal parameter extraction. They possess enhanced sensitivity towards damage monitoring as compared to accelerometers. Torsional modes perform better than bending modes for damage localization and severity quantification. Bending modes from both sensors are ineffective in locating the damage precisely. Modal strain energy (MSE) based damage index from torsional modes acquired from piezoelectric sensor performs best in terms of sensitivity towards damage location and exhibits peak severity, especially for incipient damage. The study establishes piezoelectric sensors as reliable and cost-effective modal response sensors.

Fiber-Optic Seismology

Distributed acoustic sensing (DAS) is an emerging technology that repurposes a fiber-optic cable as a dense array of strain sensors. This technology repeatedly pings a fiber with laser pulses, measuring optical phase changes in Rayleigh backscattered light. DAS is beneficial for studies of fine-scale processes over multi-kilometer distances, long-term time-lapse monitoring, and deployment in logistically challenging areas (e.g., high temperatures, power limitations, land access barriers). These benefits have motivated a decade of applications in subsurface imaging and microseismicity monitoring for energy production and carbon sequestration. DAS arrays have recorded microearthquakes, regional earthquakes, teleseisms, and infrastructure signals. Analysis of these wavefields is enabling earthquake seismology where traditional sensors were sparse, as well as structural and near-surface seismology. These studies improved understanding of DAS instrument response through comparison with traditional seismometers. More recently, DAS has been used to study cryosphere systems, marine geophysics, geodesy, and volcanology. Further advancement of geoscience using DAS requires several community efforts related to instrument access, training, outreach, and cyberinfrastructure. ▪ DAS is a seismic acquisition technology repurposing fiber optics as arrays of dynamic strain sensors at 1- to 10-m spacing over kilometers. ▪ Easy DAS installations have availed time-lapse geophysical sensing in formerly inaccessible sites: urban, icy, and offshore areas. ▪ High-frequency wavefields recorded by DAS are analyzed with array-based methods to characterize seismic sources and image the subsurface. ▪ DAS has shown low-frequency sensitivity in the laboratory and field, for slow hydrodynamic and geodynamic processes.

Distributed Fiber-Optic Dynamic Strain Sensor Based on Spectra Correlation of Rayleigh Backscattering

Distributed Fiber-Optic Dynamic Strain Sensor Based on Spectra Correlation of Rayleigh Backscattering

Dynamic Mechanochromic Optics with Tunable Strain Sensitivity for Strain‐Responsive Digit Display

AbstractA facile but highly effective mechanochromism design is reported that is based on a bilayer structure consisting of a sputter‐coated metal light‐shielding layer atop a polydimethylsiloxane (PDMS) substrate containing a fluorescent dye, featuring transverse and/or longitudinal microscale cracks when stretched. The crack opening width on the light‐shielding layer upon stretching/releasing allows the UV radiation to activate the fluorescent dye in the PDMS matrix to exhibit luminescent color, which can be easily tuned with choice of fluorescent dyes. The strain response of the crack opening width can be fine‐tuned by applying various degrees of prestretching strain in the preparation process, resulting in customizable mechanochromic responses. Three types of mechanochromic devices are prepared as representative examples. The fluorescence intensity of these devices can be tuned to either increase or decrease with an increasing strain, or first decrease and then increase within the operating strain range. Moreover, this system can be further modified into a dynamic strain sensor with digit display function, allowing the strain degree of the device to be directly read. This design strategy can be easily adopted by fellow researchers and is expected to open a brand‐new avenue to tune mechanochromic response for various dynamic adaptive optics.

Distributed Acoustic Sensing as a Distributed Hydraulic Sensor in Fractured Bedrock

AbstractDistributed acoustic sensing (DAS) was originally intended to measure oscillatory strain at frequencies of 1 Hz or more on a fiber optic cable. Recently, measurements at much lower frequencies have opened the possibility of using DAS as a dynamic strain sensor in boreholes. A fiber optic cable mechanically coupled to a geologic formation will strain in response to hydraulic stresses in pores and fractures. A DAS interrogator can measure dynamic strain in the borehole, which can be related to fluid pressure through the mechanical compliance properties of the formation. Because DAS makes distributed measurements, it is capable of both locating hydraulically active features and quantifying the fluid pressure in the formation. We present field experiments in which a fiber optic cable was mechanically coupled to two crystalline rock boreholes. The formation was stressed hydraulically at another well using alternating injection and pumping. The DAS instrument measured oscillating strain at the location of a fracture zone known to be hydraulically active. Rock displacements of less than 1 nm were measured. Laboratory experiments confirm that displacement is measured correctly. These results suggest that fiber optic cable embedded in geologic formations may be used to map hydraulic connections in three‐dimensional fracture networks. A great advantage of this approach is that strain, an indirect measure of hydraulic stress, can be measured without beforehand knowledge of flowing fractures that intersect boreholes. The technology has obvious applications in water resources, geothermal energy, CO2 sequestration, and remediation of groundwater in fractured bedrock.

Suppressing non-local effects due to Doppler frequency shifts in dynamic Brillouin fiber sensors.

Brillouin fiber sensors have traditionally been limited to low-speed or static strain measurements due to the time-consuming frequency scans required. In the past decade, a number of novel high-speed measurement techniques have been proposed to enable Brillouin-based dynamic strain sensors. In this work, we present a new mechanism, which can limit the performance of high-speed dynamic Brillouin sensors. Specifically, we show that dynamic strain induced Doppler shifts can corrupt a distributed Brillouin strain measurement by introducing non-local signals throughout the fiber. We present a model showing that these non-local signals can introduce unacceptable levels of cross-talk or even exceed the local signal strength in reasonable operating conditions and experimentally observe these signals in a standard slope-assisted BOTDA sensor. Finally, we present a modified sensor architecture to address this issue and experimentally demonstrate low-noise (2.6 nε/Hz1/2), dynamic strain measurements with significantly reduced sensitivity to this type of non-local signal.

Schottky Contacts on Polarity-Controlled Vertical ZnO Nanorods.

Polarity-controlled growth of ZnO by chemical bath deposition provides a method for controlling the crystal orientation of vertical nanorod arrays. The ability to define the morphology and structure of the nanorods is essential to maximizing the performance of optical and electrical devices such as piezoelectric nanogenerators; however, well-defined Schottky contacts to the polar facets of the structures have yet to be explored. In this work, we demonstrate a process to fabricate metal-semiconductor-metal device structures from vertical arrays with Au contacts on the uppermost polar facets of the nanorods and show that the O-polar nanorods (∼0.44 eV) have a greater effective barrier height than the Zn-polar nanorods (∼0.37 eV). Oxygen plasma treatment is shown by cathodoluminescence spectroscopy to affect midgap defects associated with radiative emissions, which improves the Schottky contacts from weakly rectifying to strongly rectifying. Interestingly, the plasma treatment is shown to have a much greater effect in reducing the number of carriers in O-polar nanorods through quenching of the donor-type substitutional hydrogen on oxygen sites (HO) when compared to the zinc-vacancy-related hydrogen defect complexes (VZn-nH) in Zn-polar nanorods that evolve to lower-coordinated complexes. The effect on HO in the O-polar nanorods coincides with a large reduction in the visible-range defects, producing a lower conductivity and creating the larger effective barrier heights. This combination can allow radiative losses and charge leakage to be controlled, enhancing devices such as dynamic photodetectors, strain sensors, and light-emitting diodes while showing that the O-polar nanorods can outperform Zn-polar nanorods in such applications.

Combining strain and acceleration measurements for random force estimation via Kalman filtering on a cantilevered structure

Several engineering applications demand in-operation dynamic forces exerted on structural components to be modeled, however there are cases in which both phenomenological and empirical estimations are more uncertain than admissible. Although direct input measurements would be the first option to solve this lack, in many instances an appropriate and feasible instrumentation is hard to be accomplished due to reduced space, especial environment conditions (e.g. submerged structures), etc. leaving a gap for new methods to be explored. Virtual sensing represents a solution for determining forces when direct measurements are not feasible or present important technical challenges. In this context, augmented-state Kalman Filter (AKF) based techniques have recently been introduced in the field of force estimation. This technique consists in adding the unknown inputs (or input parameters) to the normal state vector of the mechanical system, which usually comprises only displacement and velocity states, and estimate the augmented-state vector in a Kalman Filter scheme. One of the challenges behind a successful AKF implementation is stability, as estimates based on acceleration tend to deviate with time. Dummy measurements have been used in the past as a means to guarantee stability. In this work, online dynamic strain measurements are used in that sense and the ability of the AKF for estimating a point force random in time, is successfully evaluated. In order to validate this procedure, the present experimental approach features a cantilevered structure, instrumented with standard accelerometers and dynamic strain sensors, excited with a known force. A force sensor is used to directly measure the input force for further comparisons, which show good agreement both in time and frequency domain.

Distributed Fiber-Optic Dynamic-Strain Sensor With Sub-Meter Spatial Resolution and Single-Shot Measurement

A single-shot distributed dynamic strain sensor is proposed with high spatial resolution and broad detection bandwidth. Linear frequency modulated pulse is employed as the probe pulse, which not only solves the trade-off between spatial resolution and sensing distance, but also avoids the time-consuming stepped frequency tuning process. The matrix of Rayleigh backscattering (RBS) at multiple frequencies over the fiber is obtained using the first probe pulse. The strain variation is retrieved from the corresponding frequency shift of the following probe pulses using RBS signature matching. In the demonstrational experiment, distributed vibration measurement with 0.9 m spatial resolution, 224 $p\varepsilon /\surd$ Hz strain resolution, 5 kHz response bandwidth and 25.7 dB signal-to-noise radio is realized over 10-km sensing fiber.

Conformable Holographic Photonic Ink Sensors Based on Adhesive Tapes for Strain Measurements.

Buildings, bridges, and aircrafts are frequently exposed to fluctuation loads, which could start with a fine crack that instantly leads to unpredictable structure failures. The stationary strain sensors can be utilized, but they are costly and only detect limited deformation forms and sizes. Here, we fabricated photonic strain sensors on adhesive tapes, which can provide real-time monitoring of irregular surfaces. Holographic interference patterning was used to produce nonlinear curved nanostructures of one dimensional (1D) (900 nm × 880 nm) and two dimensional (2D) from a black dye film on a robust uniform adhesive layer and heat resistance tape. The patterned structure of the black dye was stable in broad pH environments. Diffracted light from the curved nanostructure detected the signal during structural damage, a shift or material tear of 5 με at less than 1.3 N cm-2. Additionally, the 2D nanostructure detected a surface change from x or y axis. Tilting the 1D structure within a range of 0.3° to 14.2° provided visible wavelength changes under broadband light to reveal early deflection signs. The curved nanopatterns could be also used for transferable holographic symbol design. Photonic nanopatterns on an adhesive tape could be used as a rapid response, conformable, lightweight, and low-cost dynamic strain sensor.

Negative Gauge Factor Piezoresistive Composites Based on Polymers Filled with MoS2 Nanosheets.

Nanocomposite strain sensors, particularly those consisting of polymer-graphene composites, are increasingly common and are of great interest in the area of wearable sensors. In such sensors, application of strain yields an increase in resistance due to the effect of deformation on interparticle junctions. Typically, widening of interparticle separation is thought to increase the junction resistance by reducing the probability of tunnelling between conducting particles. However, an alternative approach would be to use piezoresistive fillers, where an applied strain modifies the intrinsic filler resistance and so the overall composite resistance. Such an approach would broaden sensing capabilities, as using negative piezoresistive fillers could yield strain-induced resistance reductions rather than the usual resistance increases. Here, we introduce nanocomposites based on polyethylene oxide (PEO) filled with MoS2 nanosheets. Doping of the MoS2 by the PEO yields nanocomposites which are conductive enough to act as sensors, while efficient stress transfer leads to nanosheet deformation in response to an external strain. The intrinsic negative piezoresistance of the MoS2 leads to a reduction of the composite resistance on the application of small tensile strains. However, at higher strain the resistance grows due to increases in junction resistance. MoS2-PEO composite gauge factors are approximately -25 but fall to -12 for WS2-PEO composites and roughly -2 for PEO filled with MoSe2 or WSe2. We develop a simple model, which describes all these observations. Finally, we show that these composites can be used as dynamic strain sensors.