Strain Transducers Research Articles

Probing contact-mode characteristics of silicon nanowire electromechanical systems with embedded piezoresistive transducers

This article reports on a new method of monitoring nanoscale contacts in switches based on nanoelectromechanical systems, where the contact-mode switching characteristics can be recorded with the sensitive embedded piezoresistive (PZR) strain transducers. The devices are manufactured using state-of-the-art wafer-scale silicon-on-insulator technology featuring suspended silicon cantilevers and beams as switching elements and sub-100 nm thin silicon nanowires (SiNWs) as PZR transducers. Several different device configurations are studied, including mechanically ‘cross’-shaped (‘+’), coupled cantilever-SiNW structures, with and without local drain electrodes, and doubly clamped SiNW beams. Through detailed measurement and analysis, we demonstrate that the PZR transducers can enable detection of both mechanical and tunneling switching with multiple repeatable cycles. With the strong PZR effects in thin SiNWs, this type of device could be valuable especially for monitoring cold switching events, and when conventional direct readout of the switching events from the local gate or drain electrodes would not be efficient or sensitive, as nanoscale contacts may not be highly conductive, or may be degrading over time.

Analysis of the accuracy of fiber-optic strain transducers installed by using composite smart patches

The aim of this work is to investigate the accuracy of fiber-optic strain transducer related to the incomplete load transmission that can occur on composite smart patches used as base support for the installation of such transducer, commonly used for structural monitoring of concrete elements. In detail, since the dimensions of the transducer are generally very small compared to those of the structural element being monitored, attention is given not to the global reinforcement effect, which occurs when the transducer alters the entire strain field of the transversal section of the monitored element, but rather to the so-called local reinforcement effect, which is due to an incomplete load transmission from the analyzed structure to the transducer and results in a localized variation of the strain field, without affecting the global structural response of the monitored element. Theoretical studies carried out in this work have permitted the evaluation of a correction coefficient ( C), which allows the user the correction of the measured strain or the proper transducer calibration, according to the elastic characteristics of the materials and the geometry of the structure-transducer system. The accuracy of the proposed theoretical model has been verified by finite element method analysis, varying the main factors that influence the local load transmission. Successive experimental tests have corroborated the accuracy of the formula proposed for the correction of the measured strain. In summary, the results of this study showed that the use of thick patches made by high-modulus composite materials (carbon fiber–reinforced polymer etc.), or the use of short patches, can lead to coarse error on the strain measurement; on the contrary, for thin patches made by low-modulus composite materials (fiberglass etc.) and having sufficient base length, the error is less than ±1%.

Effect of measurement uncertainties on strain-based damage diagnostics for highway bridges

Diagnostic load rating of highway bridges using distributed strain transducers is one of the more prominent strategies applied for both periodic and continuous monitoring of highway bridges. Techniques for load rating structures using experimental measurements of neutral axis locations, girder distribution factors, and dynamic increase factors have been suggested for structural health monitoring and damage detection applications. However, practical considerations related to the influence of measurement uncertainties on the reliability of these diagnostic measures in the presence of damage have only been examined recently. This paper extends uncertainty propagation formulae previously derived for neutral axis measurements to girder distribution factors. The significance of measurement uncertainties on damage-sensitive features are then explored using experimental data from a real bridge subjected to progressive, prescribed damage. The experimental results suggest that both apparent neutral axis locations and girder distribution factors exhibit sensitivity to the damage mechanisms introduced that exceeds the measurement uncertainty. In addition, girder distribution factors are found to demonstrate the ability to locate the damage. Comparisons with finite element simulations underline the challenges associated with faithfully modeling the effects of structural damage and emphasize the need for continued full-scale validation of structural health monitoring approaches on real structures.

Measuring soil compaction and soil behavior under the tractor tire using strain transducer

Soil compaction can occur due to machine traffic and is an indicator of soil physical structure degradation. For this study 3 strain transducers with a maximum displacement of 5cm were used to measure soil compaction under the rear tire of MF285 tractor. In first series of experiments, the effect of tractor traffic was investigated using displacement transducers and cylindrical cores. For the second series, only strain transducers were used to evaluate the effect of moisture levels of 11%, 16% and 22%, tractor velocities of 1, 3 and 5km/h, and three depths of 20, 30 and 40cm on soil compaction, and soil behavior during the compaction process was investigated. Results showed that no significant difference was found between the two methods of measuring the bulk density. The three main factors were significant on soil compaction at a probability level of 1%. The mutual binary effect of moisture and depth was significant at 1%, and the interaction of moisture, velocity, and depth were significant at 5%. The soil was compressed in the vertical direction and elongated in the lateral direction. In the longitudinal direction, the soil was initially compressed by the approaching tractor, then elongated, and ultimately compressed again.

Live-Load Effect in Reinforced Concrete Box Culverts under Soil Fill

AbstractThe effects of live load in box culverts may be nearly negligible compared with the effects of dead loads when significant fill is placed above the crown of the culvert. The objective of this study was to determine the effects of live load (truck loads) on reinforced concrete box culverts classified as bridges [spans greater than 6 m (20 ft)] under soil fills of different thicknesses. The study considered the field testing of 10 existing reinforced concrete box culverts with fill depths ranging from 0.76 m (2.5 ft) to 4.1 m (13.5 ft). Instrumentation of the culverts consisted of 12 reusable strain transducers and 12 linear variable displacement transducers. The instrumentation was designed to be applied, used, and removed in one day of testing. Loaded trucks were driven over the culvert to provide live load. The results of the testing show that the live-load effect does diminish with increasing fill depth. The 2012 AASHTO LRFD bridge design specifications are overly conservative in predicting stra...

Monitoring the Mechanical and Structural Behavior of the Pavement Structure Using Electronic Sensors

AbstractOver the last decade, pavement instrumentation has become an important tool for monitoring the performance of pavement structures and materials under real climatic and traffic conditions. Stress, strain transducers, loop profilers, temperature, and moisture sensors could be reasonably used for sophisticated analysis of pavement performance. Commonly, such research projects involve only few pavement structures under accelerated loading tests. The unique experimental road section was constructed in 2007 in Lithuania. It consists of 27 different flexible pavement structures and all necessary electronic sensors. This road is affected by real climatic conditions and high‐intensity heavy vehicle traffic. The aim of such research is to continuously monitor performance of different pavement structures and to elaborate on the most suitable and economically effective pavement structures.

Design and Calibration of a Fiber-Optic Monitoring System for the Determination of Segment Joint Movements Inside a Hydro Power Dam

In hydro power dams with arch dams as water storage, the movements of the concrete segment joints are monitored. New monitoring concepts are required as most dams were updated to more efficient pumped-storage power stations within the last years, which significantly changed the loading conditions. Inside one specific dam, the largest segment joint movements were up to 2.5 mm in the past. We developed a fiber Bragg grating (FBG)-based automatic monitoring system for the simultaneous measurement of concrete segment joints. Using our fiber-optic calibration facility, several commercial FBG-based strain transducers were tested. The facility and its capabilities are described. For the investigated sensors, nonlinearity and hysteresis effects of more than 20 μm/m were found, which is much more than expected. The results clearly show the need for standards in fiber-optic sensing where minimum requirements for sensor specifications should be defined. In an extended field trial, several appropriate sensors were installed in a hydro power dam. First results show an increased number of load cycles and smaller movements for the dam in pumping mode. With the new system, these small deformations can be detected, and it will be investigated in future how they contribute to the aging of the structure.

Optical fiber strain sensor based on sandwiched long-period fiber gratings with a surface bonding layer

An optical fiber strain sensor based on sandwiched long-period fiber gratings (OFSS-SLPFG) with a surface bonding layer is proposed. The proposed OFSS-SLPFG is an etched optical fiber that is sandwiched between two thick photoresists with a periodic structure. To prevent the glue effect in the surface bonding process, where glue flows into the SLPFG structure, reducing the coupling strength, a surface bonding layer (thickness: 16 μm) is used as the base layer on the bottom of the OFSS-SLPFG. The OFSS-SLPFG is, therefore, more effective for use as a strain sensor. When external strain loading is applied, the resonant dip loss of the OFSS-SLPFG is reflected linearly. A bending strain calibration experiment is demonstrated by the four-point bending test. The results show an average linearity (R2) of 0.980, with a sensitivity of 0.00788 dB/με. This phenomenon suggests that the OFSS-SLPFG can be utilized as a sensitive strain transducer.

Structural Health and Strain Monitoring Sensing through Fourier-Based Transducers

The article reports on the design of a novel class of transducers for structural health monitoring and strain sensing designed using a Fourier-based approach. The design procedure formulates the problem considering an arbitrarily shaped distribution of the sensing surface. Interrogation of the sensors is based on the generation of guided and surface acoustic waves generated in the region surrounding the transducers. The representation of the distribution of the sensing material is analyzed and designed in the spatial Fourier domain, where the emission characteristics of the transducer in relation to the interrogating wave can be tailored to a specific application. For structural health monitoring, the sensing material distribution can be defined to provide the transducers with frequency-dependent directional properties, which can be employed as part of an interrogation scheme based on generation and processing of guided waves in the structure. For strain sensing, one-dimensional and two-dimensional grating configurations monitor frequency shifts of radiation associated to local straining of the gratings. These frequency shifts can be related to the local strain components, so that a rosette-like configuration can be implemented. The article illustrates the commonalities of the design procedure, which leads to novel Lamb wave and strain transducers, and suggests the potential integration of the two sensing modalities as a single device for health and usage monitoring of structural components.

Study of lime hemp composite precasting by compaction of fresh mix — An instrumented die to measure friction and stress state

This paper deals with the processing of lime hemp composite blocks by compaction. A rigid compression die has been developed to reach relatively high compression pressure. This has been fitted with force and strain transducers to measure the stress state during compaction. It provides some means of understanding the effect of mix design and stress level on the compressibility and homogeneity of hardened composites. It is shown that moisture globally increases both friction and packing compressibility. Proper moisture content and paste-to-shiv mass ratio (P/S) can be chosen to reduce the heterogeneity. The combination of friction at wall level and stress transmission is the most detrimental for a moisture content of 20% and a P/S=1.

Uncovering the intrinsic size dependence of hydriding phase transformations in nanocrystals

A quantitative understanding of nanocrystal phase transformations would enable more efficient energy conversion and catalysis, but has been hindered by difficulties in directly monitoring well-characterized nanoscale systems in reactive environments. We present a new insitu luminescence-based probe enabling direct quantification of nanocrystal phase transformations, applied here to the hydriding transformation of palladium nanocrystals. Our approach reveals the intrinsic kinetics and thermodynamics of nanocrystal phase transformations, eliminating complications of substrate strain, ligand effects and external signal transducers. Clear size-dependent trends emerge in nanocrystals long accepted to be bulk-like in behaviour. Statistical mechanical simulations show these trends to be a consequence of nanoconfinement of a thermally driven, first-order phase transition: near the phase boundary, critical nuclei of the new phase are comparable in size to the nanocrystal itself. Transformation rates are then unavoidably governed by nanocrystal dimensions. Our results provide a general framework for understanding how nanoconfinement fundamentally impacts broad classes of thermally driven solid-state phase transformations relevant to hydrogen storage, catalysis, batteries and fuel cells.

Novel Design of a Highly Sensitive RF Strain Transducer for Passive and Remote Sensing in Two Dimensions

A novel design of a highly sensitive wireless passive RF strain transducer is presented based on a patch antenna loaded with an open loop that is capable of sensing strain independently in two directions. An original idea of utilizing a cantilever at the gap of the open loop significantly improves the sensitivity of resonant frequency shifts. The frequency shifts in two distinct resonant modes are detected based on two dominant orthogonal modes of the patch resonators. In measurements, the prototypes achieved a sensitivity of 2.35% frequency shift per 1% strain, more than twice that of existing strain transducers of the same class. In simulations, the new design achieved a theoretical sensitivity up to four times as high as existing designs of RF passive wireless strain transducers. The ground plane allows for the sensitivity of the sensor to be independent from the applied surface. An implementation example of the passive remote sensing system based on the proposed strain transducer is also discussed as a proof-of-concept case. Based on calculations, the interrogation method in the example shows a radar cross section fluctuation of 3.8 dB corresponding to the strain induced at the sensor.

Highly birefringent dual-mode microstructured fiber with enhanced polarimetric strain sensitivity of the second order mode

We present the results of theoretical and experimental characterization of a designed and manufactured dual-mode highly birefringent microstructured fiber. We also demonstrate the measured values of polarimetric temperature and strain sensitivity of both the fundamental and second order modes. As the mode field of the second order mode has a strong interaction with the fiber air holes, we observed a significant (over two orders of magnitude) increase in the polarimetric strain sensitivity of this mode in comparison to the fundamental mode. The enhanced strain sensitivity together with the low temperature sensitivity makes our fiber very attractive for application as extremely sensitive temperature independent strain transducers.

A Piezoelectric Strain Transducer to Evaluate the Severity of Pipe Vibration

A new strain transducer developed mainly for the inspection and evaluation of piping systems with excessive vibration is presented in this paper. Fatigue failure induced by vibration is one of the most common causes of piping failures. Procedures to evaluate the severity of piping vibration are usually based on pipe velocity or displacement. Although simple and fast, these procedures do not provide precise information on the risk of piping fatigue failure. Through the measurement of pipe dynamic strains the risk of failure due to vibration could be determined more accurately; however, the measurement of strain is usually performed using the conventional strain gage method, a time-consuming technique that can be difficult to implement in certain industrial scenarios. Motivated by the need of a simple and rapid method to measure pipe dynamic strains, a piezoelectric dynamic strain transducer has been developed. This work presents a description of the piezoelectric strain transducer and the preliminary results of pipe strain measurements. The transducer can be applied directly to the pipe through magnetic bases allowing for the quick measurement of dynamic strains in many points of the pipe. The transducer signal can be read with the same commercial data collectors used for accelerometers.

Determination of small-strain stiffness of Shanghai clay on prismatic soil specimen

Although a large number of tunnels and deep excavations have been constructed in Shanghai, small-strain stiffness properties of natural Shanghai clay have rarely been reported in the literature. In this study, the degree of inherent stiffness anisotropy of natural Shanghai clay was investigated in a triaxial apparatus equipped with local strain transducers and a shear-wave velocity measurement system. Three sets of side-mounted bender elements, consisting of one transmitter and two receivers each, were installed on a prismatic specimen. Two series of triaxial tests on prismatic specimens of intact Shanghai clay were carried out under an isotropic stress state. Shear-wave velocities and hence elastic shear moduli in different planes were determined from bender element measurements. The cross-correlation method using two received signals gives rise to the most objective and repeatable results on shear-wave velocities in comparison with other commonly used methods. Intact Shanghai clay clearly exhibits inherent stiffness anisotropy in terms of its elastic shear modulus ratio (G0(hh)/G0(hv)) of about 1.2 for a mean effective stress varying from 50 to 400 kPa. The measured higher stiffness in the horizontal plane may be attributed to the stronger layering structure in the horizontal bedding plane. A unique relationship is found that relates the normalized shear moduli to the stress state in each plane by incorporating a void ratio function in the form of F(e) = e–2.6.

Microstructured fibers for sensitive strain transducers

Microstructured fibers for sensitive strain transducers

Technical Note: Evaluation of Tined Endoscopically Placed Mucosal Leads for Temporary Gastric Neurostimulation

<h3>Objective</h3> To compare larger diameter corkscrew-tined leads with previously described intracardiac pacing leads for temporary gastric neurostimulation in a canine model. <h3>Materials and Methods</h3> Two mongrel dogs underwent gastroscopy under general anesthesia, with endoscopic placement of two cardiac leads (1 mm tine diameter, 4 mm depth) placed sequentially in 1) transverse configuration in the distal antrum mucosa; 2) longitudinal (1 cm apart) configuration in gastric corpus. Stomach was then stimulated with maximal parameters to induce neutrally mediated contraction. Procedures were then repeated with larger leads (5 mm tine diameter, 8 mm length). Gastric contractions were measured with serosal strain transducers. <h3>Results</h3> Leads were placed endoscopically without difficulty. Neither lead type punctured through to the serosa of the stomach. Neither cardiac nor larger leads were capable of eliciting any gastric contractile activity with endoscopic placement either in the transverse or longitudinal orientations. <h3>Discussion</h3> While successful on the serosal side, both the cardiac leads and the larger alternative leads failed to produce stomach contraction when implanted mucosally. This may be due to the elastic nature of the mucosa, which was observed to twist around both types of leads significantly, hindering proper penetration into the muscularis. <h3>Conclusion</h3> These results suggest that the current concept of temporary gastric electrical neurostimulation via a mucosal approach must be reevaluated, as the procedure most likely does not accurately mimic electrical stimulation in the muscularis.

A comparison of three‐dimensional embedded strain transducers, compression tested within the same strain field while offset at an angle with the principal axes

Abstract: The strain tensor and principal strains were derived for a range of different patterns of three‐dimensional strain transducers. The precisions of the estimates of these values were based on the Monte Carlo simulation applied to experimental work that was conducted on strain transducers tested within the same strain field. The estimates of precision were also determined theoretically and compared with results based on experimental findings. Results suggest that the tetrahedron transducers in experimental work do not perform as well as rectangular patterns and that estimates of principal strains, which were not coincident with the axes of the transducer, showed slight discrepancies.

Deployment of a dense hybrid wireless sensing system for bridge assessment

Discussed in this paper is the deployment of a dense wireless sensor system on a short span integral abutment bridge superstructure located in St. Lawrence County, New York. The Wireless Sensor Solution (WSS) is designed specifically for diagnostic bridge monitoring providing independent conditioning for accelerometers and strain transducers. During deployment, strain measurements are obtained in real-time under ambient environmental and traffic loading. Strain transducers are placed at various locations along the girders, including the top and bottom flanges of a heavily instrumented interior girder at the mid-span and the abutment ends where the neutral axis location, section modulus, and girder moments are obtained. Results from the strain data analysis aid in quantifying the bridge response; notably, detection of end fixity, load distribution, and composite action between the girders and bridge deck. The measured responses are illustrated to propose a deterioration profile based on the level of capacity and demand.

Wireless Monitoring of a Multispan Bridge Superstructure for Diagnostic Load Testing and System Identification

Abstract: This article focuses on the deployment of a wireless sensor system (WSS) developed at Clarkson University for structural monitoring purposes. The WSS is designed specifically for diagnostic bridge monitoring, providing independent conditioning for accelerometers, strain transducers, and temperature sensors in addition to high-rate wireless data transmission and is capable of supporting large-scale sensor arrays. A three-span simply supported structure was subjected to diagnostic load testing as well as ambient vibration monitoring. A total of 90 wireless and several wired sensors, including accelerometers and strain transducers were used in the deployment. Strain measurements provided capacity and demand characteristics of the structure in the form of neutral axis locations, load distributions, and dynamic allowances which ultimately produced an inventory and operating load rating for the structure. Additionally, modal characteristics of the structure, including natural frequencies and mode shapes, were derived from measured accelerations and discussed briefly.