Hussam Saleem

Dynamic Characterization of a Soft Elastomeric Capacitor for Structural Health Monitoring

AbstractStructural health monitoring of civil infrastructures is a difficult task, often impeded by the geometrical size of the monitored systems. Recent advances in conducting polymers enabled the fabrication of flexible sensors capable of covering large areas, a possible solution to the monitoring challenge of mesoscale systems. The authors have previously proposed a novel sensor consisting of a soft elastomeric capacitor (SEC) acting as a strain gauge. Arranged in a network configuration, the SECs have the potential to cover very large surfaces. In this paper, understanding of the proposed sensor is furthered by evaluating its performance at vibration-based monitoring of large-scale structures. The dynamic behavior of the SEC is characterized by subjecting the sensor to a frequency sweep, and detecting vibration modes of a full-scale steel beam. Results show that the sensor can be used to detect fundamental modes and dynamic input. Also, a network of SECs is used for output-only modal identification of...

Novel nanocomposite technologies for dynamic monitoring of structures: a comparison between cement-based embeddable and soft elastomeric surface?sensors

The authors have recently developed two novel solutions for strain sensing using nanocomposite materials. While they both aim at providing cost-effective solutions for the monitoring of local information on large-scale structures, the technologies are different in their applications and physical principles. One sensor is made of a cementitious material, which could make it suitable for embedding within the core of concrete structures prior to casting, and is a resistor, consisting of a carbon nanotube cement-based transducer. The other sensor can be used to create an external sensing skin and is a capacitor, consisting of a flexible conducting elastomer fabricated from a nanocomposite mix, and deployable in a network setup to cover large structural surfaces. In this paper, we advance the understanding of nanocomposite sensing technologies by investigating the potential of both novel sensors for the dynamic monitoring of civil structures. First, an in-depth dynamic characterization of the sensors using a uniaxial test machine is conducted. Second, their performance at dynamic monitoring of a full-scale concrete beam is assessed, and compared against off-the-shelf accelerometers. Experimental results show that both novel technologies compare well against mature sensors at vibration-based structural health monitoring, showing the promise of nanocomposite technologies for the monitoring of large-scale structural systems.

Large-scale surface strain gauge for health monitoring of civil structures

Health monitoring of civil structures is a process that aims at diagnosing and localizing structural damages. It is typically conducted by visual inspections, therefore relying vastly on the monitoring frequency and individual judgement of the inspectors. The automation of the monitoring process would be greatly beneficial by increasing life expectancy of civil structures via timely maintenance, thus improving their sustainability. In this paper, we present a sensing method for automatically localizing strain over large surfaces. The sensor consists of several soft capacitors arranged in a matrix form, which can be applied over large areas. Local strains are converted into changes in capacitance among a soft capacitors matrix, permitting damage localization. The proposed sensing method has the fundamental advantage of being inexpensive to apply over large-scale surfaces. which allows local monitoring over large regions, analogous to a biological skin. In addition, its installation is simple, necessitating only limited surface preparation and deployable utilizing off-the-shelf epoxy. Here, we demonstrate the performance of the sensor at measuring static and dynamic strain, and discuss preliminary results from an application on a bridge located in Ames, IA. Results show that the proposed sensor is a promising health monitoring method for diagnosing and localizing strain on a large-scale surface.

Bio-inspired sensory membrane: Fabrication processes for full-scale implementation

In this paper, we investigate the influence of processing methods that dictate the performance enhancement in a nanocomposite soft capacitor. Styrene Ethylene Butylene Styrene (SEBS)/Titanium dioxide (TiO2) based nanocomposites are used as model substrates for preparing the soft capacitors. The efficiency of ultrasonic probe and high-shear melt mixing methods in dispersing TiO2 nanoparticles in SEBS polymer matrix is studied, and scanning electron microscopic (SEM) images are used to reveal fine-dispersion of TiO2 particles. After dispersion, films are prepared by compression-molding and drop-cast processing. The compression-molding method shows highly promising for engineering applications by enhancing fabrication speed, safety, and improving control over the film thickness.

Thin film sensor network for condition assessment of wind turbine blades

Existing sensing solutions facilitating continuous condition assessment of wind turbine blades are limited by a lack of scalability and clear link signal-to-prognosis. With recent advances in conducting polymers, it is now possible to deploy networks of thin film sensors over large areas, enabling low cost sensing of large-scale systems. Here, we propose to use a novel sensing skin consisting of a network of soft elastomeric capacitors (SECs). Each SEC acts as a surface strain gage transducing local strain into measurable changes in capacitance. Using surface strain data facilitates the extraction of physics-based features from the signals that can be used to conduct condition assessment. We investigate the performance of an SEC network at detecting damages. Diffusion maps are constructed from the time series data, and changes in point-wise diffusion distances evaluated to determine the presence of damage. Results are benchmarked against time-series data produced from off-the-shelf resistive strain gauges. This paper presents data from a preliminary study. Results show that the SECs are promising, but the capability to perform damage detection is currently reduced by the presence of parasitic noise in the signal.

Static characterization of a soft elastomeric capacitor for non destructive evaluation applications

A large and flexible strain transducer consisting of a soft elastomeric capacitor (SEC) has been proposed by the authors. Arranged in a network setup, the sensing strategy offers tremendous potential at conducting non-destructive evaluation of large-scale surfaces. In prior work, the authors have demonstrated the performance of the sensor at tracking strain history, localizing cracks, and detecting vibration signatures. In this paper, we characterize the static performance of the proposed SEC. The characterization includes sensitivity of the signal, and temperature and humidity dependences. Tests are conducted on a simply supported aluminum beam subjected to bending as well as on a free standing sensor. The performance of the SEC is compared against off-the-shelf resistance-based strain gauges with resolution of 1 μe. A sensitivity of 1190 pF/e is obtained experimentally, in agreement with theory. Results also show the sensor linearity over the given level of strain, showing the promise of the SEC at monitoring of surface strain.

Dynamic Characterization of a Soft Elastomeric Capacitor for Structural Health Monitoring Applications

A novel thin film sensor consisting of a soft elastomeric capacitor (SEC) for meso-scale monitoring has been developed by the authors. Each SEC transduces surface strain into a measurable change in capacitance. In previous work, the authors have shown that the performance of the SEC compares well with conventional resistive strain gauges, providing a resolution of 25 με using an inexpensive off-the-shelf data acquisition system for capacitance measurements. Here, we further the understanding of the thin film sensor by characterizing its dynamic behavior. The SEC is subjected to dynamic loads in bending mode. The study of Fourier and wavelet transforms indicates that the sensor can be used to identify dynamic inputs. Overall results demonstrate the promising capabilities of the thin film sensor at dynamic monitoring of civil structures.

Algorithm for decomposition of additive strain from dense network of thin film sensors

The authors have developed a capacitive-based thin film sensor for monitoring strain on mesosurfaces. Arranged in a network configuration, the sensing system is analogous to a biological skin, where local strain can be monitored over a global area. The measurement principle is based on a measurable change in capacitance provoked by strain. In the case of bi-directional in-plane strain, the sensor output contains the additive measurement of both principal strain components. In this paper, we present an algorithm for retrieving the directional strain from measurements. The algorithm leverages the dense network application of the thin film sensor to reconstruct the surface strain map. A bi-directional shape function is assumed, and it is differentiated to obtain expressions for planar strain. A least square estimator (LSE) is used to reconstruct the planar strain map from the sensors measurement’s, after the system’s boundary conditions have been enforced in the model. The coefficients obtained by the LSE can be used to reconstruct the estimated strain map or the deflection shape directly. Results from numerical simulations and experimental investigations show good performance of the algorithm, in particular for monitoring surface strain on cantilever plates.

Conductive paint-filled cement paste sensor for accelerated percolation

Cementitious-based strain sensors can be used as robust monitoring systems for civil engineering applications, such as road pavements and historic structures. To enable large-scale deployments, the fillers used in creating a conductive material must be inexpensive and easy to mix homogeneously. Carbon black (CB) particles constitute a promising filler due to their low cost and ease of dispersion. However, a relatively high quantity of these particles needs to be mixed with cement in order to reach the percolation threshold. Such level may influence the physical properties of the cementitious material itself, such as compressive and tensile strengths. In this paper, we investigate the possibility of utilizing a polymer to create conductive chains of CB more quickly than in a cementitious-only medium. This way, while the resulting material would have a higher conductivity, the percolation threshold would be reached with fewer CB particles. Building on the principle that the percolation threshold provides great sensing sensitivity, it would be possible to fabricate sensors using less conducting particles. We present results from a preliminary investigation comparing the utilization of a conductive paint fabricated from a poly-Styrene-co-Ethylene-co-Butylene-co-Styrene (SEBS) polymer matrix and CB, and CB-only as fillers to create cementitious sensors. Preliminary results show that the percolation threshold can be attained with significantly less CB using the SEBS+CB mix. Also, the study of the strain sensing properties indicates that the SEBS+CB sensor has a strain sensitivity comparable to the one of a CB-only cementitious sensor when comparing specimens fabricated at their respective percolation thresholds.