Self-sensing Capacity Research Articles

Investigation of carbon black nanoparticle modified cementitious composites for sensing dynamic vibro-acousto signals

Current application of intrinsic piezoresistivity of cementitious materials filled with nano conductive substances is limited in sensing quasi-static signals featuring low-frequency and high-amplitude. Its self-sensing capacity for low-amplitude and high-frequency vibro-acousto signals is not fully investigated. In this study, different doses of carbon black nanoparticles (CBN) are added in cementitious materials and their conductivity is measured to find the percolation zone where the tunnelling effect dominates the conduction mechanism, followed by the characterization of their piezoresistivity. Consequently, impact forces and crack-related energy release, which generate dynamic signals in the range of vibrational and acoustic frequencies, are individually utilized to test the behaviour of such materials for sensing dynamic vibro-acousto signals. The results show that CBN-modified cementitious composites which show excellent piezoresistivity behaviour are capable of locally self-sensing dynamic signals as sensors. Comparing with piezoelectrical sensors, the developed sensor shows higher sensitivity to crack initiation.

Bending, electroadhesion and sensing performances of single compliant electrode dielectric elastomer actuator based on SEBS gel

Dielectric elastomer actuators (DEAs) have great potential for application in soft robotics due to their ability to undergo substantial deformations, rapid response times, and high energy density when subjected to external electrical stimuli. However, the application of DEAs in the field of soft grippers is limited by their restricted direct electro-bending capability, output force, self-sensing capacity, and pre-stretching requirement. In this study, we fabricated a single compliant electrode DEA (SCE-DEA) which was made by sandwiching a compliant electrode between two layers of poly(styrene-b-ethylene-co-butylene-b-styrene) (SEBS)/white mineral oil (WO) dielectric elastomer films. The SCE-SEBS/WO was demonstrated to have the capacities of bending, sensing, and electroadhesion (EA). The SCE-SEBS/WO can be used in various soft actuator modes, such as the unipolar EA actuator, soft gripper, and soft vibrator. The grabbing mechanism of SCE-SEBS/WO-based soft grippers with opposite bipolar configuration is caused by electric field induced bending of SEC-SEBS/WO and subsequent electrostatic attraction between both SEC-SEBS/WO. The SEC-SEBS/WO based soft grippers have rapid response detachment, and ability to grasp various types of objects. Three-layer stacked SCE-SEBS/WO-60 (with 60 wt% of WO) exhibited 531 mN cm−2 of EA stress on the paper at 3.0 kV applied voltage, and the soft gripper made by four SCE-SEBS/WO-60 can successfully grab wood blocks weighing 162.4 g at 5 kV applied voltage. The sensing capacity of SCE-SEBS/WO based soft gripper was based on the bending strain dependent resistance changes of the compliant electrode. Our results provide new insights into the fabrication of DEA based soft grippers.

Effect of steel slag on the mechanical properties and self-sensing capability of ultra-high performance concrete (UHPC)

This study investigates the self-sensing capability and mechanical properties of ultra-high performance concrete (UHPC) incorporating steel slag aggregates. The experimental findings underscore the potential of steel slag as a promising industrial by-product capable of fully replacing the natural fine aggregates within UHPC. In addition, the incorporation of steel slag is shown to reduce CO2 emissions by approximately 40 kg/m3, as determined through life cycle assessment software, while simultaneously enhancing flowability when compared to UHPC incorporating natural aggregate. Although the compressive and tensile strength of UHPC with steel slag is slightly lower than that of UHPC with natural aggregates, it still demonstrates impressive performance, with compressive and tensile strengths reaching approximately 207 MPa and 14 MPa, respectively. To assess the mechanical properties and self-sensing capacity of steel slag, this study incorporates carbon fiber and multi-walled carbon nanotubes (MWCNT) into the UHPC series. The self-sensing capacity results emphasize the advantage of incorporating steel slag due to its high iron content that leads to superior conductivity compared to silica sand. Furthermore, comparisons are made with UHPC series incorporating carbon fiber and MWCNT. UHPC with steel slag demonstrates superior self-sensing capabilities compared to the silica sand series with carbon fibers, and it is comparable to UHPC with MWCNT. To quantify the self-sensing capability of UHPC with steel slag, nonlinear curve fitting is employed, resulting in highly accurate simulation outcomes. These experiments collectively highlight the multifaceted benefits of utilizing steel slag aggregate in UHPC, encompassing environmental advantages, enhanced flowability, and impressive self-sensing capabilities.

Sponge-like, semi-interpenetrating self-sensory hydrogel for smart photothermal-responsive soft actuator with biomimetic self-diagnostic intelligence

Actuators based on hydrogels are good candidates for biomimetic soft robots due to their excellent mechanical properties and flexibility. Despite great efforts in hydrogel-based actuators, their actuation process still differs from the “recognition-judgment-execution” process that conscious intelligent beings go through owing to the lack of perceptive capacity. Herein, to imitate the intelligent actuation process of conscious organisms, a fast photo-thermal responsive and self-sensory soft actuator is proposed based on sponge-like hydrogel with semi-interpenetrating (semi-IPN) polymer network of thermosensitive poly (N-isopropylacrylamide) (PNIPAM) and electronically conductive polyaniline (PANI). The proposed actuator exhibits excellent photo-thermal responsiveness enabling remotely triggered actuation controlled by near infrared light (NIR), covering all azimuths with ultrafast response rate (within 2 s) and recover rate (within 1 s). Meanwhile, the conformational transition of PNIPAM networks during the course of actuation also lead to accompanying migration of the interpenetrated PANI chains and significant variation of local conductivity (>15%), endowing the actuator with self-sensing capacity to monitor its deformations. Eventually, a smart self-adaptive actuator is designed inspired by neuromuscular in intelligent organisms, which can actively perceive external environmental stimuli and consciously regulate its own shape transformation by monitoring and analyzing electrical signal. This sensory actuating hydrogel can provide insights into the development of artificial intelligent soft robotics with higher levels of autonomy and complexity.

Health monitoring of C60 smart concrete based on self-sensing

Smart concrete is a composite material that has several advantageous characteristics such as electrical conductivity and electromagnetic shielding. It has broad application prospects in road snow and ice melting, electrical equipment grounding, structural health monitoring, and electromagnetic shielding. Thus, it is of great significance to investigate the uniaxial compressive strength (UCS) and self-sensing properties of smart concrete. In this study, two complementary properties of carbon and steel fibers were used to create functional fiber concrete. The abilities of material factors, test methods, and environmental factors to test the self-sensing capacity of concrete were investigated. Investigation of smart concrete in elastic range under stress resistivity. The results revealed that an appropriate amount of carbon and steel fibers in concrete can increase the UCS and improve self-sensing. The stress-resistivity self-sensing ability of the 0.2% carbon fiber + 0.45% steel fiber smart concrete was found to be the best. As the diameter range of the coarse aggregates increased, the self-sensing ability of the stress resistivity also increased. The environment and the measurement methods had some influence these results. When the measurement voltage was less than 5 V, the smaller the measurement voltage, the better was the self-sensing of the concrete. Concrete with a low moisture content was found to be more sensitive to stress–resistivity changes, whereas concrete with a high moisture content was more sensitive to resistivity changes under large stresses. The effect of loading speed on the self-sensing performance was relatively small. There was a good correspondence between the stress and resistivity self-sensing under cyclic unloading conditions. The resistivity decreased with an increasing load and increased with unloading, and the stability was good. The preferred carbon-steel fiber admixture for compressive-damage self-sensing concrete considering the UCS and stress-resistivity self-sensing capability was 0.2% carbon fiber+ 0.45% steel fiber. Smart concrete can be used as a stress, strain, or moisture sensor, or a load-bearing element.

4D printing Light-Driven soft actuators based on Liquid-Vapor phase transition composites with inherent sensing capability

Soft robots capable of responding to different actuation schemes are flourishing due to the appealing advantages of being highly flexible and adaptive to complex environments. However, it remains challenging to produce untether soft actuators that can sense their own motions. Herein, a novel photo-responsive liquid–vapor phase transition composite with integrated actuating and sensing performances is proposed. The composite is operated based on the principle that piezocapacitive sensing and liquid–vapor phase transition are caused by the photothermal effect of the embedded graphene plate. The composite exhibits superior and tunable optically responsive and self-sensing properties. As a proof of concept, the muscle-like actuator with effective actuating and real-time sensing feedback functions is produced. Furthermore, several “Janus” bilayer untethered actuators are fabricated via 3D printing, which can achieve a variety of light-driven programmed locomotion, such as bending, grasping, and crawling. This work holds great promise for designing and fabricating soft robots with integrated self-sensing capacity.

Experimental Investigation on the Compressive Stress-Sensing Ability of Steel Fiber-Reinforced Cement-Based Composites under Varying Temperature Conditions

This study investigates the piezoresistive (self-sensing) properties of short stainless-steel fiber-reinforced mortar under varying temperature conditions. Different reinforced mortars were produced by varying fiber and aggregate content. First, Electrical Impedance Spectroscopy (EIS) measurements were used to characterize the electrical properties of the mortar specimens. EIS measurements were performed at temperatures of 24 °C, 35 °C, and 50 °C. Second, to investigate the self-sensing capacity of the different composites, the fractional changes of electrical impedance at 1 kHz were monitored under two conditions: temperature variation alone (cooling down from 35 °C or 50 °C to room temperature), and temperature variation combined with cyclic compressive loading (up to 5 MPa). The results of the former were used to compensate for the effect of temperature variations in the latter. Both temperature and mechanical loading produced meaningful variations in the electrical impedance and piezoresistivity of the investigated composites. Conclusions are drawn with respect to the stress and temperature sensitivity of the composites. The real and imaginary parts of the electrical impedance of the mortar produced with the highest fiber volume fraction (0.01%) and higher aggregate content (volume fraction of 60%) were distinctly sensitive to temperature and stress, which suggests the possibility of using the same composite as a stress and temperature sensor.

A Hydrogel-Based Self-Sensing Underwater Actuator.

Soft robots made of hydrogels are suited for underwater exploration due to their biocompatibility and compliancy. Yet, reaching high dexterity and actuation force for hydrogel-based actuators is challenging. Meanwhile, real-time proprioception is critical for feedback control. Moreover, sensor integration to mimic living organisms remains problematic. To address these challenges, we introduce a hydrogel actuator driven by hydraulic force with a fast response (time constant 0.83 s). The highly stretchable and conductive hydrogel (1400% strain) is molded into the PneuNet shape, and two of them are further assembled symmetrically to actuate bi-directionally. Then, we demonstrate its bionic application for underwater swimming, showing 2 cm/s (0.19 BL/s) speed. Inspired by biological neuromuscular systems’ sensory motion, which unifies the sensing and actuation in a single unit, we explore the hydrogel actuator’s self-sensing capacity utilizing strain-induced resistance change. The results show that the soft actuator’s proprioception can monitor the undulation in real-time with a sensitivity of 0.2%/degree. Furthermore, we take a finite-element method and first-order differential equations to model the actuator’s bending in response to pressure. We show that such a model can precisely predict the robot’s bending response over a range of pressures. With the self-sensing actuator and the proposed model, we expect the new approach can lead to future soft robots for underwater exploration with feedback control, and the underlying mechanism of the undulation control might offer significant insights for biomimetic research.

Bio-inspired self-healing and self-sensing cementitious mortar using Bacillus subtilis immobilized on graphitic platelets

This study focuses on exploring the feasibility of immobilizing Bacillus subtilis with inert conductive Graphite nano/micro platelets (GNMPs) as the healing and sensing component of a smart mortar, respectively. The specimens are casted using variable content of GNMPs. The healing potential of smart mortar is accessed via visual inspections and mechanical recovery. Whereas, piezoresistivity of specimens incorporated with a range of GNMPs concentrations derived from theoretical percolation is analyzed under ultimate and cyclic loading to establish a correlation between strain and load with resistivity. Maximum average crack-width healing of 1.30 mm is recorded corresponding to specimens incorporated with 1.4% replacement of GNMPs. 1.5% replacement of GNMPs shows an 80.5% recovery of the compressive strength after healing. The self-sensing capacity of the composite mortar increases with increasing GNMP concentration up to 1.5%, with 1.2% replacement by GNMPs giving the highest gauge factor, making it the most strain-sensitive concentration.

Self-sensing capacity of ultra-high-performance fiber-reinforced concrete containing conductive powders in tension

This study experimentally analyzed the self-sensing performance of the tensile behavior of ultra-high-performance fiber-reinforced concrete (UHPFRC) incorporating four different types of conductive powders: carbon black (CB), nickel powder (NP), graphite powder (GP), and steel slag powder (SP). The conductive powders were incorporated by weight ratios of 5% and 10% of quartz powder (QP) which is used as a mechanical filler of UHPFRC. The results showed a decrease in tensile strength with an increase in the incorporated amount of CB, NP, and GP and an increase in tensile strength according to the increase in the incorporated amount of SP from 5% to 10%. The results of microstructure analysis by EDS mapping showed that, when incorporating 5% of CB and GP, which have a lower density than QP, conductive powders were sufficiently distributed, enabling both direct contact between conductive powders and the tunneling effect. In the case of NP and SP, which have a higher density than QP, at least 10% of NP and SP must be incorporated to achieve self-sensing performance. The tensile stress/strain versus time responses of UHPFRC with conductive powders were simulated using the measured FCRs. The pre-peak tensile behaviors were also well simulated using empirical equations based on the obtained FCR by nonlinear regression analysis. The predicted pre-peak tensile behaviors had smaller errors compared to the experimentally obtained tensile behaviors. In the case of the post-peak, only tensile stress was well simulated. As a result, 10% of NP and 5% of GP had sufficient self-sensing performance for UHPFRC with conductive powders.

Intrinsic graphene/cement-based sensors with piezoresistivity and superhydrophobicity capacities for smart concrete infrastructure

Different surface coatings were applied for graphene/cement-based sensors with self-sensing capacity to achieve an effective way for superhydrophobicity to mitigate piezoresistive instability due to various humidity in this paper. The results show that the cement-based sensors treated by the twice coating can reduce the water absorption by half and exhibit the highest water contact angle of 163.4°. The hydrophobic silane can penetrate into the cement-based sensors after the once/twice coating if 4% silane is used with isopropanol. However, it was still difficult to observe the hydrophobic groups inside the cement matrix subjected to the silane without isopropanol. On the other hand, the effect of surface coatings on the piezoresistive sensitivity was also investigated after the surface coating treatments. The sensitivity of cement-based sensor was dozens of times higher than the commercially available strain gauges. The related results will promote the practical application of superhydrophobic cement-based sensors for structural health monitoring of the smart concrete infrastructure.

Effects of filler type and aging on self-sensing capacity of cement paste using eddy current-based nondestructive detection

Eddy current-based nondestructive detection was first used for testing self-sensing cement paste in this study. The effects of three functional fillers and aging on self-sensing cement paste were observed. First, the plain mortar exhibited self-sensing ability at the earliest curing time, but its gage factor significantly decrease with ages. Second, among the three functional fillers that were studied - carbon fiber (CF), steel fiber (SF), and carbon black (CB) - the SF exhibited the highest gage factor. Third, the effect of ages on eddy current sensitivity is not obvious. The gage factor SF-reinforced specimens is high in different ages, while the gage of CF- and CB-reinforced specimens change little with ages. This study shows that eddy current-based nondestructive detection is feasible to detect stress and damage-induced variation of cement paste, and SF is the best filler to improve the self-sensing ability of cement paste.

Asymmetric bilayer CNTs-elastomer/hydrogel composite as soft actuators with sensing performance

Living creatures have the nature of actuating and sensing under external stimuli to better adapt to the environment. For instance, the combination of muscles actuation and skin sensing ensures our intelligent interaction with external environments. In terms of the soft robot, implanting sensation and actuation in one system can assist us to control it better and more efficiently. However, in the development of biomimetic materials, the simultaneous realization of actuating and sensing performance in bionic soft robots is still a great challenge. Here, we propose a novel asymmetric bilayer CNTs-elastomer/PNIPAm hydrogel composite with integrated actuating and sensing performances. Due to the excellent photothermal effect of CNTs and thermo-responsiveness of PNIPAm, under the NIR irradiation, the PNIPAm hydrogel layer would shrink and the bilayer composite would thus perform a bending deformation. At the same time, based on a piezoresistive sensing mechanism of the CNTs-elastomer, the actuating procedure can be recorded. This work may open up a new insight in the designing and fabricating intelligent biomimetic hydrogel soft robots with integrated self-sensing capacity.

Piezoresistivity deterioration of smart graphene nanoplate/cement-based sensors subjected to sulphuric acid attack

Smart cement-based sensors with self-sensing capacity have been explored for structural health monitoring (SHM) with the intrinsic piezoresistive performance. However, few studies had studied the piezoresistivity degradation of cement-based sensors after exposure to the aggressive environments, especially under sulphate acid attacks. In this study, graphene nanoplate (GNP)/cementitious composites were immersed in sulphuric acid solutions (concentrations of 0, 1%, 2%, and 3%) for 90 and 180 days. Then surface appearance, weight loss, mechanical properties, piezoresistivity and microstructure were investigated and compared before and after sulphuric acid immersion. The results show that after acid immersion, the surface deterioration and mass loss were increased, and the compressive strength was significantly decreased. As for the intact GNP/cementitious composite, the piezoresistivity exhibited excellent linearity and repeatability, demonstrating the great potential to act as intelligent cement-based sensors for SHM. After 90 and 180 days of acid immersion, the piezoresistivity was sensitive to the initial low load initially but then turned less sensitive to the later high load. The highly corroded GNP/cementitious composites exhibited porous microstructures associated with the low compressive strength. The fractional changes to resistivity (FCR) under the low load could be attributed to the compressed pores and voids filled with erosion products that would form conductive passages. In contrast, with the increase of applied load, the intact cement matrix became much denser, which in turn constrained the further development of conductive passages in the GNP/cementitious composites.

Integrated self-sensing and self-healing cementitious composite with microencapsulation of nano-carbon black and slaked lime

In this paper, multifunctional cementitious composites with integrated self-sensing and self-healing properties are developed using microencapsulation of nano-carbon black (NCB) to enclose slaked lime (SL). The results show that the cracks healing efficiency is strongly improved with NCB enclosed SL. With SL, the self-sensing capacity of NCB-cementitious composite exhibits higher and more stable piezoresistivity before or after self-healing. The NCB enclosing SL particles not only achieve excellent piezoresistivity, but also preserve SL from initial hydration. Furthermore, the remained SL can be released for further reactions in the cracks. The results provide a promising integrated multifunctional self-sensing and self-healing cementitious composite for structural health monitoring application.

Piezoresistivity enhancement of functional carbon black filled cement-based sensor using polypropylene fibre

In this study, different dosages of carbon black (CB) and polypropylene (PP) were added to develop functional cementitious composites as cement-based sensors. The results show that electrical conductivity increased with the amount of PP fibres, due to the enclosed CB nanoparticles and more conductive passages. The compressive strength slightly decreased, while the flexural strength was significantly increased with the increased amount of PP fibres. The improvement is mainly achieved by the reduced CB concentration in cement matrix and the excellent tensile strength of PP fibres. Under the cyclic compression, the piezoresistivity increased by three times for 0.4 wt% PP fibres filled CB/cementitious composite, regardless of the loading rates. The flexural stress sensing efficiency was considerably lower than that of compressive stress sensing, but it increased with the amount of PP fibres. Moreover, fitting formulas were proposed and used to evaluate the self-sensing capacity, with the attempts to apply cement-based sensors for structural health monitoring.

Mechanical strength and self-sensing capacity of smart cementitious composite containing conductive rubber crumbs

The effects of conductive rubber crumbs on the mechanical properties and self-sensing capacities of cementitious composites are investigated in this study. The rubberized cementitious composites with five different contents of conductive rubber crumbs are incorporated, ranging from 0%, 10%, 20%, 30% and 40% by mass of fine aggregate. Under the uniaxial cyclic compression, all the conductive rubber crumbs–filled cement composites exhibit excellent repeatability of piezoresistivity. The mortar with 20% conductive rubber crumbs at a water-to-binder ratio of 0.42 displayed the best piezoresistive sensitivity. Based on the relative positions of conductive rubber crumbs in the rubberized cement mortar, three conductive mechanisms were proposed for the conductive rubber crumbs, including complete isolation state, neighbouring state and the contact state. The isolation state plays a dominant role when the content of the conductive rubber crumbs is low, in which the piezoresistive behaviour is mainly controlled by the resistivity changes in cement matrix. In the neighbouring state, pores or voids in the gaps between nearby conductive rubber crumbs make the conductive rubber crumbs easier to connect, thus decreasing the resistivity under uniaxial compression. As for the contact state, the decreased contact resistance and the absence of sand between conductive rubber crumbs lead to higher resistivity changes under cyclic compression. The related results indicate that conductive rubber crumbs in cement mortar have application potentials for structural health monitoring.

Direct tensile self-sensing and fracture energy of steel-fiber-reinforced concretes

Based on the experimental tests, the tensile self-sensing and fracture energy of steel-fiber-reinforced concretes (SFRCs) were focused to investigate. Firstly, the plain mortar matrix of SFRCs produced the strain-softening behavior and no self-sensing ability. Secondly, among six fiber types with same amount 1.5 vol.-% as follows: macro twisted (T30/0.3), macro smooth (S30/0.3), macro hooked (H30/0.375), meso twisted (T20/0.2), meso smooth (S19/0.2) and micro smooth (S13/0.2), the T20/0.2 revealed the highest first gauge factor (GFcc) and first-cracking strength whereas the S30/0.3 produced the highest first-cracking strain. Thirdly, the strain-sensing capacity of SFRCs using macro twisted fiber was observed to increase with increasing of fiber content in range of 0–2.0 vol.-%. Fourthly, in evaluating three SFRC matrices with same amount 1.5 vol.-% macro hooked fibers including M1 (control), M2 (added CB), M3 (added GGBS), M2 produced a significant enhancement in both self-sensing capacity and tensile strength while M3 produced a slight enhancement of them, in comparison with M1. Finally, the fracture energy of the investigated SFRCs were comparatively evaluated as follows: the ratio between elastic fracture energy and total fracture energy was the lowest, from 0.02 to 0.08 while the ratio between localized fracture energy and total fracture energy was the highest, from 0.38 to 0.82.

Multifunctional self-sensing and ductile cementitious materials

This study focuses on the material design and electromechanical behavior of new multifunctional strain-hardening cementitious materials (MSCs) that integrate tensile ductility with self-sensing functionality. The material design was accomplished by tailoring the micro-scale electrical and mechanical parameters to achieve strongly coupled electromechanical behavior at macro-scale. Electrical impedance spectroscopy and equivalent circuit modeling were conducted to understand the effects of carbon black (CB) nanoparticles on material electrical microstructure. The results revealed that large signal-to-noise ratio and gauge factor can be achieved if the material contains sufficient amount of both partially conductive and conductive paths. Additionally, uniaxial tension, fracture toughness, and single crack opening tests explained the negative impact of CB nanoparticles on the material tensile strength and ductility, leading to material redesigning for improved mechanical behavior. Finally, the electromechanical response of the MSCs was studied under monotonic and cyclic loading, which demonstrated robust self-sensing capacity of MSCs during elastic, strain-hardening and tension-softening stages.

Effects of carbon nanomaterial type and amount on self-sensing capacity of cement paste

This study investigates the implications of carbon nanomaterial type and amount on the electrical properties of cement paste. For this, five different nanomaterials, i.e., carbon nanotube (CNT), carbon fiber (CF), graphite nanofiber (GNF), graphene (G), and graphene oxide (GO), and two different volume fractions of 0.5 and 1% were considered. In addition, the self-sensing capacity of the cement composites with nanomaterials was evaluated under cyclic compressive loads. Test results indicate that the conductivity of plain cement paste was improved by adding carbon nanomaterials. In most cases, the conductivity of the composites was reduced by an increase in curing age and a decrease in nanomaterial amount, except for CF. The composites with CNTs exhibited the best self-sensing capacity regardless of volume fraction (vf), and the order of self-sensing capacity of the composites at a vf of 1% was CNT > GO ≈ GNF > G. The composites with 0.5 and 1 vol% CFs were determined to be not appropriate for a sensor measuring compressive behaviors. The gauge factor of the composites incorporating 1 vol% CNTs was obtained as 77.2–95.5.