Electromechanical behavior of hybrid carbon/glass fiber composites with tension and bending

To understand the smart (i.e., good memory) characteristics of hybrid composites of carbon fibers (CFs) and glass fibers (GFs) with epoxy resin as a matrix, the changes in the electrical resistance of composites with tension and on bending were investigated. The electrical resistance behavior of composites under tension changed with the composition of the CF/GF, as well as with the applied strain. The fractional electrical resistance increased slowly with increasing strain within a relatively low strain region. However, with further loading it increased stepwise with the strain according to the fracture of the CF layers. The strain sensitivity of the samples increased with increasing CF weight percentage, and the samples incorporating more than 40 wt % CF showed a strain sensitivity higher than 1.54 for a single CF. The changes in the fractional electrical resistance with bending were not so dominant as those with tension. This difference was attributed to the action of two cancelling effects, which are the increasing and decreasing fractional electrical resistance due to tension and compression with bending, respectively. On recovery from a large applied bending, the fractional electrical resistance decreased slowly with unloading because of the increase of contacts between the fibers that resulted from the reorganization of ruptured CFs during the recovery. Even the composites incorporating a relatively small CF content showed an irreversible electrical resistance with both tension and bending. However, the strain sensitivity being larger with tension than with bending is ascribed to the difference in their mechanical behaviors. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 2447–2453, 2002     

Self-sensing of flexural damage and strain in carbon fiber reinforced cement and effect of embedded steel reinforcing bars

Self-sensing of flexural damage and strain in carbon fiber reinforced cement is attained by measuring the volume or surface resistance with the four-probe method and electrical contacts on the compression and/or tension surfaces. The oblique resistance (volume resistance in a direction between the longitudinal and through-thickness directions) increases upon loading and is a good indicator of damage and strain in combination. The surface resistance on the compression side decreases upon loading and is a good indicator of strain. The surface resistance on the tension side increases upon loading and is a good indicator of damage. The effectiveness for the self-sensing of flexural strain in carbon fiber reinforced cement is enhanced by the presence of embedded steel rebars on the tension side. For the same midspan deflection, the fractional change in surface electrical resistance is increased in magnitude, whether the surface resistance is that of the tension side or the compression side. The fractional change in resistance of the tension surface is increased by 40%, while the magnitude of the fractional change in resistance of the compression surface is increased by 70%, due to the steel.     

Nonmonotonic piezoresistive effect in elastomeric composite films

We compared the change of electrical resistance with elongation (piezoresistive effect) in thin films made of conductive multiwalled carbon nanotubes embedded in eight different elastomers. Two distinct forms of piezoresistive effect were observed: (i) in the “monotonic” (M) case, the film resistance always increased with the applied strain; (ii) the “nonmonotonic” (NM) case showed an initial increase in the resistance, while with further elongation the resistance began to decrease. By varying the amount of nitrile and/or styrene groups in the polymer matrix one can alter the piezoresistive effect qualitatively: composites with ～25 wt % or more of nitrile or styrene functional side groups exhibited M piezoresistance, while others, with no, or methyl side groups only, showed NM piezoresistance. Influence of the second filler (either conductive carbon black or nonconductive nanoclay) in the ternary composites on the piezoresistive effect was explored. The possibility to modify the piezoresistive behavior of the conductive elastomer composites, for example, via chemical modification of the polymeric matrix, opens up a new venue for practical applications such as diverse types of sensors and, in NM case, complex dynamical systems (bistable elements, electromechanical oscillators, etc.) in the MEMS field. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43518.     

Elastomeric ethylene copolymers with carbon nanostructures having tailored strain sensor behavior and their interpretation based on the excluded volume theory

Two ethylene/1‐butene thermoplastic elastomer copolymers were melt mixed with either multiwalled carbon nanotubes (CNTs) or thermally reduced graphite oxide (TrGO) resulting in piezoresistive composite materials. The effect of the polymer matrix, carbon nanostructure and filler concentration on the electrical behavior of the sensors was analyzed. The percolation process confirmed the relevance of these parameters as different thresholds were found depending on both the matrix and the filler. For instance, composites based on TrGO presented higher percolation thresholds than those based on CNTs. Regarding the strain sensor behavior of the electrically conductive composites, by using a matrix with a low amount of 1‐butene comonomer, higher resistance sensitivities were observed compared with the other matrix. Noteworthy, composites based on TrGO filler presented strain sensitivities one order of magnitude higher than composites based on CNT filler. These results are explained by the excluded volume theory for percolated systems. Based on these findings, polyethylene piezoresistive sensors can be designed by a proper selection of polymer matrix, filler concentration and carbon nanoparticles. © 2016 Society of Chemical Industry     

Experimental and finite element modeling of vinyl ester nanocomposites under blast and quasi‐static flexural loading

Materials used in blast, penetration, and impact loaded structural applications require high strength and toughness under high strain rate loading. 510A‐40 brominated bisphenol‐A‐based vinyl ester resin was developed and reinforced with different loadings of nanoclay and exfoliated graphite platelet to produce composites with optimal flexural rigidity, vibration damping, and enhanced energy absorption. As these reinforced polymeric materials are viscoelastic in principle, the mechanical behavior was characterized under two extremes of strain rate loading. In this article, the macroscopic response of brominated vinyl ester reinforced with 1.25 and 2.5 wt % nanoclay and exfoliated graphite platelet is considered. Air‐blast experiment was conducted by subjecting these specimens to a high‐transient pressure in a shock‐tube with flexural loading configuration. The axial response was investigated quasi‐statically in a uniaxial tension/compression test and dynamically in a compression Split‐Hopkinson bar test. The servo‐hydraulic MTS system was used to simulate the shock‐tube testing in a flexural quasi‐static loading configuration. High strain rate properties obtained from shock‐tube experiment are compared with that of characterized under the simulated quasi‐static flexural loading. Further, a computational finite element analysis model was developed in ANSYS LSDYNA to predict with reasonable accuracy the dynamic response of shock‐loaded nanoreinforced specimens. Drop in both failure strain and energy absorption was observed with the addition of nanoparticles to pristine vinyl ester. However, an improvement in energy absorption was observed in case of shock‐tube loading at high strain rates as compared to that loaded quasi‐statically. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci., 2012     

Analytical model of piezoresistivity for strain sensing in carbon fiber polymer-matrix structural composite under flexure

An analytical model is provided for the piezoresistive phenomenon of continuous carbon fiber polymer-matrix composite under flexure. This phenomenon allows strain sensing and entails reversible increase of the tension surface resistance and reversible decrease of the compression surface resistance during flexure. The model considers the surface resistance change to be due to change in the degree of current penetration. The longitudinal strain resulting from the flexure affects the through-thickness resistivity (which relates to the contact resistivity of the interlaminar interface). Good agreement is found between the model and prior experimental results, except that the calculated surface resistance on the tension side is higher than the measured value, when the magnitude of the average longitudinal strain on the surface exceeds 3 × 10⁻³.     

Multidimensional characterization of piezoresistive carbon black silicone rubber composites

Flexible and stretchable electronic skins capable of replicating the human sense of touch are a subject of active research. One of the most popular materials for force sensors in skins is carbon black (CB)/polydimethylsiloxane composite. To aid in skin design, a characterization of this composite is presented here. The sensitivity of composite resistance to uniaxial tension, compression, and shear for each CB concentration is measured and found to be similar for tension and compression, but smaller for shear, with resistance monotonically increasing with strain. In addition, under tension and compression the resistance of the material is measured both in line with and perpendicular to the axis of applied strain, and the response is found to be approximately equal in both cases. The electrical and mechanical relaxation time of the material is also measured and modeled for tension, compression, and shear. The mechanical relaxation time is found to be shorter than the electrical, with both increasing with CB concentration. However, the shortest mechanical relaxation time, 200 s, precludes a sensor with human‐like response times without an active modeling and compensation system. Finally, Young's modulus and Poisson's ratio are measured and reported for each CB concentration. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44773.     

Piezoresistive behavior of epoxy matrix‐carbon fiber composites with different reinforcement arrangements

In this work, the self‐monitoring capability of epoxy matrix‐carbon fiber composites has been studied. Different concentrations and arrangements of reinforcements were used, including random chopped, unidirectional and bi‐directional continuous carbon fibers, weaved and nonweaved. Mechanical properties were determined by uniaxial tensile tests. The composite electric to mechanical behavior was established by determining its electrical resistivity variation as a function of the stress‐strain curve. It was observed that the composites electrical resistance increased during tensile tests, a trend that indicates piezoresistive behavior. The increase was linear for the chopped reinforced composites, while it exhibits different slopes in the continuous reinforced composites. The initial smaller slope corresponds mainly to separation of the 90° oriented fibers and/or transversal cracking of the matrix, whereas the latter higher slope is caused by fiber fracture. The results demonstrated how each reinforcement configuration exhibited a unique and typical electrical response depending on the specific reinforcement, which might be appropriate either for strain‐monitoring or damage‐monitoring. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Self-sensing of flexural strain and damage in carbon fiber polymer-matrix composite by electrical resistance measurement

The self-sensing of flexural strain and damage has been demonstrated in carbon fiber polymer-matrix composite by measuring the DC electrical resistance. Upon strain in the elastic regime, the compression surface resistance decreases reversibly (due to increase in the current penetration), while the tension surface resistance increases reversibly (due to decrease in the current penetration), and the oblique resistance increases reversibly. Upon minor damage, (i) the oblique resistance after unloading decreases, (ii) the oblique resistance decreases during load increase near the start of loading, and (iii) the curve of the oblique resistance or the resistance of the tension or compression surface vs. deflection becomes nonlinear. Upon major damage, all resistances abruptly and irreversibly increase, such that the onset occurs earlier for the compression surface resistance and the oblique resistance than the tension surface resistance. The surface resistances are superior indicators of strain, whereas the oblique resistance is a superior indicator of damage.     

A comparison of constant and complex creep loading programs for several thermoplastics

Polymethyl methacrylate, polyacetal and polypropylene samples were subjected to constant-load uniaxial creep in tension and compression up to 3% strain in times up to 10⁵ sec. A higher creep resistance was obtained in compression compared with tension due to the influence of free volume on mobility. Square wave cyclic creep tests alternately in tension and equal compression for dwell times of 10, 100, 1000 sec were also conducted on each material up to a total time of 10⁵ sec. Under low cyclic creep stresses tension creep cancelled out compression creep in each successive half cycle so that there was no overall increase in creep strain as cycling proceeded. The unidirectional creep data was used successfully to predict this behavior. At higher cyclic stress levels creep strain range increased steadily throughout the test indicating a softening process. A few tests were conducted in unidirectional and cyclic stress relaxation on polymethyl methacrylate.     

Electrical,mechanical, and piezoresistive properties of carbon nanotube–polyaniline hybrid filled polydimethylsiloxane composites

The electrical, mechanical, and piezoresistive properties of ternary composites based on elastomeric polydimethylsiloxane (PDMS), carbon nanotubes (CNTs), and polyaniline (PANI) were studied and compared with those of binary PDMS–CNT composites. The presence of PANI affected the percolating network of the CNTs. At lower PANI concentrations (2.5 and 5%), the conductive network of the CNTs was constructively modified; this led to an enhancement in the conductivity in the sample containing 2% CNTs. A higher PANI content (7.5%) hindered the flow of main charge carriers through the composite. The piezoresistive response of the binary and ternary composites was studied by cyclic experiments under compression loads. In all of the samples, the electrical resistance increased monotonically up to a 10% strain. The reproducibility of the piezoresistive behavior in the binary and ternary composites provided evidence that the fillers could reversibly recover their initial position together with the PDMS chains without a significant displacement with respect to their original positions. The reduction of the piezoresistive sensibility by PANI addition was attributed to the displacement restrictions of the CNTs within the composite under pressure because of the volume exclusion of PANI particles; this maintained the probability of CNT contact and increased the possibility of the formation of new CNT conductive channels. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44780.     

应变率对乙烯基酯树脂压缩力学行为影响的研究

为研究应变率对乙烯基酯树脂压缩性能的影响,在准静态与动态试验条件下,比较分析了不同加载速率时乙烯基酯树脂的压缩力学行为.研究发现,乙烯基酯树脂的压缩性能受加载速率影响显著,具有明显的应变率效应.动态试验条件下的屈服应力比准静态下提高57.0%～85.2%,而屈服应变略有降低,其屈服形貌和破坏特征也明显不同.     

Piezoresistive effect of a carbon nanotube silicone-matrix composite

A carbon nanotube silicone-matrix composite is fabricated into a specimen with two current-electrodes and two voltage-electrodes. The specimen is sealed transversely and the four-wire-method is used to measure the piezoresistivity of the composite during compression. The electrical resistance and the resistivity of the composite increase with the increase of the absolute value of the strain during compression. The absolute value of the gage factor decreases with the increase of the carbon nanotube content. By analyzing the changes in the effective conductive path (ECP), a piezoresistive mathematical model is established. The resistance of a tunneling film (i.e. the silicone film between the adjacent carbon nanotubes where the tunneling effect occurs) attenuates with the increase of the absolute value of the strain during compression. The attenuating rate decreases with the increase of the carbon nanotube content. The ratio of the number of the tunneling films in each ECP to the number of ECPs in the composite decreases with the increase of the carbon nanotube content and increases with the increase of the absolute value of the strain during compression.     

Mechanical damage and strain in carbon fiber thermoplastic‐matrix composite,sensed by electrical resistivity measurement

The volume electrical resistivity of a unidirectional continuous carbon fiber thermoplastic (nylon‐6) matrix composite was found to be an indicator of strain and damage during repeated loading in the fiber direction. The through‐thickness resistivity irreversibly and gradually decreased upon damage (probably fiber‐matrix debonding) during repeated compression or tension. Moreover, it reversibly and abruptly increased upon matrix damage, which occurred reversibly near the peak stress of a stress cycle. In addition, the resistivity increased reversibly upon tension in every stress cycle, and decreased reversibly upon compression in every stress cycle. On the other hand, the longitudinal resistivity irreversibly and gradually increased upon damage. Moreover, it decreased reversibly upon tension in every stress cycle and increased reversibly upon compression in every stress cycle. The through‐thickness resistivity was a better indicator of damage and strain than the longitudinal resistivity.     

CFRP拉挤板材的力阻效应

采用单向拉伸试验考察了碳纤维增强塑料(CFRP)拉挤板的电阻–应变响应规律。结果表明,纵向拉应变引起CFRP拉挤板纵向电阻的增大,其力阻效应的平均灵敏度为1.87,尺寸变化是其电阻变化的主要原因;CFRP拉挤板的力阻响应具有较好的线性,其非线性误差为±1.5%;循环加载过程中,CFRP拉挤板的力阻响应平稳,重复性误差为±4.3%;经过一个周期的加载–卸载试验后,CFRP拉挤板的电阻产生不可逆的增大,其迟滞误差为±4.8%。     

Electrical and piezoresistive properties of multi-walled carbon nanotube/polymer composite films aligned by an electric field

Aligned multi-walled carbon nanotube (MWCNT)/polymer composite films are prepared by solution casting in the presence of an alternating electric field. Application of 7 kV/m at a frequency of 60 Hz to the polymer composite melt induces MWCNT alignment in the direction of the applied field, which is maintained after polymer crystallization. The electrical conductivity and piezoresistive response of electric-field-aligned and randomly oriented 0.1–0.75 wt% MWCNT/polysulfone films are evaluated. Electrical conductivity is 3–5 orders of magnitude higher for composites with electric-field-aligned MWCNTs than for randomly oriented composites. MWCNT alignment inside the polymer matrix also increases the film piezoresistive sensitivity, enhancing the strain sensing capabilities of the composite film.     

Evaluation of different conductive nanostructured particles as filler in smart piezoresistive composites

ABSTRACT: This work presents a comparison between three piezoresistive composite materials based on nanostructured conductive fillers in a polydimethylsiloxane insulating elastomeric matrix for sensing applications. Without any mechanical deformation upon an applied bias, the prepared composites present an insulating electric behavior, while, when subjected to mechanical load, the electric resistance is reduced exponentially. Three different metal fillers were tested: commercial nickel and copper spiky-particles and synthesized highly-pointed gold nanostars. These particles were chosen because of their high electrical conductivity and especially for the presence of nanosized sharp tips on their surface. These features generate an enhancement of the local electric field increasing the tunneling probability between the particles. Different figures of merit concerning the morphology of the fillers were evaluated and correlated with the corresponding functional response of the composite.     

Poly(methyl methacrylate)‐modified vinyl ester thermosets: Morphology,volume shrinkage,and mechanical properties

Thermoset materials obtained from styrene/vinyl ester resins of different molecular weights modified with poly(methyl methacrylate) (PMMA) were prepared and studied. Scanning electron microscopy and transmission electron microscopy micrographs of the fracture surfaces allowed the determination of a two‐phase morphology of the modified networks. Depending on the molecular weight of the vinyl ester oligomer, the initial content of the PMMA additive, and the selected curing temperature, different morphologies were obtained, including the dispersion of thermoplastic‐rich particles in a thermoset‐rich matrix, cocontinuous structures, and the dispersion of thermoset‐rich particles in a thermoplastic‐rich matrix (phase‐inverted structure). Density measurements were performed to determine the effect of the PMMA‐modifier concentration and curing temperature on the volume shrinkage of the final materials. The development of cocontinuous or thermoplastic‐rich matrices was not too effective in controlling the volume shrinkage of the studied vinyl ester systems. The evaluation of the dynamic mechanical behavior, flexural modulus, compressive yield stress, and fracture toughness showed that the addition of PMMA increased the fracture resistance without significantly compromising the thermal or mechanical properties of the vinyl ester networks. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Effect of external loads on damage detection of rubber‐toughened nanocomposites using carbon nanotubes sensory network

Multiwalled carbon nanotubes of 0.2% weight fraction are used as a sensory network for detecting and characterizing the damage of particulate epoxy composites under shear loading conditions. Three different weight fractions of carboxyl‐terminated butadiene acrylonitrile copolymer rubber [10 parts per hundred of epoxy resin (phr), 20 phr, and 30 phr] are used for toughening a thermoset epoxy composite. The electrical response of the specimens is measured, nearest the central shearing plane, using a four‐circumferential ring probe technique in conjunction with a high‐resolution data acquisition system. A collection of the electromechanical response results are reported with respect to the shear strain. The resistance changes observed under shear loading are related to nonlinear deformation mechanisms, void initiation, and growth around rubber particulate. With increasing rubber content, the strength of the material decreases and a greater drop in resistance is recorded as a result of decreased distance between neighboring carbon nanotubes (CNTs) due to declustering and straightening of molecular chains of host matrix. In the end, a comparison for 30 phr composites under shear loading with that of tensile and compression loading conditions is presented. For initial deformation, there is no change in resistance under shear loading condition; however, the significant resistance change can be noticed under both tension and compression. The specimen under shear loading conditions experiences smaller decrease in resistance when compared with both tension and compression. However, the decrease in resistance is higher for compression due to higher decrease in distance between neighboring CNTs. POLYM. COMPOS., 37:360–369, 2016. © 2014 Society of Plastics Engineers     

Analysis of Asymmetrical Creep of a Ceramic at 1350°C by Digital Image Correlation

The glass industry requires the use of innovative ceramics that enable for long lifetimes. At very high temperatures, one of the key parameters for ceramics is their creep resistance. The characterization of the creep behavior, usually assessed through flexural tests, can be complex when an asymmetry appears between tension and compression. To detect and quantify such asymmetrical behaviors, the use of Digital Image Correlation (DIC) is proposed. First, several challenges are to be tackled for DIC at high temperature, namely, the random pattern stability, the radiation filtering and the heat haze. They are exacerbated by the limited possibilities to heat ceramics, the nonuniform strain fields and their low levels. Beyond several experimental developments, the strain uncertainties are decreased thanks to the use of two global approaches of DIC based on ad hoc finite‐element kinematics. Last, the proposed methodology is applied to the analysis of asymmetric creep at 1350°C of an industrial zircon ceramic designed for its high creep resistance.