Preventing Fatal Fractures in Carbon-Fiber–Glass-Fiber-Reinforced Plastic Composites by Monitoring Change in Electrical Resistance

A method which monitors the changes in electrical resistance in CFGFRP (carbon-fiber–glass-fiber-reinforced plastics) composites was found to be a promising technique for foreseeing fractures and preventing fatal ones. CFGFRP composites containing two types of carbon fibers along with 31.6 vol% glass fibers were used. U-shaped copper electrodes were attached to both ends of the CFGFRP composites, using a conductive adhesive agent, and electrical resistance was measured via a two-thermal dc method. A strain gauge was attached onto a specimen to measure strain. On loading, the changes in electrical resistance increased linearly with increasing strain, and the value reached 38% for the CFGFRP composites containing 0.36 vol% PAN-based high-strength carbon fibers. A tremendous change in electrical resistance was seen for the CFGFRP composites containing 0.38 vol% pitch-based high-property carbon fibers, and the composites could maintain their shape owing to a hybrid effect after the carbon fibers fractured. The change in electrical resistance can be controlled through suitable selection of the type of carbon fibers according to their values of ultimate elongation. Permanent, residual electrical resistance was found to remain, and the changes in resistance were dependent on the maximum previous strain.     

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     

Improved strain sensing performance of glass fiber polymer composites with embedded pre-stretched polyvinyl alcohol–carbon nanotube fibers

Polyvinyl alcohol–carbon nanotube (PVA–CNT) fibers differing on their pre-stretching condition were embedded in glass fiber reinforced plastic (GFRP) composites and used as strain sensors for damage monitoring of the composite. Strain sensing of the composite was made by the in situ measurement of the embedded fiber’s electrical resistance change during the mechanical tests. Four glass fiber composite plates were manufactured; each one had embedded a different type of produced PVA–CNT fibers. The multi-functional materials were tested in monotonic tensile tests as well as in progressive damage accumulation tests. The electrical resistance readings of the PVA–CNT fibers were correlated with axial strain values, taking into account the induced damage of the composite. It has been demonstrated that increasing the fiber’s pre-stretching ratio, its electrical resistance response increases due to higher degree of the CNTs alignment in the PVA matrix. Higher fiber pre-stretching degree enables the better strain monitoring of the composite due to higher measured electrical resistance change values noticed for the same applied axial strain values. To this end, it enables for the better monitoring of the progressive damage accumulation inside the composite.     

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.     

Conductive coating on structural ceramics for strain detection utilizing electrical measurements

Strain detection in Al₂O₃ ceramics and glass plates was investigated by coating them with an electrically conducting composite (epoxy resin and needle-like SnO₂(Sb)-coated TiO₂ filler) and by measuring surface resistance during and after loading. By adding more than 6 vol%-filler, the composite became electrically conductive. Surface electrical resistance increased with increasing strain during loading, and the degree of electrical resistance change versus strain was larger when the filler volume fraction was close to the percolation critical volume fraction. In addition, when the specimens were cyclically loaded, residual electrical resistance was observed even after removing load. The value of the residual electrical resistance was dependent on the maximum strain under the stress applied. These results suggest that estimation of maximum strain is possible by measuring resistance of the composite formed on structural ceramics. Based on the results of microfracture observation, the effect of applied stress on the electrical resistance change of electroconductive composites is discussed. ©     

Through-thickness piezoresistivity in a carbon fiber polymer-matrix structural composite for electrical-resistance-based through-thickness strain sensing

Piezoresistivity (change of the volume electrical resistivity with strain) in continuous carbon fiber polymer-matrix structural composites allows electrical-resistance-based strain/stress sensing. Uniaxial through-thickness compression is encountered in fastening. As shown for a 24-lamina quasi-isotropic epoxy-matrix composite, compression results in (i) strain-induced reversible decreases in through-thickness and longitudinal volume resistivities, due to increase in the degree of through-thickness fiber–fiber contact, and (ii) minor-damage-induced irreversible changes in these resistivities, due to a microstructural change involving an irreversible through-thickness resistivity increase and an irreversible longitudinal resistivity decrease. The Poisson effect plays a minor role. The effects in the longitudinal resistivity are small compared to those in the through-thickness direction, but longitudinal resistance measurement is more practical. The through-thickness gage factor (reversible fractional change in resistance per unit strain) ranges from 2.6 to 5.1 and the reversible fractional change in through-thickness resistivity per unit through-thickness strain ranges from 1.5 to 4.0, both quantities decreasing with increasing strain magnitude from 0.19% to 0.73% due to the increasing irreversible effect. The irreversible fractional change in through-thickness resistivity per unit through-thickness strain ranges from −1.0 to −1.3 and is strain independent. The effects are consistent with the surface resistance changes previously reported for the same material under flexure.     

Electrical properties of laminated epoxy-carbon fiber composite

This work deals with the effect of temperature on the electrical properties of laminated epoxy composites containing 60% by volume of commercial unidirectional carbon fibers. The temperature was varied from 30°C to 120°C and the frequency range was from 10 Hz to 10 kHz. It is found that the impedance decreases with increasing temperature and is inversely proportional to the number of layers in the specimen. The calculated dielectric constants show a strong dependence on the frequency below 100 Hz, and attain relatively constant values for frequencies greater than 100 Hz. The decrease in impedance with increasing number of layers was explained in terms of the existence of electrical contacts and bridges both between and within the fibers, in the carbon fiber layers. The activation energy results indicate that there are two conduction mechanisms in this laminated composite.     

Polymer nanocomposites reinforced with multi‐walled carbon nanotubes for semiconducting layers of high‐voltage power cables

Polymer nanocomposites reinforced with multi‐walled carbon nanotubes (MWCNTs) have been newly introduced for semiconducting layers of high‐voltage electrical power cables. Homogeneity of the MWCNT‐reinforced polymer nanocomposites was achieved by solution mixing, and their mechanical, thermal and electrical properties were investigated depending on the type of polymer. By changing the polymer matrix, the volume resistance of the MWCNT‐reinforced polymer nanocomposites could be varied by more than four orders of magnitude. Through systematic experiments and analysis, two possible factors affecting the volume resistance were found. One is the degree of crystallinity of the polymer used and the other is the change of MWCNT morphology under strain. By increasing the degree of crystallinity above a certain level, the volume resistance linearly increased. The MWCNTs embedded in the nanocomposites gradually protruded through the surface on stretching the sample and reversibly returned back to the original positions at a relatively small strain (below 20%). Based on the criteria of tensile properties and volume resistance, a poly[ethylene‐co‐(ethyl acrylate)]/MWCNT nanocomposite was selected as the best candidate for the semiconducting layers of high‐voltage electrical power cables. Copyright © 2009 Society of Chemical Industry     

带有碳涂层的尼龙纤维增强混凝土的机敏性研究

采用PVD方法在尼龙表面沉积一层导电的碳薄膜，用带有碳层的尼龙纤维（0.4%-2.0%，按体积计增强混凝土，测试了该复合材料的应力-应变行为，结果表明：随着纤维的加入和含量的增加，混凝土的断裂从脆性断裂发展为准脆性断裂，进而发展为假延性断裂，产生假延性断裂的纤维最小体积分数为0.4%左右，在一次载菏和周期性载菏下，以电阻变化率为信号研究了该材料的机敏性，在弹性、非弹性和断裂区间有灵敏的响应；在弹性区间，信号稳定可塑，在非弹性区间，信号随材料中裂纹积累而不可逆增长；断裂时电阻值最大。     

Carbon-matrix composites with continuous glass fiber and carbon black for maximum strain sensing

Electrically conductive glass-fiber-reinforced polymer composites have been prepared by adding carbon black, and carbonization processes have been applied to the resulting matrices. The carbonized composites were found to show characteristic changes in resistance during cyclic tensile tests, in which the resistance increased in the loaded state was retained even after unloading. Pyrolysis temperature dependence of the residual phenomena was investigated in order to understand the effects of the carbonized matrix and the carbon black network. The residual behavior became more pronounced with increasing pyrolysis temperature until 500 °C, while that diminished over 600 °C. The thermal decomposition of the matrix was almost completed up to 500 °C, and the shrunk matrix coexisting with glass fibers had a residual tensile stress along the fiber direction. The matrix carbonized at higher than 600 °C showed an increase in conductivity, which disrupted the strain-sensitive percolation network and hence the resistance response. These results showed that irreversible change in the carbon black network under the internal tensile stress provided the residual phenomena.     

Effect of axial stretching on electrical resistivity of short carbon fibre and carbon black filled conductive rubber composites

The variation of electrical resistivity of carbon black and short carbon fibre (SCF) filled rubber composites was studied against the degree of strain at constant strain rate. It was found that both the degree of strain and strain rate affect the electrical resistivity of the composites. The change in resistivity against the strain and strain rate depends both on the concentration and the type of conductive filler. The incorporation of short carbon fibres (SCF) imparts higher conductivity to the composite than carbon black at the same level of loading. Composites filled with carbon black exhibit better mechanical properties than SCF filled composites. Electrical setting, ie a permanent change in electrical resistivity, was observed during extension–retraction cycles. A good correlation was found between the mechanical response and the electrical response towards strain sensitivity. The results of different experiments are discussed in the light of breakdown and formation of conductive networks in the filled rubber composites. © 2002 Society of Chemical Industry     

Acoustic emission and electrical resistance in SiC-based laminate ceramic composites tested under tensile loading

Acoustic emission and electrical resistance were monitored for SiC-based laminate composites while loaded in tension and correlated with damage sources. The ceramic matrix composites were composed of Hi-Nicalon Type S™ fibers, a boron-nitride interphase, and pre-impregnated (pre-preg) melt-infiltrated silicon/SiC matrix. Tensile load-unload-reload or tensile monotonic tests were performed to failure or to a predetermined strain condition. Some of the specimens were annealed which relieved some residual matrix compressive stress and enabled higher strains to failure. Differences in location, acoustic frequency and energy, and quantity of matrix cracking have been quantified for unidirectional and cross-ply type architectures. Consistent relationships were found for strain and matrix crack density with acoustic emission activity and the change in measured electrical resistance measured at either the peak stress or after unloading to a zero-stress state. Fiber breakage in the vicinity of composite failure was associated with high frequency, low energy acoustic events.     

Electrical properties of different types of carbon fiber reinforced plastics (CFRPs) and hybrid CFRPs

The electrical resistance (ER)–time, current–voltage and ER–temperature behaviors of carbon fiber reinforced plastics (CFRPs) reinforced with high strength (HS), medium modulus (MM), and high modulus (HM) carbon fibers were studied. At the same time, the electrical properties of hybrid CFRPs (HCFRPs) reinforced with more than one type of carbon fibers were also studied. It was shown that the value of the current influences the electrical stability and the ER measurement accuracy. In order to obtain an accurate ER measurement, the current should be larger than 1.5 mA. All the CFRPs and HCFRPs are characterized by linear current–voltage and Ohmic behaviors. A negative temperature coefficient (NTC) behavior is exhibited for all types of specimens. The NTCs of the CFRPs reinforced with HS, MM and HM carbon fibers are 0.038, 0.033 and 0.094, respectively. For the specimens of HCFRPs containing HM carbon fibers, the NTC becomes larger with increasing the volume fraction of HM carbon fibers. The good linear and reversible relationship between the change in ER and temperature indicates that the CFRPs and HCFRPs may be used as temperature sensors.     

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     

Carbon-Fiber-Reinforced Concrete as an Intrinsically Smart Concrete for Damage Assessment during Dynamic Loading

Concrete containing short carbon fibers (0.2–0.5 vol%) wasfound to be an intrinsically smart concrete that can sense elastic and inelastic deformation, as well as fracture. The signal provided is the change in electrical resistance, which is reversible for elastic deformation and irreversible for inelastic deformation and fracture. The presence of electrically conducting short fibers is necessary for the concrete to sense elastic or inelastic deformation, but the sensing of fracture does not require fibers. The fibers serve to bridge the cracks and provide a conduction path. The resistance increase is due to conducting fiber pullout in the elastic regime, conducting fiber breakage in the inelastic regime, and crack propagation at fracture.     

Improved conductivity of suspended carbon fibers through integration of C-MEMS and Electro-Mechanical Spinning technologies

Carbon microfibers suspended across carbon walls were fabricated by Electro-Mechanical Spinning and subsequent pyrolysis of a SU-8 based carbon precursor. The shrinkage and elongation of these polymer fibers during the pyrolysis process was observed to depend on the height of the supporting walls. We demonstrate that this shrinkage and elongation during pyrolysis strongly influences the resulting carbon electrical properties. Compared to fibers that retained their length during pyrolysis, conductivity was enhanced by a factor of seven after fibers were elongated four times their initial pre-pyrolysis length with a concurrent shrinkage of their diameter by half.     

长纤维增强聚合物注塑成型过程中三维网络结构的调控

以长玻纤增强聚丙烯(PP)注塑制品为研究对象,通过引入微、纳尺度第二增强相调控长纤维在注塑制品中的取向度,从而获得取向度较低的皮层结构,以诱导长纤维在注塑件空间内形成连续、均匀的三维网络结构,从而明显提高制品的力学性能.对不同尺度混杂填充长纤维注塑微观结构的比较分析结果表明:加入纳米尺度第二相碳纳米管(CNTs),在基...     

Effects of crosslinking reaction and extension strain on the electrical properties of silicone rubber/carbon nanofiller composites

Stretchable conductive silicone rubber (SR) composites are important in wearable electronic devices and the crosslinking of SR composites is necessary for their applications. But the effect of the crosslinking reaction on the electrical conductivity of SR composites is rarely reported. In this article, the effect of crosslinking reaction on the electrical conductivity of SR composites filled with conductive carbon black, carbon nanotubes, and graphene are studied. The crosslink density of SR composites increases with increasing curing time, but the electrical conductivity decreases sharply at the early stage of crosslinking, especially for SR/conductive carbon black composite, which is ascribed to the reaggregation of conductive nanofillers in SR during the crosslinking process. The elastic modulus of the three SR composites gradually increases while the elongation at break decreases with increasing curing time, and the SR/carbon black composite shows ultra-high elongation at break (1578%). In addition, SR/graphene composite is more sensitive to the extension strain than SR/carbon black and SR/carbon nanotubes composites, and its gauge factor is 414 at the strain ranges of 3–25%. This research work brings a new method to optimize the crosslinking structure of conductive SR composites.     

Introduction of carboxylic acid group to polypropylene fabric for battery separator

Carboxylic acid group was introduced by radiation-induced grafting of acrylic acid (AAc) onto a polypropylene (PP) nonwoven fabric for a battery separator. The AAc-grafted PP nonwoven fabric was characterized by IR, SEM, and TGA. The wetting speed, electrolyte retention, electrical resistance, and tensile strength were evaluated after grafting of AAc. It was found that the wetting speed, electrolyte retention, thickness, and ion-exchange capacity increased, whereas the electrical resistance decreased with increasing grafting yield. The elongation of AAc-grafted PP nonwoven fabric decreased with increasing grafting yield in dry state, whereas the elongation increased in wetting state.     

Effects of nucleating agent and processing conditions on the mechanical,thermal, and optical properties of biaxially oriented polypropylene films

The effects of nucleating agent, temperature of crystallization, and degree of machine direction (MD) orientation on the mechanical, optical, and thermal properties of biaxially oriented polypropylene (BOPP) films were investigated. Addition of nucleating agent improved only the initial tear resistance in the MD; however, the other mechanical and optical properties did not change appreciably. In the set of experiments in which the crystallization temperature was increased, the degree of crystallinity also increased. Thus, Young's modulus, yield stress, and tensile strength increased in both directions with higher degree of crystallinity. The yield strain did not change significantly, but the strain at break was higher. Although the initial tear resistance was smaller in both directions with increasing crystallization temperature, the tear propagation resistance did not change. In this case, haze and diffuse transmittance were slightly higher, but the total transmittance was constant. In the set of experiments in which the machine direction orientation was increased, the degree of crystallinity also became higher. Owing to the effects of higher crystallinity and higher MD orientation, the modulus, yield stress, and tensile strength increased, but the yield strain and tear propagation resistance did not significantly change in both directions.