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.     

Consolidation of carbon fiber laminae during polymer‐matrix composite fabrication,studied by electrical resistance measurement

The consolidation of carbon fiber epoxy‐matrix laminae during composite fabrication by lamination (involving consolidation, curing and cooling) was found to hastened and to occur to a greater extent by increasing the pressure. The consequence of better consolidation remained after curing and subsequent cooling. A higher pressure resulted in a lower throughthickness resistivity in a composite after curing and cooling. The extent of consolidation was quantitatively described by N/N_o (the ratio of the number of fiber‐fiber contacts between adjacent laminae at a given time, divided by that at the start of consolidation), which was calculated from the measured through‐thickness electrical resistivity. From the variation of N/N_o during heating in the process of consolidation, three stages of consolidation were observed. In the first stage, N/N_o increased very gradually; in the second stage, N/N_o increased abruptly; in the third stage, N/N_o increased at a moderate rate.     

Thermoplastic matrix phase transitions in a carbon fiber composite,studied by contact electrical resistivity measurement of the interface between two unbonded laminae

Measurement of the contact electrical resistivity of the interface between two unbonded laminae of a continuous carbon fiber thermoplastic (nylon-6) matrix composite during heating provides a method of thermal analysis that is sensitive to the glass transition and melting of the thermoplastic matrix. The phase transitions result in peaks in the resistivity, due to matrix molecular movement.     

Interlaminar damage in carbon fiber polymer-matrix composites, studied by electrical resistance measurement

Interlaminar thermal damage in continuous carbon fiber polymer-matrix composites was monitored in real time during thermal cycling by measurement of the contact electrical resistivity of the interlaminar interface. Damage was accompanied by an abrupt increase of the resistivity for a thermoset-matrix composite, and by an abrupt decrease of the resistivity for a thermoplastic-matrix composite. Both phenomena are due to the effect of matrix damage on the chance of fibers of one lamina touching those of an adjacent lamina. The damage involved matrix molecular movement in the thermoplastic case, but not in the thermoset case.     

Real‐Time damage detection in thermoplastic‐based composite materials with embedded multi‐mode optical fiber sensors

In this paper we describe the use of an inexpensive standard communication‐grade optical fiber for the purpose of detecting damage in glass fiber reinforced thermoplastic composite materials and hybrid fiber metal laminates based on the same composite constituent. These multi‐mode, step‐index optical fibers were embedded at different locations within the composite structure to assess their ability for structural health monitoring. In this study, the specimens were loaded under a three‐point bend test configuration, Results from the investigation have shown that these optical fibers are capable of detecting the peak failure load in the host material when embedded at locations where structural failure initiates at peak load. The output from the optical fibers was shown to be sensitive to the applied loading conditions suggesting that they can also be used to monitor structural loading. Optical micrographs of selected specimens were obtained by sectioning the specimens at the fracture location to provide confirmation of the optical fiber results.     

Mechanical properties and dynamic mechanical analysis of thermoplastic‐natural‐rubber‐reinforced short carbon fiber and kenaf fiber hybrid composites

The hybridization of thermoplastic natural rubber based on carbon fiber (CF) and kenaf fiber (KF) was investigated for its mechanical and thermal properties. Hybrid composites were fabricated with a melt‐blending method in an internal mixer. Samples with overall fiber contents of 5, 10, 15, and 20 vol % were subjected to flexural testing, and samples with up to 30% fiber content were subjected to impact testing. For flexural testing, generally, the strength and modulus increased up to 15 vol % and then declined. However, for impact testing, higher fiber contents resulted in an increment in strength in both treated and untreated composites. Thermal analysis was carried out by means of dynamic mechanical analysis on composites with 15 vol % fiber content with fractions of CF to KF of 100/0, 70/30, 50/50, 30/70, and 0/100. Generally, the storage modulus, loss modulus, and tan δ for the untreated hybrid composite were more consistent and better than those of the treated hybrid composites. The glass‐transition temperature of the treated hybrid composite was slightly lower than that of the untreated composite, which indicated poor damping properties. A scanning electron micrograph of the fracture surface of the treated hybrid composite gave insight into the damping characteristics. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Piezoresistivity in continuous carbon fiber polymer‐matrix composite

Piezoresistivity involving the volume resistivity of a continuous unidirectional carbon fiber epoxy‐matrix composite in the fiber direction decreasing reversibly upon tension in the fiber direction was observed by the four‐probe method, due to an increase in the degree of fiber alignment. Use of the two‐probe method resulted in measurement of the contact resistance rather than the volume resistance. The contact resistance increased reversibly upon tension.     

Continuous carbon fiber polymer‐matrix composites and their joints,studied by electrical measurements

Continuous carbon fiber polymer‐matrix composites and their Joints, as studied by DC electrical measurements, are reviewed. The resistance gives information on the microstructure and allows the self‐sensing of strain, damage and temperature. In the case of composites with dissimilar fibers in adjacent laminae, the Seebeck effect allows temperature sensing, using the interface between laminae as a thermocouple junction. The resistance in the through‐thickness direction can be apparently negative, due to entropy‐driven electron backflow. The longitudinal resistance allows sensing of the glass transition and melting of the thermoplastic polymer matrix. The quality of composite‐composite joints obtained by adhesion or fastening, and of composite‐concrete joints obtained by adhesion, is revealed by resistance measurements.     

Sensing delamination in a carbon fiber polymer-matrix composite during fatigue by electrical resistance measurement

Delamination in a crossply [0/90] continuous carbon fiber polymer-matrix composite was sensed in real time during fatigue by measuring the electrical resistance of the composite in the through-thickness direction. Upon 0° tension-tension fatigue at a maximum stress of 57% of the fracture stress, the resistance irreversibly increased both in spurts and continuously, because of delamination, which started at 33% of the fatigue life. The resistance increased upon loading and decreased upon subsequent unloading in every cycle, thereby allowing strain sensing. The minimum resistance at the end of a cycle irreversibly increased during the first 0.1% of the fatigue life. The resistance became noisy starting at 62% of the fatigue life, at which delamination occurred rapidly and the fraction of laminate area delaminated reached 4.3%.     

Glass transition and melting behavior of carbon fiber reinforced thermoplastic composite,studied by electrical resistance measurement

DC electrical resistance measurement was applied to investigate the glass transition and melting behavior of carbon fiber reinforced nylon‐6 composite. The electrical resistance exhibited temperature dependencies that were attributed to the matrix molecular movements associated with structural transitions. The electrical resistance was affected by the degree of crystallinity and the thermal oxidative degradation, which were governed by the thermal history. The resistance results are consistent with differential scanning calorimetry (DSC) results. The resistance is more sensitive to the glass transition than DSC.     

Mechanical,electrical, and dielectric properties of polyvinylidene fluoride/short carbon fiber composites with low‐electrical percolation threshold

In this investigation, polyvinylidene fluoride (PVDF)/short carbon fiber (SCF) composites have been prepared by solution casting technique to enhance electrical and dielectric properties with very low‐electrical percolation threshold (0.5 phr SCF). The effect of SCF content on mechanical, thermal and morphological properties of the composites have also been investigated. The mechanical properties of the composites are found to reduce compared to neat PVDF due to poor polymer–filler interaction which can be concluded from FESEM micrographs showing poor bonding between PVDF and SCF. The PVDF/SCF composites exhibit either positive temperature coefficient effect of resistivity or negative temperature coefficient effect of resistivity depending on the loading of SCF in the polymer matrix. The change in conductivity during heating–cooling cycle for these composites shows electrical hysteresis along with electrical set. The melting point of the composites marginally increases with the increase in fiber loading in PVDF matrix as evidenced from DSC thermograms. X‐ray diffraction analysis reveals the crystallinity of PVDF decreases with the increase in SCF loading in matrix polymer. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39866.     

Effect of strain rate on cement mortar under compression, studied by electrical resistivity measurement

The electrical resistivity of cement mortar increased monotonically during compression. An increase in strain rate caused the resistivity at any strain level to decrease, in addition to causing the resistivity at failure to decrease. This means that the microstructural change that occurred continuously during loading decreased with increasing strain rate.     

Biaxial compression in carbon-fiber-reinforced mortar, sensed by electrical resistance measurement

The stress-sensing behavior of carbon-fiber-reinforced mortar under biaxial compression was found to be different with that under uniaxial compression. For both the case of uniaxial compression and the case of biaxial compression, with an increase in the compressive stress, the fractional change in resistance in any direction decreases gradually at first, and then increases with increasing stress. But the stress level at which the fractional change in resistance starts to increase, referred to as the critical stress in this paper, varies with the loading style and the direction in which the resistance is measured. The critical stress related to the biaxial compression and the stress direction is larger than that related to the uniaxial compression and the stress direction. But on the other hand, the critical stress related to the biaxial compression and the direction other than the two stress directions is less than that related to the uniaxial compression and the direction other than the stress direction. The piezoresistivity of carbon-fiber-reinforced mortar under biaxial compression is more sensitive than that under uniaxial compression.     

Effect of heating on the structure of carbon fiber polyphenylenesulfide-matrix composite,as studied by electrical resistance measurement

Heating and cooling were found to affect the structure of carbon fiber polyphenylenesulfide-matrix composites, as shown by their effects on the DC electrical resistance of the composite in the fiber direction. Matrix flow during first heating above T_g and thermal stress buildup during cooling in any thermal cycle caused the resistance to increase, whereas isothermal crystallization at 180°C caused the resistance to decrease. Thermal stress buildup was reversible upon thermal cycling. Prior annealing at 180°C diminished the matrix flow because of improved fiber-matrix bond, and it increased the matrix crystallinity. Annealing at 180°C for 15 h or more was sufficient to eliminate the matrix flow.     

Mechanical properties,electrical resistivity and piezoresistivity of carbon fibre-based self-sensing cementitious composites

Carbon fibre (CF) is one of the most effective materials in improving the conductivity of the composites by developing a conductive network within the matrix, which also enhanced the piezoresistivity behaviour of the cementitious composites and has a potential application for structural health monitoring. A systematic study of the effect of sizing condition and fibre length on the piezoresistivity behaviour of cementitious composites by adopting unsized CF with the length of 3, 6 and 12 mm, and desized CF of 6 and 12 mm as functional fillers. Each type of CF was added at four different weight fractions of 0.1, 0.3, 0.5 and 0.7% to determine the optimal fibre content. Electrical resistivity and piezoresistivity tests were conducted for samples before and after drying treatment to evaluate the effect of water content on electrical properties. Besides, fresh properties of the fresh mixture, mechanical properties and microstructure of the composites were also investigated. Results showed that unsized CF is more effective in enhancing composites flexural strength and reducing the electrical resistivity, which also showed a stronger bonding with the cement matrix and also demonstrated a better dispersive ability. In terms of piezoresistivity behaviour, for a given fibre length, desized CF showed higher sensitivity and repeatability compared to unsized CF; however, the signals showed more noise. The best piezoresistivity behaviour was obtained for composites containing 3 mm CF at 0.7 wt%, which showed a fractional change in resistivity (FCR) value of approximately 70%. An equation was developed, which can successfully describe the relationship between the FCR of cementitious composites containing CF and the applied external stress.     

Effects of the temperature,humidity, and stress on the interlaminar interface of carbon fiber polymer-matrix composites,studied by contact electrical resistivity measurement

The interlaminar interface (i.e., the interface between laminae) of continuous carbon fiber polymer-matrix structural composites was monitored in real time during dynamic changes in temperature, humidity and stress by measurement of the contact electrical resistivity of the interface. The stress was compressive, in the direction perpendicular to the interlaminar interface. Temperature, humidity and stress were all found to have reversible effects on the resistivity, due to the effect of temperature on the probability of the jump of an electron from one lamina to the adjacent one, and the effects of humidity and stress on the extent of contact between fibers of adjacent laminae. In addition, due to damage, temperature caused the resistivity to increase whereas stress caused the resistivity to decrease.     

碳纤维和钢渣水泥基复合材料导电性及损伤识别研究

在水泥基复合材料中参入碳纤维和钢渣,以研究复合水泥基材的导电性能,结果表明,碳纤维能够显著改善水泥基材的导电性,碳纤维体积率为1.2%时,其导电性能最佳,且碳纤维对试件导电稳定性有显著影响;在碳纤维水泥基材中参入钢渣并不会显著影响试件导电性,会显著影响导电稳定性;抗压和抗折试验结果表明利用导电性判断构件是否损伤(开裂)及损伤程度在试验上是可行的。     

The pull-out and failure of a fiber bundle in a carbon fiber reinforced carbon matrix composite

The interfacial failure is examined for a unidirectionally reinforced carbon fiber/carbon matrix composite. A novel tensile test is conducted which realizes the processes of interfacial debonding and subsequent pull-out of a fiber bundle from the surrounding composite medium. The critical stress at the onset of delamination cracking is related to the fracture energy (the critical energy release rate for mode II cracking). A force-balance equation of a fiber bundle, which is quasi-statically pulled-out of the composite socket, is formulated in terms of the inter- and intra-laminar shear strengths of the composite. This equation is successfully used to estimate the delamination crack length along the debonded fiber bundle, as a function of the stress applied to the bundle.     

Mechanical and electrical properties of aligned carbon nanotube/carbon matrix composites

To synthesize carbon nanotube/carbon matrix (CNT/C) composites rivaling or exceeding the mechanical and electrical properties of current carbon fiber/carbon matrix composites, it is essential to align carbon nanotubes in the composite. In this work, we fabricated CNT/polyacrylonitrile (PAN) precursor composites with high degree of CNT alignment, and carbonized and graphitized them at high temperatures. Carbonizing the precursor composites significantly improved their elastic modulus, strength, and electrical conductivity. The matrix was uniformly carbonized and highly graphitized. The excellent mechanical and electrical properties make the CNT/C composites promising for many high temperature aerospace applications.     

The electrical conductivity of polypropylene and nickel-coated carbon fiber composite

The effect of filler content, temperature, and frequency of the applied electric field on DC and AC electrical conductivities of composites of nickel-coated carbon fibers and polypropylene are considered. The impedance behavior and the dielectric properties of these composites were studied in the low frequency range 10 Hz–30 kHz. It was found that the volume electrical resistivity shows filler content and temperature dependence. The calculated activation energy of the thermal rate-process decreases with the filler concentration, while the shielding effectiveness increases. The overall observed permittivity of the composites increases with the filler concentration, and the dielectric behavior is discussed in terms of the space charge, electronic and interfacial polarization within the covered frequency range. It was observed that the AC conductivity is nearly independent of the frequency below 100 Hz and increases with frequency above this range. Finally, it was concluded that addition of nickel-coated carbon fibers could alter the electrical conduction mechanism and the polarization process of the polymeric matrix.