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A detailed experimental study is performed for piezo resistance damage sensing on conductive glass fiber/epoxy composites under mode-I fracture conditions. The conductive composites are fabricated by homogeneously dispersing carbon nanotubes (CNTs) within the epoxy matrix and electro-flocking short carbon fibers onto the laminates along with a vacuum infusion process. A parametric study is done on the in-situ damage sensing properties by varying the carbon fiber lengths (150 µm and 350 µm) and the carbon fiber areal densities (500, 1000, 1500, and 2000 fibers/mm2). The change in resistance is captured with a four-point probe measuring methodology by measuring the resistance through the thickness of the composite. The crack initiation toughness value of the composites containing carbon fibers showed improvement over control composites. Composites containing 350 µm length carbon fibers and 2000 fiber/mm2 not only showed the best crack initiation toughness but also provided sensitive network for detecting crack growth.

Electrical and shear behaviour of electrically conductive glass fibre/epoxy composites is studied under interlaminar shear loading. A well-connected network is developed by dispersing carbo...

In this study, an experimental investigation has been performed to understand the electromagnetic interference-shielding effectiveness (EMI-SE) of glass fiber/epoxy laminated composites embedded with carbon nanotubes (CNTs) and Fe3O4 nanoparticles, reinforced with micro carbon fibers along the thickness direction. Micro carbon fibers were reinforced along the thickness direction between the laminates using an electro-flocking process and a vacuum infusion process used to fabricate the composites. The EMI-SE of the composites was measured in the X-band frequency range (8–12 GHz). The effect of carbon fibers of three different lengths (80 µm, 150 µm, and 350 µm) with two different fiber densities (1000 and 2000 fibers/mm2) and two different amounts of Fe3O4 nanoparticles (0.5 and 1 wt.%) on total SE, absorption, and reflection was investigated. Due to the synergetic effect of Fe3O4 nanoparticles, CNTs, and carbon fibers, the final EMI shielding of the composites was mainly dominated by the absorption process. The absorption was more pronounced in the composites of longer carbon fibers with improved electrical conductivity. The presence of Fe3O4 nanoparticles also enhanced total SE values with improved magnetic permeability. The composite with micro carbon fibers of 350 µm length and 2000 fibers/mm2 density with 1 wt.% of Fe3O4 nanoparticles showed the maximum value of total SE.

The electro-fracture studies on jute/epoxy composites are conducted for monitoring damage under mode-I interlaminar fracture loading condition. A well-connected electrical network is built inside t...

An experimental study was performed to compare the electro-mechanical response of three-dimensionally conductive woven carbon fiber/epoxy laminated composites under quasi-static uniaxial tensile and in-plane shear loading conditions. Three-dimensional (3D) electrical network was generated in these composites by embedding carbon nanotubes (CNTs) of 0.025 wt% in the epoxy and reinforcing short carbon fibers (150 μm and 350 μm long) with a fiber density of 1000 fibers/mm2 between the laminates using electro-flocking process. CNTs are effectively dispersed in the epoxy matrix using a combination of ultrasonication and shear mixing techniques. A compression molding fabrication technique was employed to fabricate composite materials. For all composite types, in general, the in-situ electrical response showed a trend of initial decrease in resistance and later increase in resistance for tensile loading. However, no noticeable increase in electrical resistance was observed until failure of the composite under shear loading conditions. Both CNTs and short carbon fibers made new contacts with neighboring carbon fiber laminates for entire duration of shear loading even though the composite was undergoing progressive failure.

A comprehensive experimental parametric study is performed to investigate the electro-mechanical response of novel three-dimensional conductive multi-functional carbon/epoxy composite materials. Three-dimensional conductive network is generated by embedding multi-wall carbon nanotubes in the epoxy matrix and reinforcing short carbon fibers between the carbon fabric laminates. An open mold compression method is utilized to fabricate the composite materials. Wet electro-up-flocking technology is employed to reinforce 150-µm and 350-µm length carbon fibers vertically at varying densities (500, 1000, 1500, 2000 fibers/mm2) between each laminate to analyze the effects these parameters have on electrical resistivity, tensile properties, and electro-mechanical response to quasi-static tensile loading. A high-resolution four-point circumferential ring probe is used to obtain electrical measurements. The resistivity of the composites having 150 µm flocked carbon fibers did not show significant change with increase in flock density; however, composites of 350-µm length carbon fibers showed a clear decrease in resistivity by a factor of 10 when the flock density increased from 500 to 2000 fibers/mm2. The electro-mechanical response of composites without short carbon fibers is inconsistent and jagged compared to that of flocked composites. The composites having 350-µm long carbon fibers showed a longer duration of initially decreasing resistance due to the applied tensile load when compared to that of 150 µm flocked carbon fibers. However, composites with no short carbon fibers registered maximum value of percentage change in resistance at the break point of tensile loading.

An experimental study is performed to investigate the electro-mechanical response of three-dimensionally conductive multi-functional glass fiber/epoxy laminated composites under quasi-static tensile loading. To generate a three-dimensional conductive network within the composites, multi-wall carbon nanotubes are embedded within the epoxy matrix and carbon fibers are reinforced between the glass fiber laminates using an electro-flocking technique. A combination of shear mixing and ultrasonication is employed to disperse carbon nanotubes inside the epoxy matrix. A vacuum infusion process is used to fabricate the laminated composites of two different carbon fiber lengths (150 µm and 350 µm) and four different carbon fiber densities (500, 1000, 1500, 2000 fibers/mm2). A four circumferential probe technique is employed to measure the in-situ electrical resistance of composites under tensile load. Although composites of both carbon fiber lengths showed significant decrease of sheet resistance under no mechanical load conditions, composites of 350 µm long carbon fibers showed the lowest resistivity of 10 Ω/sq. Unlike the resistance values, composites of 350 µm carbon fibers showed a significant decrease in Young’s modulus compared to 150 µm counterparts. For the electro-mechanical response, composites containing carbon fibers of 150 µm long demonstrated a maximum value of percentage change in resistance. These results were then compared to both 350 µm and no added carbon fibers under quasi-static tensile loading.

A comprehensive experimental investigation was performed to study the dynamic compressive constitutive response of novel auxetic Kevlar®/epoxy laminated composites. Strain rate response was investigated using the split Hopkinson pressure bar (SHPB) test setup. Laminated composites were fabricated using the vacuum infusion process. Short Nylon fibers were flocked between the laminates with different flock densities and flock length. For obtaining dynamic force equilibrium in SHPB experiments, a copper pulse shaper was used to increase the rising time of incident pulse. To have a comparison, woven Kevlar®/epoxy composites were also characterized at similar strain rates. In addition, quasi-static tests were also performed on both woven and auxetic laminated composites for completeness of the study. For quasi-static loading conditions, auxetic composites showed higher peak strain and lower peak stress compared to woven composites. For non-flocked composites, both auxetic and woven composites showed rate dependency. Woven composites provided 353% increase in peak stress when the strain rate increased from 1200 s−1 (low) to 3300 s−1 (high). However, in the same conditions, auxetic composites showed only 155% increase in peak stress. For different flocking conditions, woven composites showed rate dependency for all strain rates, but auxetic composites demonstrated rate dependency only from low to medium strain rates. Both auxetic and woven composites experienced shear failure under quasi-static compression, where auxetic composites failed at higher shear angle of 37°, but woven composites had a failure angle of 30°. For impact loads, under no flocking condition, woven composites did undergo severe edge failure at all strain rates, but auxetic composites showed a sign of edge failure only at high strain rates. With the flocking condition, auxetic composites had through thickness shear failure and woven composites experienced splitting and fibrillation of Kevlar® fibers.

Numerical studies were performed to determine electrical contacts in carbon nanotubes (CNTs)-embedded epoxy under quasi-static tensile loading conditions. A realistic CNT network in epoxy was generated by random orientation and waviness by employing the Monte Carlo method. Two kinds of contacts between CNTs, namely Type-I (overlap) and Type-II (in-plane), were considered in generating a conductive network. A commercial finite element package, COMSOL, was used to determine the distance between these contacts and variation of these contacts during elastic deformation under tensile loading. Numerical predictions were later compared with the analytical results derived from experimental findings. The effect of initial f-ratio and the amount of waviness of CNTs on the variation of f-ratio (which is determined as a ratio of Type-II contacts to total contacts) was investigated as a function of axial strain. The initial f-ratio as well as the extent of waviness showed a significant effect on the change in the type of contacts with increasing axial strain values.

Experimental study was performed to investigate fracture and impact properties of novel Auxetic Kevlar® laminated composites. For comparison, standard Kevlar® woven composites with and without polyurethane treatment were also considered in this study. For these three composites, short nylon fibers of two different fiber lengths and three different fiber densities were flocked between laminates. Vacuum infusion process along with optimized compaction was employed to fabricate composites. The double cantilever beam configuration was used to investigate the fracture properties. The Auxetic Kevlar® composites showed a significant improvement of 225% in fracture toughness compared to regular woven Kevlar® composites. Furthermore, the initiation toughness was increased by 577% with the application of flocking in Auxetic Kevlar®. During impact testing, the Auxetic Kevlar® reinforced composites showed a significant reduction in damaged area compared to woven counterpart. On the other hand, the reduction in damaged area influenced the reduction in impact energy absorption.

Natural fibers are lighter, less expensive, and less abrasive than synthetic fibers such as glass or carbon, and provide a resource that is both renewable and biodegradable. This study uses surface treatments and flock fiber based Z-axis reinforcement technology to improve the strength and fracture toughness properties of natural fiber composites (NFCs), thus increasing their ability to bear loads. The enhancement of performance properties will expand applications of NFCs in industries such as automotive and housing/construction industries. NFCs will also provide a more sustainable, environmentally friendly, and inexpensive structural material alternative over traditional resin reinforced glass or carbon fiber composite materials.

A detailed experimental study was performed on the piezo-resistive damage sensing of novel electrically conductive inter/intra glass/carbon fiber hybrid epoxy laminated composites reinforced with micro carbon fibers along the thickness direction between the laminates using electro-flocking technique. The electrical resistance is determined by four-circumferential probe measurements under tensile and flexure loads. Intralaminar hybrid composites’ mechanical and electrical properties showed a dependency on layup angle, with glass fibers along the loading direction showed the highest initial resistance value (1180 Ω) and ultimate tensile strength (400 MPa). Intra-ply composites with [±45/∓45] layup orientation demonstrated the lowest ultimate tensile strength (57 MPa) and the highest elongation at break (9.7%). Mechanical and electrical properties of interlaminar hybrid composites showed a dependency on the number of carbon fiber laminates. More carbon fiber laminates within the composite layup produces a lower initial resistance value (0.37 Ω) and higher ultimate tensile strength (247 MPa). In both intra and inter composites cases, with the addition of short carbon fibers, the initial resistance decreased. In some cases, with the addition of short carbon fibers the failure mechanisms changed. Composites of all types experienced much higher percentage change in resistance under flexural loading due to severe interlaminar failure compared to corresponding cases under tensile loading conditions. These results are useful for designing composite structures with emphasis on structural health monitoring.

Damage monitoring of conductive glass fiber/epoxy laminated composites is performed by observing the electro-fracture response under dynamic mixed-mode fracture loading. A three-dimensional conductive network is generated in these composites by embedding carbon nanotubes (CNTs) in the matrix and reinforcing short carbon fibers (80 µm, 150 µm, and 350 µm) between the laminates. A modified split Hopkinson pressure bar setup is used to load the mixed-mode open notch flexure (MONF) specimens. A modified four-point probe technique coupled with a high-frequency data acquisition system is employed to capture the electrical response within process zone of the fracture specimens. A high speed video camera is used to capture the dynamic crack propagation. Due to the bending effect, a decreasing trend in resistance change is observed before the damage initiation in composites with no carbon fibers. However, short carbon fiber reinforced composites show a minimal change in resistance before the crack initiation due to less amount of CNTs in them and moreover short carbon fibers did not change their position during elastic deformation. After crack initiation, the rate of increase of resistance is almost five times higher in composites containing short carbon fibers of 80 µm than to those of 150 µm.

Theoretical modeling and experimental validation for electro-shear behavior of carbon nanotubes (CNTs) embedded epoxy under quasi-static shear loading are performed. The theoretical modeling incorporates electron tunneling between CNTs and the electrical resistance change due to shear deformation of the epoxy matrix between CNTs. Two different contact types (head to head and overlap) and three different deformation connections (Type-I, Type-II, and Type-III) are considered to model the electrical network. Using the constitutive relations, the model relates the matrix deformation between the CNTs to the macroscale deformation of the composite. To validate the modeling results, experiments are performed using a four circumferential ring probe method. A parametric study is performed to investigate the effect of the total number of initial contact types, ratio of contact types, and number of potential Type-II deformation connections on electrical response. The theoretical predictions are validated with experiments for the first two stages of deformation and an optimal combination of deformation connections are obtained to have a close match with experimental findings.

Experiments are performed to understand the electro-bending behavior of conductive jute/epoxy curved composites for monitoring of damage under four-point bending. A well-connected electrical network in the laminated curved composite is obtained by wet flocking micro carbon fibers between laminates and dispersing carbon nanotubes (CNTs) within the matrix. By embedding only 0.1 wt% of CNTs into the composites, the effects of carbon fiber length and density on electro-bending behavior is studied. The normalized curved beam strength (curved beam strength/thickness) of composites marginally increased for a case of carbon fiber length of 350 µm and fiber density of 500 fibers/mm2 compared to composites with no carbon fibers, however for all other cases, it showed slight decrease. The electro-bending behavior of composites of carbon fiber length of 150 µm showed higher percentage resistance change both at crack initiation point as well as at failure of specimen compared to those of carbon fiber length of 350 µm. The longer fiber (350 µm) composites provided rich conductive network and formed new connections to compensate the increase in resistance due to damage growth.

An experimental study was conducted to detect the onset of damage in epoxy embedded carbon nanotubes (CNTs) using a novel combination of in situ electrical resistance and acoustic emission ...

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., 2014. © 2014 Society of Plastics Engineers

This study examines the effects of preform architecture, surface treatment, and z-directional micro-fiber reinforcement on the interlaminar fracture performance of jute/epoxy laminated composites. Laminated composites were fabricated using an optimized vacuum infusion process. Fracture characterization studies showed surface treatments increased fracture toughness as a result of improved interfacial adhesion. Unidirectional preforms were found to decrease fracture toughness as compared to plain weave preforms due to reduced inter-ply interaction. The addition of z-direction reinforcement increased the composites’ Mode-I fracture toughness by 80% and increased interlaminar shear strength (ILSS) at the cost of decreased tensile strength and modulus.

Low fiber volume fraction and uncertain void content are continuing problems in natural fiber composites with continuous reinforcement. This study addresses these problems through the optimization of the Vacuum Infusion (VI) fabrication process for use with jute fabric reinforcement. The effects of reinforcement weave architecture, surface treatment, and fabrication method on the performance of jute fiber/epoxy laminated composites are subsequently characterized. Alkali and silane surface treatments were applied to enhance the fiber/matrix bonding. Wicking tests showed silane coupling produced jute yarn with zero water wicking and increased epoxy wicking. Extension of this treatment to reinforcing fabrics significantly reduced the moisture absorption of jute/epoxy composites. Constituent analysis showed traditional hand-layup fabrication method results in composites with unacceptably high void content. For this reason, the VI method was employed and optimized. The addition of pre-compaction and deadweight compaction steps resulted in composites having a near-zero void content and a very good fiber volume fraction. Mechanical testing showed that the studied surface treatments improved tensile modulus in jute/epoxy composites, regardless of reinforcement architecture. Unidirectional preforms increased tensile strength and modulus due to reduced fabric crimp. POLYM. COMPOS., 35:310–317, 2014. © 2013 Society of Plastics Engineers

A detailed experimental study is conducted using multi-walled carbon nanotubes (MWCNTs) as a sensory network to understand damage initiation and growth within the process zone of epoxy particulate composites under opening mode fracture loading. Thermoset epoxy resin composites are toughened with three weight fractions of carboxyl-terminated butadiene acrylonitrile copolymer (CTBN) rubber (10 phr, 20 phr and 30 phr). Single-edge notch tension (SENT) geometry and far-field tensile loading conditions are used for this study. A modified four-point probe methodology coupled with a high-resolution data acquisition system is employed to capture the electrical response within the process zone of fracture test specimens. For each composite, the damage initiation and growth associated with linear and non-linear deformation is analyzed and discussed with respect to the change in crack tip opening displacement (CTOD). Although the electrical response did not differ qualitatively with the addition of both 10 phr and 20 phr rubber to epoxy, addition of 30 phr of rubber showed a significantly different response both qualitatively and quantitatively. This is the result of several mechanisms such as disentanglement and stretching of epoxy matrix molecules, micro-crack blunting, debonding of rubber particles, void growth and propagation.

Nano-characterization studies using atomic force microscopy (AFM) was conducted to study the effect of accelerated ultra violet (UV) and thermal degradation on mechanical properties of Nylon textile fibers. Nylon fibers were exposed to UV radiation for six different hours using Q-UV Panel Weatherometer. The exposed fibers were molded in an epoxy plug for nanaoindentation using AFM. Progressive nanoindentation from surface to the center of the fiber was used to investigate the effect of degradation on gradation of Young’s modulus across fiber cross-section. it was identified that UV degradation decreases the Young’s modulus from center to surface of the fibers up to 144 hours of exposure. Reduction of Young’s modules at surface was greater than the center implying more deterioration at the surface. To investigate thermal degradation effect on Nylon fibers, the fibers were exposed to 175oC for four weeks. initial experimentation indicates that the thermal exposure causes gradual reduction of Young’s modules. Wide angle x-ray spectroscopy (WAXS) and Fourier Transform infra Red Spectroscopy (FTIR) are used to correlate the fiber chemical and micro-structured changes to the variation of nano-mechanical properties.

A series of dynamic compressive experiments were performed to experimentally investigate the electrical response of multi-wall carbon nanotube (CNT)-reinforced epoxy nanocomposites subjected to split Hopkinson pressure bar (SHPB) loading. Low-resistance CNT/epoxy specimens were fabricated using a combination of shear mixing and ultrasonication. Utilizing the CNT network within, the electrical resistance of the nanocomposite was monitored using a high-resolution four-point probe method during each compressive loading event. In addition, real-time deformation images were captured using high-speed photography. The percent change in resistance was correlated to both strain and real-time damage. The results were then compared to previous work conducted by the authors (quasi-static and drop weight impact) in order to elucidate the strain rate sensitivity on the electrical behavior of the material. Furthermore, the percent change in conductivity was determined using a Taylor expansion model to investigate the electrical response based on both dimensional change as well as resistivity change during mechanical loading within the elastic regime. Experimental findings indicate that the electrical resistance is a function of both the strain and deformation mechanisms induced by the loading. The bulk electrical resistance of the nanocomposites exhibited an overall decrease of 40–65% and 115–120% during quasi-static/drop weight and SHPB experiments, respectively.

A comprehensive experimental study is performed to investigate the electro-mechanical response of jute/epoxy composites for in situ damage detection under in-plane shear loading conditions. By embedding carbon nanotubes (CNTs) in epoxy matrix and also reinforcing carbon fibers (CFs) using electro-flocking process between the jute laminates, a three-dimensional sensory network is generated inside the composites. A parametric study is conducted to examine the effect of two different weight percentages of CNTs (0.025% and 0.1%), two different CFs lengths (150 and 350 μm), four different CF flock densities (500, 1000, 1500, and 2000 fibers mm−2), and two different laminate orientations ((0-0-0-0)T and (0-90-0-90)T) on the generation of electrical conductive network and its role on damage detection. The electrical resistance values are measured using four circumferential ring probes measurement system. Results indicated that (0-0-0-0)T orientation showed 400% improvement in interlaminar shear strength compared to (0-90-0-90)T composites. CF flocking between the laminates experienced maximum increase of 50% in shear strength compared to non-flocked composites. For all composite types, there is no change in resistance during the elastic deformation of shear loading. Beyond elastic deformation, the increase in CF length and its density showed similar electrical response. The increase in CNTs concentration and laminate orientations did not show significant change in electrical response under shear loading conditions.

Alkali and Silane surface treatments based off of work published previously (J Appl Polym Sci 71(4):623–629, 1999; Polym Eng Sci 49(7):1253–1272, 2009; Mater Sci Eng A 508(1–2):247–252, 2009) were applied to plain weave and unidirectional jute fabric to improve epoxy compatibility and reduce moisture affinity. Efficacy of treatments was proven with the use of wicking tests. Studies performed on the hand-layup method showed unacceptable void content. Laminated jute/epoxy composites were fabricated using Vacuum Infusion to create void-free samples. This process was improved through optimization for use with jute and the addition of a pre-compaction step to increase fiber volume fraction from 25 % to a maximum of 40 %. Mechanical testing on composites fabricated with raw and treated jute fabrics showed a 300 % increase in elastic modulus for treated jute fabric over neat epoxy resin. Moisture absorption testing (ASTM D570) showed a significant improvement in moisture resistance for silane-treated fabrics.

A detailed experimental study is conducted to understand damage initiation and growth in epoxy particulate composites using a multi-wall carbon nanotube (MWCNTs) conductive network under two different loading conditions: (a) quasi-static shear and (b) fracture. Two different particulates (a) Cenospheres (aluminum silicate hollow spheres), and (b) carboxyl-terminated butadiene acrylonitrile copolymer (CTBN) rubber of three different volume fractions (10%, 20% and 30%) and mass fractions (10phr, 20phr and 30phr) respectively are used in thermoset epoxy resin composites. First, MWCNTs are well dispersed in an epoxy matrix using ultrasonication, and later the above particulates are added and shear-mixed into the solution to prepare composites. A v-notch rail shear specimen configuration for shear experiments, and single edge notch tension (SENT) configuration for fracture are considered in this experimental study. A four-point probe methodology along with high-resolution data acquisition is employed to capture electrical-resistance response of network changes associated with non-linear deformation, damage initiation and growth within composites under said loading conditions. It is identified from experiments that the electrical response associated with the above mechanisms is quite different with the addition of particulates compared to that of epoxy with no particulate.

We describe a new class of hydrogels based on fiber reinforced hydrogel composites with a cartilage-like structure. In analogy to the spinning of a spider web, a pultrusion system is developed to spin micron -diameter fibers from polymer solution in order to build three dimensional patterned fibrous structures. Impregnating the fibrous construct with epoxy -amine hydrogel forms fiber -reinforced hydrogel composites. The fibrous construct improves the strength, modulus and toughness of the hydrogel and also const rains the swelling. By altering the construct geometry (fiber diameter, density and polymer solution) and studying the effect on mechanical properties we will develop the understanding needed to design strong hydrogels for biomedical devices and soft machines.

We report a novel method for synthesizing electrically conductive nano-brush (CNB) by physical grafting of organic conducting polymers on carbon nano tubes (CNT). The objective for this synthesis is to produce nano tubes having a CNT stem coated with flexible electronic conducting polymers. The nano-brush is to be blended into common polymers (e.g. epoxy, polyurethane, and poly(vinyl alcohol)) to form electrically conductive composite material. The flexible organic conducting polymer in CNB is a potentially sensitive electronic probe for mechanically induced nano-deformation of the composite because it is molecularly entangled with the host polymers. The electrical networks of CNB embedded in polymeric composites are potentially useful as in situ sensors for monitoring material deformation with an unprecedented level of sensitivity. The composite material is “smart” in the sense that it self-reports the structural “health” before load induced material failure.Our method for grafting conducting polymer does not require chemical reactions with the surface atoms of the carbon nano tube. We used physical adsorption to graft electronic conducting polymer to CNT. We first synthesized a water-soluble electronic conducting polymer which is a molecular complex between poly(acrylic acid) and polyaniline. The CNT solid were dispersed and un-bundled by sonnication in the conducting polymer solution. Due to the high affinity between CNT and the conducting polymer, the surface of CNT can be fully covered with the conducting polymer. The experimental data is consistent with a structure of nano brush with high density of conducting polymers grafted on the CNT surface.

Two different resistance measurement techniques are used in an epoxy material reinforced separately with carbon black (CB) micro-particles and carbon nano-tubes (CNTs) to evaluate the effectiveness of both the techniques and type of reinforcement on damage detection under uni-axial tensile loading. Two techniques, namely traditional four-point probe (FPP) and fourcircumferential ring probe (FCRP) are employed and a constant current is applied through outer probes. The resulting voltage drop between inner probes is measured using a commercial high resolution electrometer based system. Since current density distribution in both techniques is different, the measured change in resistance (both qualitatively and quantitatively) is also different. In addition to change in current density due to different techniques, the size of conductive reinforcement also has significant impact on both current distribution and further change in resistance. CB reinforced epoxy showed very high percentage change in resistance against CNTs reinforced epoxy for both techniques. It was identified that CNTs reinforced epoxy showed no significant difference for both FPP and FCRP methods. However, for CB reinforced epoxy, significant difference in percentage change in resistance was observed for both resistance measurement methods.

Quasi-static and dynamic compressive behavior of three different types of hydrogels used for soft tissue applications are tested using a modified split Hopkinson pressure bar. Three kinds of hydrogels: (a) Epoxy hydrogels, (b) Epoxy hydrogels reinforced with definite orientation of three-dimensional polyurethane fibers and (c) fumed silica nano particles reinforced hydrogels with different weight fractions are considered in this study. Swellability of the all the hydrogels considered are studied and controlled by mixing different ratios of jeffamines and epoxides. The three dimensional pattern of the fibers are generated by a rapid robo-casting technique. Split Hopkinson pressure bar (SHPB) was used for dynamic loading and a pulse shaping technique was used to increase the rising time of the incident pulse to obtain dynamic stress equilibrium. A novel liquid environment technique was implemented to observe the dynamic behavior of hydrogels when immersed in water. Experiments were carried out at dynamic loading conditions for different strain rates with and without water environment. Results show that the hydrogels are rate sensitive. Also the yield strength of hydrogels decreased and elongation percent increased when they were immersed in water.

An experimental investigation was conducted to study the effect of quasi-static and dynamic compressive loading on the electrical response of multi-wall carbon nanotube (MWCNT) reinforced epoxy nanocomposites. An In-situ polymerization process using both a shear mixer and an ultrasonic processor were employed to fabricate the nanocomposite material. The fabrication process parameters and the optimum weight fraction of MWCNTs for generating a well-dispersed percolation network were first determined. Absolute resistance values were measured with a high-resolution four-point probe method for both quasi-static and dynamic loading. In addition to measuring the percentage change in electrical resistance, real-time damage was captured using high-speed photography. The real-time damage was correlated to both load and percentage change in resistance profiles. The experimental findings indicate that the bulk electrical resistance of the nanocomposites under both quasi-static and dynamic loading conditions initially decreased between 40% - 60% during compression and then increased as damage initiated and propagated.

Damage sensing of epoxy particulate composites was investigated using multi-wall carbon nanotubes (MWCNTs) under quasi-static uniaxial tensile loading. Two types of particulates, namely (a) aluminum silicate hollow microspheres (cenospheres), and (b) liquid carboxyl-terminated butadiene acrylonitrile (CTBN) rubber were considered in this study. The influence of three different volume fractions of cenospheres (10%, 20% and 30%) and three different weight fractions of CTBN resin (10, 20 and 30 phr) on the electromechanical response was studied. A four-circumferential ring probe (FCRP) technique was employed to measure the electrical resistance of the test specimen as a function of the axial strain. The resistance–strain curve is compared with a simultaneously measured mechanical stress–strain curve. The electromechanical measurement show additional stages of material deformation not readily revealed from the mechanical stress–strain curve. The resistance change associated with the unfolding of entangled polymer chains and further straightening of the polymer chains decreased the distance between CNTs, causing improved electron hopping in all composites except 30% cenospheres composite. The U-shaped electrical response demonstrated by both 20 and 30 phr rubber composites exploited the CNT sensory network successfully by providing early warning of composite failure due to micro-crack propagation which resulted in breaking of the CNT network.