K. Lawrence DeVries

Review of self-sensing of damage and interfacial evaluation using electrical resistance measurements in nano/micro carbon materials-reinforced composites

Nondestructive evaluation methods have been utilized to detect and to prevent structural damage in research and development. Most such detection methods used expensive external sensors to detect damage. This paper explores the use of a less expensive electrical resistance measurement method for damage and strain sensing resulting from electrical signal variations, induced by stresses or shape changes in conductive materials. This method of damage sensing was performed first on carbon fibers composites, and in this study, its use is extended to conductive nanoparticles composites. Self-sensing can also be used to evaluate the interfacial properties of fiber-reinforced polymer composites. This electrical resistance measurement method had several advantages compared to other nondestructive evaluation methods such as better stability, lower cost, and being rather simple. Future plans are to include studies of this nondestructive method into the manufacturing and robotic fields.

New method for interfacial evaluation of carbon fiber/thermosetting composites by wetting and electrical resistance measurements

Interfacial properties were evaluated for carbon fiber (CF) with different thermosetting polymeric matrices in composites. CF tow was wet by phenolic or epoxies, and the interfacial adhesion evaluated by electrical resistance changes. The interfaces between two types of CF tows with phenolic resin and three types of epoxies were investigated. The change in electrical resistance was found to depend on the wettability of CF by the polymer resins, with the more obvious resistance changes being associated with better wettability. The electrical resistance changes were measured 20 min after the polymer resin was dropped on the CF tow. To confirm the relationship between changes in resistance and interfacial properties, both interfacial shear stress (IFSS) and interlaminar shear stress (ILSS) were also measured. The results of these mechanical measurements were generally consistent with the electrical resistance measurements in that the materials with high electrical resistance also exhibited high IFSS and ILSS.

Surface control and cryogenic durability of transparent CNT coatings on dip-coated glass substrates

Transparent carbon nanotube (CNT) coatings were deposited on boro-silicate glass substrates by dip-coating. Ultraviolet-visible (UV) spectra, surface resistance measurement, and the wettability tests were used to investigate the optical transmittance and electrical properties of these CNT coatings. The changes in electrical and optical properties of these coatings were observed to be functions of the number of dip-coating cycles. The surface resistance of the CNT coated substrates decreased dramatically as the number of dip-coatings was increased, whereas the increases in the CNT layer thickness beyond that for the first dipping cycle had little effect on the transparent-properties. Static contact angle measurements proved to be an effective means for evaluating the surface morphology of CNT coatings. The interfacial durability of the CNT coatings on a glass substrate was much better than that of ITO coatings over the temperature range from -150°C to +150°C.

Dispersion and Related Properties of Acid-Treated Carbon Nanotube/Epoxy Composites using Electro-Micromechanical, Surface Wetting and Single Carbon Fiber Sensor Tests

Studies of dispersion and related properties, in carbon nanotube/epoxy composites, were conducted using electro-micromechanical and wettability tests. Specimens were prepared from neat epoxy as well as composites with untreated and acid-treated carbon nanotube (CNT). The degree of dispersion and its standard deviation were evaluated by turbidity of the dispersing solution, as well as by volumetric electrical resistivity. Acetone was a better dispersing solvent than purified water and various acid treatments of the CNT also enhanced dispersion. Contact resistivity responded differently with dispersion degree. The apparent Youngs modulus was higher for composites with acid treated CNT. The interfacial shear strength between a single carbon fiber and CNT/epoxy was lower than that between a single carbon fiber and neat epoxy. This difference is attributed to increased viscosity and decreased bonding availability in the matrix due to the added CNT. The optimum CNT treatment, for maximizing interfacial adhesion while maintaining good electrical conductivity was the sulfuric acid treatment. The CNT composites can also sense micro-damage in terms of the stepwise increments of electrical resistivity combined with acoustic emission.

Mechanical and electrical properties of electrospun CNT/PVDF nanofiber for micro-actuator applications

Electrospun polyvinylidene fluoride (PVDF)-containing carbon nanotubes (CNT) were prepared for use in fabricating actuator materials. Actuating displacement was measured in an electrochemical environment. The electrospun nanofibers were arranged using a drum-type collector, and morphology was investigated using a field emission-scanning electron microscope. The uniformity of dispersion of CNT in the PVDF nanofibers was monitored by electron probe X-ray micro-analysis. Tensile strength and electrical resistivity results were used as an indication of the state of alignment. The electrospun CNT/PVDF nanofiber sheets exhibited better mechanical and electrical properties in the arranged direction. The efficiency and electrical capacities of electrospun CNT/PVDF nanofiber sheet were compared with those of cast PVDF sheets for use in actuator applications in electrochemical environments. The electrospun CNT/PVDF nanofiber sheets exhibited much better actuator performance than PVDF sheets, which are attributed to their superior electrical properties. Highlights (1) The interfacial durability of CNT/PVDF nanofibers was enhanced to increase contact area by reinforcing CNT. (2) The efficiency of CNT/PVDF actuators was improved due to interfacial properties. (3) Thin thickness drum-type collector was made to enhance nanofiber alignment. (4) The arranged CNT/PVDF nanofibers exhibited better mechanical and actuating displacements.

To improve interfacial and mechanical properties of carbon fiber–modified nano-SiC–epoxy composites using dispersion and wetting control

Significant improvements in mechanical properties (particularly stiffness) result from the appropriate addition of micro-carbon fibers in the nano and heterostructures of modified nano-SiC-filled epoxy matrix composites. The optimum dispersion conditions were found to be significantly dependent upon both the amount of nano-SiC filler and the sonication time. To investigate these dispersion effects, composites were fabricated with five different nano-SiC filler concentrations and compared to the untreated composite. Changes in electrical capacitance were used as a measure of the comparative degree of dispersion in these nano-SiC–epoxy composites. FE-SEM was used to observe the interfacial changes for the different surface conditions, and the mechanical damage was evaluated by inspection of fractured surfaces. Optimal conditions of dispersion, interfacial adhesion, and aspect ratio of the modified nano-SiC fillers were found to improve the composites’ mechanical properties.

Improvement in mechanical properties of recycled GF prepreg with CNT reinforced composites using a spray coating method

Dispersion and shape of nanoparticles, as well as interfacial conditions, add significantly to difficulties in composite manufacture. In the work reported here, an innovative method of recycling composites using out-of-date prepreg was investigated in which the carbon nanotube (CNT) on the prepreg was optimally coated. Nanocomposites utilizing the out-of-date prepreg were coated with CNT and fabricated by a sheet molding method. CNT nanofillers were observed to be uniformly dispersed on epoxy prepreg by spray coating. The mechanical and interfacial properties of these CNT coated nanocomposites were improved over those of more conventionally manufactured carbon fiber/epoxy composites. The CNT nanofillers were embedded at the epoxy and fiber interface, as a result of etching of the epoxy prepreg surface by a CNT dispersion solution which enhanced interfacial reactivity.

Evaluation of optimal dispersion conditions for CNT reinforced epoxy composites using cyclic voltammetry measurements

The optimum dispersion time of nanoparticles is important for obtaining uniform dispersion of fillers or other additives in a matrix. In this study, the optimal dispersion time of carbon nanotube (CNT) in a matrix was investigated using cyclic voltammetry (CV), measurement for different dispersion methods and times. In addition, the mechanical properties of CNT composites manufactured using different dispersion methods were evaluated by tensile and flexural tests. The CV and mechanical test results were correlated to the dispersion condition of CNT in the composites. It was found that tip-type sonication resulted in better dispersion than bath-type sonication. Improved CNT dispersion resulted in composites with both enhanced CV measurements and improved mechanical properties. In the study reported here, improvements in dispersion were generally accompanied by higher electrical currents. This suggests that the CV measurement method is an effective tool for determining optimal dispersion times, for different CNT dispersion processes.

A new strategy of carbon fiber reinforced plastic drilling evaluation using thermal measurement

Carbon fiber reinforced plastic composites are being increasingly used for more applications. The evaluation of hole-drilling in these composites remains a difficult and unsolved problem. The drilling process for multi-material composites is quite different than those for their more conventional metal counterparts. In the research reported here, the drilling ability and durability of drills used for drilling carbon fiber reinforced plastic composites, made with a diamond layer was deposited by chemical vapor deposition are evaluated with different r/min. Spindle speed was fixed reasoning based in abundant previous works. A thermal camera and related techniques were used to evaluate chip removal, temperature measurement inside the hole, associated thermal damage during the drilling operation, in an effort to ascertain optimal drilling conditions. Currently, chemical vapor deposition diamond drills appear to produce better holes at lower r/min.

Stress and Cure Sensing of Single-Shape Memory Alloy (SMA) Fiber/Epoxy Composites using Electro-micromechanical Technique

SMA (shape memory alloy) is well known to change the microstructure from martensite to austenite with either temperature or stress. Stress and temperature response were investigated for SMA fiber/epoxy composites using an electro-micromechanical technique during curing. SMA fiber can be used in practical applications, including stress or cure-monitoring sensors, due to its inherent shape recovery properties, i.e., it exhibits a shape memory effect (SME) when subjected to applied stress or temperature. Superelasticity was observed for SMA fiber/epoxy composites under cyclic stress–strain curve. Under a certain stress, the original modulus was reduced steeply but then recovered under further stress. A sudden transitional change in electrical resistance was also observed around 87°C with an increase of temperature. Under cyclic loading the stress was suddenly leveled-off with a certain stress, which resulted in a different stress hysteresis repeatedly with two differing matrices and surface treatment. In SMA fiber/epoxy composites, residual stress of single-SMA fiber with and without embedding epoxy matrix exhibited the incomplete and complete recovery, respectively, during the curing process. Interfacial effect between SMA fiber and matrix can be important factor for practical applications including feasible sensing and actuator.