Engineering
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Strain Sensing Based on Multiscale Composite Materials Reinforced with Graphene Nanoplatelets
Chapters
Summary November 7th, 2016
The integration of conductive nanoparticles, such as graphene nanoplatelets, into glass fiber composite materials creates an intrinsic electrical network susceptible to strain. Here, different methods to obtain strain sensors based on the addition of graphene nanoplatelets into the epoxy matrix or as a coating on glass fabrics are proposed.
Transcript
The overall goal of this procedure is to achieve self-sensing properties in composite materials for structural health monitoring. This method can help answer key questions in the sea automatic detection fail, such as quantification of a strain and damage in place of off-shore wind farms and prediction of their server life. The main advantage of this technique is that damage can be detected by the structural component itself.
Though this method can provide insight into structural damage of composite materials, it can also be applied to other systems such as biomechanical analysis during injury recovery. To begin the preparation, in a ductless fume hood, hand-mix 24 grams of functionalized graphene nanoplatelets with DGEBA monomer. To disperse the f-GNPs into the monomer, first sonicate the mixture with a Probe Sonicator for 45 minutes.
Then, calender the mixture three times, increasing the roller speed each time. After dispersion, weigh the mixture. Heat the mixture to 80 degrees Celsius while stirring, and then place the mixture under vacuum.
Degas the mixture while stirring for 15 minutes. Weigh out the hardener in a 100 to 23 weight ratio of DGEBA monomer to hardener. Remove the mixture from vacuum, and stop magnetic stirring.
Add the hardener, and stir by hand until homogeneous. After degassing and hand-mixing, keep the f-GNP epoxy material at 80 degrees Celsius under magnetic stirring. Then, clean a steel plate with acetone.
Using fabric scissors, cut 14 layers of glass fiber fabric to the desired dimensions. Warm the glass fabric to 80 degrees Celsius in an oven. Cut two squares of antiadherent polymer film in the same dimensions as the fabric.
Place the film on the clean steel plate. Using a brush, apply a layer of f-GNP-filled epoxy to one square of the polymer film. Carefully place one square of warmed glass fiber plastic on the epoxy-covered film, so the epoxy and film are completely covered.
Use a de-airing roller to compress the materials. Continue applying layers of epoxy and fabric, compressing each time, until all remaining fabric has been used. Apply one last layer of epoxy, and place the remaining square of antiadherent polymer film on top of the laminate.
Cure the laminate in a hot plate press with increasing pressure. In a ductless fume hood, prepare a one to one mixture of sizing agent to distilled water. Add to this 7.5 grams of f-GNPs.
Remove the mixture from the fume hood, and disperse the nanoplatelets by probe sonication. Cut out 14 squares of glass fiber fabric. Use a Dip Coater to coat the glass fabric with the f-GNP-filled sizing.
Then, dry the coated fabric in a vacuum oven. Clean a steel plate with acetone. Place a square of antiadherent polymer film of the same dimensions as the glass fiber fabric squares on the steel plate.
Place the 14 squares of coated glass fiber fabric onto the polymer film, ensuring that the edges are aligned. Then, place the plate and fabric in a vacuum bag. Using sealant tape, seal the vacuum bag.
Pre-heat the bag in an oven at 80 degrees Celsius. Degas the DGEBA monomer under vacuum while stirring. Add hardener in a 100 to 23 monomer to hardener weight ratio and stir until homogeneous.
Connect a vacuum pump to the vacuum bag and perform leak checks. Connect a length of polymer tubing to the bag and submerge the open end in the epoxy resin. Turn on the vacuum pump to draw the resin into the bag.
Once the glass fabric pile is completely soaked with epoxy resin, turn off the pump, and seal the bag. Cure the laminate in the bag in an oven at 140 degrees Celsius for 8 hours. Then, remove the cured laminate from the vacuum bag.
To begin preparation for strain testing, machine the laminate samples. Then, clean the sample surfaces with acetone. Using silver acrylic conductive paint, draw two narrow lines 20 millimeters apart on each sample.
Lay fine copper wires flat along the wet silver paint to act as electrodes. Once the paint has dried, fix the wires in place with hot-melt adhesive. Configure a mechanical test machine for a flexural test.
For each sample, measure the width and thickness with calipers, before placing the sample in the machine. Set the test speed and start position appropriately for the size and location of the sample. Connect the electrical contacts to a multimeter and measure the initial electrical resistance between the contacts.
Run the flexural test, monitoring the resistance throughout. To prepare for the strain test, cut f-GNP glass fiber fabric to 10 millimeter wide strips. And to fix copper wires to the fabric with silver conductive paint and hot-melt adhesive.
Next, attach the f-GNP glass fiber fabric bands to the thumb and first three fingers of a nitrile glove using hot-melt adhesive. Measure the initial electrical resistance across the fabric on each finger. Perform a sequence of finger bending while recording the electrical resistance.
Begin by bending the thumb, then index, then middle, then ring fingers. Bend all fingers simultaneously. Then, with increased speed, repeat the sequence from thumb to ring finger, and back.
Incorporation of f-GNPs into glass fiber increases the electrical conductivity of the material, which is effected by induced strain. The normalized electrical resistance increases with increasing flexural strain. Breakage of glass fibers at the point of failure disrupts the electrical network, which is seen as a jump in normalized resistance.
When f-GNP glass fiber fabric is applied to the fingers of a nitrile glove, the bending of each finger, and the duration of the movement can be tracked by changes in resistance. Subjecting the f-GNP epoxy and f-GNP glass fiber composite materials to compressive and tensile strain induced slightly different patterns of change in electrical resistance. Below a certain level of compressive strain, normalized electrical resistance gradually decreases with increasing strain.
Beyond that, the resistance increases. However, the normalized electrical resistance increases with tensile strain throughout. After its development, this technique paved the way for researchers in the field of sensing, monitoring material to explore damage in structural components.
After watching this video, you should have a good understanding of how prepare self-sensing composite material, and how monitoring it in real time.
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