Optimizing the Manufacturing Process of Glass Fabric/ Epoxy Composites with Nanoadditives.
Author: Infain Cruz 11/3/13
Mentor: Dr. Feridun Delale, ME
Abstract Glass fabric reinforced composites are becoming of great importance in many applications such as in the automotive and aerospace industry. This type of composite material is of great importance because of its unique properties compared to metals used in the industry today. Some of the advantages of using this material are its high strength, ease of fabrication, low cost, and impact resistance. In our research we are developing lightweight multi-functional energy absorbing composites with nanoadditives such as nanoclay and carbon nanotubes. These composite materials will be manufactured with glass fabric reinforced with the addition of nanoadditives in order to enhance the composite’s mechanical and thermal properties. Tests have been conducted to compare composites with and without the nanoadditives. We will enhance the multiple properties through various techniques. Techniques we are currently using are optimizing the manufacturing process, ballistic tests and shock tube tests for high velocity energy absorption, flame retardancy tests, tensile tests, drop weight tests for low velocity energy absorption, and temperature tests. The purpose of these various techniques is to understand the properties and determine the potential application of this composite material. We have also developed sensing techniques in order to detect damages experienced by these composites under each test. In this paper we will learn about the manufacturing techniques used in our research, the assembling process of piezoelectric sensors implemented into our composites, and provide a comparison between the mechanical properties of our material with metals such as metal alloys.
Introduction and Theory In this research we will study and help develop lightweight multi-functional energy absorbing composites with nanoadditives. These composite materials will be composed of glass fabric reinforced materials with the addition of nanoclay and carbon nanotubes in order to enhance the composite’s properties such as mechanical and thermal properties. Tests have been conducted to compare composites with and without the nanoadditives. We will enhance the multiple properties through various techniques. Techniques we are currently using are optimizing the manufacturing process, ballistic tests and shock tube tests for high velocity energy absorption, flame retardance, tensile tests, drop weight tests for low velocity energy absorption, and temperature tests. The purpose of these various techniques is to understand the properties and determine the potential application of this composite material. We also developed sensing techniques in order to detect damages experienced by these composites under each test. For example, during ballistic tests we analyze ballistic penetration by embedding circular sensors (circular shape reduces stress concentrations within the panel) into our composites and measure the energy absorbed with the assistance of electronics and a high-speed camera. This composite material could be used in the military in order to improve both bulletproof vests and military tanks. In this paper we will learn about the manufacturing techniques used in our research, the assembling process of piezoelectric sensors implemented into our composites, and provide a comparison between the mechanical properties of our material with metals such as steel. Glass fabric reinforced composites are becoming of great interest in many applications such as in the automotive and aerospace industry. This type of composite material is of great importance because of its unique properties compared to metals used in the industry today. Some of the advantages of using this material are its high strength, ease of fabrication, and low cost. Another advantage is that it is an ideal material to use because it is light weight and stronger or as strong as most metals.
Figure 1 Image of Matrix and Fiber Phase. Callister, William D., and David G. Rethwisch. Materials Science and Engineering. Hoboken, NJ: Wiley, 2011. Print.
Glass fabric/fiber reinforced composite materials normally consist of two components each with unique properties. These materials consist of a matrix phase and fiber phase. The matrix phase is usually an adhesive such as epoxy resin. The purpose of this layer is to bind the fibers together and provide ductility to the material. Not only does it introduce flexibility but it also protects the individual fibers from surface damage, mechanical abrasion, and chemical reactions all of which could weaken the strength of the material. Also, the purpose of the matrix
phase is to behave as a medium to transmit load to the fiber. The fiber phase consist of either polycrystalline or amorphous and have a small diameter (approximately 3 to 20µm). This phase is essential to the material because it provides extra strength. The fiber’s elastic modulus is typically higher than the harden polymer. Figure 1 shows a schematic of both the matrix and fiber phase after the composite material cures we see how the matrix binds the fibers creating one solid body.
Figure 2 Image of before and after dipole moment. Callister, William D., and David G. Rethwisch. Materials Science and Engineering. Hoboken, NJ: Wiley, 2011. Print.
The sensors we embed in our composite materials are piezoelectric sensors. These sensors are of great interest in our research because it allows us to detect damages after each test we perform. The sensors under pressure causes a phenomenon called the piezoelectric effect. Essentially specific types of nonmetals, typically crystals and ceramics, tend to have this unique property in which a mechanical input results in an electrical response and electrical input results in a mechanical distortion. Figure 2 shows a simple schematic of this phenomenon at the atomic level. Initially, the “green” anion is perfectly centered in the middle of the lattice structure which means that the structure is non-polar. In the second image we notice that the anion has shifted closer to one of the cations resulting in polarization. As stress is applied on these piezoelectric sensors its lattice structures create dipole moments resulting in electrons to transfer towards the surface of the sensors. In the case of our sensors, after several ballistic tests on our composite panels with embedded sensors the impact of each projectile produces a wave propagation in which suddenly applies pressure on the sensors. As a result, these sensors produce a voltage output in which enable us to measure these damaged regions.
Materials and Equipment Nanoclay is a clay mineral used in many composite material applications. Applying nanoclay into our multifunctional composite material is one of our techniques to enhance the material’s properties. The scientific name of nanoclay is known as montmorillonite which consist of inorganic minerals. As shown in the figure, nanoclay structure is composed of many elements such as oxygen, silicon, aluminum, and a compound called hydroxyl. Essentially, these components create plate like atomic structures which is of great interest to manufacturers and researchers due to its strength and thickness and length ratio (approximately 1nm thickness and 10µm length). For our research, we are using nanoclay because it enhances the composite’s strength, decreases gas permeability, and fire retardance when mixed with polymers such as epoxy.
Figure 3 Shows the atomic structure of Montmorrilonite also known as nanoclay. Image from Nanocor.
Glass fabrics provide a wide range of properties in which becomes an ideal material when manufacturing composite materials. Glass fabrics are simply compiled fiber glass layers with high strength and flexibility. According to BGF industries, some various properties of glass fabric are high heat and fire resistant, durable, and economical. This material serves a great purpose because of its unique properties and affordability.
Figure 4:Shows the type of glass fabric we used in our experiment. Image from Nanocor
The Material Test System (MTS) 810 is a mechanical instrument in which is used to perform both monotonic and dynamic tests. The instrument is capable of applying up to 22,000lbs of force according to MTS. The chamber shown allows testing specimens under high, low, and room temperatures. Also, another beneficial feature is the hydraulic actuated clamps in which minimizes specimen bending during tests preventing any contamination to the experiment.
Fig5: MTS 810 (left) and hydraulic actuated clamps inside of chamber (right).
Experimental Setup and Methodology The following experimental setup and methodology is known as the vacuum infusion method. This fabrication technique has been continuously modified by my mentors and I in order to optimize the manufacturing process of our composite panel. This technique is composed of laid dry materials and vacuum pressure is applied to remove air and introduce a polymer resin to the laminate through vacuum tubes. The goal of our experiments is to manufacture and develop the best technique to build a glass fabric composite panel with multifunctional properties such as light weight, high strength, high impact resistant, and fire retardant. With each experimental trial we anticipate improvements and successful data. The following procedure will provide a visual insight of our manufacturing process. Place a clean flat metal plate and clamp the plate onto a table for perfect stability and avoid warps (make sure the top surface of the plate is clean). Once we have a nearly flat and leveled surface we place an air sealant tape (yellow tape shown below) around the outer edges in order to prevent air from entering the bottom surface of the panel creating air bubbles. We have to make sure we do not remove the paper covering the air sealant before placing the first plastic layer because we want to prevent air contaminates from sticking creating minute passages for air. Next, we place strips of cotton around the inner edge of the air sealant tape. The purpose of placing these cotton strips is to absorb air bubbles left after we vacuum this bottom layer.
Figure 4: Shows the setup for the bottom surface of the composite panel
We connect the electric motor and vacuum pump where these devices will remove the air. Once both devices are connected we will attach the tubes from the vacuum pump onto the metal plate as shown in figures 5 and 6. All excess resin will flow inside of this vacuum pump and evaporate.
Motor
Vacuum Pump
Figure 5 Shows the electric motor (left) and vacuum pump (right) used to remove the air and resin out of the panel.
Figure 6 We connect tubes from the vacuum pump to the metal plate with the air sealant tape around the tube as shown below. This tube will remove the air from the bottom of the panel.
Once the tube is positioned we place the first bag over the air sealant tape. The bag must be placed one side at a time in order to have a flat surface as shown below. Before proceeding, we also must apply pressure around the edges to seal potential air holes. This first bag is the bottom surface of the panel and we want this surface to be as flat as possible and avoid any warps on the bottom surface of the panel. Once the bag is checked we can turn on the electric motor and begin removing air. If a “hissing” sound is heard quickly search for the air hole and apply pressure. If everything is done accordingly we will notice the bag flatten and stick to the metal plate indicating air has been removed. We will now look at the pressure gauge on the vacuum pump to measure the pressure. The pressure reading we want is approximately -29inHg and once this reading is achieved we will measure any drop in pressure within 10 minutes. If there is a 1inHg drop within this time we will have to check for any air leak or perform the bagging process over.
Figure 7
After the pressure is measured correctly we record the pressure for the first bag. Next, the fabric peel ply, flow channels, and sensors are placed. The figure shown below shows another layer of air sealant tape and cotton strips. Once these materials are placed we begin placing the fabric. Note this step varies depending on which type of panel we are interested in testing. For instance, we might place fabric with or without sensors. While placing this fabric a mask to prevent inhaling the fibers is required and gloves to prevent contact with the oil of our skin.
Figure 8 Fiber glass placed on the center of panel.
The next layer we apply is the peel ply also known as the release fabric. It is a synthetic cloth that we drape over our epoxy surface as the epoxy sets up. The purpose of this layer is to give the final harden surface a smooth finish. Since glass fabric and epoxy do not provide that smooth finish, manufacturers use this technique. Not only does applying peel ply smooth the glass fabric/epoxy surface but it also improves the adhesion process.
Figure 9: The Peel Ply placed over the fiberglass.
Place the flow channel over the peel ply. The purpose of this layer is to provide the resin a guide or “channel” to spread across the panel. The placement of this layer is significant to the manufacturing process because it helps the dispersion of the resin. Once this layer is in place, place tape on each side of the peel ply and flow channel in order to keep each unit in place. Next, place air sealant tape and cotton strips near the edges. Note do not place these strips above strips covered over by the vacuum bag this may cause air leakage through the core layer.
Figure 10 Images showing the flow channel placed over the peel ply.
In this next step we need to insert the vacuum tubes to the second layer. In contrast to the first layer, we will place four vacuum tubes in this layer because we are driving epoxy resin with vacuum. We basically cut three strips of vacuum tubes and join them with tape as shown below. Once this task is accomplished we place and tape two vacuum tubes to one side of the panel and one of the tubes with spiral geometry will provide the uniform dispersion. This technique we use is to obtain uniform dispersion and we want uniform dispersion because we want a uniform configuration.
Figure 11: One of the sides with vacuum tubes. This is the technique we use to supply epoxy to the panel. The clamps shown in orange are used to clamp the tubes once all of the epoxy is dispersed inside of the panel; prevents air from entering.
The next pair of vacuum tubes was placed at the other end of the panel. These tubes (being assembled by one of my mentors Bruno Zamarano) is attached to the AirTech vacuum pump in which will remove the air creating a vacuum pressure inside the panel and essentially this pressure difference drives the epoxy from one end to the other. Once the epoxy reaches this end, as shown below, any excess epoxy will be driven inside of the AirTech pump and evaporate. Also, the tubes connected to the pump need to be sealed with sealant tape.
Figure 12: Tube connected to the vacuum pump placed on the panel. Air sealant tape (yellow) used to seal potential air leakage at connection joints.
We place the last vacuum bag over the previous layers. Compared to the first vacuum bag we want to apply less tension to this bag. The same procedure for the first vacuum bag is applied to this bag. All edges must be tightly sealed before epoxy can be poured. Once the air leakage test is checked, epoxy can be used to begin the matrix process.
Figure 13: The placement of the final bag.
This image shows the complete vacuum infusion technique. We see how the epoxy is driven by vacuum from left to right. In our experiments, we want the epoxy distribution as shown below because it covers a larger area. The light shade indicates that the epoxy is traveling much faster under the fabric layers than through them indicated by the dark shaded region. This is expected since the epoxy has fewer barriers to travel through below. This experimental procedure was done without nanoadditives but the procedure is the same for 1%, 3%, and 5% nanoclay.
Dark region Light region
Figure 14: Clear indication of the difference speed of the epoxy travelling above or below the core fibers.
Assembled Piezoelectric Sensors Cut several pieces if electrical wires. These wires will transmit the voltage from the piezoelectric sensor to our lab’s computer.
Figure 15: Electrical wires.
In order to prevent burning the electrical wires with the solder we apply flux where the soldering will take place. The purpose of the flux is to also prevent impurities penetrating the joining such as dirt and oxidation.
Figure 16: Electrical wires dipped in flux.
Once the wires are dipped into the flux we use an electrical soldering iron at 700˚C to solder the tip. Again, we apply this technique to solder the wire on to the sensors with ease.
Figure 17: Wires are being soldered.
We will next apply a small amount of flux on the center of both sides of the piezoelectric sensor where we will solder the electrical wires and grab each sensor individually for the soldering process. Once this is done the assembling process is done.
Figure 18: Electrical wire soldered on the center of sensor.
In the case of manufacturing panels with embedded sensors the manufacturing process is the same but excessive caution is taken for this type of experimentation. The image below shows a composite panel with embedded sensors after a ballistic experimentation. As we can see from figure 19 the sensors are embedded near the perimeter (where the black line is drawn). The inner region shows the damaged or the region of impact of four bullets at different velocities.
Figure 19 (Left) Panel with embedded sensors with damage due to ballistic impact. (Right) Cables serving as a channel from the sensors to the lab computer.
Tensile Test Using the MTS 810 we performed several tensile tests experiments. The composite panels were manufactured and cut to 1 in by 10 in size which is standard according to the American Society for Testing and Materials (ASTM) standards for this type of experimentation. The ends of the specimens are 1/16th in thick in order to allow uniform stress distribution. The figure shown below shows the standard shape for the specimens. For each experiment we conducted, we used specimens with 0%, 1%, 3%, and 5% nanoclay. The specimens were placed inside the environmental chamber at temperatures of -54˚C (-65˚F), -20˚C (-4 ˚F), room temperature, 49 ˚C (120 ˚F), and 71 ˚C (160 ˚F) in order to have uniform temperature distribution. The rate of displacement we used was 5mm/min which is also standard according to ASTM. This displacement is actually the speed of which the clamps will pull the specimen. At higher velocities the specimens will fracture faster therefore we will not completely understand its maximum mechanical properties. The purpose of performing these experiments is to understand the mechanical properties. These experimental results will allow us to compare the mechanical properties under extreme temperature conditions and room temperature. Also, compare these properties with and without nanoadditives under these same conditions.
Figure 20: Standard ASTM “Dog Bone” shape. Specimens with nanoclay are darker than the specimen with 0% nanoclay.
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Figure 21: Specimen with 0% Nanoclay Stress (MPa) vs Strain(µԑ) at several different temperatures.
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Strain(µԑ) Figure 22: Specimen with 1% Nanoclay Stress(MPa) vs Strain(µԑ) at several different temperatures.
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Strain(µԑ) Figure 23: Specimen with 5% Nanoclay Stress(MPa) vs Strain(µԑ) at several different temperatures.
Discussion of Results The results shown above shows 0% percent nanoclay behaves much stronger at low temperatures than at high temperatures. According to our results this material without nanoclay is ideal under stress in a cold environment. The results show at low temperatures the material gains stiffness whereas at high temperatures stiffness lowers. Also, we note at lower temperatures the specimen’s failure strain is higher. The results for 1% and 5% nanoclay are similar to 0% in terms of strength increasing at low temperatures. Interestingly, 0% and 1% nanoclay results show 600MPa as the ultimate stress and approximately .04 µԑ where fracture occurs. At 71˚C, the 0% nanoclay specimen fractures at about 350MPa whereas the 1% nanoclay fractured at 400 MPa which indicates that the nanoclay increases strength at high temperatures although not as high in low temperature environments. In contrast, the 0% nanoclay is slightly stronger in room temperature than 1%. As expected we looked forward to see a distinguishable difference between 5% and the lower percentages of nanoclay. According our results, specimens with 5% nanoclay had an ultimate stress of approximately 700MPa at -54˚C about 100MPa higher than both 0% and 1%. Also, the strength and the failure strain increased at -20˚C. However, the strength of the 0% and 5% nanoclay are similar under room temperature conditions at approximately 500 MPa but the 0% specimen failure strain is higher than both 1% and 5% under 49˚C. Below is tabulated data showing the ultimate strength for commonly used steel alloys at room temperature. This roughly estimated data gives a sense of how much stronger our material is compared to one of the strongest materials used in the industry. Steel Type Ultimate Strength (MPa) 1020 400 1040 380 4140 520 Table1: Data obtained from the text Material Science and Engineering Note: Data may vary according to the type of work done on the steel (annealing, hot rolling, etc)
Conclusions Composite materials and nanoadditives are becoming the solution to many engineering applications because of their multiple properties. The purpose of our research is to enhance these properties such as mechanical and thermal through a variety of experimentations. For instance, we were interested in the mechanical properties therefore we performed many tensile tests to obtain the stress versus strain relationship by applying knowledge of mechanics of materials . We also wanted to understand how these panels behaved under high-velocity impacts therefore we performed ballistic experiments. Glass fabric composite materials are beneficial not only because it is multifunctional but because it can be built on site for relatively low cost and many techniques can be used such as vacuum infusion. However, nanoadditives such as nanoclay are beneficial because of its unique atomic structure and strength. Also, mixing nanoadditives with a polymer matrix binds the fibers into specific orientations providing greater strength capability. Due to confidentiality the results and images for ballistics were preserved and not placed in this paper. Through the course of this research, time was invested to not only optimize the manufacturing process, but understand all aspect of our material. Many experiments were conducted such as the tensile test in order to understand the potential application of this composite material. During this research, we learned how to adapt to constant changes in our approach in order to design the best panels with the resources we had. The time duration for each panel would vary depending on which percentage of nanoclay we were manufacturing. The higher the percentage of nanoclay the longer the waiting time because the epoxy and nanoclay are highly viscous when mixed therefore dispersion through the panel is slow. Also, we learned that epoxy resin cures quickly which was one of our challenges because the dispersion mechanism at the panel station had to be prepared before the epoxy mixture was poured. Once the epoxy and nanoclay were mixed it had to be quickly dispersed in the panel before curing. According to our results, adding nanoclay enhances the mechanical properties. Although we cannot see a large improvement under high temperature, we see under low temperature conditions the material increases its stiffness therefore absorbing more energy under stress. We also see the material elongating further with higher percentage of nanoclay which is a significant improvement of the material’s strength. According to the text Material Science and Engineering, we realize that our material’s ultimate stress is higher than commonly used steel alloys such as steel 1020, 1040, and 4140. In conclusion, this paper provided insight about the manufacturing process we used to optimize our composite materials, assembling method for embed sensors, and a comparison between the mechanical properties of our material with steel alloys.
References Callister, William D., and David G. Rethwisch. Materials Science and Engineering. Hoboken, NJ: Wiley, 2011. Print. "Nanocor® Leading-Edge Developer of Nanoclay Technologies for Plastics." Nanocor® Leading-Edge Developer of Nanoclay Technologies for Plastics. N.p., n.d. Web. 17 Mar. 2013.
