The ultrasonic treatment of composite materials.
It's gunna go something like this (I can type as fast as I can speak, actually a bit faster):
In here to show you what has been done to gain a visual understanding of ultrasonic processing of composite materials. Also, coensiding with that we will go through some of the machining difficulties. As mentioned earlier, we delt with three fiber reinforcements in an epoxy matrix: s-glass, carbon-fiber, and aramid. S-glass is fairly easy to cut, with only minor decrease in tool life, high speed steel even works fairly well with s-glass. The biggest bother is the dust, which is itchy. Carbon-fiber isn't as easy to machine, with about 1000X the abrasivess of medium carbon steel. Particulate and VOCs range depending on who you talk to, but its a good idea when cutting carbon-fiber to keep a mask on. With carbon-fiber delaminations need to be monitored, because of its stiff nature. Aramid/Kevlar fibersare the most difficult, they tend to fray and dull tooling extremely fast. High speed steel, tungsten carbide and even diamond saws do not provide clean cuts and good tool life. A waterjet is the best way to cut these.
So here is S-glass under a stereomicroscope. Fairly difficult microstructure to view using a stsreomicroscope, because of the simularity in color between the resin and the fibers. Treated specimen are seen at the top, untreated at the bottom.
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This is a closer view of the treated S-glass, at the break post-bending. It clearly shows the bi-directional layers and will prepare you as we move closer.
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The increase of thickness was determined to be from premature gelling of the resin due to the heat from the transducer. Heat speeds curing, and the energy from the transducer when placed too long is kicking off the resin... its starting to gel before we can pull a vacuum on it. However, you do see increased resin saturation through inflated lamina thickness (more resin soaked into fibers), which is good. The resin pooling between layers, is bad, however. These sort of battle eachother, but the increased resin penetration doesn't offset the magnitude of the thickness, brittle resin layer in bending.
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Another key goal with ultrasonic treatment was to reduce void content. Air voids are bubbles deposited throughout the matrix that are stress concentrations, and cause lack of cohesion between layer. From this photograph at 50x, you can see an example of a void. It may or may not have been induced during machining, but there are some that likely are actual air voids, well micro-airvoids, in later slides. Ultrasound NDT should be used to evaluate specimen if void content is of question.
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Here is treated carbon fiber specimen, also with a thickness increase.Fractured carbon-fiber specimen showed the typicall britlle nature of the material with increased stiffness causing a two-stage break in three point bending.
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This is a closer 20 micron shot of the carbon-fiber failure in bending.
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The aramid fibers are the easiest to view using the stereomicroscope. White areas are on the 90, yellow on the o-deg fabric directions. These were cut using a waterjet, which produced a clear view at this magnification.
Next Slide: Kevlar's toughnass may be seen here, a classic example of fibular-failure modes.
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Untreated specimen on your left, treated on your right. Notice the increase in resin penetration on the treated specimen, yet also a thick film of resin inbetween layers.
Next Slide: Next specimen were cold mounted and coated for use in a scanning electron microscope. Higher magnification, 3D view,
Next slide: Treated specimen shoed areas with decreased voids and others with increased. Hand application is likely the problem, because when you lift up the transducer it will displace air moved or pulled wet surface. We need to develop a press to take the human element out of this.
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Now we are at 10um in scale on the SEM, focused on S-glass fibers. On the left is treated S-glass, well penetrated with resin, and on the right treated glass on the 0-deg plane, which has stripped resin.
Next Slide: Here we see an air void, a micro-airvoid. 10um scale right now, in these air void regions fibers tend to seperate and brittle resin rich areas form around the boundaries.
This is a 1um shot of the side of a micro-bubble.
Next Slide: A problem area in S-glass, where the transducer was lifted.
Next Slide: This is a good visual of carbon-fiber delaminations. Layers of parallel fibers have seperated from the perpendicular fibers, leaving a long thin delaminated area.
We are at 1um.
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10um scale, view is of carbon-fiber micro-delamination. Layers of parallel fibers have seperated from the perpendicular fibers leaving a long thin delamination area.
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Zooming out we put this into perspective. The black bit in the upper left is the micro-delaminated viewed earlier. On the macroscopic level these are not visible and therefore go un-noticed during visual inspection.
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