Here's some information from the CompositesWorld link that I posted earlier in the thread:
The biggest challenge, Spencer reports, was developing a manufacturable design that “would produce a consistent part with no wrinkles, voids or delaminations.” And without use of an autoclave. Spencer opted for a layup strategy that combines alternating placement of prepreg carbon fiber/epoxy unidirectional fabrics in the axial direction, with wet winding of carbon fiber/epoxy in the hoop direction, for a total of 480 plies. The carbon fiber is standard-modulus Grafil 37-800 (30K tow), supplied by Mitsubishi Chemical Carbon Fiber & Composites Inc. (Irvine, CA, US). Prepreg was supplied by Irvine-based Newport Composites, now part of Mitsubishi Chemical Carbon Fiber & Composites Inc. The wet-winding epoxy is Epon Resin 682 from Hexion Inc. (Columbus, OH, US). The curing agent is Lindride LS-81K frLindau Chemicals Inc.cals (Columbia, SC, US).
....
After layup and winding was complete, the cylinder was bagged with cellowrap and then cured in an oven at 137°C for 7 days. There was no postcure. Spencer says initial assessment of the cured cylinder shows that it has porosity of <1%. As CW went to press, the cylinder was being prepared for machining to cut it to length, square up the ends and bond it to the titanium end caps.
That alone was so disturbing to read the first time, that I was kind of glad that the article didn't get into how the "two titanium interface caps" were "bonded to each open end of the hull."
...and then there was this:
“They basically said, ‘This is the pressure we have to meet, this is the factor of safety, this is the basic envelope. Go design and build it,’” Spencer reports. And he was given six weeks in which to do it.
https://www.compositesworld.com/art...#carouselae323cca-6ad8-4de8-9b66-75a9cb083bbf
Hi
Shepherd, thank you for posting this, I missed it. That's a good read.
As
The Man already pointed out, the 'Titan' carbon fiber pressure hull
wasn't put in an autoclave. An autoclave could've helped remove air bubbles.
But even if the CF cylinder hull was put in an autoclave, there are other major problems.
PROBLEM OF DISSIMILAR MATERIALS; CARBON FIBER AND EPOXIES
According to the CW article,
prepreg carbon fiber/epoxy unidirectional fabrics in the axial direction,
was used with alternating wet winding of carbon fiber/epoxy in the hoop direction. Why is that a problem? This needs to be verified as I don't see that the prepreg used isn't specifically mentioned, so it's quite plausible it uses different carbon fiber, and different epoxy resins, from the wet-winding carbon fiber/epoxy/curing agent.
This means that the different materials will shrink and expand at different rates. That can lead to delamination and cracks in the carbon fiber/epoxy.
PROBLEM: CUTTING THE CYLINDER EXPOSES BARE FIBER
The CW article said, "the cylinder was being prepared for
machining to cut it to length,
square up the ends..." and the cylinder was to be bonded to the titanium end caps.
Cutting the carbon fiber/epoxy cylinder would expose bare, carbon fibers. The machining would likely cause a situation in which air can get into the ends of the cylinder because of tiny cracks in the CF/epoxy, and in-between the 'hairs' of the carbon fiber. If the machined cut ends, and any work done to "square up the ends" didn't fully encapsulate the carbon fibers without air bubbles or pockets...the cut ends would likely make air bubbles.
It appears different carbon fiber was used, likely different epoxies, and different layup methods that likely didn't form a cohesive whole. I don't recall seeing alternating placement or layers of prepreg with the hoop direction wet windings of carbon fiber.
The CW article doesn't doesn't say if the ends of the carbon fiber cylinder fully encapsulated the cut & exposed carbon fibers. I don't know if the "square up the ends" included using sandpaper on the carbon fiber. If so, that was likely a no-no. You need to stop air bubbles or pockets from happening.
The ends of the carbon fiber/epoxy cylinder appear to be HIGHLY-PROBLEMATIC as to how it was cut and machined, then epoxied into the titanium (interface) end caps. The carbon fiber/epoxy cylinder ends could have deformed under pressure so as to come into contact with 'sharp corners' of the titanium (interface) end caps? I don't know. So many different materials, so many chances for air bubbles. Different materials/methods means delamination, leading to cracking.
And it's quite possible that
air bubbles were likely whipped into the epoxy that was used to bond the carbon fiber/epoxy cylinder to the titanium (interface) end caps. Air bubbles are problematic as the materials cycle with pressure and temperature, as to potentially cause failure.
Again, I don't know what caused 'Titan's' catastrophic implosion, but the use of different materials and construction methods used in the joinery areas....really, really, really, cry out.
