JohnV wrote:I read this bombshell post by a carbon fiber materials Engineer I can't credit his name because it was not supplied . So as it was posted public domain I see this as fair use . The people who built the sub should be up on criminal negligence resulting in death .
Quote " The fundamental design flaw for this vessel is that the designers forgot that "You can't push a rope." All fiber composites have their greatest strength in pure tension. Such materials are ideal for pressure tanks, which have a hoop stress of pure tension. The Titan, however, was not a pressure tank, it was a type of vacuum chamber, where the hoop stress is pure compression "
The CEO had a degree in Aerospace engineering so he would be very familiar with CF.
Steel is stonger in tension compared to compression. You can hang good weight from coat hangar wire don't expect it to hold anything up if loaded in compression. Beams and posts loaded in compression have to be carefully considered for buckling failure. Concrete is really good in compression sucks in tension so they put steel in for tensile loads.
A good article on the build of this sub it wasn't the first carbon fibre one they have a link to that.
Composite submersibles: Under pressure in deep, deep waters
Published 5/10/2017
Manned deepsea exploration calls for a highly engineered composites solution that saves weight and preserves life — at 6,500-psi service pressure.
Metallic hulls, however, because they are not buoyant in designs for depths of more than 2,000m, present challenges when it comes to managing ballast for ascent and descent. In particular, metal-hulled craft require the use of syntactic foam attached to the outside of the craft to achieve neutral buoyancy.
OceanGate CEO Stockton Rush says the company had been evaluating the potential of using a carbon fiber composite hull since 2010, primarily because it permits creation of a pressure vessel that is naturally buoyant and, therefore, would enable OceanGate to forgo the use — and the significant expense — of syntactic foam on its exterior. So, for Cyclops 2 OceanGate decided to avoid the metallic hull altogether and began a search for a manufacturer that could help it develop a composite hull.
Spencer Composites’ president Brian Spencer signed a contract with OceanGate for the Cyclops 2 hull in early January 2017 and was presented with very basic — but challenging — performance parameters: Length, 2,540 mm; outside diameter, 1,676 mm; service pressure, 6,600 psi; pressure safety factor, 2.25. “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.Thus, in the event of catastrophic failure of a submersible at any depth greater than even 250m, deepsea water pressure would instantly kill every passenger on board. And this is the primary concern of OceanGate and, by extension, Spencer Composites. Cyclops 2 faces potential failure in any one of three structures: the composite hull, the titanium end caps and the acrylic viewport.
The viewport, says Rush, because it is acrylic, fails optically long before it fails structurally — and in this case, catastrophically — thus the crew will detect a problem visually first.
Initial design work indicated that the hull, to be rated for 4,000m depth with a 2.25 safety factor, should be 114 mm thick or 4.5 inches, which OceanGate opted to round up to 5 inches (127 mm) to build in an additional safety margin.More here worth a read-
https://www.compositesworld.com/articles/composite-submersibles-under-pressure-in-deep-deep-waters
A Factor of Safety 2.25 is ballpark for aircraft which is what Aerospace engineers like the former CEO use.
With working load of 6600 PSI a FOS of 2.25 gives 14,850 PSI breaking load.
Those who have studied engineering would be saying WTF the Factor of Safety should be minimum 4-5 for something like this. With FOS of 4 = 26,400 PSI breaking load with FOS 5 = 33,000 PSI.
The problem with increasing FOS is increased weight and cost due to using more materials.
Carbon fibre is excellent for fatigue strength compared to steel/alloys provided it has a decent factor of safety to keep stress levels down.
It wasn't a backyard build as the fake news reported i don't think Spencer Composites are to blame they provided what was asked.
When engineers design something the question they ask is what are the consequences of failure then they decide on a factor of safety to make sure it stays together. Sometimes they add a bit to compensate for manufacturing flaws.
Nothing wrong with materials used in this sub the Factor of safety was far to low at 2.25.