Disaster Dan.
Tourist submarine failure.
I should create another blog just on my take on disasters from poor engineering.
So that Ocean Gate "Titan" implosion. Another regulations are for losers megalomaniac type guy. Pay attention Elon Musk. A carbon fiber hull section failed instantly killing 5 people.
I am currently working as a design engineer in composite vessels as in tanks and that sort of thing. The biggest external pressure I have to deal with is 1 atmosphere or about 15 psi. (14.7 typically) The wreck of the Titanic is deep enough for about 6000 psi or 400 times as much. 15 is hard enough to make work, but lets set that aside.
A CF drum shape is made with a method called filament winding. A mold is prepared called a mandrel upon which fine filaments of fiber are wound after being saturated in resin usually an epoxy-vinyl-ester. This method creates a dense well formed compact section of very good strength. There are two basic variations on this scheme.
The first variation is called helical winding where the feed of fiber is moved back and forth as the mandrel rotates to create a mesh like net of fiber. This is less compact but is provides some strength in two directions those being axial and circumferential. Circumferential direction is also called the "hoop" axis. Think of a wooden barrel and the steel hoops around it.
The second variation is called ortho winding where there are two types of fiber. One is the hoop axis fiber the other is a uni-directional fabric where the main fibers run at 90 degrees to the winding fibers. The hoop fibers act to compress and compact the "uni" fibers to make a dense matrix of material. The two types are alternated as the product is wound.
OK that is all good. One thing to take from this is that both methods make a product that is anisotropic.
Anisotropic ADJECTIVE:
An object or substance having a physical property that has a different value when measured in different directions. A simple example is wood, which is stronger along the grain than across it.
The material has one set of properties axially and another circumferentially. CF composite has a grain like wood.
OK class, a filament wound cylinder has different strength and stiffness in the two major directions of it's geometry whichever method is used to wind it.
The TITAN submersible had titanium hemispheres on each end to cap the passenger space. Titanium is a metal which is generally and effectively Isotropic. That means it's properties are the same in all directions. Titanium is strong and relatively light compared to say steel.
The first problem in combining two different materials is that you have two different materials.
The second is that those materials have different properties and one has different properties on two different axes.
Titanium has a "young's modulus" (YM) of 120 GPa. (gigaPascals).
CF composite has a modulus of from 70 to 220 GPa. You have to figure that out by testing of samples for a given laminate specification.
That modulus is the key factor to buckling failure from compression for any given geometry. Spheres are stronger than cylinders for a given material thickness due to the two way curvature of the surface.
The thickness of both materials and the YM determine how much the shell will compress or shrink under load. The thickness of the cylinder must carefully be proportioned to shrink at EXACTLY the same rate under load as the titanium ends or a secondary flexural stress will occur where they join. That stress can be huge.
I do Finite Element analysis of things like that and it is these joins and details that are the hard part to make work. My designs are driven by the secondary stresses for the most part.
I have no idea who designed this vessel. I heard they uses Finite Element Analysis. It has to be used properly to be correct. It is possible to fiddle constraints and such to make things appear to work. Whatever and whoever did the design it was wrong.
In my work I am required to use a factor of safety of 5 to 1 for loads like that. It is apparent the FOS here was close to 1 and as more dives were made it got smaller. A higher FOS is expensive and they were running a business there.
At the very least the person who must take the blame/responsibility went down with the ship.
May his customers rest in peace.
