On the Loss of the “Titan” Submersible

We will all know more,  after the Coast Guard inquiry is completed.  The picture shows the layout and characteristics of the vessel.  The cylindrical portion was a tube made of carbon composite,  with a wall thickness of 5 inches.  The domes on each end were thick titanium metal,  bolted to metal rings bonded to the carbon composite tube. 

The end domes were recovered more-or-less intact,  along with other large chunks that include some of the aft exterior plating,  and some exterior-mounted equipment items.  This suggests it was the carbon composite pressure hull tube that failed in the catastrophic implosion,  as no large chunks of that tube have been seen as recovered items. 

Composites made with layers of woven cloth must be compressed during cure,  to avoid massive porosity issues.  This is usually done by vacuum-bagging the layup for cure.  The stronger the vacuum,  and the thinner the part,  the lower the porosity.  That porosity weakens the part,  and gives it a finite life under cyclic loading,  due to the voids locally concentrating stresses. 

Parts made by continuous fiber winding are not vacuum-bagged,  but avoid the massive porosity of woven-cloth parts,  at the cost of requiring multiple fiber winding directions to handle all the forces applied in various directions.  Some porosity is inherent,  just as in the properly vacuum-bagged woven cloth parts.

I suggest that composite material was the wrong choice for a deep submergence vehicle pressure hull! 

There is time for sea water to infiltrate into the porosity voids during the descent and subsequent bottom time. At those depths,  sea water is slightly compressible (by around 1.5 to 2% of volume).  It does not have time to fully percolate back out during the ascent,  so to one extent or another,  it swells and causes local cracking around the porosity voids,  weakening the part.  The part gets more damaged internally and thus a little bit weaker,  with each dive cycle.  Sooner or later,  it will fail.

The problem here is sea water infiltration under high pressure into the inherent porosity of the part,  something that does not happen in more ordinary conditions.  Metals do not suffer this porosity infiltration mechanism,  although they are subject to fatigue. 

Update 7-7-2023:  Just to be clear,  composites do not “fatigue” the way metals do (agglomerating atomic lattice misalignments leading to cracks,  with stress concentrations at the crack tips propagating them rapidly further).  But there is a limited life in composites due to accumulating internal damage with cyclic loading.  This is not as well understood as metal fatigue,  but nevertheless,  it is quite real. 

It appears to have something to do with stress concentrations about voids in the matrix (leading to cracks),  and about other voids at the fiber-matrix interfaces (leading to delaminations).  What I suggest here in this article is a seawater infiltration effect at high pressures that greatly compounds the already-existing cumulative damage problem. 

The infiltrated seawater cannot get out fast enough during the ascent,  but as sea pressure reduces,  it expands,  forcing both these types of voids wider,  and making the damage they do  much worse.  This would not be so much of a problem at the “ordinary” depths of submarines,  where the seawater is essentially incompressible.  But at very deep depths,  the seawater actually is compressible,  and you can incur significant damage,  as it expands faster than it can get out of the material,  during even a single ascent. 

To reduce that risk,  you use only materials with no internal porosity.  And that rules out composites!  Which is exactly why I said composites were the wrong material choice,  for the pressure hull of a deep-diving submersible.