Questions Than Answers, Part 2
The loss of buoyancy in the bow and the effect of gravity on the unsupported after third of the stern was creating a massive bending moment around Titanic's new centre of buoyancy. At around 2:18 a.m., the stresses concentrated on the upper starboard-side shell plating exceeded the yield strength of the steel. A crack began underneath the expansion joint and propagated rapidly down the side of the ship, moving from opening to opening (portholes, shell doors, etc.) in the shell plating. When the crack reached the turn of the bilge and the more robust double-bottom structure, it continued along the path of least resistance by changing its direction of propagation from the vertical to one paralleling the tank top. The separation of bow and stern would quickly spread across the top decks to the port side, where a crack (or cracks) began at the top of the structure and then propagated downward, much like on the starboard side.
Seconds later, the hull began to fracture in earnest as bending stresses were relieved by the break. The double-bottom structure was separated from the upper works in a "greenstick fracture" effect, meaning that as the hull structure fractured above, the stout double-bottom structure resisted the bending forces and instead pulled away from the weaker structure as a more intact piece. As the bow and stern sections tore apart deck by deck, the stern was allowed to slowly settle into a more nominal (horizontal) position. Jim likens the tearing of the hull to a "party popper," where the middle section of the ship was caught between two opposing sides...some of the intervening decks pulled forward with the bow, some aft with the stern. The steady settling of the bow would tend to arrest the hull's rotation to port prior to the break, but as the bow tore free, the unsupported stern took on a more pronounced roll to port, throwing people on the after well deck against the port rail. This twisting motion caused an approximate seventy-foot section of the double-bottom to tear transversely from starboard to port.
Jim's interior exploration of the bow section in 2001 and 2005 revealed that the lower decks up to and including C Deck flooded slowly. Beds and washstands still sit in their original positions with little evident damage. A and B Decks, on the other hand, are chaotic, indicating damage wrought by explosive hydrodynamic effects. Short of a comprehensive flooding survey, this is our best indication of how much buoyancy remained in the forward superstructure at the time of the break. When stresses were relieved by the break, the after, more buoyant, end of the bow section would have risen temporarily (not readily apparent in the illustration, as this effect was realised after the rendering was done). The same could be said for the forward end of the stern. This would have accelerated both the stem's downward arcing and the separation of the seventy-foot section of double-bottom structure.
The increasing separation between bow and stern would pull the upper decks apart (due to time and scope restrictions, I did not have time to render the shredding of the middle section; rather, I broke the section up in major chunks to act as placeholders for later animation), leaving the still-intact double-bottom structure as the only consistent link between the two sections. Because of massive flooding into the area of the break, the hull's centre of buoyancy rapidly moved aft and up, away from the separating double-bottom section, sparing it from extreme compressive effects as the upper structure above tore itself apart from tension.
In fact, though this section was originally compressed between frames 25 and 26 (at the point where the double-bottom floors begin to rise in height to form the engine bedplates), the downward arcing movement of the flooded bow and the initial upward movement of the buoyant stern would actually pull it back into tension. A combination of both compressive and tensile stresses is seen in the double-bottom sections, possibly corroborating this theory.
Approximately 2:19 a.m. Much of the tearing apart of the middle section, even the area of the break itself, was just below the surface of the water as the hull continued to settle. Because the break was so low in the water, it may have gone unnoticed by some observers, depending on their individual vantage points. If one were to take Lightoller's vantage point — in the water at some point above the submerged Officers' Quarters, looking aft and down the length of the ship, as shown in the stem-on view — one can understand why he insisted during the enquiries that the ship sank intact. He honestly couldn't see the break.