9-30-08

September 30, 2008

Dick:

Sorry to be so tardy getting back to you, my job has kept me going at a fast pace.

Following is my understanding of the scenario you described relative to the separation of bottom shell plates from the center vertical keelson (CVK) and adjacent vertical keelsons (VK) on the Edmund Fitzgerald. Also, I believe you indicated that this separation was a recurring problem.

Separation of the bottom shell plates from the VK caused the shell plates to bow downward in a concave manner away from the VK. Shipyard workers pulled the bottom shell plates up to the bottom of the VK and re-welded the shell plates in place. When the pulling/lifting apparatus was removed, the welds fractured and the bottom shell plates again separated from the VK.

I might have an idea why the welds parted once the pulling/lifting apparatus was removed after re-welding. The idea centers around two concepts called Yield Point and Modulus of Elasticity. These concepts are best understood using a cap screw (bolt) as an analogy. When the hex nut is tightened on a bolt clamping parts together, the bolt is placed in tension and is being elongated or stretched. The bolt will elongate and return to its original length and strength any number of times as long as it is not over tightened reaching the Yield Point. If the Yield Point is reached just one time, the bolt will never again return to its original length and strength, but will only continue to elongate or stretch.

The Yield Point concept is relative to the Modulus of Elasticity of the material from which the bolt is made. Simply stated, metals (in this case steel) will stretch, but only so far, and will eventually loose their elastic properties and just continue to stretch (increase in length) until the material pulls apart.

When the bottom shell plates separated from the VK, the plates were no longer restrained by attachment to the VK. It might be possible that the separated portion of the shell plates were occasionally placed in tension with enough force from torsional inputs acting about the ships longitudinal axis to reach the Yield Point and exceed the Modulus of Elasticity of the material from which the plates were made. If so, the separated portion of the shell plates would elongate and "grow" by very small amounts with each tension force input of sufficient magnitude causing the shell plates to bow down even more. In other words, the separated portion of the shell plates were getting longer.

When the shipyard workers pulled the bottom shell plates up to the bottom of the VK, they may have been working with a plate that was actually ever so slightly longer than the plate was originally. If so, trying to re-attach the now longer plate into the same position into which a shorter plate had been originally attached, would be akin to placing two pounds of sand in a one pound can.

Pulling with enough force to bring the now longer plates up to the bottom of the VK would place unbelievable compression stresses into the bottom plates perhaps even resulting in small amounts of buckling. Once the pulling/lifting apparatus was removed, all of the above stresses would be transferred directly into the welds. If the tensile strength of the welding electrodes used was less than the stresses pulling the shell plates away from the VK, the welds would part immediately. This scenario would make it almost impossible to successfully re-weld the bottom shell plates to the bottom of the VK.

Keep in mind once the Yield Point is reached, the tensile forces required to continue elongation are much less than the forces required to reach the Yield Point initially. As a result, once the Modus of Elasticity was exceeded, lesser tensile forces would be required to elongate the shell plates the next time and every time thereafter. If this was the case, as the bottom shell plates continued to elongate by small amounts, the continuing elongation placed greater and greater stresses on the welds of the next VK attached to the bottom shell plates. As elongation continued and the stresses increased, it might have been possible that the next attached VK welds would part and the whole grim affair continued to play out again and again resulting in catastrophic failure.

Just some thoughts, let me know what you think.

Sincerely,

Roger D. Stone

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