Brittle Fracture of Ship Side Shell

Brittle Fracture 

At the beginning of  World War II, the US Maritime Administration created two ship designs for emergency mass production. The Liberty Ship cargo vessel and the T2 tanker for liquid petroleum products. About 2710 “Liberty Ships” were produced between 1941 and 1945 at about 18 shipyards across the United States. The average time to produce a Liberty ship was 42 days. The record shortest time for producing was less than 5 days. The ships were 440 feet long, 56 feet wide, used steam boilers and reciprocating engines to produce a speed of 11 knots.

 About 533 T2 tankers were built between 1940 and 1945. There were some variations in the design of T2 Tankers but the standard design was 501 feet long, 68 foot beam used a steam turbine and single shaft for propulsion and was capable of 16 knots.  

Liberty ships and T2 tankers, made a significant contribution to transporting war materials. For both ships, the mass production standard used welded construction methods and assembly of prefabricated sections to achieve rapid construction times. About 1031 of early Liberty ships produced, failed due to cold weather induced brittle fracture of the ship’s side shell. (Statistics vary but about 30%.) More than 200 Liberty ships sank or were damaged beyond repair some by breaking in half. A similar experience was shared with T2 tankers.

T2 tanker USS Saugatuck (AO75)

Liberty ship in her element, at sea.

Failure analysis identified:

     Steel used in ship construction, below a critical temperature is subject to transition from elastic deformation (ductile) behavior to brittle behavior. (see internet “Nil Ductility Transition Temperature.)

     Brittle fracture cracks propagating in welded structures can propagate rapidly across large distances.

     Sharp corners in structures may concentrate stress and contribute to failure.

     Impurities in the metal can contribute to brittle fracture.

     Brittle fracture can occur at stress levels well below metal yield point.

In 1952 a Coast Guard investigation board stated high sulfur content made the steel brittle. 

T2 tanker Schenectady broke in two at its dock shortly after launching due to metallurgical problems.

Definitions (properties of steel)

     Stress – Force applied to compress or pull apart (tension) a steel structure.

     Elastic deformation – Elongation of metal when a tensile stress is applied below yield point. Metal will return to its original dimension when force is removed.

     Ductility – measurement of materials ability to undergo deformation when stressed.

     Plastic deformation – When stressed past yield point, metal  will not return to its original dimension once force is removed. There is some permanent deformation.

     Yield point – transition between elastic deformation and plastic deformation.

     Strain – measurement of how much a metal deforms when a stress is applied.

     Brittle failure – rapid crack propagation in stressed metal with little ductility. The stress level may be below yield point and may result from micro-structure stress of a weld.

 

Brittle failure can occur in any large continuous box like structures and is not restricted to ships (see internet Great Molasses Flood of 1919).

Ships are designed, constructed and operated in accordance with industry standards. Standards identify and correct problems that have been revealed by prior ships. Using industry standards is required for insurance coverage. Standards are produced by engineering societies, government and other industry groups such as the American Bureau of Shipping. Standards for structures limit stress to a fraction of yield. Even if there is an unexpected stress greater than what is anticipated, there is a margin to safety before yield point. The structure may deform slightly but will return to its original dimensions. Unfortunately the steel selection standards applied to early Liberty ship steel failed to require proper cold temperature transition properties.  

The ship “John Brown” is one of two known survivor Liberty ships. It is currently a museum ship located in Baltimore MD.

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