How The Liberty Ships of WWII Contributed to the Study of Fracture Mechanics

During World War II (WWII), the United States Government planned the construction of 2,751 naval cargo ships. Out of this number, a total of 2,710 ships were completed between 1941 and 1945. Eighteen different shipyards throughout the United States of America were used for fabrication/construction. Many of these ships, especially those nicknamed “Liberty Ships”, were extremely important to the development and understanding of fracture mechanics because of the materials that were used in their construction and the disastrous consequences due to the material properties in cold water. Many lessons were learned from these failures including, but not limited to, the need for proper selection of materials and the necessity of testing of these materials to determine their chemical and physical properties. [1]

Liberty Ships were based on a British design that was adapted by the US. They were 441.5 feet long, could travel at speeds up to 13.2 miles per hour, had a range of 23,000 miles, and had a capacity of 10,685 long tons. Many of these ships were lend-leased to Britain and the Soviet Union to help replace the vessels that were sunk by the German U-boats. These ships contained a stern-mounted 4 inch deck gun for use against submarines and had a variety of anti-aircraft guns as well. [1]

The US chose this design, over others, because of its relatively simplistic design and the low cost of construction. They were designed to have a five (5) year service life but many lasted much longer.  These ships were designed for batttle not beauty. Franklin D, Roosevelt referred to these ships as “a dreadful looking object” and were referred to as “ugly ducklings” by Time magazine. Today only a handful of Liberty Ships remain, two of which are used as museums.[1]

Little was known about fracture mechanics during WWII. Liberty Ships were welded together from modular sections that were constructed first and the constructed in an assembly line fashion. In the beginning of the war, the ships took as long as 230 days to construct. By the end of the war the average time for construction was only 42 days and one ship was built in as little as 5 days. In 1943, as many as 3 ships were constructed per day. Many of these ships were named after famous Americans. Below are pictures showing the different phases of construction of a Liberty Ship at the Bethlehem-Fairfield Shipyard in Baltimore, Maryland. [1]

Day 2 : Laying of the keel plates

Day 6 : Bulkheads and girders below the second deck are in place

Day 10 : Lower deck being completed and the upper deck amid ship erected

Day 14 : Upper deck erected and mast houses and the after-deck house in place

Day 24 : Ship ready for launching

The US was attempting to build Liberty Ships at a faster rate than the German U-boats could destroy them. This would ensure that when the German U-boats attacked a US or British ship, there would be a ship ready to replace it. [4]

Hull and deck cracks were reported to be common on many of the early Liberty Ships and it was estimated that there were around 1,500 brittle fractures on navy ships. 12 Liberty Ships broke in half with no warning. 3 of the ships which broke in half were Liberty Ships including the SS John P. Gaines in which 10 lives were tragically lost. The SS Schenectady, a T2 Tanker, is probably the most well known of the Liberty Ships due to its sudden and catastrophic failure. [1]

The SS Schenectady successfully completed its sea trials in January 1943. It returned to the harbor in still weather and almost freezing 39° F water when... "Without warning and with a report which was heard for at least a mile, the deck and sides of the vessel fractured just aft of the bridge superstructure. The fracture extended almost instantaneously to the turn of the bilge port and starboard. The deck side shell, longitudinal bulkhead and bottom girders fractured. Only the bottom plating held. The vessel jack-knifed and the center portion rose so that no water entered. The bow and stern settled into the silt of the river bottom." [2] [5]

The responsibility for the safety of merchant vessels fell to the US Coast Guard (USCG). A board of Investigation into the construction, design, and fracture of welded ships was requested by the USCG and was established in April 1943. The Board of Investigation published the final Report in 1946 which concluded that 970 of the 4694 merchant ships welded during the emergency ship building program sustained fractures due to the presence of notches in the steels which were sensitive at lower operating temperatures. It was also concluded that the current specifications in use were “not sufficiently selective to exclude” these types of steels. [5]

The Charpy V-notch test, which measures the absorbed energy, was used to examine the properties of failed ships’ steel plates. This test was one of the few standardized fracture tests available at the time. The results of these tests were found to correlate well with “the observed crack initiation, propagation and arrest behavior of the ship steels”. The 15 foot-pound or 20 Joule Charpy transition temperature began to be used “as a reference as it appeared to define the highest temperature at which brittle fracture initiation would occur in ship quality steels” in the early 1950s. [5]

Continued research demonstrated that the “critical temperature for brittle fracture initiation corresponded to higher Charpy energy values when modifications to alloying elements, grain size, deoxidation methods and normalizing heat treatments were made. Hence the approach to brittle fracture avoidance could not be based on a simple fixed reference Charpy energy level”. [5]

SS Schenectady

Originally, engineers thought faulty welds were the cause of the failures.  However, after nearly 5 decade of research and investigation, the USCG engineers were finally able to determine that poor selection of steels, due to lack of knowledge of fracture mechanics, which wasn’t well understood in the 1940s, was the real cause and now say “it was a metallurgy problem” [3]

The ships’ hulls were butt-jointed and welded together, and the welds were sound. Unfortunately, “the pouring, rolling, and shaping processes used to make the steel left an undesirable chemistry. Sulfur content was too high; manganese was too low”. That undesirable chemistry ultimately weakened the strength of the steels used on Liberty Ships. The materials grew brittle in cold temperatures and caused the annealed areas adjacent to the welds to become weaker and caused the welds to crack. This is now known as a brittle fracture. [3]

In the case of the SS Schenectady, the brittle fracture cracks rapidly propagated down both the port and starboard sides of the hull, all the way to the ship’s keel which also fractured. The ship’s mid-section turned skyward, while the ship’s ends plunged deeper into the water. [3]

Brittle fracture is different than other structural steel failures in that the material behaves elastically right up until the point of fracture. This is a grave concern for engineers because the material, when tested for tensile strength, does not reach the point on a stress-strain curve where it would permanently deform. Rather, it approaches the limit of its elasticity and catastrophically fails without notice. [3]

Even without the benefit of this modern knowledge of fracture mechanics, the builder of the SS Schenectady, Kaiser Shipyards, resolved the problem and went on to build an estimated 500 additional ships for the war. None of those ships were reported to have a brittle fracture incident. [3]

The US Government repaired the SS Schenectady in four months for and estimated cost of $2,700,000. "In those months the SS Schenectady could have been helping America win the war". The ship was recommissioned in April of 1943. The ship was used until 1962 when it was eventually scrapped.  The US Government learned a valuable lesson out of these brittle fracture failures, of which the SS Schenectady was the worst. To prevent these fractures from happening again, the government slowed down future production and used the lessons learned for material selection and fracture mechanics. [4]

In conclusion, the brittle fracture failure of many ships, including the SS Schenectady, contributed significantly to the study of fracture mechanics. Because of these hard-learned lessons, the US Government now specifies the chemical and physical properties, such as Sulphur content and material toughness respectively, for materials used on all ships. By mandating the proper selection of materials for modern military ships, the US has successfully prevented the recurrence of the disastrous brittle fractures that were seen on ships like the SS Schenectady during WWII. [5]

References:

1. https://en.wikipedia.org/wiki/Liberty_ship
2. https://www-mdp.eng.cam.ac.uk/web/library/enginfo/textbooks_dvd_only/DAN/fracture/maritime/maritime.html
3. https://www.designnews.com/author.asp?doc_id=276818
4. https://prezi.com/nlddcqblzno4/ss-schenectady/
5. https://www.twi-global.com/news-events/case-studies/schenectady-t2-tanker-165/