Group 23:
AMY O’CONNELL
RONAN DYLAN MURPHY
NIALL MINIHAN
BEN TOOMEY

Many of the Liberty Ships constructed by the US during World War II failed primarily due to the brittle fracture of the steel. This report will introduce the Liberty
Ships, explore the reasons for failure, describe the brittle fracture problem, show developments in the field and show how failures could have been avoided in the
Liberty Ships.
During World War II the U.S. Government supplied the UK with around 2,700 urgently needed cargo ships christened ‘The Liberty Ships’. They measured 442 feet in length and could carry 10,000 tonnes of cargo at eleven knots carrying much needed supplies across the Atlantic. With German U-Boats destroying cargo ships at allegedly three times the rate of production, the need for rapidly constructed ships was essential. A US construction engineer Kaiser developed a new and rapid method of ship construction by welding steel plates as opposed to the traditional method of riveting. A huge workforce was required for the massive production and altogether up to thirty thousand inexperienced workers came to the shipyard to build the Liberty
Ships.
Of the 2700 cargo ships built using this method, approximately one in seven sustained fractures with 90 serious conditions, 20 fractures and 15 completely broken in two.
Brittle fracture was an unknown phenomenon at the time. Whilst cracks in steel plates were commonplace, they would not have been recognised as brittle cracks and the riveted structures traditionally used, acted as natural crack-arresting boundaries. The all-welded hulls allowed paths of continuous crack propagation through the welded joints. This meant that a small crack could traverse the entire hull and result in almost instantaneous breaking of a ship in two.
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The drastic failure rates of the Liberty Ships left the US Government baffled. Initially the disappearance of ships in the North Atlantic was blamed on German Navy torpedo attacks. However the huge failure statistics of the North Atlantic were literally nonexistent in the warm waters of the South
Pacific. It was not until 1947 when a ship broke in two whilst docked in the cold waters at Boston Harbour that the serious situation could be properly analysed with the ship pieces intact and salvageable.
Failures occurred mostly in winter months and cold waters. These events opened a new field of study, that of brittle-fracture.
There are three factors which contribute to brittle fracture; high stress concentrations, low temperatures and a high strain rate (rapid rate of loading). These do not need to occur all at the same time.
In 1963 two researchers in the U.S. Naval Research laboratory published the first authoritative document on the phenomenon of ductile to brittle crack propagation in steel. They devised a method of quantifying the temperature at which steel changes from ductile to brittle fracture modes named the Nil Ductility Transition. A certain piece of steel can be loaded to its ultimate tensile strength and will stretch 20-40% before it breaks if the temperature of the steel is above the Nil Ductility Transition
(NDT) temperature. Below the NDT temperature when a piece of steel is loaded to only the yield strength (approximately ½ of the ultimate strength) the piece of steel will crack in a brittle manner, like glass. Once the crack starts to run, it will only stop when it runs out of steel.
In the case of the ship in Boston Harbour, the stresses reached the yield strength at a location where a tiny crack existed on the deck of the ship in 2°C weather. The NDT of steel used to build the ship was determined after the failure to be 10°C. Because of that, the crack propagated in a brittle manner and literally ran around the ship near simultaneously and the ship broke in two.
Nowadays the brittle fracture risk of a material is tested using impact tests. Notchedbar tests evaluate the relative toughness of steel by measuring the energy absorbed by the material when it is struck and fractured. The two main types of test-bar used are the Charpy and Izod specimens. They have specific dimensions and have a V notch cut into them. The Charpy test involves striking the bar behind the V notch with varying loads/rates of strain. The results will determine the ductile-to-brittle transition when plotted on a graph. An example of such a graph is shown below.
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As was demonstrated in the Liberty Ships, and is shown in the graph above, in cold temperatures steel only requires low impact energy to cause brittle failure.
WELDING Unskilled welding can lead to micro-cracks being formed within the weld and/or creates areas of high stress concentration prone to cracking. If the steel is heated before welding occurs this allows the weld to cool slowly and can prevent this problem. Cracks due to welding on the Liberty Ships were due to inexperienced labour. The main problem with welding is that it allows a crack to propagate across the ship completely. Once the problem was identified, US designers riveted higher toughness steel arrester plates to points of concern.
SQUARE HATCH – The need for a quick and simple design resulted in square hatches. This caused huge stress concentrations at the corners. When this problem was identified curved reinforcing strips were welded to each corner of the hatches.
COLD ATLANTIC WATERS – It was the mixture of high stress concentrations, micro cracks and steel below its NDT that resulted in the massive failures.
Once the design changes were put in place the fracture rate reduced to 5%.
There are many factors which contribute to brittle fracturing. The three most important ones are the materials used (whether or not they are suitable), how the structure is manufactured (if the correct methods of construction are used) and the design of the structure itself (does the design amplify the already large stresses).
The materials used should not only have sufficient toughness for the expected loading but must also be able to perform at the lowest operating temperature envisaged.
Very strict methodology should be employed during the welding that will ensure that the steel in the vicinity of the weld will not be embrittled. Also the flaws and
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discontinuities must be within acceptable specified sizes. (Experienced welders should be used). Or alternatively employ rivet joints as oppose to welding, as this will ensure crack- arresting plate boundaries are present.
The designers of the structure must ensure that any notches, holes or sudden changes in cross section must be minimized as much to ensure that the stresses are not amplified.
At the time of construction of the liberty ships the engineers and designers were puzzled as to why the ships were failing like they were. This was due to the fact that fracture mechanics principals were not documented properly yet.
However many years later it became clear that the reasons for the failures were:
1) The material used did not have sufficient toughness especially at low temperatures.
2) The fact that the structure was completely welded as oppose to riveted meant that crack arresting plate boundaries were not present.
3) The standard of the weld joints in general were poor due to the inexperienced welders, this meant that micro cracks were present.
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