Liberty Ships – Brittle Fracture
October 21, 2011
Liberty Ships – Brittle Fracture
Rebecca Galvin, Ciarán O’Shea Brady, Victor Raucent,
Zhe Su.
Abstract. This report looks into the method by which a series of U.S. cargo ships, the Liberty
Ships, which were used during World War II, failed suddenly. It aims to provide a background
to the events that occurred, an analysis of the reasons for their occurrence and finally
information on the measures taken to resolve the problems.
Trinity College Dublin.
Liberty Ships – Brittle Fracture
October 21, 2011
Background.
As part of the Allies’ war effort during the Second World War the materials,
munitions and supplies were vital to sustaining the battle against the Germans.
A very successful bombing campaign by Germany on allied ships had inflicted
significant damage on the Allies’ war effort. As a result, in 1941, President
Roosevelt announced that $350million would be provided to support a shipbuilding program – the objective being to ‘ Build ships faster than the enemy
could sink them’ In pursuit of the objective the American’s decided to use the
method of welding rather the established conventional method of riveting
believing that this would speed up the process. These ships were called The
Liberty Ships.
In addition to increasing the speed of construction, the use of welding also
decreased construction costs. The numbers of skilled labourers required to carry
out welding on the ship’s hull and deck was significantly less than the numbers
required to build using rivets.
The components for the Liberty Ships were manufactured in various locations
around the U.S. and then transported to the one site for assembly. Also helping to
increase
the
rate
of
construction.
September 17th 1941 saw the launch of the new generation of American cargo
ships and in the subsequent 4 years, 2,751 ships were built in American
shipyards.
Soon after their introduction to water, many of the ships experienced cracking
along the hull and deck.
Three of the Liberty Ships
even broke in half without
any prior warning. The SS
John P. Gaines was one such
case with 10 people loosing
their lives when the ship sank
in November 1943.
These problems were, at first,
attributed to the shipyards
where
construction
had
taken place and the relatively
inexperienced work force
employed. The new welding
techniques also came under scrutiny. Following this, Constance Tipper of
Cambridge University identified and was able to demonstrate that brittle
fracture was in fact the cause of failure.
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Liberty Ships – Brittle Fracture
October 21, 2011
Brittle Fracture
In its most basic terms, brittle fracture is the failure of a material where no
apparent plastic deformation occurs before fracture. Brittle fracture usually
occurs before a material has reached its yield strength. Without even analysing
this form of failure in relation to the Liberty ships, it is clear that brittle fracture
is potentially very dangerous. The sudden nature of this phenomenon provides
no warning that a material is near failure. The Liberty ships are therefore a great
example of how what seems like a small-scale problem can turn into a large-scale
disaster.
There are three factors which contribute to brittle fracture. These are:
 high stress concentrations,
 low temperatures, and
 a high strain rate (rapid rate of loading).
It is not necessary for all three factors to occur at the same time for brittle
fracture to happen.
The low temperatures of the water were a major factor in the failure of the ships
in our case study. Nearly all the failures occurred in the cold waters of the North
Atlantic, whereas the ships stationed in the South Pacific remained intact. This
mystery was later explained by Constance Tipper of Cambridge University. She
demonstrated that there is a critical temperature below which the fracture mode
in steel switches from ductile to brittle. This brittle failure occures in the linearelastic region, without any visible elastic deformation, and while absorbing very
little energy.
The ships stationed in the North Atlantic were then susceptible to brittle fracture
as the water temperature was below this critical temperature. The Charpy test
evaluates the relative toughness of steel by measuring the energy absorbed by
the material when it is struck and fractured. It 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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Liberty Ships – Brittle Fracture
October 21, 2011
The welding techniques used in the ship production also caused controversy.
There were issues with inexperienced labourers who had been drafted in to
increase ship production for the WWII effort. It was likely that unskilled welding
caused micro-cracks to be formed within the weld itself. It could also have
caused areas of high stress concentrations, which would have led to brittle
fracture. Once this problem was identified, brittle fracture could be prevented
by heating the steel before welding, and then allowing the weld to cool slowly.
At the time of the Liberty ships disasters, brittle fracture was an unknown
phenomenon. It was not until the British-built Empire Duke was used for testing
purposes that the reasons for failure started to become more clear. Constance
Tipper established the ships were not failing due to the fact that they were
welded rather than riveted. The brittle fracture was actually being caused
because the grade of steel used failed due to embrittlement. However, this issue
was exacerbated by the welding, as they allowed the cracks to run unimpeded
along the hull of the ships. If the hulls had been riveted rather than welded, the
impact of these fractures would have been reduced. Probably the most common
type of crack was one that began at the square corner of a hatch which coincided
with a welded seam. In this case, both the corner and the weld acted as stress
concentrators. This stress concentration caused localised areas of high stress,
which contributed to the brittle fracture of the ships. One contingency measure
for these cracks was to rivet steel arrester plates to areas of high stress
concentration. The Victory ship was an upgrade in ship design which used these
arrester plates to maintain a less stiff and stronger ship design that was better
able to deal with fatigue.
Summary
The Liberty Ships were not designed as normal ships. The Liberty Ships had an
all-welded hull, whereas the traditional ships where made with riveted joints. Of
2751 ships, 400 suffered fractures, 90 were reported serious. The reason for the
disastrous fractures on liberty ships was a mystery at first. This was due to the
fact that fracture mechanics principals were not documented properly at that
time. However many years later it became clear that the reasons for the failures
were:
- The material used did not have sufficient toughness especially at low
temperatures.
- The standard of the weld joints in general were poor due to the
inexperienced welders, this meant that micro cracks were present.
- The all-welded construction which eliminated crack-arresting plate
boundaries which are present in riveted joints.
For the main reasons, which lead to above three problems, solutions fell into
three categories:
- Improvements to shipyard practice,
- Retrofits of the completed ships,
- Changes to design.
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