4A6(1) Sructures : Project
LIBERTY SHIPS FAILURES –
BRITTLE FRACTURE
Group 23:
Maxime Gonzalez
Sylvain Ruggis
Stéphanie Luquet
Neil Bonner
Introduction
This report concerns the failure of many of the Liberty Ships during the Second World
War. The cause of failure, brittle fracture, is explained. The lessons learned from these
disasters are explained and also suggestions are given on how to prevent brittle fracture.
The Liberty Ships
Liberty Ships were lightly armed cargo vessels built in the US for transporting
desperately needed supplies across the U-boat infested Atlantic to a beleaguered Europe
in World War II. These ships were all-welded, measured 442 feet long, and they could
carry 10,000 tons of cargo at eleven knots (12 mph). Some 2700 vessels were built from
1942 until the end of the war. Such huge numbers were possible only through
prefabricated all-welded construction to a standard design, together with a massive
investment of capital, materials and workers.
Thirty thousand workers, largely inexperienced, came to the shipyard to build
these ships. Eventually 3,700 women were employed as tackers and burners,
machine workers, pipe coverers, spray
painters, and crane operators.
At the start of the program some 30% of
Liberty Ships suffered catastrophic
fracture.
The Brittle – Fracture Problem
During World War II a great deal of
attention was directed to the brittle
failure of welded Liberty ships and T-2
tankers. Some of the ships broke completely in two, while, in other instances, the fracture
did not completely disable the ship. Most of the failures occurred during the winter
months. Failures occurred both when the ships were in heavy seas and when they were
anchored at dock. These calamities focused attention on the fact that normally ductile
mild steel can become brittle under certain conditions.
The following factors contribute to a brittle type of fracture:
1.
2.
3.
4.
A triaxial state of stress,
A low temperature,
A high strain rate or rapid rate of loading.
The effect of stress concentrations such as notches and cracks
All of these factors do not have to be present at the same time to produce brittle fracture.
A triaxial state of stress, such as exists at a notch, and low temperature are responsible for
most service failures of the brittle type. However, since these effects are accentuated at a
high rate of loading, many types of impact tests have been used to determine the
susceptibility of materials to brittle behavior. Steels which have identical properties when
tested in tension or torsion at slow strain rates can show pronounced differences in their
tendency for brittle fracture when tested in a notched-impact test.
Since the ship failures occurred primarily in structures of welded construction, it was
considered for a time that this method of fabrication was not suitable for service where
brittle fracture might be encountered. The design of a welded structure is more critical
than the design of an equivalent riveted structure, and much effort has gone into the
development of safe designs for welded structures. It is important to eliminate all stress
raisers and to avoid making the structure too rigid. To this end, riveted sections, known as
crack arresters, were incorporated in some of the wartime ships so that, if a brittle failure
did occur, it would not propagate completely through the structure.
Ductile-to-brittle transition
Notched-bar Impact Tests are methods for evaluating the relative toughness of
engineering materials. They measure the energy absorbed by the high strain rate fracture
of a standard notched specimen, and can be used as an economical quality control method
to assess the notch sensitivity and impact toughness of engineering materials.
Charpy Test and the Izod Test exist as 'standard' notched-bar tests. Variables including
the size and shape of the specimen and notch, the orientation at which the specimen is
impacted, and the range of speeds at which the bar is impacted are specified so that test
results will always be comparable.
The notched-bar impact test can be used to determine whether or not a material
experiences a ductile-to-brittle transition as the temperature is decreased. In such a
transition, at higher temperatures the impact energy is relatively large since the fracture is
ductile. As the temperature is lowered, the impact energy drops over a narrow
temperature range as the fracture becomes more brittle.
If a material experiences a ductile-to-brittle transition, the temperature at which it occurs
can be affected by the variables mentioned earlier, namely the strain rate, the size and
shape of the specimen and the relative dimensions of the notch.
Many of the Liberty Ships failures occurred at low temperatures, i.e. in North Atlantic
waters, which shows that the temperature was a very important factor in their failure.
Preventing Brittle Fracture
Brittle fracture can be prevented by eliminating the critical factors which produce it. The
primary factors are the material used in construction, the methods used in construction,
and the design of the structure. The material must be chosen to have an adequate
toughness for the anticipated loading and the lowest operating temperature envisaged.
When welding is used in construction, procedures must be used that will not embrittle the
steel in the vicinity of the weld. The flaws and discontinuities must be checked for and
kept within acceptable specified sizes. The design of the structure must eliminate any
notches, holes or sharp changes in cross section that may elevate stress and encourage
crack initiation and propagation.
Summary/Conclusion
The general technological fraternity was unaware of Fracture Mechanics principles when
these ships were designed, and the reason for the disastrous fractures was a mystery since
conventional safety assessments were unremarkable and the extremely short lives ruled
out conventional fatigue as the culprit. It later became clear that the failures could be
attributed to:
-
The all- welded construction which eliminated crack- arresting plate boundaries
which are present in riveted joints
The presence of crack- like flaws in welded joints performed by inexperienced
operators pressed into service by the exigencies of the program.
The use of materials whose low resistance to crack advance (toughness) was
further reduced by low temperatures.