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 these ships broke completely in two, while, in other instances,
the fracture did not completely disable the ship. Most of the failure 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.
A broad research program was undertaken
to find the causes of these failures and to prescribe the remedies for their
future prevention. In addition to research designed to find answers to a
pressing problem, other research was aimed at gaining a better understanding of
the mechanism of brittle fracture and fracture in general. While the brittle
failure of ships concentrated great attention to brittle failure in mild steel,
it is important to understand that this is not the only application where
brittle fracture is a problem. Brittle failures in tanks, pressure vessels,
pipelines, and bridges have been documented as far back as the year 1886.
Three basic factors contribute to a
brittle-cleavage type of fracture. They are
- a triaxial state of stress,
- a low temperature, and
- a high strain rate or rapid rate of
loading.
All three 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. A great deal of research has since demonstrated
that welding, per se, is not inferior in this respect to other types of
construction. However, strict quality control is needed to prevent weld defects
which can act as stress raisers or notches. New electrodes have been developed
that make it possible to make a weld with better properties than the mild-steel
plate.
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.
Notched-bar Impact Tests
Various types of notched-bar impact
tests are used to determine the tendency of a material to behave in a brittle
manner. This type of test will detect differences between materials which arc
not observable in a tension test. The results obtained from notched-bar tests
are not readily expressed in terms of design requirements, since it is not
possible to measure the components of the triaxial stress condition at the
notch. Furthermore, there is no general agreement on the interpretation or
significance of results obtained with this type of test.
A large number of notched-bar test
specimens of different design have been used by investigators of the brittle
fracture of metals. Two classes of specimens have been standardized for
notched-impact testing. Charpy bar specimens are used most commonly in the
United States, while the Izod specimen is favored in Great Britain.
The Charpy specimen has a square cross
section (10x10 mm) and contains a 45° V notch, 2 mm deep with a 0.25 mm root
radius. The specimen is supported as a beam in a horizontal position and loaded
behind the notch by the impact of a heavy swinging pendulum. The specimen is
forced to bend and fracture at a high strain rate on the order of 103
s-1. The Izod specimen, which is used rarely today, has either a
circular or square cross section and contains a V notch near the clamped end.
The principal measurement from the
impact test is the energy absorbed in fracturing the specimen. After breaking
the test bar, the pendulum rebounds to a height which decreases as the energy
absorbed in fracture increases. The energy absorbed in fracture, usually
expressed in joules, is rending directly from a calibrated dial on the impact
tester.
The notched-bar impact test is most
meaningful when conducted over a range of temperatures so that the temperature
at which the ductile-to-brittle transition takes place can be determined.
The principal advantage of the Charpy
V-notch impact test is that it is a relatively simple test that utilizes a
relatively cheap, small test specimen. Tests can readily be carried out over a
range of subambient temperatures. Moreover, the design of the test specimen is
well suited for measuring differences in notch toughness in low-strength
materials such as structural steels. The test is used for comparing the
influence of alloy studies and heat treatment on notch toughness. It frequently
is used for quality control and material acceptance purposes.
Instrumented Charpy Test
The ordinary Charpy test measures the
total energy absorbed in fracturing the specimen. Additional information can be
obtained if the impact tester is instrumented to provide a load-line history of
the specimen during each test. With this kind of record it is possible to
determine the energy required for initialing fracture and the energy required
for propagating fracture. It also yields information on the load for general
yielding, the maximum load, and the fracture, load.
Because the root of the notch in a
Charpy specimen is not as sharp as is used in fracture mechanics tests, there
has been a trend toward using standard Charpy specimens which arc precracked by
the introduction of a fatigue crack at the tip of the V notch. These precracked
specimens have been used in the instrumented Charpy test to measure dynamic
fracture toughness values (KId).
Significance of Transition-Temperature Curve
The chief engineering use of the Charpy
test is in selecting materials which are resistant to brittle fracture by means
of transition-temperature curves. The design philosophy is to select a material
which has sufficient notch toughness when subjected to severe service conditions
so that the load-carrying ability of the structural member can be calculated by
standard strength of materials methods without considering the fracture
properties of the material or stress concentration effects of cracks or flaws.
The transition-temperature behavior of a
wide spectrum of materials falls into the three categories. Medium- and
low-strength fcc metals and most hep metals have such high notch toughness that
brittle fracture is not a problem unless there is some special reactive chemical
environment. High-strength materials (s0 >
E/150) have such low notch toughness that brittle fracture can occur at nominal
stresses in the elastic range at all temperatures and strain rates when flaws
ace present.
High-strength steel, aluminum and
titanium alloys fall into this category. At low temperature fracture occurs by
brittle cleavage, while at higher temperatures fracture occurs by low-energy
rupture. It is under these conditions that fracture mechanics analysis is useful
and appropriate.
The notch toughness of low- and
medium-strength bcc metals, as well as Be, Zn, and ceramic
materials is strongly dependent on temperature. At low temperature the fracture
occurs by cleavage while at high temperature the fracture occurs by ductile
rupture. Thus, there is a transition from notch brittle to notch tough behavior
with increasing temperature. In metals this transition occurs at 0.1 to 0.2 of
the absolute melting temperature Tm,
while in ceramics the transition occurs at about 0.5 to 0.7 Tm.
A well-defined criterion is to base the
transition temperature on the temperature at which the fracture becomes 100
percent cleavage. This point is known as nil ductility temperature (NDT). The
NDT is the temperature at which fracture initiates with essentially no prior
plastic deformation. Below the NDT the probability of ductile fracture is
negligible.
Metallurgical Factors Affecting Transition Temperature
Changes in transition temperature of
over 55°C (100°F) can be produced by changes in the chemical composition or
microstructure of mild steel. The largest changes in transition temperature
result from changes in the amount of carbon and manganese. This transition
temperature is lowered about 5.5°C (10°F) for each increase of 0.1 percent
manganese. Increasing the carbon content also has a pronounced effect on the
maximum energy and the shape of the energy transition-tempera lure curves.
The Mn/C ratio should be at least 3/1
for satisfactory notch toughness. A maximum decrease of about 55°C (100°F) in
transition temperature appears possible by going to higher Mn/C ratios.
Phosphorus also has a strong effect in
raising the transition temperature. The role of nitrogen is difficult to assess
because of its interaction with other elements. It is, however, generally
considered to be detrimental to notch toughness.
Nickel is generally accepted to be
beneficial to notch toughness in amounts up to 2 percent and seems to be
particularly effective in lowering the ductility transition temperature.
Silicon, in amounts over 0.25 percent, appears to raise the transition
temperature. Molybdenum raises the transition almost as rapidly as carbon, while
chromium has little effect.
Notch toughness is particularly
influenced by oxygen. For high-purity iron it was found that oxygen contents
above 0.003 percent produced intergranular fracture and corresponding low energy
absorption.
Grain size has a strong effect on
transition temperature. An increase of one ASTM number in the ferrite grain size
(actually a decrease in grain diameter), results in a decrease in transition
temperature of 16°C (30°F) for mild steel. Decreasing the grain diameter from
ASTM grain size 5 to ASTM grain size 10 can change the 10 ft/lb Charpy V-notch
transition temperature from about 39°C to -33°C (70°F to -60°F).
The energy absorbed in the impact test
of an alloy steel at a given test temperature generally increases with
increasing tempering temperature. However, there is a minimum in the curve in
the general region of 200 to 320°C (400 to 600°F). This has been called 260°C
(500°F) embritilement, but because the temperature at which it occurs depends on
both the composition of the steel and the tempering time, a more appropriate
name is tempered-martensite embrittlement.
Drop-Weight Test and Other Large-Scale Tests
Probably the chief deficiency of the
Charpy impact test is that the small specimen is not always a realistic model of
the actual situation. Not only does the small specimen lead to considerable
scatter, but a specimen with a thickness of 10 mm (0.394 in) cannot provide the
same constraint as would be found in a structure with a much greater thickness.
The most logical approach to this
problem is the development of tests that are capable of handling specimens at
least several inches thick. The development of such tests and their rational
method of analysis has been chiefly the work of Pellini and his coworkers at the
Naval Research Laboratory. The basic need for large specimens resulted from the
inability to produce fracture in small laboratory.
The first development was the
explosion-crack-starter test which featured a short, brittle weld bead deposited
on the surface of a 14x14x1 in steel plate. The plate was placed over a circular
die and dynamically loaded with an explosive charge. The brittle weld bead
introduces a small natural crack in the test plate similar to a weld-defect
crack. Tests are carried out over a range of temperature and the appearance of
the fracture determines the various transition temperatures. Below the NDT the
fracture is a flat fracture running completely to the edges of the test plate.
Above the nil ductility temperature a
plastic bulge forms in the center of the plate, but the fracture is still a flat
elastic fracture out to the plate edge. At still higher temperature the fracture
does not propagate outside of the bulged region. The temperature at which
elastic fracture no longer propagates to the edge of the plate is called the
fracture transition elastic (FTE). The FTE marks the highest temperature of
fracture propagation by purely elastic stresses. At yet higher temperature the
extensive plasticity results in a helmet-type bulge. The temperature above which
this fully ductile tearing occurs is the fracture transition plastic (FTP).