Brittle Fracture and Impact Testing: Part One

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

1. a triaxial state of stress,
2. a low temperature, and
3. 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 (K_Id).

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 T_m, while in ceramics the transition occurs at about 0.5 to 0.7 T_m.

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).